Selective glycogen depletion in skeletal muscle fibres of man following sustained contractions

1. Six healthy males performed sustained contractions with different tensions related to their maximal voluntary contraction (MVC). The isometric exercise consisted of efforts to extend the knee when flexed at an angle of 90°. 2. Biopsy samples were taken from the lateral portion of M. quadriceps femoris before and after different periods (6–45 min) during a series of sustained contractions. Total glycogen content was determined on each muscle sample. In order to evaluate whether the glycogen depletion occurred preferentially in slow twitch (ST) or fast twitch (FT) fibres, serial sections of the muscle samples were stained for myofibrillar ATPase and glycogen (PAS reaction). 3. In all experiments a selective glycogen depletion was observed. At low tensions, the ST fibres and at higher tensions the FT fibres became glycogen depleted. The critical tension at which this conversion in glycogen depletion from ST to FT fibres took place was 20% MVC. 4. It is concluded that at sustained contractions of less than 20% MVC there is a major reliance upon ST fibres and above that level a primary dependence upon FT fibres. It is further suggested that restriction of blood flow and thus availability of oxygen at forces higher than 20% MVC may be the explanation for the present findings.