There is a lot of debate at the moment about the subject of “over training” i.e. over working muscle groups compared to conventional thought regarding how often and how hard we should be training. (Volume vs. Load vs. Frequency) 

Imagine your body as a glass of water, the space in the glass representing your potential work capacity for the week. Every time you train you add water to the glass, every time you rest the glass drains slightly but never fully draining the water added when you trained, as you continue to train and rest the glass will continue to fill.