The principle of variety is ensuring training sessions use multiple training types and methods, as well as exercises within these methods. Variety is needed in training, not only to prevent boredom, but also to ensure complete and full development of fitness. An example would be for a Rugby League player to use various training types throughout the week, such as aerobic, strength, flexibility and anaerobic training, all of which are needed to play the sport as well as possible. Within these the sessions would use a variety of methods, so that the anaerobic training was not always short interval training, but also included other forms of resistance training, or Plyometrics. Furthermore, interval-training sessions would not always be the same, but would include a variety of exercises. This could include: shuttle runs, repeated tackling for 1 min, followed by a 2 min rest x10, and 20 sec agility runs followed by 40 sec rests x20.
For aerobic training variety would include changes to the training method, between fartlek, circuit, continuous, and aerobic interval. It would also include variations within this training, so that the continuous training was not always doing a 10 km flat run on a treadmill, but included outside runs, or possibly some bike work.
For resistance training variety would mean mixing the training sessions up, using free weights and machine weights and also adding some elastic or hydraulic training as well. I means when you train your chest muscles you would include incline, flat and decline activities to ensure the entire pectoral muscle was trained. You might use hydraulic machines to ensure the gains also occurred at a faster speed of movement.