Steven J. Fleck, PhD
Q: Some guys in my gym do several exercises for the same muscle group per training session, while others do only one exercise per muscle group per session. The guys doing only one exercise say it’s not necessary to do more to cause maximal hypertrophy gains, but those performing several exercises claim more than one exercise is necessary. Who’s right?
A: There is no direct evidence that multiple exercises for the same muscle group per training session result in greater size gains than doing only one exercise per muscle group. However, the rationale behind the need for more than one exercise is based in muscle fiber recruitment patterns. Obviously, if a muscle fiber is not recruited during a training session, it will make no adaptation, such as an increase in size, for that session. The ultimate goal of performing multiple exercises for a certain muscle group is to make sure all, or at least as many muscle fibers as possible, are recruited during one or more of the exercises performed.
One aspect of muscle fiber recruitment order is commonly termed the size principal. This refers to the idea that slow-twitch muscle fibers are the first fibers recruited to do an activity, while fast-twitch fibers are recruited after the majority of slow-twitch fibers have been recruited. This means that if small or moderate amounts of force are needed to perform a weight training exercise, predominantly slow-twitch fibers will be recruited. Fast-twitch fibers will only be recruited if the slow-twitch fibers cannot generate enough force to lift the weight.
The size principal indicates that perhaps the easiest way to ensure recruitment of all your muscle fibers, including the fast-twitch fibers, is to lift a heavy weight. By lifting a heavy weight it’s ensured that slow-twitch fibers will not be able to generate enough force to successfully complete the task. You’ll have to recruit your fast-twitch fibers. The need to lift a relatively heavy weight to bring about maximal gains is supported by a recent statistical analysis of sport science studies that indicated advanced lifters must train with 80 percent of one rep max (1-RM) to bring about maximal strength gains (Rhea et al. 2003). So, no matter how many exercises per muscle group are performed per training session, it’s important that the intensity or percent of 1-RM used is relatively high. This assumes that sets are carried to volitional fatigue or at least close to failure.
It appears that the muscle fiber recruitment order is relatively fixed for many movements, including resistance exercises. This means not only will slow-twitch fibers be recruited before fast-twitch fibers, but also that there will be a specific order in which certain slow-twitch and fast-twitch fibers are recruited. Thus, with certain exercises, certain muscle fibers— normally fast-twitch fibers— will be very difficult to recruit. In a practical sense this means, for example, that a standing arm curl will have a different muscle fiber recruitment pattern of the biceps than a machine arm curl.
Sport science data indicates that when body position in an exercise is changed, the order of muscle fiber recruitment also changes (Grimby and Hannerz 1977; Matheson et al. 2001). For example, the magnitude of muscle fiber recruitment of different portions of the quadriceps is different during performance of the leg press compared to a squat (Escamilla et al. 2001) and different between one type of quadriceps exercise and another (Matheson et al. 2001). Similarly, the magnitude of recruitment of different areas of the abdominal musculature is different with different abdominal exercises (Willett et al. 2001). So, there is sport science information demonstrating that different exercises do recruit muscle fibers in a slightly different order and to a greater or lesser extent in different areas of a particular muscle.
Again, the idea of performing different exercises for the same muscle group is to help insure that as many muscle fibers as possible are recruited and therefore undergo physiological adaptations, such as an increase in size. There is sport science information demonstrating that the order and magnitude of muscle fiber recruitment varies with different exercises. Hence, there’s a sound rationale for the use of several different exercises for the same muscle group to bring about maximal size gains. (Med & Sci in Sports & Exer, 33:1552-566; J Physiol (London) 264:867-879; Med & Sci in Sports & Exer, 33:1713-725; Med & Sci in Sports & Exer, 35:456-464, 2003; J Strength & Cond Res, 15:480-85).