SPINAL AND SUPRASPINAL MECHANISMS AFFECTING TORQUE DEVELOPMENT AT DIFFERENT JOINT ANGLES

Introduction: We examined the neural mechanisms responsible for plantar flexion torque changes at different joint positions. Methods: Nine subjects performed maximal voluntary contractions (MVC) at 6 ankle-knee angle combinations [3 ankle angles (dorsiflexion, anatomic position, plantar flexion) and 2 knee angles (flexion, full extension)]. Neural mechanisms were determined by V-wave, H-reflex (at rest and during MVC), and electromyography during MVC (RMS), normalized to the muscle compound action potential (V/M-sup, H-max/M-max, H-sup/M-sup and RMS/M-sup) and voluntary activation (VA), while muscle function was assessed by doublet amplitude. Results: MVC and doublet amplitude were significantly lower at plantar flexion (P < 0.01), while VA was significantly lower at dorsiflexion and full knee extension (P < 0.05). V/M-sup and RMS/M-sup were significantly lower at knee extension (P < 0.01), while H-sup/M-sup was not affected by joint angle. Conclusions: These results indicate that joint positions leading to muscle lengthening produce reduced neural drive, due mainly to supraspinal mechanisms.