Arterial Oxygen Content Influences Exercise Performance via Effects on Limb Fatigue

Abstract

PURPOSE: It is well known that arterial oxygen content (CaO2) is positively correlated with whole body exercise performance. We previously demonstrated, by comparing the effects of ΔCaO2 at equal exercise work rates and durations, that CaO2 is negatively correlated with peripheral locomotor muscle fatigue. Accordingly, we investigated the role of locomotor muscle fatigue as a determinant of the effects of ΔCaO2, on exercise performance. METHODS: On separate days, 8 healthy, trained males performed four 5 km cycling time trials (TT) [inspired O2 fraction (FIO2) / CaO2 (ml O2·dl blood−1): 0.15/17.6 (TTHYPO); 0.21/20.9 (TTNORM); 0.28/22.6 (TTISO); 1.0/24.4 (TTHYPER)], as well as 3 constant work-load trials (314 ±13 W; FIO2 / CaO2: 0.15/18.2, 0.21/21.3, 1.0/24.5) to the limit of exhaustion. Quadriceps muscle fatigue was assessed via evoking quadriceps twitch force (Qtw) through magnetic stimulation of the femoral nerve (1–100 Hz) pre- and 2.5-min post-exercise. We determined that this method for peripheral locomotor muscle fatigue assessment was highly reproducible within and between days (coefficient of variation <6%) and highly sensitive to small variations in exercise duration at the same work rate. RESULTS: With each increment in CaO2 from hypoxia to hyperoxia, time trial performance (time required to completion and mean power output) was increased significantly (e.g. 483 s / 270 W with TTHYPO vs. 438 s / 349 W with TTHYPER, p <0.05). Despite these wide differences in time trial performance with varying CaO2, the amount of exercise-induced quadriceps fatigue (% ΔQtw) was nearly identical (ΔQtw pre- to post-exercise = −27 to −29%, p = 0.4). During the constant work-load trials, varying CaO2 resulted in a wide range in time to exhaustion (4.5 ±0.4 min in FIO2 0.15 to 19.4 ±3.9 min in FIO2 1.0; p <0.01), however, similar levels of peripheral locomotor muscle fatigue (ΔQtw −26 to −27%, p = 0.8) were observed following exhaustive exercise. CONCLUSIONS: These findings indicate that a certain critical level of peripheral locomotor muscle fatigue is an important regulated variable during exercise which determines the level of motor output and therefore exercise performance, i.e. power output and time to task completion. Thus, we propose that variations in CaO2 and O2 transport to muscle affect time trial performance secondary to the rate of quadriceps fatigue development, which in turn may feed back to higher motor areas of the central nervous system to modulate or titrate motor output so as to prevent “excessive” development of peripheral locomotor muscle fatigue beyond a critical threshold.