As previously intimated, the body adapts to the demands of a training session or period of training. Variation means providing a change in training stimulus through manipulating or changing something in the training programme. If training continues without variation, then the body may cease to adapt and may become stale or over-trained (Hakkinen et al, 1985; Bompa and Haff, 2009). An example of this would be the player who undergoes the same resistance training programme from early pre-season all the way throughout the in-season. While positive adaptation may occur in the early stages of the programme, over time, the monotony of the programme may be counter-productive (Hakkinen et al, 1985; Kraemer, 2008). 

Variation does not have to be a huge change in training as you can achieve variation with subtle adjustments to the training programme. For example, in a strength training programme changing the weight lifted, changing rest times, or changing the sets and repetitions are all examples of variation in a training programme.