Reflex reciprocal facilitation of antagonist muscles in spinal cord injury

Study Design: Electromyographic study in complete and incomplete spinal cord injury (SCI). Objective: To examine the changes in the pattern of reciprocal inhibition between agonist and antagonist muscles in SCI. Settings: Sensory Motor Performance Program, Rehabilitation Institute of Chicago, IL, USA. Methods: Tendon taps were delivered manually with an instrumented hammer to the tendons of the tibialis anterior and soleus muscle in positions of full-ankle dorsiflexion and plantarflexion in eight subjects with complete SCI and eight subjects with incomplete SCI. Electromyographic activity (EMG) was recorded from ankle dorsiflexor and plantarflexor muscles. Tapping force was also recorded by a force sensor mounted to the tendon hammer, indicating the stimulus onset. Measures of re. ex EMG magnitude and re. ex latency were obtained for both agonist and antagonist muscles. The ratio of antagonist to agonist EMG was computed based on normalized EMG. Results: Substantial re. ex responses occurred in both the stretched muscle and in its antagonist. The re. ex in antagonist, which we term 'reciprocal facilitation (RF)', was most evident in subjects with incomplete SCI. The magnitude of RF was consistently greater than re. ex responses in agonist muscles under all test conditions. The latency of the RF was comparable to that of monosynaptic re. ex response. Conclusions: Following SCI, reciprocal organization of segmental reflexes at the ankle is often partially or completely suppressed, allowing re. ex activation in antagonist muscles to be manifested. Possible mechanisms underlying these changes in neural organization are discussed. Sponsorship: This study was supported by Spinal Cord Research Foundation, the Paralyzed Veterans of America.