The aim of this contribution is to examine some limitations in our understanding of the physiological mechanisms underlying the neural and muscular responses to electrical stimulation (ES) and to present a number of problems which limit the comparisons among the results obtained in different laboratories (such as the large varieties of methodological approaches used for muscle stimulation and of testing procedures aimed to describe and quantify the neural and muscular responses). With such an approach, we aim to provide a conceptual foundation for subsequent studies in this area. In exercise and sport, ES is used to assess the neuromuscular function in vivo, but more importantly it is used to promote strength-training like adaptations. Skeletal muscles benefit from ES use, despite it is applied in an empirical manner, often based on personal experience or manufacturers claims rather than on scientific evidence. In general, ES current parameters are poorly reported and there is a considerable heterogeneity between the different studies. This is due to the fact that researchers and clinicians tend to consider the different forms of electrical stimulation as a whole, irrespective of the species (human versus animal), of the stimulation model, of the type, size and location of electrodes, of the stimulus parameters and of the muscle being stimulated. We contend that the key factor for optimizing ES is muscle tension, i.e., the level of evoked force (with respect to maximal voluntary force), which should be maximized - whenever possible - via an appropriate manipulation of the two main ES current parameters: frequency (50-100 Hz) and intensity (as high as possible). Surface ES activates motor units without any specific sequencing related to unit types (i.e., random or disorderly recruitment), which implies that some fast units can be activated even at relatively low levels of force. Such peculiarity of ES recruitment inevitably entails some disadvantages (e.g., onset and extent of muscle fatigue) but also several advantages, particularly for athletes requiring high levels of muscle strength and power, but also for individuals presenting a selective impairment of fast muscle fibers (elderly subjects). Despite ES is commonly viewed as a technique to induce muscle contractions with a negligible contribution of the central nervous system, several lines of evidence indicate a considerable involvement of different neural structures during ES. In accordance with this contention, it has even been suggested that ES would provide a multimodal bombardment of the central nervous system, which results in increased cortical activity as well as in spinal motoneuron recruitment. This reflexive recruitment, which activates motor units in the normal physiological recruitment order, could be maximized through simple experimental manipulation of stimulus pulse duration (1 ms) and frequency (>50 Hz). Such physiological phenomenon remains however to be demonstrated for commonly stimulated muscles such as the quadriceps femoris. ACKNOWLEDGEMENT: Supported by Compagnia di San Paolo Project “NICEM” (MAM).
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