MODULATION OF SHORT-LATENCY STRETCH REFLEXES DURING HUMAN HOPPING

To gain insight into central and peripheral reflex control mechanisms in moving humans we have investigated short latency stretch reflex act ivity in m. triceps surae during two legged hopping. The objectives we re: ii) to compare movement induced short latency stretch reflexes in soleus and medial gastrocnemius (MG) muscles, (2) to determine the rel ationship between the size of these reflexes and the muscle spindle st retch velocities, and (3) to compare the size of the movement induced short latency stretch reflexes and the H-reflexes simultaneously. Six well-trained healthy male subjects participated and they hopped at thr ee different work rates. Surface electromyogram (EMG) and H-reflexes w ere recorded during hopping. Muscle spindle length changes were estima ted as the difference between estimated origin-to-insertion length cha nges and tendon length changes. The important findings were that durin g hopping: iii movement induced short latency stretch reflexes were ob served consistently in soleus, (2) the EMG amplitude of this stretch r eflex was negatively correlated with the estimated peak muscle spindle stretch velocity (r(s) = -0.52, P < 0.02), and (3) the amplitude of t he soleus H-reflex at touchdown did not change in parallel with the st retch reflex. The negative correlation observed between the stretch re flex and the estimated peak muscle spindle stretch velocity in soleus is opposite to the basic velocity sensitive behaviour of stretch refle xes mechanically elicited during resting conditions. Possible control mechanisms are discussed. Additionally, muscle spindle length changes estimated from changes in the skeletal movements (joint angles) should be inferred cautiously because of tendon compliance, especially at hi gh tendon forces.