Adaptation of human skeletal muscle to nine weeks of intermittent resistance training
Twelve persons were randomly assigned to a training group (n=6) or a control group (n=6). The training consisted of intensive, intermittent elbow extensions three times a week at 55 per cent of 1RM for nine weeks. Needle biopsies were taken from the lateral head of the triceps muscle one week before and 2-3 days after termination of the training period. Following training, the mean percentage of type I fibres in m. triceps brachii had increased from 42.4 per cent to 58.2 per cent (P < 0.03) and the mean percentage of type IIB fibres had decreased from 22.8 to 8.1 per cent (P < 0.03). There was no change in distribution of type IIA fibres. The mean diameter of type I fibres increased from 56.0 to 64.2 μm (14.6%) (P < 0.05), type IIA diameter from 65.3 to 76.5 μm (17.1%) (P < 0.03). Type IIB fibre diameter showed no significant change. NADH-TR activity in all fibre types was unchanged following training. We observed no significant changes in the control group. Others  have found similar results in the same muscle group after continuous, low intensity training. Taken together, these results suggest that increased amount of contractile of activity may be more important than the type of activity in transforming fast muscle fibres to slower fibres in an untrained non-postural muscle.
Dahl, Hans A.