Purpose: To examine the interrelationship of pulse duration and pulse frequency on torque production and muscle fatigue in both impaired and non-impaired skeletal muscle of men and women.
Methods: Individuals with [n = 14 (6 females), 38 ± 13 yr; 175 ± 11 cm; 76 ± 20 kg] and without [n = 14 (6 females), 29 ± 8 yr; 175 ± 9 cm; 74 ± 14 kg] spinal cord injury (SCI) participated. Muscle torque was recorded during a series of NMES-induced isometric muscle contractions using different combinations of pulse durations and frequencies. Additionally, two different muscle fatigue protocols (20 and 50 Hz/200µs) were utilized to elicit repeat isometric muscle contractions (1s on and 1s off × 3 min).
Results: There was a statistically significant linear trend for pulse charge (the product of pulse frequency and pulse duration) on isometric torque production in participants without (p < 0.001, η2 = 0.79), and in participants with SCI (p < 0.001, η2 = 0.66), with higher total pulse charge generating higher torque values. Participants with SCI had significantly greater muscle fatigue for both muscle fatigue protocols (p < 0.05).
Conclusions: NMES protocols should consider using longer pulse durations with lower frequencies to maximize force production for individuals with SCI. However, because mechanisms of muscle fatigue may be different for impaired muscle when compared to non-impaired muscle, further studies on protocols to offset fatigue are warranted.
Keywords: Electrical stimulation; Force production; Muscle fatigue; Spinal cord injury.