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

Purpose 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 (m<(V)over dot>O-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 30 degrees, 60 degrees and 90 degrees of flexion (0 degrees = 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) alpha. m<(V)over dot>O-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 90 degrees and 60 degrees (decreased ApEn, increased DFA alpha, both P < 0.001), but not 30 degrees. m<(V)over dot>O-2 increased at all joint angles (P < 0.001), though the magnitude of this increase was lower at 30 degrees compared to 60 degrees and 90 degrees (both P < 0.01). There were significant correlations between torque complexity and m<(V)over dot>O-2 at 90 degrees (ApEn, r = - 0.60, P = 0.049) and 60 degrees (ApEn, r = - 0.64, P = 0.035; DFA alpha, rho = 0.68, P = 0.015). Conclusion The lack of correlation between m<(V)over dot>O-2 and complexity at 30 degrees was likely due to low relative task demands, given the similar kinetics of m<(V)over dot>O-2 and torque complexity. An inverse correlation between m<(V)over dot>O-2 and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths.

Automated summary

The study showed that the correlation between torque complexity and metabolic rate may change as joint angle decreases, and muscle length during high-intensity contractions has an impact on this correlation.