MECHANICAL AND FATIGUE PROPERTIES OF WRIST FLEXOR MUSCLES DURING REPETITIVE CONTRACTIONS AFTER CERVICAL SPINAL-CORD INJURY

Objectives: Force generation and fatigue properties of wrist flexor mu scles were examined in subjects with chronic (> 1 year) cervical spina l cord injury (SCI, n = 16), and also in a control group of able-bodie d (AB, n = 9) subjects. Design: Using surface electrodes, wrist flexor muscles were stimulated with 126 trains of 26 stimuli at a frequency of 40Hz. The offset of each train was followed by a 1.5-second pause, for a total fatigue-test time of approximately 4.2 minutes. Isometric wrist flexion force was measured with a strain gauge. Setting: This st udy was conducted at a research and rehabilitation center for spinal c ord injury. Main Outcome Measures: Force profiles were analyzed for th e maximum (peak) amplitude, the rise time, and the time constant of re laxation. Results: At the outset, the average peak isometric measured in the SCI group was approximately one half that of the AB subjects. A lthough the relative decline in force with repeated stimulation was co mparable between groups, the slowing of relaxation rate was much more pronounced in the SCI group. Conclusions: These findings are consisten t with alterations in the metabolic profiles of wrist flexor muscles i n the SCI group, probably reflected their altered activation pattern. When designing stimulation protocols for optimizing force and fatigue resistance in muscle left partially-paralyzed after spinal cord injury , particular care must be taken to allow adequate time for complete mu scle relaxation, to avoid overdriving of the muscle and a loss of func tional capacity. (C) 1995 by the American Congress of Rehabilitation M edicine and the American Academy of Physical Medicine and Rehabilitati on