Influence of load type on presynaptic modulation of Ia afferent input onto two synergist muscles

The present work was designed to investigate presynaptic modulation of Ia afferents in the extensor (ECR) and flexor carpi radialis (FCR) when the two muscles acted as synergists during radial deviation to either support an inertial load (position task) or exert an equivalent constant torque against a rigid restraint (force task). H reflexes were evoked in the ECR and FCR by stimulating at the elbow level (1-ms duration) the radial and median nerves, respectively. Conditioning stimulation was applied to the median and radial nerves at the elbow level to assess presynaptic inhibition of homonymous Ia afferent input (D1 inhibition) from the ECR and FCR, respectively. The ongoing presynaptic inhibition of heteronymous Ia afferents that converges onto ECR and FCR motor neuron pools (heteronymous Ia facilitation) was assessed by stimulating the median nerve at the wrist level (palmar branch) prior to the stimulus applied over the radial or median nerve. The heteronymous monosynaptic Ia facilitation was greater (P < 0.05) during the position task (ECR 121%; FCR 147%) compared with the force task (ECR 115%; FCR 132%), and was paralleled by the depression of D1 inhibition (P < 0.05) during the position task (ECR 75.4%; FCR 79.0%) compared with force task (ECR 58.7%; FCR 58.8%). These data indicate that Ia presynaptic inhibition is reduced during the position task relative to the force task. Such differential modulation of Ia afferent input onto the motor neuron pool likely reflects the requirement to heighten reflex responsiveness during the unstable task of maintaining limb position.