**4.5 Progression**

Gains in strength are usually rapid and substantial following commencement of PRT, with 10-15% increases in strength typically observed each week for the first 8 weeks of training in healthy, previously untrained individuals (Evans, 1999). Initially these improvements are due to enhanced neural factors i.e. improved motor unit recruitment, firing rate and synchronisation (Sale, 2003), with muscle hypertrophy contributing from about week 4 onwards (Sale, 2003). In order to maintain the maximal muscle fibre recruitment necessary for optimal increases in strength and muscle hypertrophy to occur, progressively higher loads need to be lifted. This increase in resistance (in accordance with increases in strength) to maintain a constant relative intensity is termed "progressive overload", and is a fundamental principle of all exercise training regimes.

Whilst marked responses to training are expected in untrained or deconditioned individuals, after an extended period of training the "law of diminishing returns" applies i.e. as an individual's fitness improves and he/she approaches their genetic ceiling it becomes harder to achieve further fitness gains. Consequently, when PRT is prolonged, plateaus in training response should be anticipated. The usual way of dealing with this situation is to manipulate the training program variables (types of exercises, training intensity, number of sets and/or repetitions, rest period between sets), so that the body is challenged by an unfamiliar training stimulus.
