For NMES vs. PAS (Table 6), at 10 minutes, there was no heterogeneity in results between studies, I2 = 0%, with the overall pooled effect (n = 56) in favor of NMES, although not statistically significant (p = 0.07). At 15 minutes, there was “considerable” heterogeneity (I2 = 85%) (according to Cochrane, an I2 > 75% is considered “considerable” (16)), with the overall pooled effect (n = 37) in favor of NMES, although not statistically significant (p = 0.22). At 20 minutes, there was no heterogeneity in results between studies, I2 = 0%, with the overall pooled effect (n = 43) statistically in favor of NMES (p = 0.007). At 25 and 30 minutes (no meta-analysis were performed as only 1 study used in each), results significantly favor NMES at 25 minutes (p < 0.00001), but not at 30 minutes (p = 0.87).
For NMES vs. ACT (Table 7), at 10 minutes, there was no heterogeneity in results between studies, I2 = 0%, with the overall pooled effect (n = 56) statistically in favor of ACT (p = 0.0006). At 15 minutes, there was “considerable” heterogeneity (I2 = 90%), with the overall pooled effect (n = 37) in favor of ACT, although not statistically significant (p = 0.26). At 20 minutes, there was “considerable” heterogeneity (I2 = 84%), with the overall pooled effect (n = 43) statistically in favor of ACT (p = 0.009). At 25 and 30 minutes (no meta-analysis was performed as only 1 study used in each), results significantly favor ACT at 30 minutes (p < 0.00001), but not at 25 minutes (p = 0.40).
Blood lactate was investigated during the recovery intervention period in 6 of the 13 studies. Of these, 4 were classified as high quality (10,15,22,27) and 2 as medium quality (8,35). When NMES was compared with PAS recovery, 4 of the 6 studies showed that NMES had a significant BLa-lowering effect compared with PAS recovery (2 were classified as high-quality studies (15,27) and 2 as medium-quality studies (8,35)). Only 2 studies (10,22) found no significant BLa-lowering effects of NMES compared with PAS recovery. Based on these and the results of the meta-analysis for NMES vs. PAS and ACT recovery, there is strong evidence (level 1) that NMES is effective for lowering postexercise BLa compared with PAS recovery. When NMES was compared with ACT recovery, only 1 medium-quality study (35) showed that NMES had a significant BLa-lowering effect compared with ACT recovery, whereas 3 high-quality studies (15,22,27) found that ACT recovery had a significant BLa-lowering effect compared with NMES. One high quality (10), found no significant BLa-lowering effects between NMES and ACT recovery. Therefore, there is no evidence (level 4) that NMES is effective for lowering postexercise BLa compared with ACT recovery.
Performance parameters were investigated in 11 of the 13 studies. Of these, 7 were classified as high quality (10,15,17,21,22,30,33), 3 as medium quality (31,32,35), and 1 as low quality (36). When NMES was compared with PAS recovery, only 1 low-quality study (36) showed that NMES had a significant positive effect on performance parameters compared with PAS recovery. There were no significant differences for performance parameters found for NMES compared with PAS recovery for all of the other 10 studies. Therefore, there is no evidence (level 4) that NMES is effective for enhancing performance compared with PAS recovery.
When NMES was compared with ACT recovery, only 1 medium-quality study (35) showed that NMES had a significant positive effect on performance parameters compared with ACT recovery. There was 1 high-quality study (15) that showed that NMES had a significant negative effect on performance parameters compared with ACT recovery. There were no significant differences for performance parameters found for NMES compared with ACT recovery for all of the other 8 studies (1 study (36) did not use an ACT recovery intervention). Therefore, there is weak evidence (level 3) that NMES is ineffective for enhancing performance compared with ACT recovery.
