A set is a grouping of repetitions that is followed by a rest interval. The number of sets performed in a workout is one of the factors affecting the total volume (sets X repetitions X resistance) of exercise. Therefore, it must be consistent not only with the individual’s strength goals but also with his or her current level of fitness.
Generally speaking, it is accepted that multiple sets are more beneficial for developing strength and muscle mass. In fact, this stance is supported in guidelines set by the National Strength and Conditioning Association (Pearson, Faigenbaum, Conley, and Kraemer 2000) and the American College of Sports Medicine (Kraemer et al. 2002). Single sets are effective for building strength for beginning weightlifters or for maintaining strength during periods when it is necessary or desired to reduce the volume performed. Beginners starting with a single-set program should progressively increase the number of sets to make continued adaptations in strength.
When designing a workout, one should consider the number of sets performed per exercise, the number of sets per muscle group, and the total number of sets for the workout. The number of sets per exercise typically varies depending on the strength training program. Most programs designed for the intermediate to advanced weight trainer incorporate between three and six sets per exercise. This set range is considered optimal for increasing strength. How many sets one should perform per muscle group is a question that is most applicable to bodybuilding-type training, in which numerous exercises are performed for each muscle group. This is in opposition to strength training programs for conditioning athletes, which may typically involve only one exercise per major muscle group. The number of sets per muscle group may range from 3 to 24 but ultimately depends on the number of exercises performed for that muscle group, the number of muscle groups trained in that workout, the intensity used, and where the person is in his or her training cycle. The total number of sets performed for a workout may vary from about 10 to 40, depending on the type of training and the number of sets per exercise. Care must be taken so that not too many total sets are performed, particularly when intensity is high, since these variables greatly influence total work. Performing too much total work over time stresses the body and can lead to overtraining. Although defining how much work is too much is a difficult task because many factors are involved, such as the person’s training experience and genetics, general recommendations can be made. Typically doing more than 20 sets per muscle group for an extended period can lead to overtraining. In addition, doing more than 40 sets per workout, even when multiple muscle groups are trained in that workout, can lead to overtraining if done too frequently.
As for any other acute variables of training, the number of sets should be manipulated to prevent stagnation of training adaptations. The most important variable of training that influences the number of sets that should be performed is intensity (the amount of weight lifted). The greater the intensity, the greater the stress placed on the muscle, and thus the lower the number of sets that should be performed. Therefore, the total number of sets in a training cycle should vary inversely with training intensity. In fact, training with too many total sets can be detrimental to the adaptations of strength training and lead to overtraining.
This is an excerpt from Encyclopedia of Muscle & Strength.