Physiological changes in skeletal muscle as a result of strength training.

The picture of training that emerges is of a process that can be divided into a number of phases. In the first phase there is a rapid improvement in the ability to perform the training exercise such as lifting weights which is the result of a learning process in which the correct sequence of muscle contractions is laid down as a motor pattern in the central nervous system. This phase is associated with little or no increase in the size or strength of individual muscles. The learning process appears to be very specific in that lifting weights makes better weight lifters but not better sprinters. The second phase is an increase in the strength of individual muscles which occurs without a matching increase in the anatomical cross-section. The mechanism for this is not clear but could be a result of increased neural activation or some change in the fibre arrangement or connective tissue content. The third phase starts at a point where scientific studies usually end, at about 12 weeks when non-athletic subjects are beginning to tire of the repeated training and testing. After this point, if training continues, there is probably a slow but steady increase in both size and strength of the exercised muscles. The stimulus for these changes remains enigmatic but almost certainly involves high forces in the muscle, probably to induce some form of damage that promotes division of satellite cells and their incorporation into existing muscle fibres. Our information on the effect of long-term training comes primarily from observations on elite athletes whose physique may well be the result of genetic endowment or the use or abuse of drugs. For the athlete or patient hoping to increase muscle size by weight training the best combination of intensity, frequency and type of exercise still remains a matter of individual choice rather than a scientific certainty.