Measurements of Perceptions of Pain or Exertion (Ratings)
Ratings of perceptions of pain and/or perceptions of exertion were investigated in 9 of the 13 studies. Of these, 5 were classified as high quality (10,15,22,30,33), 3 as medium quality (31,32,35), and 1 as low quality (36). When NMES was compared with PAS recovery, 4 studies showed that NMES had a significant positive effect on ratings of pain and/or exertion compared with PAS recovery. Of these, 2 were classified as high quality (10,30), 1 as medium quality (31), and 1 as low quality (36). There were no significant differences for ratings of pain or exertion for all of the other 5 studies. Therefore, there is strong evidence (level 1) that NMES is effective for enhancing ratings of pain or exertion performance compared with PAS recovery. When NMES was compared with ACT recovery, only 1 medium-quality study (35) showed that NMES had a significant positive effect on ratings of pain or exertion compared with ACT recovery. There was 1 high-quality study (23) that showed that NMES had a significant negative effect on ratings of pain and/or exertion compared with ACT recovery. There were no significant differences for ratings of pain and/or exertion for all of the other 7 studies. Therefore, there is no evidence (level 4) that NMES is any more effective than ACT recovery for improving perceptions of pain or improving perceptions of exercise exertion, either during or after a recovery intervention period.
The overall findings of this systematic review of previous studies that have investigated the use of NMES for the purpose of enhancing postexercise recovery suggest that NMES in not more effective than traditional recovery intervention modalities for enhancing subsequent performance parameters. However, caution should be exercised when interpreting these findings because of the heterogeneity that exists among study protocols, NMES parameters used, and the quality rating of some of the important protocol procedures. From the 13 studies that were included for analysis, quality assessment rating revealed that although some were rated strongly, others were only rated medium (unclear risk of bias) or weak, particularly with regard to reporting of protocol details and investigator bias. In addition, some studies that showed overall positive results were either poorly controlled or assessed few outcomes.
The majority of studies analyzed in this review used RXTs, which as previously stated are the most appropriate for these types of studies, especially if sample sizes are small. The procurement of a suitable QAT for these studies proved very challenging, which was not that surprising considering that the acquiring of a suitable QAT for rating RXT's can often be problematic, especially as there is a large heterogeneity in the reporting of RXTs, possibly reflecting the lack of standards with the field (25). It was decided that a revised version of “The Cochrane Collaboration's tool for assessing risk of bias” would be more suitable for these studies, especially as the design of this tool allowed scope for modification of the tool to make it more specific to these type of RXT studies. This strategy, recommended by Moher et al. (26), has been adopted by previous researchers who have conducted systematic reviews (6,14,19).
Regarding the overall findings of the 13 studies, 9 found a positive effect for NMES for at least one of the outcome variables measured. However, of these 9 studies, a positive effect for a performance parameter outcome measure was only found in only 2 studies (35,36), one of which was rated by the investigators as having a high risk of bias (weak quality rating), with the other study rating as an unclear risk (medium quality) because of several fundamental protocol issues found.
Four of the 9 mentioned studies (8,15,27,35) found that NMES had a positive BLa-lowing effect during the recovery intervention period compared with PAS recovery. Although the results of the meta-analysis showed that there were heterogeneity between studies and although there was a consistent trend for NMES to be associated with lower BLa vs. PAS, the sample sizes were small and the only significant effects were seen at 20 minutes. In addition, 2 of the aforementioned studies (8,27) did not perform a postintervention exercise bout to assess its effects on subsequent performance. This could raise a question over these results, especially as lowering BLa alone may not result in a subsequent performance enhancement. This is because, despite traditional viewpoints to the contrary, lactate is no longer viewed as a major contributor to muscle fatigue (1). In support of this view, 1 study (15) showed that, despite BLa decreasing significantly faster during the recovery intervention period with NMES compared with PAS recovery, there were no significant performance differences between both groups for the postrecovery intervention exercise bout.
