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

Purpose Joint angle is a signifcant determinant of neuromuscular and metabolic function. We tested the hypothesis that previously reported correlations between knee-extensor torque complexity and metabolic rate (mVȮ 2) would be conserved at reduced joint angles (i.e. shorter muscle lengths).
Methods 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 fexion (0º=extension). Torque and surface EMG were sampled continuously. Complexity and fractal scaling of torque were quantifed using approximate entropy (ApEn) and detrended fuctuation analysis (DFA) α. mVȮ 2 was determined using near-infrared spectroscopy.
Results 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. mVȮ 2 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 signifcant correlations between torque complexity and mVȮ 2 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).
Conclusion The lack of correlation between mVȮ 2 and complexity at 30º was likely due to low relative task demands, given the similar kinetics of mVȮ 2 and torque complexity. An inverse correlation between mVȮ 2 and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths.