Several studies have demonstrated that neuromuscular electrical stimulation (NMES) may lead to increased quadriceps strength in healthy1,2 and injured3–5 populations. NMES training intensity, defined most often as the ratio of NMES-induced torque to torque produced by a maximum voluntary isometric contraction (MVIC) (expressed as percent MVIC),6 is thought to be the primary determinant of the effectiveness of NMES treatments.7,8 This belief is based on the established dose-response relationship, which indicates that NMES training intensity is positively related to strength gains.1–4,9 Consequently, clinicians should use the maximum possible NMES training intensity.8 However, a sufficient NMES training intensity is difficult to achieve and maintain due to limitations such as muscle fatigue,8,10–12 spatially limited motor unit recruitment,8,12,13 and patient discomfort associated with the electrical stimulus and subsequent involuntary contraction.6,8,11–15 Strategies with the potential to minimize these limitations have been examined extensively.16–24 Unfortunately, many techniques supported by empirical evidence cannot be incorporated easily or are inaccessible in clinical settings,15 so additional strategies are needed.10
The Kneehab XP (Theragen LLC, Leesburg, VA) is an electrical stimulator that incorporates a novel clinically applicable strategy referred to as multipath technology. The stimulator is designed to enhance motor unit recruitment via improved patient comfort and spatial distribution of the stimulus, leading to stronger NMES-induced contractions and minimizing muscle fatigue.25–27 Conventional NMES (c-NMES) devices transmit an electrical current from one electrode to another via a single fixed path, whereas the novel device transmits an electrical current with altered pulse durations between four large electrodes integrated within a neoprene thigh garment via two separate channels.12,15,26,27 Due to its unique current distribution method, this device has been referred to as multipath NMES (m-NMES) and has gained a significant amount of attention in research.12,15,26–31
Feil et al.29 observed greater quadriceps strength 6 weeks after an anterior cruciate ligament repair in a group of patients receiving m-NMES treatments compared to a group that received c-NMES treatments. The authors reported that the mechanisms by which m-NMES outperformed c-NMES remain unclear. Two basic studies12,15 were subsequently performed to determine if the mechanisms responsible for the outcomes observed by Feil et al. were the result of the m-NMES device by comparing fatigue-related outcomes and discomfort under m-NMES and c-NMES conditions. Although some improved outcomes under the m-NMES condition were reported, the conditions in each study differed substantially with respect to the current distribution method (eg, multipath vs single fixed path) and electrode size (eg, large vs small). Therefore, previous studies12,15 were limited in their ability to determine if the novel multipath current distribution method was the primary mechanism responsible for the improved outcomes.
Evidence-based practice requires that clinicians incorporate the best current evidence and their clinical expertise when making clinical decisions. Therefore, scientific examinations of commercially available modalities, such as the novel m-NMES device, are needed. Further investigations of the m-NMES device, with an emphasis on the influence of its novel current distribution method, are warranted to enhance the related evidence currently available to clinicians. Therefore, the purpose of our study was to compare the effects of m-NMES and c-NMES on outcomes related to fatigue and discomfort. We hypothesized that fatigue would occur during each treatment, but greater fatigue would occur while using c-NMES. We also hypothesized that discomfort would be greater while using c-NMES and discomfort levels would decrease during each treatment.
We performed a single-blind counterbalanced crossover study with two independent variables (NMES condition at two levels: m-NMES and c-NMES; time at 17 or 18 levels: based on number of NMES-induced repetitions) and five dependent variables (percent decline in MVIC, percent decline in NMES-induced torque, percent decline in torque-time integral (TTI), total torque-time integral (T-TTI), and self-reported discomfort). We randomly assigned participants to one of two permutations designed to counterbalance the session order in which the c-NMES and m-NMES treatment conditions were performed by having them draw a number.
We determined our target sample size with a priori power analyses using G*Power software (version 18.104.22.168).32 A convenience sample of 21 participants from the university and community completed all four study sessions. To be included, participants were required to be healthy, recreationally active males between the ages of 18 and 35 years. Recreationally active was operationally defined as participation in some form of physical activity (eg, strengthening-related activities, jogging, running, cycling, swimming, tennis, etc) for at least 20 minutes twice a week. NMES tolerance and motor thresholds have been shown to differ between individuals with a body mass index (BMI) above and below 30 kg/m2,33; thus participants also had to have a BMI of 30 kg/m2 or less to be included. This study was approved by the university's institutional review board and participants provided written informed consent. To facilitate participant recruitment, we incentivized participants via a lottery for a chance to win one of four $50 gift cards.
