This is a popular theorem that’s been around forever, but I’d like to approach it in a different way. Maybe in a way that can help you be more practical in your training. I’m all for and all about achieving great levels of strength and endurance in all possible areas and as much as is possible all at the same time. But you must still be intelligent in the muscular, mental and structural recovery of your body. Here are some examples of what I mean:
Squats and Deadlifts:
Pretty much everyone will structurally favor one of these lifts. I.e., your physical leverages and build give you greater aptitude at one or the other. For most people working these lifts in conjunction will help them get stronger at both. However, working both heavily at the same time may prove difficult to recover from. Also for certain people, extremely heavy work in the lift that does not favor them structurally tends to cause them injury. This may be from genetics, but it is also accentuated by the stress placed on the body of working these exercises intensely together. It may not always however by necessary and may be at times, more productive, to work one exercise heavy and the other one light. By concentrating on one exercise heavy you get the real strength and power benefits of very intense work. But by doing the other exercise light you can maintain the development that you get from that exercise as well as the nerve efficiency of continuous practice and the benefit of combining the two exercises without the potential of injury, the excess strain and recovery and risk of over training.