When we are training physically we will be tiring both the muscles and the central nervous system every time we train. Dependent on the intensity and duration of that training we will need time to recover. Get it right and your training will continue heading in a positive direction, get it wrong and you may hit a plateaux or worse still a decline.
Too short or too long a recovery period and you will probably get weaker over time. It really is about getting in the ‘Goldilocks Zone’, where you recover enough to reap the benefits of your last session in the next. With each session being a little step up on the last.