We used a Quickset 4 Biodex dynamometer (Biodex Medical Systems, Inc., Shirley, NY) to measure and record isometric knee extension torque during all voluntary and NMES-induced contractions at a sampling rate of 100 Hz. During all contractions, participants were seated in the dynamometer chair with the seat back tilted at 85° and the dominant leg was secured within a lever arm fixed at 60°. We aligned the axis of rotation of the dynamometer to the anatomical axis of the test knee, and the lower leg was secured in the fixed lever arm by an ankle strap placed 2 to 3 cm above the lateral malleolus (Figure 1).15 We calibrated the dynamometer to the manufacturer's specifications prior to beginning the study to ensure reliable measurements.15
Isokinetic dynamometer set-up. Participant performing voluntary isometric contraction with the lever arm fixed at 60°.
We applied all c-NMES treatments using the same Sonicator Plus 940 stimulator (Mettler Electronics Corp., Anaheim, CA). To maintain consistency across the two NMES conditions, we set the c-NMES parameters as similarly as possible to the parameters used with the Kneehab XP program 6 (Table 1). We used four self-adherent electrodes to deliver the c-NMES current (two 5 × 9 cm [Metron Bolingbrook, IL], one 10.79 × 17.78 cm [TENS Products, Grand Lake, CO], and one 7 × 14 cm electrode [SME Inc., Wilmington, NC]; Figure 2). To guide the placement of the c-NMES electrodes, we manually identified motor points using a pencil electrode (Mettler Electronics XK2; Active Forever, Scottsdale, AZ) following the procedures outlined by Gobbo et al.13 Based on the results of a recent study that identified seven motor points of the quadriceps,34 we selected four commonly identified motor points to guide the c-NMES electrode placement. Furthermore, the selected motor points allowed us to place the c-NMES electrodes in a similar fashion to the m-NMES electrode configuration, because they were located on the proximal and distal vastus lateralis, proximal rectus femoris, and distal vastus medialis (Figure 2).
Parameters of Neuromuscular Electrical Stimulation Conditions
Conventional electrode configuration.
We applied all m-NMES treatments using the same Kneehab XP stimulator, but we assigned each participant a separate Kneehab XP garment with integrated electrodes. We integrated the m-NMES electrodes into the neoprene garment and placed it on the dominant thigh according to the manufacturer's recommendations.35 We set the stimulator parameters to program 6 during all m-NMES treatments (Table 1).
Participants reported at the same time of day (± 2 hours) on four separate occasions and each session lasted approximately 1 hour. Each participant's dominant leg served as the leg of interest throughout the study (13 right, 1 left). We also instructed participants to report well hydrated and to refrain from strenuous activities for 12 hours prior to reporting.
The first two sessions served as familiarization sessions and were separated by 24 to 48 hours, whereas the last two sessions served as test sessions and were separated by 48 to 72 hours. Each session began with a standardized warm-up.21 Participants rested for 8 minutes following the warm-up while we identified the motor points using the pencil electrode method and cleaned the leg of interest with an alcohol-free wipe. Although motor point identification was not necessary for the m-NMES condition because the electrodes were integrated within the garment, we identified motor points in each session in an effort to blind participants to the treatment condition.
Participants performed MVICs of the quadriceps for 6 seconds in duration, and we followed the methods described in Table A (available in the online version of this article) to ensure reliable measurements. Participants rested for 5 minutes prior to performing the NMES procedures, during which we placed the Kneehab XP garment with integrated electrodes or the c-NMES electrodes over the participant's shaved dominant thigh. We also placed an empty Kneehab XP garment over the c-NMES electrodes in an effort to blind participants to the treatment condition.15 Participants were exposed to both forms of NMES during the two familiarization sessions by performing 10 NMES-induced quadriceps contractions using each NMES device (a total of 20 contractions). The NMES parameters used in the familiarization sessions mirrored those used during the test sessions, except we allowed participants to control the stimulus intensity throughout the familiarization sessions. In previous studies, we observed that participants were willing to use greater stimulus intensities if they were allowed to control the level of intensity themselves. We anticipated that participants would tolerate greater intensities as they became acclimated to the stimulus,36 so we encouraged participants to increase the stimulus intensity between contractions and between familiarization sessions to maintain a maximum comfortable stimulus level during each contraction (eg, highest stimulus intensity that does not cause pain).37
Maximal Voluntary Isometric Contractions (MVICs) Procedures
The NMES procedures for the second familiarization day were similar to the first, except that the devices were used in a reverse order from the pattern used during the initial session. Participants were required to tolerate a stimulus intensity sufficient to produce a NMES-induced contraction of 30% MVIC during the subsequent test sessions (days 3 to 4). To evaluate whether a participant was capable of tolerating the required stimulus intensity, we identified the greatest peak torque observed over the 10 contractions of each condition during the second familiarization session. We expressed the NMES-induced peak torque observed under each NMES condition as a percentage of the participant's pretest MVIC peak torque measured in the second familiarization session (day 2). If participants were unable to tolerate a stimulus intensity sufficient to produce a NMES training intensity of 30% MVIC or greater, we excluded them from further participation.
