Fatigue-induced changes in knee-extensor torque complexity and muscle metabolic rate are dependent on joint angle

Joint angle is a significant determinant of neuromuscular and metabolic function. We tested the hypothesis that previously reported correlations between knee-extensor torque complexity and metabolic rate ([Formula: see text]) would be conserved at reduced joint angles (i.e. shorter muscle lengths).

Eleven participants performed intermittent isometric knee-extensor contractions at 50% maximum voluntary torque for 30 min or until task failure (whichever occurred sooner) at joint angles of 30º, 60º and 90º of flexion (0º = extension). Torque and surface EMG were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA) α. [Formula: see text] was determined using near-infrared spectroscopy.

Time to task failure/end increased as joint angle decreased (P < 0.001). Over time, complexity decreased at 90º and 60º (decreased ApEn, increased DFA α, both P < 0.001), but not 30º. [Formula: see text] increased at all joint angles (P < 0.001), though the magnitude of this increase was lower at 30º compared to 60º and 90º (both P < 0.01). There were significant correlations between torque complexity and [Formula: see text] at 90º (ApEn, r =  - 0.60, P = 0.049) and 60º (ApEn, r =  - 0.64, P = 0.035; DFA α, ρ = 0.68, P = 0.015).

The lack of correlation between [Formula: see text] and complexity at 30º was likely due to low relative task demands, given the similar kinetics of [Formula: see text] and torque complexity. An inverse correlation between [Formula: see text] and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths.