Muscle Fiber Type
All of our muscles are composed of Type 1 and Type 2 muscle fibers. Type 1 muscle fibers are often referred to as slow twitch, while Type 2 muscle fibers are often termed fast twitch. Type 1 fibers are characterized by smaller size, less force capacity and more endurance capacity. They are the dominant muscle fibers in endurance activities such as long distance running, cycling and swimming. Type 2 fibers are characterized by larger size, more force capacity and less endurance capacity. They are the dominant muscle fibers in power activities such as sprinting, jumping and throwing.

Due to the physiological differences, Type 1 fibers fatigue more slowly and can complete more repetitions with a given resistance. Conversely, Type 2 fibers fatigue more quickly and can complete fewer repetitions.

First Study
Consider the results of a research study in which 87 men and women performed as many repetitions as possible with 75 percent of their maximum resistance. Specifically, all of the subjects were tested for the maximum weight load they could perform one time on the nautilus 10-Degree Chest machine. After a five-minute rest, they completed as many repetitions as possible with 75 percent of their maximum weight load.

As illustrated in Figure 1, most of the participants performed between eight and 13 repetitions. This median group represented normal individuals with a relatively even mix of Type 1 and Type 2 muscle fibers. At the left side of the chart are a few subjects who completed fewer than eight repetitions. These are excellent power athletes (sprinters, jumpers) who typically have a higher percentage of Type 1 (low endurance) muscle fibers. At the right side of the chart are a few subjects who completed more than 13 repetitions. These are outstanding endurance athletes (marathoners, triathletes) who typically possess a higher percentage of Type 1 (high endurance) muscle fibers.

It would seem that individuals who have an even mix of Type 1 and Type 2 muscle fibers may obtain best results by training with about 10 repetitions per set. It would also appear that individuals who have predominately Type 2 muscle fibers may obtain best results by training with fewer repetitions per set. Conversely, individuals with predominately Type 1 muscle fibers should train with more repetitions per set to achieve maximum results.

Second Study
This observation was supported by a follow-up study in which the middle-endurance athletes trained with nine to 11 repetitions per set, the low-endurance athletes trained with six to eight repetitions per set, and the high-endurance athletes trained with 12-14 repetitions per set. As presented in Table 1, all of the athletes made similar improvements in muscle strength after eight weeks of training with a personalized repetition protocol. These results indicate that predominantly Type 2 muscles respond well to low-repetition training. Type 1 muscles respond well to high-repetition training and evenly mixed muscles respond best to mid-repetition training.