The NMES procedures during the two test sessions were similar to the familiarization sessions with the following exceptions: participants performed 18 NMES-induced contractions using a single NMES device (c-NMES or m-NMES) during each test session; we standardized the stimulus intensity using a target training intensity of 30% MVIC and did not manipulate it within each test session; and participants performed posttest MVIC procedures immediately following the NMES-induced contractions during each test session.
Prior to beginning each test session's NMES condition, we determined the stimulus intensity required to produce the target torque output of 30% MVIC, which was selected because it fell within the therapeutic window.36 We increased the stimulus intensity until the targeted torque output of 30% MVIC was achieved. If a participant reported reaching a maximum comfortable intensity prior to this point, we immediately decreased the intensity and the participant recovered for 30 seconds. We repeated this process a maximum of three times and, if the participant was unable to tolerate a sufficient stimulus intensity by the third attempt, he or she was excluded from further participation. After 50 seconds of rest, participants completed the assigned NMES condition and we frequently encouraged the participants to “relax and allow the machine to do all of the work” during the NMES-induced contractions. We did not manipulate the initial stimulus intensity used during the test sessions over the course of each NMES treatment condition. Immediately after completing the assigned NMES treatment condition, participants performed a single 6-second posttest MVIC. We elected to use a single repetition in an effort to limit recovery from the NMES-induced contractions.
Percent Decline in MVIC Torque. A change in peak MVIC torque is considered the gold standard for assessing fatigue.38,39 Therefore, we expressed each participant's posttest MVIC peak torque as a percent decline relative to their pretest MVIC peak torque, and the subsequent percent decline served as a fatigue-related outcome measure for our study.
Percent Decline in NMES-Induced Torque. NMES-induced fatigue is also frequently assessed by measuring the decline in NMES-induced torque over the course of a treatment.12,15,18–20,40–44 Accordingly, we expressed the peak torque produced during the NMES-induced contractions for each test session as a percent decline relative to the peak torque produced during the initial NMES-induced contraction of each test session.
Percent Decline in TTI. A decline in TTI observed during NMES-induced contractions has also been used as an index of NMES-induced fatigue because it has been suggested to represent isometric work.11,17,45 Therefore, the data necessary to calculate the TTI (eg, torque and duration of torque recording; expressed as Newton-meter seconds [Nm*s]) were measured and recorded during the NMES-induced contractions. We imported the data files into an analysis software package (AcqKnowledge 4; Biopac Systems, Inc., Goleta, CA), which calculated the TTI of each contraction. We expressed the TTI of the NMES-induced contractions during the test sessions as a percent decline relative to the initial contraction of each test session.
T-TTI. We considered the T-TTI to be an index of the total amount of isometric work performed under each condition, and it also represented a fatigue-related outcome. We calculated the T-TTI for each condition by summing the individual TTI data from each contraction.
Self-Reported Discomfort. We used a 100-mm horizontal visual analog scale (VAS) to measure self-reported discomfort levels during each NMES condition. As is common during NMES studies, the descriptors at each end of the scale were “no discomfort” (0 mm) and “worst possible discomfort” (100 mm).12,15,17,21,22,46 We asked participants to “rate your level of discomfort by making a vertical tick mark on the line” following each NMES-induced contraction, and we measured the distance (mm) from the “no discomfort” anchor to the vertical mark made on the horizontal line to obtain self-reported discomfort levels. When used to assess NMES-induced discomfort in a sample of healthy individuals, the VAS showed a high inter-session test–retest reliability.46
We analyzed the data using the Statistical Package for Social Sciences (version 23.0; IBM Corporation, Armonk, NY). We performed a series of two-way repeated measures analysis of variance (ANOVA) on three of the outcome measures (percent decline in NMES-induced torque, percent decline in TTI, and self-reported discomfort). In the event that the assumption of sphericity was violated, we followed the Greenhouse–Geisser procedure for correcting degrees of freedom. In the event of a significant time main effect, we performed post-hoc pairwise comparisons using a Bonferroni procedure to maintain family-wise error rate. Due to the number of NMES-induced contractions, we performed a large number of post-hoc pairwise comparisons. To simplify the results, we reported only the significant pairwise comparisons deemed to be clinically important (eg, first contraction to demonstrate a significant decline relative to the second and final contraction).
We performed a dependent t test on the other outcome measures (percent decline in MVIC and T-TTI). In addition, we performed dependent t tests to examine any potential baseline differences between the two conditions with respect to pretest MVIC, initial NMES training intensity (% MVIC), and initial TTI. We also calculated test–retest reliability (intraclass correlation coefficient [ICC](2,1))47,48 and measurement precision (standard error of measurement [SEM])49 estimates for pretest MVIC measurements.
