Motor unit behavior during clonus.
Medial gastrocnemius surface electromyographic activity and intramuscular electromyographic activity were recorded from six individuals with chronic cervical spinal cord injury to document the recruitment order of motor units during clonus. Four subjects induced clonus that lasted up to 30 s while two subjects induced clonus that they actively stopped after 1 min. Mean clonus frequency in different subjects ranged from 4.7 to 7.0 Hz. Most of the 166 motor units recorded during clonus (98%) fired once during each contraction but at slightly different times during each cycle. Other motor units fired during some clonus cycles (1%) or in bursts (1%). When 59 pairs of units were monitored over consecutive clonus cycles (n = 5-89 cycles), only 8 pairs of units altered their recruitment order in some cycles. Recruitment reversals only occurred in units that fired close together in the clonus cycle. These data demonstrate that orderly motor unit recruitment occurs during involuntary contractions of muscles paralyzed chronically by cervical spinal cord injury, providing further support for the importance of spinal mechanisms in the control of human motor unit behavior.