Four of the 9 mentioned studies (10,30,31,35) found a benefit on subjective ratings of muscle pain, yet only one of these studies (35) showed a direct performance benefit as a result. One study (32) found that NMES significantly lowered creatine kinase (CK) blood levels at 72 hours after eccentrically damaging exercise compared with PAS recovery. However, there were no significant differences for either performance parameters or ratings of muscle pain at any of the postexercise time points in their study.
The findings for subsequent performance parameters are very significant, especially as performance enhancement is likely the most important factor considered when using recovery intervention modalities, particularly for sporting populations. Yet, only 2 studies (35,36) found any performance benefit of using NMES, both of which had potential protocol limitations (Table 5) resulting in being classified as having medium and high risk of bias, respectively. The positive findings for ratings of muscle soreness in the 3 aforementioned studies are somewhat more encouraging, as although in most cases there were no significant positive effects on performance parameters measured, the perceived benefit of positive psychological effects on recovery should not be dismissed (10).
A major observation with these studies is the considerable heterogeneity that exists between the study protocols, particularly for NMES parameters (Table 3). Regarding the NMES parameters, this is not that surprising as there is still no definitive consensus on what are the optimal parameters that should be used with NMES for postexercise recovery (28). The large variation of different NMES devices that are used by investigators is likely a significant factor for this heterogeneity, especially in relation to variables such as electrode size and shape, pulse intensity and shape, and pulse frequency.
The mean frequency used by these studies was 4.7 Hz, with a range from 1 to 8 Hz. These are within the expected range of frequencies that are normally used for inducing subtetanic muscle contractions. Neuromuscular electrical stimulation used for the purpose of enhancing postexercise recovery is characterized by the use of low-frequency, (relatively) high-intensity stimulation to induce light muscle contractions, as opposed to high-frequency, low-intensity stimulation that is normally used for sensory level stimulation (3).
The mean pulse duration used by studies was 320 microseconds, with a range from 125 to 500 microseconds. Three of the studies (15,27,36) did not report pulse durations used. The general consensus on the optimal pulse durations that should be used for the purpose of exercise recovery is not definitive, but as shown from previous research, it is normally between 100 and 500 microseconds. It is believed that if the pulse duration is too narrow, it can result in insufficient muscle activation because of a minimum time required for the swell intensity to create an action potential within the stimulated motor neurons (13). Alternatively, if the pulse duration is too wide, the proportionately deeper and more intensive muscle stimulation can be accompanied by undue discomfort because of the increased presence of algesic substances as the pulse duration rises (13). Overall, it is probably very difficult to recommend a specific range at which pulses durations should be fixed for recovery intervention protocols. This is likely because of the considerable heterogeneity that exists between NMES devices (such as electrode size and positioning) and parameters used between different studies which accounts for a lack of consensus.
Only 9 of the 13 studies provided details about the intensity of stimulation used, although specific details were not entirely clear in all 9 studies. For example, one of the studies (27) stated that their NMES device was capable of achieving a maximum output intensity of 35 mA. They did not specifically state the range of values that all of their subjects used, instead stating that the intensity was “typically” increased to an intensity of 7–10 to elicit a strong comfortable contraction, with 10 being the highest setting (35 mA) on a scale of 1–10. One of the studies (36) did not disclose any information on the intensity of stimulation, only that it was used at “moderate intensity.”
Despite intensity of stimulation arguably being the most important NMES parameter (21), there is currently no consensus on what is the optimal intensity that should be used with NMES, when used during recovery from exercise. This present position is not helped by the fact that (a) there is considerable heterogeneity that exists between study protocols with regard to NMES devices (electrode size and positioning) and parameters used; and (b) there are large interindividual differences into how people respond to NMES, which makes it very difficult to use similar NMES intensities for everybody. As previously mentioned, the reasons for this heterogeneity are multivariant and likely include factors such as individual perceptions of discomfort and levels of subcutaneous adipose tissue (20). Therefore, it is very difficult to definitively state what the optimal intensity of stimulation for the postexercise recovery should be. However, it is known that the higher the intensity of stimulation, the greater the number of motor units that will be activated and the deeper the level of muscle contraction attained. Therefore, it is likely that an increased intensity of stimulation will result in a greater muscle pump effect because of greater muscle activation, which in turn should increase muscle metabolite removal at a faster rate. However, because of the associated problems of increasing intensity, such as perceptions of discomfort and muscle fatigue, a balance clearly needs to be found between increasing muscle activation and reducing the likelihood of increasing muscle fatigue.
