Alterations in human muscle fibre type distribution induced by acute exercise.

Fibre type distributions in the vastus lateralis muscles of six male subjects (age 18 to 22 years) have been compared at rest and during exercise. Exercise consisted of one leg cycling at 60% VO2 max (one leg) for 120 min. The increased contractile activity was associated with a 28.8% (p less than 0.05) decrease in the distribution of Type I fibres in the exercised leg. This change in fibre type distribution was manifested early in the exercise (15 min), and was also evident in muscle samples obtained after 60 and 120 min of activity. Reductions in the Type I fibre distribution was accompanied by an increase in the Type II fibres, specifically the Type IIA distribution (p less than 0.05). Comparable alterations in the specific fibre distribution were also found in the non exercising leg. These observations indicate that alterations in the muscle cell induced either directly or indirectly by the increased contractile activity interact with normal pre-incubation conditions to effect changes in the stability of the myofibrillar ATPase reaction. Specifically, it appears that a percentage of the Type I fibre population becomes acid labile and alkali stable.