The slow component of oxygen uptake during intense, sub-maximal exercise in man is associated with additional fibre recruitment

Single muscle fibre metabolites and pulmonary oxygen uptake ((V)over dotO(2)) were measured during moderate and intense, sub-maximal exercise to test the hypothesis that additional fibre recruitment is associated with the slow component of (V)over dotO(2). Seven healthy, male subjects performed 20 min moderate (MOD, similar to50% of (V)over dotO(2,max)) and intense (INT, similar to80% (V)over dotO(2,max)) cycling at 70 rpm. Glycogen content decreased significantly in type I and IIa fibres during INT, but only in type I fibres during MOD. During INT, creatine phosphate (CP) content decreased significantly both in types I and II fibres in the first 3 min (DeltaCP: 16.0+/-2.7 and 16.8+/-4.7 mmol kg(-1) d.w., respectively) and in the next 3 min (DeltaCP: 16.2+/-4.9 and 25.7+/-6.7 mmol kg(-1) d.w., respectively) with no further change from 6-20 min. CP content was below the pre-exercise level (mean-1 SD) in 11, 37, 70 and 74% of the type I fibres after 0, 3, 6 and 20 min of INT, respectively, and in 13, 45, 83 and 74% of the type II fibres. During INT, (V)over dotO(2) increased significantly by 6+/-1 and 4+/-1% in the periods 3-6 and 6-20 min, respectively (Delta(V)over dotO(2,(6-3min)): 0.14+/-0.02 l min(-1)), whereas (V)over dotO(2) was unchanged from 3 to 20 min of MOD. Exponential fitting revealed a slow component of (V)over dotO(2) during INT that appeared after similar to2.6 min and amounted to 0.24 l min(-1). The present study demonstrates that additional type I and II fibres are recruited with time during intense sub-maximal exercise in temporal association with a significant slow component of (V)over dotO(2).