Heterogeneity of Muscle Deoxygenation Kinetics During Repeated Bouts of Heavy Exercise

Abstract

Since there is substantial heterogeneity in muscle blood flow, motor unit recruitment, and fiber type distribution within and across muscles, it is likely that considerable heterogeneity of muscle oxygenation exists across the exercising muscle, particularly in transient states where metabolic rate is changing rapidly. Performance of prior heavy exercise, where elevated levels of blood flow are sustained for a period of time, might decrease the muscle oxygenation heterogeneity by priming the vasodilatory responses to muscle contractions. PURPOSE: To test the hypothesis that the degree of heterogeneity in the matching of quadriceps microvascular O2 requirement to O2 delivery ratio (VO2/Q) would be lower after prior heavy exercise. Moreover, we anticipate that the primary time constant of VO2 will not be affected by prior heavy exercise. METHODS: Seven healthy male subjects (23.7 ±5.5 years) performed repeated bouts of 6 min heavy cycle exercise, separated by 6 min unloaded exercise. Pulmonary VO2 was measured breath-by-breath and changes in deoxygenation [deoxy-(Hb+Mb)] were assessed simultaneously at ten different sites of the quadriceps using multi-channel near-infrared spectroscopy. RESULT: Prior exercise had no significant effect on the time constant of the Phase II VO2 (Bout 1: 28.8 ± 6.3 vs. Bout 2: 26.4 ± 6.3 s), but reduced the amplitude of the slow component during subsequent heavy exercise. The time delay of deoxy-(Hb+Mb) for bout 1 was longer than for bout 2 (12.8 ±1.8 vs. 9.4 ±1.2s, p < 0.01). The time constant of deoxy-(Hb+Mb) primary component (τ) for bout 1 was faster than for bout 2 (11.1 ± 1.9 vs. 15.5 ± 1.3 s, p < 0.01). Prior exercise resulted in a reduction in the dispersion of τ of deoxy-(Hb+Mb) (standard deviation of 10 sites, Bout 1: 1.9 vs. Bout 2: 1.3 s). CONCLUSION: These results suggest that the reduction of the amplitude of the VO2 slow component during subsequent heavy exercise might be associated with 1) less heterogeneous VO2/Q and 2) an enhanced muscle O2 delivery (presumably by the increased- and/or faster capillary blood flow), which in turn might have led to decreased recruitment of type II fibers.