Neuromuscular and biomechanical coupling in human cycling: modulation of cutaneous reflex responses to sural nerve stimulation.

This study tested the hypothesis that the modulation of cutaneous reflexes during human cycling would be dependent on muscle biomechanical function and phase of leg movement. The coupling between neuromuscular (electromyographic, EMG), kinetic and kinematic responses to brief innocuous (75% of the pain threshold PnT) and noxious (125% PnT) sural nerve stimulation were studied. Stimuli were delivered pseudorandomly at eight equidistant (45 degrees) positions of the crank cycle. Peak ipsilateral middle latency EMG reflex responses were calculated between 70 and 130 ms post stimulus in Biceps Femoris (BF), Rectus Femoris (RF), Tibialis Anterior (TA) and Soleus (SOL). Peak torque, knee and ankle joint angle changes were calculated between 140 and 220 ms post stimulus to quantify net kinetic and kinematic reflex modulation. Reflex responses were predominately suppressive during early activation of all muscles and facilitatory during BF and TA muscle inactivation. EMG reflex responses in monoarticular lower leg muscles TA and SOL were well correlated with ankle angle in dorsi/plantaflexion, whereas the correlation between reflex modulation in biarticular upper leg muscles (BF and RF) and knee angle changes in flexion/extension was weaker. Stimulation provoked significant ankle eversion over the whole crank cycle for both stimulus intensities, which was correlated with TA and BF EMG reflex responses. Torque modulation followed EMG and kinematic changes in a movement phase-dependent manner. Reflex magnitude was stimulation intensity-dependent. Supplementary nociceptive activation may contribute for this increase. We conclude that sural nerve stimulation during human cycling evokes distinct reflex responses in muscles operating around the knee (BF and RF) and the ankle (TA and SOL). These reflexes are modulated in a phase-dependent manner depending on muscle biomechanical function to generate energy for limb and crank propulsion during a specific region in the cycle. This modulation contributed to a specific adaptation of joint motion and force production in order to maintain task performance.