Quadriceps Muscle Length Differentially Affects Exercise Capacity, the Torque‐Duration Relationship, and Neuromuscular Fatigue

ABSTRACT

To determine the effects of muscle length on exercise capacity, critical torque/W′ and neuromuscular fatigue. Thirteen subjects performed 60 isometric maximal voluntary contractions (MVC) over a period of 5 min, at five knee angles to manipulate maximal torque (i.e., optimal, −15°, −30°, +15° and +30°). Exercise capacity was quantified by total impulse. Critical torque was calculated as the mean torque of the last 30s of exercise, and W′ as the impulse done above critical torque. Cardiometabolic and ventilatory responses were measured. MVC, potentiated and interpolated twitches were performed to quantify neuromuscular fatigue, muscle contractility, and voluntary activation failure. Compared to optimal muscle length (i.e., the length producing the greatest torque‐output), total impulse was reduced in all conditions ( p  < 0.001) but −15°. W′ was reduced in every condition whereas critical torque was reduced only in +15° and +30° ( p  < 0.001). Neuromuscular fatigue, contractile impairments, and oxygen cost of contraction were reduced in −15° and −30°. In contrast, no difference in the degree of neuromuscular and contractile fatigue was found at +15° and +30° exercise end. However, the rate at which neuromuscular and contractile fatigue accumulated, as well as the oxygen cost of contraction were increased. Compared to optimal length, muscle lengthening and shortening produced symmetrical effects on W′, suggesting that maximal torque plays an important role in determining W′. In contrast, the asymmetrical effects of changing muscle length on critical torque, neuromuscular fatigue, and its mechanisms suggest that the role of maximal torque in determining these variables is limited.