Electrical stimulation and neuromuscular fatigue in healthy and chronic post-stroke populations

by 1961- Doucet, Barbara Mollere

Neuromuscular electrical stimulation (NMES) is effective for recovery of motor function following injury or pathology, however, NMES can impart rapid fatigue and the specific parameters of stimulation that maximize force output and delay the onset of fatigue remain unclear. Variable stimulation patterns have been previously examined, and in most cases, are administered using submaximal stimulation levels that target a limited number of muscle fibers. Responses obtained using this method may not be representative of motor behavior of the whole muscle. To determine if stimulation intensity impacted the fatigue response, we investigated three different variable stimulation patterns and compared the effects at submaximal as well as supramaximal intensities. Ten young healthy individuals were tested with three 3-minute intermittent stimulation patterns applied to the thenar muscle at both intensities: 1) a 20 Hz pattern, 2) a pattern of 90s at 20 Hz followed by 90s of a gradual increase from 20 Hz to 40 Hz; and 3) a pattern of 90s of 20 Hz followed by 90s of doublets. Significant differences were found overall between the submaximal and supramaximal stimulation intensity conditions. At submaximal intensities, the doublet pattern showed a significantly higher FTI (1.40 ± 0.02 kN•s vs. 1.05 ± 0.01 kN•s for 20 Hz and 1.17 ± 0.01 kN•s for 20 20-40 Hz), higher average forces (7.89 ± 2.79 N vs. 5.99 ± 0.87 N for 20 Hz and 6.66 ± 0.93 N for 20-40 Hz) and a higher peak force overall (11.02 ± 0.88 N vs. 8.30 ± 0.27 N for 20 Hz and 9.35 ± 0.25 N for 20-40 Hz). At supramaximal intensities, the doublet pattern - 34 - showed the lowest FTI, 1.54 ± 0.01 kN•s vs. 1.62 ± 0.01 kN•s for the 20 Hz and 1.69 ±0.01 kN•s for the 20 20-40 Hz pattern. Variable stimulation patterns administered to the thenar muscle of young healthy individuals showed significantly different results at submaximal and supramaximal intensities. Consideration of parameters and intensities should be given when investigating the motor benefits of NMES and when using this modality in clinical settings. - 35 -