Within‐train neuromuscular propagation varies with torque in paralyzed human muscle

Abstract

Electromyographic (EMG) recordings may serve an important role in predicting torque during repetitive activation of paralyzed muscle. We compared the initial M-wave to the subsequent M-waves of the same train under fatigued and recovered conditions in the paralyzed human soleus muscle. Sixteen individuals with chronic (n = 13) or acute paralysis (n = 3) had the tibial nerve activated before and after a repetitive supramaximal stimulation protocol. The mean within-train M-wave amplitude and median frequency increased approximately 20%, whereas the duration decreased approximately 15% compared with the initial M-wave of each train. During fatigue, there was a linear decrease in the difference between the initial M-wave amplitude and subsequent train ( approximately 20% to 8%). Following fatigue, this difference recovered to approximately 12%. The difference between the M-wave train average and the initial M-wave for amplitude, duration, and median frequency closely followed torque (Pearson correlations = 0.99, 0.94, and 0.98, respectively) during fatigue. We conclude that the difference between the later-occurring M-waves (average of the train) and initial M-wave is large when muscle torque is high and less when torque is low and, therefore, predicts torque during activation of paralyzed muscle. This difference in the within-train M-wave amplitude, duration, and median frequency may reflect a mechanical change, such as muscle shortening and increased muscle cross-sectional area during isometric contractions. Electromyographic feedback may assist in the optimization of neuromuscular electrical stimulation of paralyzed muscle.

Decreased central fatigue in multiple sclerosis patients after 8 weeks of surface functional electrical stimulation

“…It should be noted that the current study did not disclaim the influence of peripheral fatigue on patients with MS. Lenman et al suggested that patients with MS suffered from higher levels of peripheral fatigue than nondisabled subjects [5]. A previous study showed that in paralyzed muscle, twitch potentiated more in low frequency fatigue status than in fresh status [32]. Although they did not reach statistical significance, the results of the current study showed that the twitch force potentiated before decreasing (Figure 2(c)), suggesting that the subjects might have been in a peripheral fatigue status before the voluntary fatigue protocol was applied.…”

High-frequency submaximal stimulation over muscle evokes centrally generated forces in human upper limb skeletal muscles

“…Irrespective of the exact mechanism, this finding has important practical implications for functional electrical stimulation (FES) in patient populations. Although there are potential benefits from low-frequency FES in the upper limb after spinal cord injury, the benefits do not occur in all patients, due to an inability to achieve high muscle power and due to fatigue of any evoked forces (7,20). Because the long-pulse-width, high-frequency stimulation trains elicit the enhanced forces via afferent mediated stimulation of motoneurons, they have the advantage of potentially activating the motoneurons according to the size principle (23,24).…”

Robot-Assisted Passive Exercise for Ankle Hypertonia in Individuals with Chronic Spinal Cord Injury

“…One may argue that the amplitude of M waves should be reduced after ES due to fatigue. However, previous research reported that the reduction of M waves after fatigue was related to neuromuscular transmission failure and that they recovered within 5 min after fatigue [45,46]. In this study, the M wave was measured at 10 min post-treatment, at which time the neuromuscular transmission should have been fully recovered.…”