Knee Angle Alters Quadriceps Fatigue and Critical Torque

Key Takeaways

• In 13 physically active adults performing repeated maximal isometric knee extensions, total impulse was highest at the individualized optimal knee angle and was lower at −30°, +15°, and +30°.
• W′ was lower at every nonoptimal knee angle, falling by about 16% to 45% from the optimal position as the quadriceps was either shortened or lengthened.
• Absolute critical torque was preserved at shortened muscle lengths but was lower at +15° and +30°, showing an asymmetrical response around the optimal angle.
• Shorter muscle lengths produced less contractile fatigue, whereas longer muscle lengths raised oxygen cost and accelerated fatigue accumulation without increasing end-exercise fatigue magnitude versus the optimal angle.

Quadriceps performance and fatigue do not read the same way at every knee angle, and a fall in maximal torque may not reflect the same physiology when the muscle is shortened versus lengthened. That distinction matters when torque-based exercise tolerance is interpreted from isometric testing, because the balance between force production, fatigue development, and work capacity can shift around the angle of peak torque.

In a within-subject repeated-measures experiment, 13 physically active adults completed six visits over 2 to 3 weeks, including one familiarization visit and five randomized exercise visits separated by at least 48 hours. They performed 60 maximal isometric knee-extension contractions over 5 minutes in 3-second on, 2-second off cycles at five individualized knee angles centered on the optimal torque-producing angle, which averaged 76.2° ± 4.6°. The conditions were optimal, −15°, −30°, +15°, and +30°; the negative offsets shortened the quadriceps and the positive offsets lengthened it. Exercise capacity was defined as total impulse, critical torque as the mean torque of the last 30 seconds, and W′ as the impulse above critical torque. Maximal voluntary contraction (MVC), potentiated twitches, voluntary activation, surface electromyography (EMG), cardioventilatory variables, blood lactate, and near-infrared spectroscopy (NIRS) were tracked through exercise and recovery, allowing each participant to serve as their own control.

Total impulse was highest at the optimal angle and at −15°, then fell by about 12% at −30°, 21% at +15°, and 37% at +30° versus optimal. Torque in the final 30 seconds appeared to plateau across conditions, supporting the end-exercise estimate of critical torque. Absolute critical torque was reduced only at +15° and +30°, while shortened lengths preserved it, so similar losses of maximal torque did not translate into the same loss of exercise capacity when they came from shortening versus lengthening.

W′ was greatest at the optimal angle and was lower at every nonoptimal condition, decreasing by about 16% to 45% relative to optimal. Shortened muscle lengths produced smaller end-exercise declines in MVC and twitch-based contractile indices, whereas longer lengths reached a similar end-exercise fatigue magnitude as the optimal angle but accumulated fatigue faster during the task. Voluntary activation declined over time without between-condition differences, while vastus lateralis and vastus medialis activation and biceps femoris coactivation increased from the shortest to the longest lengths. Cardioventilatory and metabolic responses were otherwise similar across conditions, most NIRS measures did not differ by muscle length, and the clearest accompanying change was a higher oxygen cost of contraction at +15° and +30°.

These findings describe an isometric knee-extensor model in healthy, physically active adults rather than injured populations, dynamic exercise, or sport-specific tasks. The within-subject design reduced between-person confounding, but the sample was small and restricted to one muscle group and one exercise format. The authors linked the smaller fatigue response at shortened lengths to less contractile impairment rather than to differences in voluntary activation, and they linked the lower critical torque at longer lengths to a higher oxygen cost of contraction.

Changing quadriceps muscle length altered exercise capacity, critical torque, W′, and fatigue through different patterns around the optimal angle. The authors concluded that symmetrical effects on W′ but asymmetrical effects on critical torque and fatigue suggest maximal torque alone did not fully account for critical torque behavior or neuromuscular fatigue in this model.

Clinician Questions

How was critical torque defined in this quadriceps knee-angle experiment?

Critical torque in the 5-minute all-out quadriceps protocol was defined as the mean torque of the last 30 seconds of exercise. Final-30-second torque slopes were not different from zero across conditions, which supported use of that end-exercise plateau estimate.

Which parts of neuromuscular fatigue changed with shorter versus longer quadriceps muscle lengths?

Shorter quadriceps lengths produced smaller end-exercise declines in MVC and twitch-based contractile indices, while voluntary activation declined similarly across conditions. Longer lengths reached a similar end-exercise fatigue magnitude as the optimal length but accumulated fatigue faster during exercise.

What physiological signals did not explain the knee-angle differences in exercise capacity?

Overall cardiometabolic and ventilatory measures were not different between knee-angle conditions, and most NIRS indices were also unchanged by muscle length. Outside torque and fatigue measures, the clearest accompanying physiological difference was a higher oxygen cost of contraction at +15° and +30°.