Abstract
BackgroundNeuromuscular electrical stimulation (NMES) is commonly used to activate skeletal muscles and reverse muscle atrophy in clinical populations. Clinical recommendations for NMES suggest the use of short pulse widths (100–200 μs) and low-to-moderate pulse frequencies (30–50 Hz). However, this type of NMES causes rapid muscle fatigue due to the (non-physiological) high stimulation intensities and non-orderly recruitment of motor units. The use of both wide pulse widths (1000 μs) and tendon vibration might optimize motor unit activation through spinal reflex pathways and thus delay the onset of muscle fatigue, increasing muscle force and mass. Thus, the objective of this study was to examine the acute effects of patellar tendon vibration superimposed onto wide-pulse width (1000 μs) knee extensor electrical stimulation (NMES, 30 Hz) on peak muscle force, total impulse before “muscle fatigue”, and the post-exercise recovery of muscle function.MethodsTendon vibration (Vib), NMES (STIM) or NMES superimposed onto vibration (STIM + Vib) were applied in separate sessions to 16 healthy adults. Total torque-time integral (TTI), maximal voluntary contraction torque (MVIC) and indirect measures of muscle damage were tested before, immediately after, 1 h and 48 h after each stimulus.ResultsTTI increased (145.0 ± 127.7%) in STIM only for “positive responders” to the tendon vibration (8/16 subjects), but decreased in “negative responders” (−43.5 ± 25.7%). MVIC (−8.7%) and rectus femoris electromyography (RF EMG) (−16.7%) decreased after STIM (group effect) for at least 1 h, but not after STIM + Vib. No changes were detected in indirect markers of muscle damage in any condition.ConclusionsTendon vibration superimposed onto wide-pulse width NMES increased TTI only in 8 of 16 subjects, but reduced voluntary force loss (fatigue) ubiquitously. Negative responders to tendon vibration may derive greater benefit from wide-pulse width NMES alone.
Highlights
Neuromuscular electrical stimulation (NMES) is commonly used to activate skeletal muscles and reverse muscle atrophy in clinical populations
A positive versus negative responder to tendon vibration analysis was undertaken where positive responders to tendon vibration were defined as participants who responded with a greater torque-time integral (TTI) after stimulation train (STIM) + Vib and negative responders to tendon vibration as participants who showed
It appears that negative responders to tendon vibration will show a decrease in total muscle contractile work if tendon vibration is added and in these cases tendon vibration superimposed onto wide-pulse width NMES may represent a disadvantage and the use of NMES alone would be more beneficial to elicit a high muscle force production
Summary
Neuromuscular electrical stimulation (NMES) is commonly used to activate skeletal muscles and reverse muscle atrophy in clinical populations. Strength training poses an increasing challenge for people with a neurological condition, such as people with spinal cord injury (SCI) who have limited ability, or are unable, to voluntarily activate their muscles Due to this limitation, neuromuscular electrical stimulation (NMES) has been conventionally used in clinical practice, in the form of functional electrical stimulation (FES), i.e. a continuous, prolonged stimulation at low-to-moderate frequencies (30–50 Hz) paired simultaneously or intermittently with a functional task (e.g. cycling) [8]. FES exercise has been shown to slow muscle weakening or even increase muscle strength as well as reduce the rate of skeletal muscle atrophy and weakness and improve physical health in people with a SCI [9,10,11,12] Such interventions evoke only low relative muscle forces [9, 13] and may not optimally stimulate neuromuscular strength and mass increases [14]. Instead the imposition of a higher load on the muscle with intermittent rest periods to allow continuous higher force output would be preferable [15]
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