METABOLIC RESPONSE OF TYPE-I AND TYPE-II MUSCLE-FIBERS DURING REPEATED BOUTS OF MAXIMAL EXERCISE IN HUMANS

Nine male subjects performed two bouts of 30-s maximal isokinetic cycl ing. Each bout of exercise was per formed at 80 revolutions/min and wa s separated by 4 min of recovery. Mixed-muscle phosphocreatine (PCr) r esynthesis during recovery (88.1 +/- 6.1%) was positively correlated w ith the restoration of total work production during bout 2 (r = 0.80, P < 0.05). During bout I, ATP and PCr utilization were greater in type II compared with type I fibers (P < 0.01 and P < 0.05, respectively). The subsequent 4-min period of recovery was insufficient to allow tot al restoration of ATP and PCr in type II fibers, but restoration of AT P and PCr in type I fibers was almost complete. During the second bout of exercise, ATP and PCr utilization were reduced in type II fibers ( P < 0.01), without a corresponding change in type I fibers, and perfor mance was also significantly reduced. The reduction in work capacity o bserved during bout 2 may have been related to a slower resynthesis, a nd consequently a reduced availability, of ATP and PCr in type II fibe rs.