The temporal structure, or complexity, of muscle torque output decreases with neuromuscular fatigue. The role of central fatigue in this process is unclear.
Purpose: We tested the hypothesis that caffeine administration would attenuate the fatigue-induced loss of torque complexity.
Methods: Eleven healthy participants performed intermittent isometric contractions of the knee extensors to task failure at a target torque of 50% maximal voluntary contraction, with a 60% duty factor (6-s contraction, 4-s rest), 60 min after ingesting 6 mg·kg caffeine or a placebo. Torque and surface EMG signals were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) α scaling exponent. Global, central, and peripheral fatigue was quantified using maximal voluntary contractions with femoral nerve stimulation.
Results: Caffeine ingestion increased endurance by 30% ± 16% (mean ± SD; P = 0.019). Complexity decreased in both trials (decreased ApEn, increased DFA α; both P < 0.01), as global, central, and peripheral fatigue developed (all P < 0.01). Complexity decreased significantly more slowly after caffeine ingestion (ApEn, -0.04 ± 0.02 vs -0.06 ± 0.01 (P = 0.004); DFA α, 0.03 ± 0.02 vs 0.04 ± 0.03 (P = 0.024)), as did the rates of global (-18.2 ± 14.1 vs -23.0 ± 17.4 N·m·min, P = 0.004) and central (-3.5 ± 3.4 vs -5.7 ± 3.9 %·min, P = 0.02) but not peripheral (-6.1 ± 4.1 vs -7.9 ± 6.3 N·m·min, P = 0.06) fatigue.
Conclusions: Caffeine ingestion slowed the fatigue-induced loss of torque complexity and increased the time to task failure during intermittent isometric contractions, most likely through central mechanisms.