Redefining Physiologic Predictors of Endurance Performance with Measures of Skeletal Muscle Oxygenation: E pluribus unum

Abstract

PURPOSE: To scrutinize the ability of classic measures of endurance performance vs. NIRS-derived measures of skeletal muscle oxygenation to predict 25 km cycling time trial performance. METHODS: 14 participants (4 f / 10 m) underwent 3 sequential exercise bouts on a cycle ergometer while fitted with NIRS devices over each vastus lateralis: 1) A warmup consisting of 2 sequential 7.5-min bouts of 50 and 100 W at 80 rpm to determine gross efficiency (GE), exercise economy (EC), and delta efficiency (DE) across the same absolute workloads followed by an incremental max test to volitional fatigue (VF) used to discern 3 separate measures of ventilatory threshold (VT), maximal rates of whole-body oxygen consumption (VO2max) and maximal aerobic power (Wmax); 2) A warmup consisting of 2 sequential 7.5-min bouts at 80 rpm corresponding to 15% and 30% Wmax for GE, EC, and DE across the same relative workloads followed by a 60 sec ramp to a sustained 110% Wmax until VF to verify VO2max; and 3) A 25 km TT. Ventilatory measures were sampled throughout bouts 1 and 2. RESULTS: Stepwise and multiple linear regression analyses revealed that from the classic variables only mean VTVO2 (2.59 ± 0.53 l O2·min-1, p = 0.019) and EC at 15% Wmax (47 ± 10 W, p = 0.030) explained (adj R2 = 0.463; p = 0.013) 25 km TT performance (46:40 ± 04:36 min:sec) variance whereas the change in skeletal muscle oxygenation (ΔSmO2; -6.9 ± 6.1%; p = 0.001) from 50 to 100 W and the deoxygenated hemoglobin and myoglobin index (HHb) at Wmax (9.57 ± 1.56; p = 0.044) were the best NIRS-derived variables to describe TT performance (adj R2 = 0.751; p < 0.001). When combining all variables, skeletal muscle measures provided a superior physiologic explanation of 25 km TT performance versus those classically used to describe endurance performance (VTVO2, EC @ 15% Wmax, HHbmax, and ΔSmO2 at p = 0.083, 0.056, 0.008, and 0.005, respectively). CONCLUSION: These results demonstrate that measures reflecting the balance of skeletal muscle O2 delivery and utilization during exercise are superior to classic measures of whole-body aerobic capacity, ventilatory threshold, and/or the efficiency of exercise at predicting 25 km time trial (TT) cycling performance.