Thi thử VSTEP 10 reading test 2 part 2

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 3

It’s 1:45 a.m, and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and focused, he’s had two cups of coffee in the last three hours and is now downing a popular energy drink – one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy – and sometimes their efficiency – to one of humankind’s oldest stimulants: caffeine. [A]

The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world’s most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but also in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine: afternoon tea in the U.K., the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that in many cultures marks the start of the day.

Caffeine is present in many of the foods or drinks we consume, but is it really good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. “Without adequate sleep – the typical eight hours – the human body will not function at its best, physically, mentally, or emotionally.” Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.

Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases. [B]
A number of scientists, including Roland Griffiths, a professor at the Johns Hopkins School of Medicine in the U.S believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors: Their moods fluctuate from high to low; they get mild to severe headaches; or they feel tired or sad when they can’t have a coffee, a soft drink, an energy drink, or a cup of tea. To minimize or stop these feelings, users must consume caffeine – a behavior Griffiths says is characteristic of drug addiction.

Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed moderately – for example, having one or two small cups of coffee (about 300 milligrams of caffeine) per day. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one’s mood. Research has also shown that some caffeinated drinks – specifically certain teas – have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer. [C]

Moreover, despite its nearly universal use, caffeine has rarely been abused. “With caffeine, overuse tends to stop itself,” says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, “you get uncomfortable, and you don’t want to continue.” [D]

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.

PASSAGE 4

The human body contains muscle, a soft tissue made of protein filaments. The filaments move and flex in different ways to make the muscles contract and expand, supporting the body and producing motion. There are two primary types of muscle – skeletal (“voluntary”) muscle and smooth (“involuntary”) muscle. While the differences between skeletal muscle and smooth muscle are taught in elementary schools around the world, few of us were taught the differences between the two types of skeletal muscles: slow-twitch muscles and fast-twitch muscles. Slow-twitch and fast-twitch muscles are named for the relative speeds at which they twitch, or contract. The two types of muscle have different structures, compositions, actions, and functions and work together to keep our bodies in motion.

Slow-twitch muscles, officially called Type I muscles, are the muscles that provide endurance. They conduct oxygen and contain blood capillaries that give them a red color, so they are sometimes referred to as "red muscles." Slow-twitch muscles do not contract with significant force, but this is offset by the fact that they can contract for longer periods of time. Slow-twitch muscles process fats and carbohydrates to use as fuel for aerobic activity, carrying oxygen for endurance activities, such as long-distance running and cycling.
Fast-twitch muscles, officially called Type II muscles, are the muscles that provide strength and speed. There are two subtypes of fast-twitch muscles. Type IIa muscles are called intermediate fast-twitch muscles, and perform both aerobic and anaerobic functions equally. Type IIb muscles are what are traditionally thought of as fast-twitch muscles. Fast-twitch muscles do not conduct oxygen, and are not red in color but white. They perform anaerobic activity, not aerobic, but contract quickly and forcefully. Fast-twitch muscles are the muscles that build through weightlifting and other use because of the strength of contraction, and are useful in activities requiring speed and force, such as sprinting.

In general, the muscle fibers in our skeletal muscles are divided evenly between slow-twitch and fast-twitch muscles, but each individual’s specific mix of types of muscles is determined genetically. It may be true that we are “born runners,” or weightlifters, or swimmers because of our skeletal and muscle structures. However, humans have the ability to change our bodies and prevail over genetics by altering which form of muscle is prominent in our bodies. The more we work one of the two types, the more dominant that type will become. This can be seen in professional and elite athletes, who have muscle structures that reflect the different demands of their particular sports. For example, sprinters’ bodies and long-distance runners’ bodies reflect the fact that sprinters use fast-twitch muscles more predominately and long-distance runners use slow-twitch muscles more predominately. This change does not happen overnight, however, and is the result of years of specific and intentional training.

This is not to say that elite athletes only focus on one type of muscle and ignore the other type. Even endurance athletes need well-developed fast-twitch muscles, and sprint and weight athletes need well-developed slow-twitch muscles. Many athletes focus on working both types of muscles specifically, through different forms of athletic training, to maintain balance and support for their sports. They also support the development of balanced muscle types through diet, as the two types of muscle are fueled by different nutrients.