The Effect Of Muscle Oxygenation On Neuromuscular Efficiency And Force Complexity

Limiting oxygen delivery during steady-state exercise gradually increases muscle excitation at submaximal forces, suggesting a time-dependent effect of oxygenation on neuromuscular efficiency (NME). Whether sustained reductions in muscle oxygen saturation (SmO2) from rest impacts NME and motor output characteristics of subsequent voluntary contractions is unknown. PURPOSE: We tested the hypothesis that sustained reductions in SmO2 from rest would decrease NME and force complexity of submaximal contractions. METHODS: Twenty-nine participants (12 M/17F, 22 ± 3 yrs) performed submaximal dorsiflexion under control conditions (CON) and at 70%, 60%, and 50% SmO2. Leg circulatory occlusion was used to elicit target SmO2 of the tibialis anterior muscle at rest. After 3 minutes at the target SmO2 (via cuff-pressure manipulation), participants performed 6 contractions (6 s contraction, 14 s relaxation) at 50% maximal voluntary contraction (MVC). NME was calculated as force divided by the root mean square of electromyography signals normalized to MVC (N-RMS). Force complexity was assessed by detrended fluctuation analysis (DFA) and approximate and sample entropy (ApEn and SampEn). Data from the final three contractions were averaged, and comparisons were made using linear mixed-effects models and Tukey’s tests. RESULTS: There was a significant effect of SmO2 on N-RMS and NME (both, p < 0.001; Figure 1) as well as DFA, ApEn, and SampEn (all, p < 0.05). NME was significantly lower in all desaturated conditions compared to CON (all, p < 0.05, Figure 1). Greater desaturation led to greater decreases in NME (all p < 0.05; Figure 1) except for 50% (n = 9). At 60%, DFA was higher (+2%; p < 0.05) and ApEn and SampEn were lower (-32% and -30%; both, p < 0.05) than CON. CONCLUSIONS: Reducing SmO2 decreases subsequent NME and force complexity, supporting that muscle oxygenation influences muscle activation characteristics and motor unit control of submaximal voluntary contractions.