The 13 studies were rated for quality assessment to assess whether they were considered to be of high, medium, or low quality, that is, having a low, unclear, or high risk of bias associated with the study protocol (Table 5). Each study was rated using a modified QAT that was designed to be specific to these types of controlled studies.
Although the majority of studies stated that they used a random allocation to determine the order of the recovery intervention modalities, none of these studies reported their method of random allocation used. This absence of detail makes it unclear if there were any risk of bias associated with their respective randomization procedures, which is why details about the method of sequence generation are recommended (16). In addition, 2 of the studies either did not randomize or were confusing as to whether randomization was used. Although one of these studies (36) only performed 1 recovery intervention session for each of their studies, in their protocol design, they did not randomize which leg received the NMES treatment. That is, in all cases, the right leg received NMES and left leg received PAS recovery. Another possible consideration with their design protocol, apart from the issue of randomization, may be that systemic factors make it more difficult to assess the direct effects of NMES on the stimulated limb, especially as direct systemic blood flow to and from the limb during stimulation was not controlled. That is, because as suggested by the authors, NMES can exert systemic and peripheral effects, the use of NMES on one limb and not the other would not only directly affect the stimulated limb but also affect systemic blood flow as a whole, which could carry over into the opposite leg, thus confounding results. The other study (35) did not appear to use a randomized process to select the order of their recovery intervention. However, they did not make it definitively clear whether they used a randomization process.
Regarding blinding procedures for the recovery intervention protocols used, no study implemented an NMES sham for any of their interventions. This does provide a limitation to studies and must be considered when interpreting results found. However, it is important to recognize that implementing an effective sham NMES intervention for studies of this nature would likely be difficult.
With regard to participant's familiarization sessions before the implementation of the recovery intervention protocols, only 3 of the studies had implemented a familiarization session before the first recovery intervention session. However, it must be noted that for 4 of the other studies that did not implement a familiarization session (27,30,31,35), they all used highly trained athletes performing exercises that were very familiar and specific to their everyday activities. This should make it far less likely that familiarization effects could interfere with the data collection, compared with, for example, if they were untrained populations or performing unfamiliar exercises to them. The implementation of a familiarization session was not applicable to 2 of the studies (22,32) because they used nontrained populations who performed exercise protocols that were designed to induce muscle soreness.
The use of a washout period was not applicable to the 2 studies that only had 1 testing session (8,36). Most of the other studies clearly reported the washout periods between sessions, which appeared to be adequate in almost all cases. One study (27) reported that the “3 sessions were separated by a minimum of 24 hours and were all completed within 3 weeks.” The use of 24 hours, although quite short to allow full recovery from high-intensity exercise, was probably adequate in this case as (a) they were using trained swimmers performing a familiar exercise without a large eccentric component and (b) the only outcome measure was BLa, which returns to resting levels within 90 minutes after very high-intensity exercise.
In general, the populations used for the studies were appropriate for their respective research protocols. That is, most of the studies used trained sports-specific populations who were performing exercise bout(s) and recovery intervention periods very relevant to their sporting discipline. Therefore, these populations were generally very representative of the type of sporting populations which were being targeted. In addition, because they were specifically trained for the exercises being performed, this would likely dramatically decrease the likelihood of a familiarization effects or inadequate washout period(s) affecting the data recorded. Conversely, of the studies that used nontrained populations, the aim of their studies was to induce muscle soreness and damage that made these populations better suited for these situations than using trained athletic populations, as they were unaccustomed to the exercises undertaken. However, despite the appropriateness of the populations used for these studies, only 7 of the 12 studies adequately reported the recruitment procedures, which meant that the other studies were at a higher risk of selection bias.
