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

<jats:title>Abstract</jats:title><jats:sec> <jats:title>Purpose</jats:title> <jats:p>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 (<jats:inline-formula><jats:alternatives><jats:tex-math>$${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula>) would be conserved at reduced joint angles (i.e. shorter muscle lengths).</jats:p> </jats:sec><jats:sec> <jats:title>Methods</jats:title> <jats:p>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. <jats:inline-formula><jats:alternatives><jats:tex-math>$${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> was determined using near-infrared spectroscopy.</jats:p> </jats:sec><jats:sec> <jats:title>Results</jats:title> <jats:p>Time to task failure/end increased as joint angle decreased (<jats:italic>P</jats:italic> &lt; 0.001). Over time, complexity decreased at 90º and 60º (decreased ApEn, increased DFA <jats:italic>α</jats:italic>, both <jats:italic>P</jats:italic> &lt; 0.001), but not 30º. <jats:inline-formula><jats:alternatives><jats:tex-math>$${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> increased at all joint angles (<jats:italic>P</jats:italic> &lt; 0.001), though the magnitude of this increase was lower at 30º compared to 60º and 90º (both <jats:italic>P</jats:italic> &lt; 0.01). There were significant correlations between torque complexity and <jats:inline-formula><jats:alternatives><jats:tex-math>$${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> at 90º (ApEn, <jats:italic>r</jats:italic> =  − 0.60, <jats:italic>P</jats:italic> = 0.049) and 60º (ApEn, <jats:italic>r</jats:italic> =  − 0.64, <jats:italic>P</jats:italic> = 0.035; DFA <jats:italic>α</jats:italic>, <jats:italic>ρ</jats:italic> = 0.68, <jats:italic>P</jats:italic> = 0.015).</jats:p> </jats:sec><jats:sec> <jats:title>Conclusion</jats:title> <jats:p>The lack of correlation between <jats:inline-formula><jats:alternatives><jats:tex-math>$${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> and complexity at 30º was likely due to low relative task demands, given the similar kinetics of <jats:inline-formula><jats:alternatives><jats:tex-math>$${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> and torque complexity. An inverse correlation between <jats:inline-formula><jats:alternatives><jats:tex-math>$${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mover> <mml:mtext>V</mml:mtext> <mml:mo>˙</mml:mo> </mml:mover> <mml:mtext>O</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths.</jats:p> </jats:sec>