THE METABOLIC RESPONSES OF HUMAN TYPE-I AND TYPE-II MUSCLE-FIBERS DURING MAXIMAL TREADMILL SPRINTING

1. Muscle biopsy samples were obtained from the vastus lateralis of si x healthy volunteers before and after 30 s of treadmill sprinting. A p ortion of each biopsy sample was used for mixed-fibre metabolite analy sis. Single fibres were dissected from the remaining portion of each b iopsy and were used for ATP, phosphocreatine (PCr) and glycogen determ ination. 2. Before exercise, PCr and glycogen contents were higher in type II fibres (79.3 +/- 2.7 and 472 +/- 35 mmol (kg dry matter (DM))( -1), respectively) compared with type I fibres (71.3 +/- 3.0 mmol (kg DM)(-1), P < 0.01 and 375 +/- 25 mmol (kg DM)(-1), P < 0.001, respecti vely). 3. Peak power output was 885 +/- 66 W and declined by 65 +/- 3% during exercise. Phosphocreatine and glycogen degradation in type II fibres during exercise (74.3 +/- 25 and 126.3 +/- 15.8 mmol (kg DM)(-1 ), respectively) was greater than the corresponding degradation in typ e I fibres (59.1 +/- 2.9 mmol (kg DM)(-1), P < 0.001, and 77.0 +/- 14. 3 mmol (kg DM)(-1), P < 0.01, respectively). The decline in ATP during exercise was similar when comparing fibre types (P > 0.05). 4. Compar ed with previous studies involving similar durations of maximal cyclin g exercise, isokinetic knee extension and intermittent isometric contr action, the rates of substrate utilization recorded in type I fibres w ere extremely high, being close to the rapid rates observed in this fi bre type during intense contraction with limb blood flow occluded.