The Muscles

Our muscles are composed of a collection of different types of fibers with various functions for speed (fast versus slow), fatigue ability (easily fatigued versus fatigue resistant), physical support, and other features. This neuromuscular variation is one feature that enables humans to perform effectively in so many different sports requiring very different movements at different paces. The ability to swim, bike, and run in one event is an example of this feature.

The different types of muscle fibers are assembled in each of our muscles side by side in a seemingly random order. The different fibers are used when called upon by the brain. For example, during walking almost all the muscle action is from aerobic fibers. If you quicken the pace and even jog, faster moving aerobic fibers are enlisted. As you begin to increase your speed to higher intensity, more anaerobic fibers take over. But with faster activity the slower-moving aerobic fibers reduce their action.

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

In some animals, like chickens, different whole muscles have more exclusive fiber types. For example, the chicken’s leg and thigh contain mostly red (“dark”) aerobic muscle fibers; they are used for slow, constant walking. The “breast” muscles, however, are composed of mostly anaerobic fibers—the white meat—used for fast, powerful, short-lived flapping of the wings.

In humans, the percentage of different fibers in a given muscle varies between individuals. Certain muscles of elite endurance athletes contain up to 90 percent aerobic fibers, where sprinters may have as little as 25 percent, with the remainder being anaerobic. Some anaerobic fibers also have the potential for significant aerobic function, as some of the great sprinters can be trained to make even greater endurance athletes. Knowing the percentage of aerobic and anaerobic muscle fibers will do little in ascertaining a person’s performance outcome. This is because the muscle itself is not the determining factor in performance, but the sum of the brain, muscle, and metabolic activity and their efficiencies, along with lifestyle factors. For endurance athletes, the lifestyle effect, including training, more than makes up for any individual difference in fiber type and genetics. While your particular fiber makeup is influenced by genetics, your training helps determine the type and efficiency of those fibers. If the goal is reaching your athletic potential, emphasis should obviously be placed on training, diet, and other lifestyle factors.