Abstract
BackgroundTranscutaneous neuromuscular electrical stimulation (NMES) can be applied as a complementary intervention to regular exercise training programs. A distinction can be made between high-frequency (HF) NMES and low-frequency (LF) NMES. In order to increase understanding of the mechanisms of functional improvements following NMES, the purpose of this study was to systematically review changes in enzyme activity, muscle fiber type composition and muscle fiber size in human lower-limb skeletal muscles following only NMES.MethodsTrials were collected up to march 2012 and were identified by searching the Medline/PubMed, EMBASE, Cochrane Central Register of Controlled Trials, CINAHL and The Physical Therapy Evidence Database (PEDro) databases and reference lists. 18 trials were reviewed in detail: 8 trials studied changes in enzyme activities, 7 trials studied changes in muscle fiber type composition and 14 trials studied changes in muscle fiber size following NMES.ResultsThe methodological quality generally was poor, and the heterogeneity in study design, study population, NMES features and outcome parameters prohibited the use of meta-analysis. Most of the LF-NMES studies reported significant increases in oxidative enzyme activity, while the results concerning changes in muscle fiber composition and muscle size were conflicting. HF-NMES significantly increased muscle size in 50% of the studies.ConclusionNMES seems to be a training modality resulting in changes in oxidative enzyme activity, skeletal muscle fiber type and skeletal muscle fiber size. However, considering the small sample sizes, the variance in study populations, the non-randomized controlled study designs, the variance in primary outcomes, and the large heterogeneity in NMES protocols, it is difficult to draw definitive conclusions about the effects of stimulation frequencies on muscular changes.
Highlights
Regular exercise training programs consist of a combination of aerobic and strengthening exercises for developing and maintaining muscular endurance and strength, respectively [1]
A single session of neuromuscular electrical stimulation (NMES) is sufficient to stimulate molecular-level responses, which are indicative of the initiation of myogenic processes in skeletal muscle, while an additional NMES session, was sufficient to induce an increase in the concentration of total ribonucleic acid (RNA) [16], most likely representing an increase in muscle protein synthesis
The purpose of this study is to systematically review changes in enzyme activity, muscle fiber type composition and muscle fiber size in human lower-limbs following a NMES programme
Summary
Regular exercise training programs consist of a combination of aerobic and strengthening exercises for developing and maintaining muscular endurance and strength, respectively [1]. Some studies suggest preferential or selective activation of fast motor units with NMES [19,20], whereas others suggest that motor unit recruitment during NMES reflects a non-selective, spatially fixed, and temporally synchronous pattern rather than in a reversal of the physiological voluntary recruitment order [17]. These diverse results could have been related to differences in protocols and stimulated muscles [14]. In order to increase understanding of the mechanisms of functional improvements following NMES, the purpose of this study was to systematically review changes in enzyme activity, muscle fiber type composition and muscle fiber size in human lower-limb skeletal muscles following only NMES
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