Effects of stimulation frequency versus pulse duration modulation on muscle fatigue

Abstract

During functional electrical stimulation (FES), both the frequency and intensity can be increased to increase muscle force output and counteract the effects of muscle fatigue. Most current FES systems, however, deliver a constant frequency and only vary the stimulation intensity to control muscle force. This study compared muscle performance and fatigue produced during repetitive electrical stimulation using three different strategies: (1) constant pulse-duration and stepwise increases in frequency (frequency-modulation); (2) constant frequency and stepwise increases in pulse-duration (pulse-duration-modulation); and (3) constant frequency and pulse-duration (no-modulation). Surface electrical stimulation was delivered to the quadriceps femoris muscles of 12 healthy individuals and isometric forces were recorded. Muscle performance was assessed by measuring the percent changes in the peak forces and force-time integrals between the first and the last fatiguing trains. Muscle fatigue was assessed by measuring percent declines in peak force between the 60 Hz pre-and post-fatigue testing trains. The results showed that frequency-modulation showed better performance for both peak forces and force-time integrals in response to the fatiguing trains than pulse-duration-modulation, while producing similar levels of muscle fatigue. Although frequencymodulation is not commonly used during FES, clinicians should consider this strategy to improve muscle performance.

Effect of adjusting pulse durations of functional electrical stimulation cycling on energy expenditure and fatigue after spinal cord injury

“…We have clearly documented that longer pulse duration, but not stimulation duration, resulted in a greater evoked and normalized torque compared with the shorter pulse duration, even after controlling for the activated muscular cross-sectional areas (CSAs) [14]. Others have clearly documented that increasing pulse duration to 600 µs results in increasing the evoked torque of the knee extensor [9,[15][16]. It is still unclear whether skeletal muscle atrophy and infiltration of intramuscular fat interfere with the capacity of adjusting pulse durations to increase the evoked torque following SCI [19].…”

“…The fatigue tests were done before, immediately after, and 48 to 72 h after each cycling session by establishing force-frequency curves [15][16]. A lift (ArjoHuntleigh; Malmö, Sweden) mounted in the ceiling was available to transfer subjects onto the isokinetic dynamometer chair (Biodex Medical Systems Inc; Shirley, New York) in order to measure torque generated by the knee extensor muscle group.…”

“…The frequency order was randomly assigned to each participant [15][16]. Subjects were seated with the hip flexed at 90 and the knee flexed at 90 (where 0 corresponds with full knee extension).…”

“…We and others have previously demonstrated that altering electrical stimulation parameters can influence motor unit recruitments and reduce neuromuscular fatigue [10,[13][14][15][16]. Increasing the amplitude of the current and pulse duration has been shown to increase skeletal muscle recruitment but does not interfere with the extent of fatigue during stimulation [10].…”

Relationship between peak and mean amplitudes of the stimulating output voltage for functional control of the knee by spinal cord patients and healthy volunteers

“…All FES profiles (Table 1) were applied to each volunteer randomly, one profile per day, over four testing days with a minimal interval of two rest days (Kesar et al, 2008;Marion et al, 2010;Smith et al, 1997;Stock et al, 2010) between the tests to avoid physiological interference between consecutive protocols. The volunteer was positioned on an adapted chair with the hip and knee angles set to 70° (Matsunaga et al, 1999) and 90°, respectively, as illustrated in Figure 1.…”