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 muscles were examined in subjects with chronic (> 1 year) cervical spinal cord injury (SCI, n = 16), and also in a control group of able-bodied (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 study was conducted at a research and rehabilitation center for spinal cord injury.

MAIN OUTCOME MEASURES

Force profiles were analyzed for the maximum (peak) amplitude, the rise time, and the time constant of relaxation.

RESULTS

At the outset, the average peak isometric measured in the SCI group was approximately one half that of the AB subjects. Although the relative decline in force with repeated stimulation was comparable between groups, the slowing of relaxation rate was much more pronounced in the SCI group.

CONCLUSIONS

These findings are consistent with alterations in the metabolic profiles of wrist flexor muscles in 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 muscle relaxation, to avoid overdriving of the muscle and a loss of functional capacity.