Does neuromuscular electrical stimulation strengthen the quadriceps femoris? A systematic review of randomised controlled trials

Devices for neuromuscular electrical stimulation (NMES) are increasingly used by individuals without specific injuries and are standard equipment in most physical therapy practices. The most often stimulated muscle group is the quadriceps femoris. We designed a systematic review and meta-analysis of randomised controlled trials to determine whether NMES is an effective modality for strength augmentation of the quadriceps femoris. A full content search for randomised controlled trials was performed in Medline, Embase, Cinahl, the Cochrane Controlled Trials Register and the Physical Therapy Evidence Database. Maximum volitional isometric or isokinetic muscle torque in Nm was used as main outcome measure. Thirty-five trials were included and evaluated. A fundamental distinction was made between the trials using subjects with unimpaired quadriceps femoris muscles and the trials using post-injury or post-operative subjects. In the unimpaired quadriceps subgroup, meta-analyses were performed for the comparisons 'NMES versus no exercises' and 'NMES versus volitional exercises'. All other comparisons were evaluated descriptively. The included trials were generally of poor quality and meta-analytic data indicate that publication bias may be present. The evaluated data suggest that, both for the unimpaired and impaired quadriceps, NMES makes sense compared with doing no exercises but volitional exercises appear to be more effective in most situations. Based on the available evidence, NMES may only be preferred over volitional training for within-cast muscle training and perhaps in specific situations where volitional training does not receive sufficient patient compliance. Further research should be directed toward identifying the clinical impact at activity and participation levels and the optimal stimulation parameters of this modality.