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 (V̇O2) were measured during moderate and intense, sub-maximal exercise to test the hypothesis that additional fibre recruitment is associated with the slow component of V̇O2. Seven healthy, male subjects performed 20 min moderate (MOD, ~50% of V̇O2,max) and intense (INT, ~80% V̇O2,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 (ΔCP: 16.0±2.7 and 16.8±4.7 mmol kg−1 d.w., respectively) and in the next 3 min (ΔCP: 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, V̇O2 increased significantly by 6±1 and 4±1% in the periods 3–6 and 6–20 min, respectively (ΔV̇O2,(6−3min): 0.14±0.02 l min−1), whereas V̇O2 was unchanged from 3 to 20 min of MOD. Exponential fitting revealed a slow component of V̇O2 during INT that appeared after ~2.6 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 V̇O2.