Conflicting Muscle Groups

It is virtually impossible to isolate a single particular muscle group by itself - especially the larger ones. Large muscle groups are best trained with multi-joint compound free weight movements which can require many secondary and stabilizing muscle groups for assistance. For example, the squat primarily works the quads but also greatly employs the calves, hamstrings, lower back and glutes as well as the entire upper body in order to support and stabilize the weighted barbell. This is very important to consider when designing a proper training split to avoid overtraining.

Quads are best trained on a separate day because squats and other overall leg movements (such as leg presses) will be required for maximum growth and much energy will be expended. The only rule of thumb for training quads is the hamstrings and lower back must not be sore or overtrained. Lower back and hamstrings should be trained together since the exercise for best overall development of both is deadlifts. Since deadlifts require almost as much energy as squats they should be performed when the quads are strong.

There are two major categories of muscle groups in the upper body - push and pull muscles. Push muscle groups move the weight away from the body and include chest, shoulders and triceps. Chest training requires both the shoulders and triceps to be strong but shoulder training only requires triceps for the most part. Pull muscle groups move the weight toward the body and include the entire upper back and biceps. The upper back can be trained together with the lower back or with other muscle groups of the upper body but almost all require the assistance of the biceps. There are many variations of upper body splits that work well and it really just takes some trial and error to find the right one (one size does not fit all). And finally the finishing muscle groups - calves, abs and forearms recover quite quickly and can be trained with most other groups but not more than twice per week.