Fatigue of Paralyzed and Control Thenar Muscles Induced by Variable or Constant Frequency Stimulation

Abstract

Muscles paralyzed by chronic (>1 yr) spinal cord injury fatigue readily. Our aim was to evaluate whether the fatigability of paralyzed thenar muscles (n = 10) could be reduced by the repeated delivery of variable versus constant frequency pulse trains. Fatigue was induced in four ways. Intermittent supramaximal median nerve stimulation (300-ms-duration trains) was delivered at 1) constant high frequency (13 pulses at 40 Hz each second for 2 min); 2) variable high frequency (each second for 2 min). The first two intervals of each variable frequency train were 5 and 20 ms. The remaining pulses were evenly distributed in time across 275 ms. The number of pulses varied for each subject such that the force time integral in the unfatigued state matched that evoked by a constant 40-Hz train; 3) constant low frequency (7 pulses at 20 Hz each second for 4 min); and 4) variable low frequency (each second for 4 min). The pulse pattern was the same as that for variable high frequency except that the force-time integral was matched to that produced by the constant low-frequency stimulation. These same experiments were performed on the thenar muscles of five able-bodied control subjects. The variable high-frequency trains used to fatigue paralyzed and control muscles had an average (+/- SE) of 12 +/- 2 and 10 +/- 1 pulses, respectively. Variable low-frequency trains had 7 +/- 1 and 6 +/- 1 pulses, respectively. Significant mean force declines of comparable magnitude (to 20-25% initial fatigue force or to 13-21% initial 50 Hz force) were seen in paralyzed muscles with all four stimulation protocols. The force reductions in paralyzed muscles were always accompanied by significant increases in half-relaxation time and decreases in force-time integral, irrespective of the stimulation protocol. Significant force decreases also occurred in control muscles during each fatigue test. Again, these force declines were similar whether constant or variable pulse patterns were used at high or low frequencies (to 40-60% initial fatigue force or to 29-36% initial 50 Hz force). The force reductions in control muscles were significantly less than those seen in paralyzed muscles, except when constant high-frequency stimulation was used. The variations in stimulation frequency, pulse pattern, and pulse number used in this study therefore had little influence on thenar muscle fatigue in control subjects or in spinal cord-injured subjects with chronic paralysis.

Minimizing muscle fatigue through optimization of electrical stimulation parameters

“…[18,38] Nevertheless, other researchers have also used a threshold of 50%. [34,39,40] Because In the present results, at lower levels of injected PA×PD, a low signal-to-noise ratio combined with low security in torque production resulted in low bilateral symmetry (see Table 1) and higher sensitivity to measurement error (e.g., when low torque levels [< 10 Nm] are generated in Figure 3). However, our measurement precision was sufficiently high at higher levels of injected PA×PD, where both torque production and fatigue were substantial that has higher clinical relevance.…”

“…Presently, we applied our protocol to FT and TTI, as two previously used fatigues indices in the literature. [18,20,[33][34][35][36][37] A drop of MT from its maximum (100%) to 70.7% (3 dB) was chosen since, in the control engineering literature, when the system output drops by 3 dB, its performance is considered altered. [33] This value has been used in other studies.…”

A novel modulation strategy to increase stimulation duration in neuromuscular electrical stimulation

“…23 Doublet stimulation has also been demonstrated to provide both positive and negative effects on fatigue, depending on the protocol. [24][25][26][27] In all the aforementioned studies, the stimulation intensity (amplitude) was kept constant, whereas frequency was modulated. Our study has examined the use of varied frequency and intensity stimulation to improve the closed-loop performance of the quadriceps femoris muscle group in terms of limb position tracking error.…”

Electrical stimulation of transplanted motoneurons improves motor unit formation

“…7B). Use of fewer pulses may reduce fatigue (Garland et al 1988;Thomas et al 2003a). Even though reinnervated MG muscles often produced a fraction of the force of uninjured muscles (ϳ5% uninjured, on average, in groups receiving 1 h of stimulation with short MRT; Fig.…”