Author(s): JEM, A., YOGEV, A., NELSON, H., KOEHLE, M.S., Institution: UNIVERSITY OF BRITISH COLUMBIA, Country: CANADA, Abstract-ID: 747

INTRODUCTION:
Near-infrared spectroscopy (NIRS) is used to indicate the balance of local oxygen (O2) delivery and O2 uptake in exercising muscle. Muscle oxygen saturation (SmO2) generally decreases proportionally to exercise intensity and duration, and increases during recovery. Reoxygenation represents a relative excess in O2 delivery during recovery while O2 uptake declines as the local metabolic milieu is restored. Reoxygenation is typically evaluated in a single working muscle, where SmO2 will reach peak values above resting baseline (hyperaemia) within 1-3 minutes after maximal exercise. We sought to compare reoxygenation kinetics between working and accessory muscles during an incremental cycling exercise step test (IET) as a function of increasing intensity. We hypothesised that reoxygenation would be faster in working muscle, slower in accessory muscle, and slower overall with increasing intensity.
METHODS:
Twenty-one trained cyclists (10 F, 11 M) performed two IET trials at 1.0+0.5 W·kg-1 per 5-min stage, with 1 min rest between stages, to maximal tolerance. Moxy NIRS sensors (Fortiori Design LLC, Hutchinson, MN, USA) were placed on vastus lateralis (VL), rectus femoris (RF), erector spinae (ES), and deltoid (DL) muscles. Reoxygenation kinetics were evaluated during rest intervals as the time to recover half the SmO2 amplitude from the end of work to the peak SmO2 value (half recovery time, HRT in seconds). A linear mixed effects model was used to analyse HRT with fixed effects for trial, relative intensity (% peak workload), and muscle site, with random effects of slope and intercept by participant. Post hoc estimated marginal means were contrasted across intensity at 50%, 75%, and 100%, and between muscle sites.
RESULTS:
Reoxygenation kinetics were generally slower (HRT was greater) with increasing exercise intensity beyond 50% in the VL, RF, and ES muscles (each p<0.01), but not DL (p>0.05). VL had the lowest between- and within-participant variation and recovered faster than other sites (HRT model estimates [95% CI]: VL=9 [6, 11], 12 [9, 14], 17 [13, 21] sec at 50, 75, 100% intensity, respectively). RF, ES, and DL were progressively slower at all intensities (p<0.001), except for ES and DL at 100% (ES=38 [34, 42] vs DL=40 [36, 45] sec; contrast=3 [-3, 9] sec, p=0.68).
CONCLUSION:
In a cycling IET, reoxygenation kinetics are faster in working muscle than accessory muscle, and generally slower overall as systemic metabolic demands increase with intensity. The VL is the primary working muscle in cycling and is consistently prioritised for recovery, as might be expected where motor recruitment and metabolic demand are highest. Slower recovery and higher variability in other muscle sites hint at heterogeneities in accessory muscle recruitment strategies and systemic competition for cardiac output. Integrating NIRS responses across working and accessory muscles may help to reveal more about local contributions and limitations to performance.