Estimated Kinetics Of Muscle Capillary Blood Flow During Recovery From Exercise

Abstract

Following the onset of exercise, we have previously shown that the estimated kinetics of muscle capillary blood flow (Qcap) appear to be tightly coupled with the kinetics of muscle oxygen uptake (VO2m) in healthy subjects. This coupling would facilitate adequate oxygen delivery during the adjustment phase. It is currently unclear if a similar coupling is maintained during recovery from exercise. PURPOSE To test the hypothesis that the kinetics of muscle capillary blood flow (Qcap) would be similar to VO2m kinetics during recovery from moderate and heavy intensity cycling exercise. METHODS Nine subjects performed three bouts (6 min. each) of constant work rate exercise on a cycle ergometer at a work rate corresponding to 90% lactate threshold (LT) VO2 followed by one bout (8 min.) at a work rate halfway between LT and peak VO2 (50% Δ). Each exercise bout was separated by 6 min. baseline cycling (20 watts), and the procedure was repeated on another day. Pulmonary oxygen uptake (VO2p) was monitored breath-by breath, while muscle oxygenation (deoxy-hemoglobin concentration [HHb]) of the vastus lateralis was determined by surface NIRS. Assuming that the primary component of VO2p during phase II described VO2m and that [HHb] kinetics represented muscle capillary O2 extraction (CaO2 – CvO2), the Fick equation was rearranged to estimate the temporal characteristics of Qcap (i.e., Qcap(t) α VO2m(t)/[HHb](t)). RESULTS The recovery kinetics of Qcap (mean response time, MRT) were significantly slower than the kinetics of VO2m (τ) for both moderate (Qcap: 36.1 ± 8.6 s; VO2m: 27.8 ± 5.3 s; p<0.05) and heavy (Qcap: 48.7 ± 21.8 s; VO2m: 32.32 ± 6.67 s; p<0.05) exercise intensities. In addition, the kinetics of Qcap tended to be (P = 0.068) slower following heavy intensity exercise than after moderate exercise. CONCLUSIONS In contrast to the onset of exercise, the estimated kinetics of Qcap during recovery were slower than the kinetics of VO2m. This may serve to ensure that O2 delivery to the muscle is in excess of O2 extraction during recovery. Supported by AHA Grant-in-Aid #0151183Z