Electrical stimulation and neuromuscular fatigue in healthy and chronic post-stroke populations
Neuromuscular electrical stimulation (NMES) has been shown to be 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. Frequency, intensity, and pulse pattern are a few of the parameters that can be manipulated to achieve desired outcomes. Strong evidence supports the use of higher frequencies of stimulation to maximize performance of fatigued or paralyzed muscle. Likewise, several studies advocate the use of varied pulse patterns, rather than constant pulses, to maximize force output as well. Much of the research literature regarding NMES use with a pathological population involves spinal injured individuals. Much less is known about the effect of NMES on motor recovery, especially when delivered through varied pulse patterns, in the post-stroke population. The three studies contained herein addressed these issues. In Study 1, submaximal and supramaximal intensities of two variable frequency stimulation patterns and one constant frequency pattern were applied to the thenar muscles of a young healthy population. Results showed marked differences in force output between the two intensities. Submaximal stimulation enhanced the effect of the variable pulses and a greater force response was produced. In Study 2, the same three patterns were applied to the thenar muscles of a post-stroke group and an age-matched able-bodied group at submaximal intensities. Again, differences in force output were evident between the healthy and pathological group, and the variable pulses may have further depressed already weakened hemiplegic muscle. Study 3 compared the effects of a NMES rehabilitation program using a high (40 Hz) and a low (20 Hz) frequency to determine if task-specific improvements were related to frequency used. Results showed that those in the high frequency condition demonstrated greater improvements in strength, dexterity and force accuracy; those in the low frequency condition showed improvements in motor endurance. The results of this work suggest that the intensity, frequency and stimulation pattern of NMES used have a significant impact on the resultant muscle contraction and functional skills gained following stroke and should be carefully considered when implementing a clinical regimen for motor recovery.