Surface electromyography (sEMG) is a popular research tool in sport and rehabilitation sciences. Common study designs include the comparison of sEMG amplitudes collected from different muscles as participants perform various exercises and techniques under different loads. Based on such comparisons, researchers attempt to draw conclusions concerning the neuro- and electrophysiological underpinning of force production and hypothesize about possible longitudinal adaptations, such as strength and hypertrophy. However, such conclusions are frequently unsubstantiated and unwarranted. Hence, the goal of this review is to discuss what can and cannot be inferred from comparative research designs as it pertains to both the acute and longitudinal outcomes. General methodological recommendations are made, gaps in the literature are identified, and lines for future research to help improve the applicability of sEMG are suggested.

Non-technical summary In the medial gastrocnemius muscle of cats, the longitudinal size (3-4 cm) of the territory of motor units is large (∼60%) relative to the muscle length. In the human medial gastrocnemius, the size of motor unit territories is unknown. By comparing intramuscular and surface electromyograms we show that, when subjects stand at ease, the motor units active in the medial gastrocnemius have small territories. They extend no longer than 4 cm and no less than 1 cm along the longitudinal axis (∼25 cm long) of the muscle. Physiologically, the small territories of motor units give the nervous system a mechanism for the independent activation of sub-volumes of the medial gastrocnemius muscle.Abstract In cat medial gastrocnemius (MG), fibres supplied by individual motoneurones (muscle units) distribute extensively along the muscle longitudinal axis. In the human MG, the size of motor unit territory is unknown. It is uncertain if the absolute size of muscle unit territory or the size relative to the whole muscle is most comparable with the cat. By comparing intramuscular and surface electromyograms we tested whether muscle units extend narrowly or widely along the human MG muscle. Due to the pennation of the MG, if individual motoneurones supply fibres scattered along the muscle, then action potentials of single motor units are expected to appear sparsely on the surface of the skin. In nine healthy subjects, pairs of wire electrodes were inserted in three locations along the MG muscle (MG 60% , MG 75% and MG 90% ). A longitudinal array of 16 surface electrodes was positioned alongside the intramuscular electrodes. While subjects stood quietly, 55 motor units were identified, of which, significantly more units were detected in the most distal sites. The surface action potentials had maximum amplitude at 4.40 ± 1.67 (mean ± S.D.), 8.02 ± 2.16 and 11.63 ± 2.09 cm (P < 0.001) from the most proximal surface electrode, for motor units in the MG 60% , MG 75% and MG 90% locations, respectively. Single motor unit potentials were recorded by five consecutive surface electrodes, at most, indicating that muscle units extend shortly along the MG longitudinal axis. It is concluded that relative to the whole muscle, and compared with the cat, muscle units in human MG are localised. The localisation of muscle units might have implications for the regional control of muscle activity. Abbreviations ARV, average rectified value; MG, medial gastrocnemius.

This article is the first section of a review work structured in three parts and concerning a) advances in surface EMG detection and processing techniques, b) recent progress in surface EMG clinical research applications and, c) myoelectric control in neurorehabilitation. This article deals with the state of the art regarding a) the electrode-skin interface (equivalent circuits, skin treatment, conductive gels), b) signal detection modalities, spatial filters and front-end amplifiers, c) power line interference removal, separation of propagating and non-propagating potentials and removal of outliers from surface EMG signal maps, d) segmentation of surface EMG signal maps, e) decomposition of surface EMG into the constituent action potential trains, and f) relationship between surface EMG and force. The material is presented with an effort to fill gaps left by previous reviews and identify areas open for future research.

The surface electromyography (sEMG) records the electrical activity of muscle fibers during contraction: one of its uses is to assess changes taking place within muscles in the course of a fatiguing contraction to provide insights into our understanding of muscle fatigue in training protocols and rehabilitation medicine. Until recently, these myoelectric manifestations of muscle fatigue (MMF) have been assessed essentially by linear sEMG analyses. However, sEMG shows a complex behavior, due to many concurrent factors. Therefore, in the last years, complexity-based methods have been tentatively applied to the sEMG signal to better individuate the MMF onset during sustained contractions. In this review, after describing concisely the traditional linear methods employed to assess MMF we present the complexity methods used for sEMG analysis based on an extensive literature search. We show that some of these indices, like those derived from recurrence plots, from entropy or fractal analysis, can detect MMF efficiently. However, we also show that more work remains to be done to compare the complexity indices in terms of reliability and sensibility; to optimize the choice of embedding dimension, time delay and threshold distance in reconstructing the phase space; and to elucidate the relationship between complexity estimators and the physiologic phenomena underlying the onset of MMF in exercising muscles.