Are they effective?

- Written by Nicola A. Maffiuletti, Switzerland and Grégory Dupont, France

INTRODUCTION AND RATIONALE

Transcutaneous electrical stimulation (ES) consists of delivering small electrical pulses via electrodes that are positioned on the skin, usually around skeletal muscle motor points or painful body areas. Depending on electrical current (frequency and intensity) and electrode characteristics (size, position), two major ES categories can be distinguished:

1. ‘Sensory’ ES (no muscular contractions are evoked). Conventionally called transcutaneous electrical nerve stimulation. This is mainly delivered using relatively low current intensities (at or below the sensory threshold) in an attempt to relieve pain via spinal circuitry transmitting pain (gate theory) and endorphin release.
2. ‘Motor’ ES. Conventionally called neuromuscular electrical stimulation. This elicits visible muscular contractions (stimulations are delivered at or above the motor threshold), using either tetanic or sub-tetanic stimulations, whose clinical/physiological consequences are improved neuromuscular function and enhanced peripheral blood flow.

In reality, this distinction is not always respected and considerable confusion continues to surround the main physiological effects, methodological aspects and clinical/sports applications of transcutaneous ES.

Some individual and team sport athletes use motor ES modalities as a complement to their training/rehabilitation programmes for:

1. improving muscle strength during the pre-season,
2. maintaining/improving muscle function while injured/after an injury and
3. restoring physical performance after intense exercise (i.e. for accelerating recovery).

There is some evidence to suggest that motor (tetanic) ES is effective to increase muscle strength in athletes (for a review see Seyri and Maffiuletti¹), and to preserve muscle mass during prolonged periods of inactivity². Surprisingly, however, the growing interest in applied research on tetanic ES observed in the last few years has corresponded to a reduced use by athletes and reduced development by manufacturers. At the same time, but with premature physiological background, subtetanic ES has gained popularity in sportspeople as a potentially effective strategy to accelerate post-exercise recovery.

This review article aims to assess the effectiveness of motor subtetanic and sensory ES as a recovery modality for athletes by following a simplified and non-systematic approach. Because the post-exercise decline in physical/sports performance is due to an impairment in neuromuscular and/or psychomotivational function, and because recovery modalities are designed to restore neuromuscular function and/or psychomotivational function to the pre-exercise level as quickly as possible, we preferred to provide separate definitions, analyses and interpretations for ‘physiological’ and ‘perceptual’ recovery throughout the article. We therefore considered original research studies:

1. published in peer-reviewed journals,
2. comparing the effectiveness of ES-related recovery strategies to passive recovery or other recovery modalities (at least two conditions),
3. focusing on athletes or healthy subjects,
4. having quantified at least one physiological and/or perceptual variable of recovery.

In order to examine the effectiveness of ES-related strategies, we arbitrarily classified the studies as having demonstrated that recovery with ES was “less effective”, “equally effective” and “more effective” compared to passive rest and to other recovery modalities (submaximal exercise, cold water immersion, contrast water therapy, compression garments and also placebo ES in one study).

DOES ELECTROSTIMULATION IMPROVE PHYSIOLOGICAL RECOVERY?

In the context of this article, physiological recovery was considered successful when neuromuscular function, as measured objectively (strength, power or physical performance outcomes), was fully restored to pre-exercise levels. The rationale for the use of motor ES to promote physiological recovery is based on the assumption that evoked muscular contractions are able to enhance peripheral blood flow mainly due to the muscle pump effect. In turn, this would accelerate metabolites removal, improve substrate availability, decrease the inflammatory response and reduce muscle damage that could contribute to restoring the neuromuscular determinants of muscle strength/power and thus physical performance (cf. left portion of Figure 1).

In reality, subtetanic ES of the calf muscles and of the plantar muscles of the foot are able to enhance peripheral blood flow with respect to passive rest conditions, however active exercise (voluntary dynamic muscle contractions) is even more effective than ES to increase the ejected venous volume³. This observation fits well with recent recovery studies in which ES was found to be equally or more effective than passive rest in reducing post-exercise blood lactate concentration, while ES was less effective than active recovery modalities such as submaximal swimming and cycling (Table 1). It is nevertheless worth remembering that blood lactate should not be considered the best indicator of metabolite removal.

Four studies evaluated the changes in serum creatine kinase concentration, as an objective marker of muscle damage in the days consecutive to an exhaustive exercise bout (including eccentric exercise) and demonstrated that ES was equally or more effective than passive recovery, and equally effective compared to submaximal exercise (Table 2).

When evaluating the effectiveness of ES recovery-related strategies on muscle strength/power, physical performance and even on the neuromuscular determinants of muscle strength (e.g. muscle activation, muscle contractility), only 3 out of 19 studies demonstrated that ES was more effective than passive recovery (and only for one variable), while all the other studies were unable to detect a difference between the two modalities in healthy active individuals, recreational sportsmen and professional athletes (Table 3). ES was found to be equally effective compared to other recovery modalities in all but one study, in which better effectiveness of ES compared to active recovery (submaximal concentric contractions) was observed following isokinetic fatiguing exercise in male elite judo athletes.

DOES ELECTROSTIMULATION IMPROVE PERCEPTUAL RECOVERY?