The preintervention exercises chosen to induce fatigue were generally appropriate and well controlled, both in terms of their intensity and duration. However, there were some studies that used questionable exercise protocols. One such study (31) was generally a well-conducted study. However, despite using a very appropriate specific population for their study, they used a Futsal match as their preintervention exercise. Because of the nature of such an exercise session, it would have been extremely difficult to standardize the level of fatigue induced by the sessions. This is because of the lack of control over exercise variables such as exercise intensity and durations of times spent walking, jogging sprinting, and moving in multiple movement planes. Therefore, this makes it very difficult to determine to what extent the subsequent recovery intervention modalities affected the outcome measures, especially if the preintervention exercise sessions were not strictly controlled for all major variables between the multiple testing days.
Another study (36) used a preintervention exercise bout in their second study of hiking single or multiple loops of a course in a hill range, depending on subject fitness level. However, the trained status of subjects was not stated in their study methodology. They also did not control the postexercise bout period before the implementation of the recovery intervention modality, as they stated that “participants drove back to the center, which took approximately 10 minutes.” Although, there was only 1 session involved, which meant controlling the variables for subsequent sessions was not an issue, because the protocol procedures were poorly controlled, such practices would likely increase the likelihood of bias occurring.
The recovery intervention protocols were adequately detailed in almost all of the studies, with the majority of the studies implementing the modalities for 20–25 minutes. One of the studies (36) implemented their NMES protocol for a considerable longer duration than the other studies (60 minutes) for both parts of their study. However, they gave virtually no details about parameters used, which makes it very difficult to assess its appropriateness.
Statistical analyses were appropriate and well reported for the majority of the studies, with one notable exception (36), who disclosed very little detail regarding their statistical analysis. Because none of the studies stated whether they had performed a power calculation to determine sample size needed, it was assumed that they did not perform one. The use of power calculations to estimate sample sizes is recommended when designing research protocols, especially as it decreases the chance of obtaining an underpowered result. However, where statistically significant differences are found between groups in a trial, then the power and sample size are by definition adequate even if there was no a priori sample size calculation performed.
Despite NMES often being commercially marketed as an effective modality for enhancing recovery from exercise, the overall findings of this review seem to provide insufficient evidence to support this. Although there seems to be good evidence to show that NMES can have a positive BLa-lowering effect compared with PAS recovery, as well as positive effects on subjective ratings of pain and overall well-being, there is no evidence to support its use for enhancing subsequent exercise performance compared with traditional recovery methods. Although the beneficial effects of NMES on subjective measures of pain and feelings of well-being should not be discounted and may provide some justification for its use in some populations, the lack of evidence regarding its effects on actual athletic performance is likely the most important factor to consider for athletic populations.
For athletes who currently use, or are considering the use of NMES for the purpose of enhancing recovery from exercise, there are some important factors that need to be considered: (a) there is considerable heterogeneity of existing research protocols that have investigated NMES as an recovery modality, in terms of the NMES parameters used, mode of exercise, and duration of recovery periods and (b) when using NMES, considerable individual variability can exist in the stimulation intensity required. This can be because of factors such as adipose tissue variability, which can affect current to the stimulated region, as well as variability in an individual's perception of pain or discomfort when using NMES. This likely explains why there is no universal recommendation on the optimum NMES intensity that should be used during recovery from fatiguing exercise and why this needs to be selected subjectively on an individual basis. However, intensity likely needs to be high enough to induce sufficient muscle activation (for muscle pump effect) to promote metabolite clearance, without being too high, that will cause muscle fatigue.
The authors thank Craig Denegar and Eamonn Delahunt for their helpful input into the construction of this study. No external sources of funding were used for the construction of this review.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
risk of bias; best evidence synthesis; meta-analysis; subjective ratings; blood lactate; performance parameters