Increases in myofibrillar ATPase intermediate human skeletal muscle fibers in response to endurance training

The response of human muscle triceps brachii (TB) to training, which consisted of skiing with sledges 500 miles over a period of 36 days, has been investigated by means of histochemical fiber typing (myofibrillar ATPase, n = 7). Muscle biopsies were taken in the right TB during pretraining, and in the right as well as in the left TB after training. The percentage of type 2 fibers and intermediate fibers (type 2C and 1B) in the right TB decreased and increased, respectively, by 13%. The fiber type distribution in the left TB after training was very similar to that in the right TB after training. This concordance indicates that the change in fiber type distribution was a result of the training per se, and not related to any eventual effect of the biopsy-sampling.