In the context of this article, perceptual recovery was considered successful when psychomotivational factors, as measured subjectively (basically, by replying to simple questions such as “what is your level of recovery following this intervention?”, by ranking the perceived effectiveness of different recovery modalities using scales and scores or by quantifying the perceived level of energy and enthusiasm), were fully restored to pre-exercise levels. The rationale for the use of motor and sensory ES to promote perceptual recovery is based on theoretical models rather than on scientific evidence, as the biological basis of the analgesic and psychological effect of ES is not known. The underlying theories are the gate control theory, which proposes that pain transmission through small afferent fibres would be selectively blocked at the spinal cord level by the ES-induced activation of large afferents – the endorphin-release theory and the placebo theory. In turn, the neurophysiological/psychological mechanisms underlying these theories would be able to attenuate the perception of pain and tiredness (both locally and at a whole-body level), for example by reducing exercise-induced muscle soreness, which could ultimately result in recovered psychomotivational function and thus successful perceptual recovery (Figure 1, RHS).

The effects of ES recovery-related strategies on delayed onset muscle soreness (referred to as muscle pain in some studies) induced by heavy exercise (including eccentric exercise) have been compared to other recovery modalities in 12 studies. Subsensory (micro-current ES), sensory and subtetanic ES has been found to be more effective than passive rest in three studies (a placebo ES condition was used in one instance), and more effective than active water exercise in only one study, while in all the other instances ES was equally effective compared to both passive and active recovery modalities (Table 4).

More interestingly, ES recovery-related strategies have been found to be more effective than passive rest for restoring psychomotivational factors in three out of four studies conducted on highly-trained cyclists and team sport athletes. In the same way, ES has been demonstrated to be more effective compared to other recovery modalities (submaximal exercise, contrast water therapy and compression garments) for perceptual recovery in three out of five studies (Table 5).

CONCLUSIONS AND CONSIDERATIONS

We conclude that ES recovery-related strategies are:

• unlikely beneficial for improving physiological recovery compared to both passive rest (including a placebo condition in one study) and other recovery modalities;
• likely beneficial for improving perceptual recovery compared to passive rest, and possibly beneficial for improving perceptual recovery compared to other recovery interventions.

In other words, there is no justification to expect any physiological benefit from ES for post-exercise recovery purposes, while restoration of some psychomotivational factors – likely mediated by a placebo effect of ES⁴ – appear to be enhanced when ES is used as a recovery modality compared to several passive and active recovery strategies.

Medical device manufacturers continuously introduce new electrical stimulators on the market with futuristic current characteristics, claiming superior effectiveness compared to previous systems, but with no preliminary physiological validation. ES users are therefore faced with considerable confusion, particularly with respect to the multitude of parameters, protocols and potential applications, and their choice is often based on convenience. We believe that, in order to ensure solid credibility of newly developed units, their physiological (and eventually also perceptual) effectiveness should be scientifically demonstrated at all levels of our simplified model (Figure 1) e.g. by showing that ES is able to increase blood flow using ultrasonography, to accelerate metabolites removal by quantifying blood lactate concentration and to improve neuromuscular function by evaluating muscle strength/power and eventually sport-related performance (rather than only one of these mechanisms as it is the case for currently available units).

Based on subjective belief rather than on scientific evidence we propose that in order to maximise potential physiological benefits of ES for recovery purposes (with respect to passive rest), the following methodological precautions should be observed:

• ES would be better combined with submaximal voluntary dynamic contractions (such as toe curls) whenever possible, in order to maximise blood flow increase,
• ES would be better applied distally (to the calf or foot muscles and eventually to the common peroneal nerve⁵) rather than proximally (to the quadriceps muscle belly) to maximise the muscle pump effect,
• ES would be better used in weight-bearing and contact sports to reduce signs and symptoms of muscle damage,
• ES would be better delivered with low doses but for long-term periods (such as overnight)⁶, during travel or by means of electrostatically charged self-adhesive membranes⁷).

ES recovery-related interventions represent an alternative and potentially useful way of restoring sports performance after intense exercise because they are practical, relatively cheap and quite beneficial for promoting perceptual (not physiological) recovery. Many athletes are, however, still reluctant to apply ES to their muscles mainly because of the discomfort induced by the electrical current, and therefore its use for recovery purposes should be proposed on an individual basis (and according to expectations). There is limited research in this area, particularly on upper limb muscles and in female athletes, and further efforts are required to demonstrate the physiological effectiveness and the biological basis of the analgesic and psychological effect of ES.

WATCH DR MAFFIULETTI'S PRESENTATION AT ASPETAR'S POST-EXERCISE RECOVERY CONFERENCE BELOW

A 40 MINUTE Q&A SESSION WITH ALL RECOVERY EXPERTS WHO PRESENTED AT THIS CONFERENCE CAN BE VIEWED HERE

Nicola A. Maffiuletti

Neuromuscular Research Laboratory

Schulthess Clinic

Zurich, Switzerland

Grégory Dupont

University of Lille Nord de France

Ronchin

LOSC

Lille, France

Contact: nicola.maffiuletti@kws.ch

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Image via Michael Ziemann