The following is an exclusive excerpt from the book Strength Training, Second Edition, published by Human Kinetics. All text and images provided by Human Kinetics.
Periodization of training (see chapter 3) is based on the principles stated previously—that different loads (light, moderate, or heavy) or power requirements recruit different types and numbers of motor units. On a light training day, you would allow some muscle fibers to rest by recruiting fewer of them than on a heavy training day. For example, if your maximal lift (1RM) for one dumbbell biceps curl is 100 pounds (45.4 kg), then 10 pounds (4.5 kg) of resistance represents only about 10 percent of your maximal strength in the biceps curl exercise. Performing 15 repetitions of the dumbbell biceps curl with 10 pounds would activate only a small number of your motor units in the biceps. Conversely, performing a biceps curl with 100 pounds would require all of the available motor units.
The size principle’s order of recruitment ensures that low-threshold motor units are predominantly recruited to perform lower-intensity, long-duration (endurance) activities, whereas the higher-threshold motor units are used only to produce higher levels of force or power. This helps delay fatigue during submaximal muscle actions because the high activation threshold for the highly fatigable Type II motor units is not reached unless high levels of force or power are needed; instead, mainly the lower-threshold, fatigue-resistible Type I motor units are recruited. In conjunction, higher-threshold motor units will be recruited only when enough total work has been performed to dramatically reduce the glycogen stores in the lower-threshold motor units. However, this typically has not been observed with resistance exercise because the activity does not tend to reduce muscle glycogen stores significantly. When force production needs are low to moderate, motor units can be alternately recruited to meet the force demands (asynchronous recruitment). This means that a motor unit may be recruited during most of the first repetition of a set with a light weight and then not (or only minimally) recruited during the second repetition. This ability to rest motor units when submaximal force is needed also helps to delay fatigue. When velocities are very slow and loads are very light—as in super-slow training—this type of recruitment may predominate during the exercise, leaving many muscle fibers not stimulated and thus primarily promoting endurance.
Recruitment order is important from a practical standpoint for several reasons. First, in order to recruit Type II fibers and thus achieve a training effect in these fibers, the exercise must be characterized by heavy loading or demands for high power output. Second, the order of recruitment is fixed for many movements, including resistance exercise; if the body position changes, however, the order of recruitment can also change and different muscle fibers can be recruited (e.g., in a flat vs. an incline bench press). The magnitude of recruitment of different portions of the quadriceps also varies among different types of leg exercises (e.g., a leg press vs. a squat). Order and magnitude of recruitment may contribute to strength gains being specific to a particular exercise. The variation in recruitment order provides some evidence to support the belief held by many strength coaches that a particular muscle must be exercised using several different movement angles to develop completely.
Not every person has the same complement of motor units available; thus, not every person has the same strength potential. This, along with differences in the total number of muscle fibers available, allows for differences in force and power capabilities among individuals. These differences are determined largely by genetics; however, various forms of endurance and resistance training as well as detraining can slightly change fiber type composition. The effects of detraining are seen especially with the loss of Type II motor units during aging. Some people and some muscles, such as abdominal muscles, may have only low-threshold motor units predominantly comprising Type I muscle fibers, thus limiting their capability to produce power and force. The type, number, and size of muscle fibers in the motor unit dictate the functional abilities of that individual motor unit and, eventually, the functional abilities of the whole muscle.
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