Effect of tendon vibration during wide-pulse neuromuscular electrical stimulation (NMES) on the decline and recovery of muscle force

Abstract

Background: Neuromuscular 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.

Methods: Tendon 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.

Results: TTI 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.

Conclusions: Tendon 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.

Keywords: Meuro-rehabilitation; Muscle damage; Muscle fatigue; Muscle function; Muscle stimulation; Muscle strength.

Fig. 1

Picture showing the electrodes position on the thigh muscles and the placement of tendon vibration during one of the sessions. Patellar tendon vibration was applied with a vibration device (Deep Muscle Stimulator, Las Vegas, NV, USA) to mechanically vibrate the tendon. The tip of the vibration device was maintained at a steady pressure in a fixed position on the tendon immediately distal to the inferior border of the patella. This position was marked on the skin, and covered by a thin (1 mm thickness) soft pad to minimize pain or abrasion

Fig. 2

Torque production (Nm) for a positive and a negative responder during STIM + Vib. Last: last contraction before target fatigue. Target torque = 20% MVIC. Target Fatigue = 60% of target torque

Fig. 3

a Percentage difference between STIM and STIM + Vib in torque-time integral (Nm·s) for positive and negative responders (145.0 ± 127.7% and −43.5 ± 25.7%). b Mean torque-time integral (TTI; Nm·s) for positive responders and negative responders for STIM (1201.2 ± 321.9 Nm·s and 2402.6 ± 497.7 Nm·s) and STIM + Vib (2757.2 ± 1329.8 Nm·s and 1344.0 ± 674.6 Nm·s). *Significant difference from STIM (P < 0.05)

Fig. 4

Torque production (Nm) for a positive responder during STIM + Vib and STIM. Last: last contraction before target fatigue. Target torque = 20% MVIC. Target Fatigue = 60% of target torque

Fig. 5

Changes in peak isometric voluntary contraction torque (MVIC) across time (PRE, POST, 1H and 48H). ^#Significant difference from PRE (P < 0.05) for STIM. Mean values ± standard error (SE). Inset: Percentage change in MVIC from PRE to POST in STIM and STIM + Vib conditions. *Significant difference from PRE (P < 0.05). Mean change ± SD