DISTINCT PATTERNS OF MOTOR UNIT BEHAVIOR DURING MUSCLE SPASMS IN SPINAL-CORD INJURED SUBJECTS

Surface electromyograms (EMG) and force were recorded during repeated involuntary spasms of paralyzed triceps surae muscles of four men with chronic cervical spinal cord injury. The firing rates of 78 medial ga strocnemius (MG) motor units also were recorded intramuscularly with t ungsten microelectrodes. Spasms typically involved a relatively rapid rise, then a more gradual fall in triceps surae EMG and torque. Motor unit firing rates either increased and then decreased with the spasm i ntensity (54%) or were relatively constant (26%), firing mainly at 2-1 0 Hz. The remaining units (20%) produced trains that included one or s everal doublets. Mean peak spasm firing rates were 18 +/- 9 Hz (mean /- SD) for rate modulated units and 11 +/- 10 Hz for units with little or no rate modulation. Some motor units fired at rates comparable wit h those recorded previously during maximum voluntary contractions perf ormed by intact subjects. Others fired at rates below the minimum usua lly seen when normal units are first recruited (<6 Hz). Doublets (inte rspike interval <10 ms) often repeated every 123-333 ms, or were inter spersed in trains firing at low steady rates (<11 Hz). This study show s that rate coding for many motor units appears to be similar whether descending motor input is intact or whether it has been reduced severe ly by spinal cord injury. In contrast, rate modulation in other units appears to depend mainly on voluntary motor commands.