To examine the magnitude of the differences, we calculated Cohen's f and d effect sizes.50 We calculated Cohen's d effect sizes corresponding to within-groups comparisons using the equation suggested by Cumming,51 which uses the average standard deviation of the paired data as the standardizer. Because d statistics are believed to overestimate the population effect size, Cumming51 recommended that an unbiased Cohen's d (dunb) also be provided. Accordingly, we calculated dunb values using the equation provided by Cumming.51 We interpreted Cohen's f values as small (f = 0.10 to 0.24), medium (f = 0.25 to 0.39), and large (f ≥ 0.40 large), and we interpreted Cohen's d effect sizes as small (d = 0.20 to 0.49), medium (d = 0.50 to 0.79), and large (d ≥ 0.80).50
While inputting values necessary to maintain adequate power (1 − ß 0.80) and detect a medium to large effect size (d = 0.650, f = 0.325),43 the dependent t test power analysis revealed a target sample size requirement of 17 participants, whereas only 12 participants were required for the two-way repeated measures ANOVA. We selected a medium to large effect size because we believed that any statistically significant differences with small to medium effect sizes would lack clinical relevance with respect to the outcomes of our study. Other authors have selected a large effect size (eg, Cohen's f = 0.68) for an a priori power analysis while examining similar dependent variables during a NMES study,22 which further supports our selection of a medium to large effect size as appropriate for this study. Although we initially exceeded our target sample of 17 participants, the data of 7 participants were excluded. As a result, our final sample consisted of 14 participants (age = 23.7 ± 4.8 years; height = 175.3 ± 6.4 cm; mass = 78.7 ± 11.6 kg; BMI = 25.4 ± 2.8 kg/m2).
Prior to analyzing the data, we assessed the tenability of the applicable statistical assumptions using the procedures described in Table B (available in the online version of this article). There were no significant differences with respect to baseline measurements across the two conditions (Table 2), and the test–retest reliability and measurement precision estimates of the pretest MVICs were within acceptable limits (ICC(2,1) = 0.957; 95% confidence interval [CI]: 0.855, 0.983; SEM = 13.35 Nm).52 Furthermore, these values are similar to those reported by Park and Hopkins53 and Jenkins et al.54 when assessing healthy populations.
Baseline Comparisons Across Conditions
Percent Decline in MVIC Torque. The dependent t test revealed that the percent decline in MVIC torque following the NMES treatments was not significantly different across conditions (t13 = 1.086; P = .149; d = 0.310; 95% CI for effect size: −0.276, 0.884; dunb = 0.292; Figure 3).
Percent decline in MVIC torque. Error bars indicate 95% confidence intervals calculated using a critical t value, as has been recommended.51 MVIC = maximal involuntary isometric contraction; m-NMES = multipath neuromuscular electrical stimulation; c-NMES = conventional neuromuscular electrical stimulation
Percent Decline in NMES-Induced Torque. For the percent decline in NMES-induced torque, the repeated-measures ANOVA revealed no significant condition by time interaction (F2.6,34 = 0.849; P = .464; f = 0.255) or condition main effect (F1,13 = 0.052; P = .411; f = 0.063). However, there was a significant time main effect (F2.9,38.1 = 192.156; P < .001; f = 3.857; Figure 4). Post-hoc analyses revealed that the decline was significantly greater by the sixth contraction (difference = 10.9 ± 7.5%; P < .001; d = 1.559; 95% CI for effect size: 0.978, 2.125; dunb = 1.515) and it remained significantly greater for each subsequent contraction (18th contraction, difference = 54.0 ± 13.7%; P < .001; d = 5.004; dunb = 4.863).
Percent decline in NMES-induced torque. *First contraction with a significantly greater decline relative to contraction two (P < .001), which occurred regardless of group. Error bars represent 95% confidence intervals calculated using the equation for within-participants design that has been recommended. 55 NMES = neuro-muscular electrical stimulation; c-NMES = conventional neuromuscular electrical stimulation; m-NMES = multipath neuromuscular electrical stimulation
Percent Decline in TTI. For the percent decline in TTI, the repeated-measures ANOVA revealed no significant condition by time interaction (F2.4,30.6 = 1.223; P = .313; f = 0.306) or condition main effect (F1,13 = 0.182; P = .338; f = 0.119). However, there was a significant time main effect (F2.7,34.8 = 276.330, P < .001; f = 4.607; Figure 5). Post-hoc analyses revealed that the decline was significantly greater by the fifth contraction (difference = 9.04 ± 7.7%; P = .004; d = 1.123; 95% CI for effect size: 0.654, 1.579; dunb = 1.091) and it remained significantly greater for each of the subsequent contractions (18th contraction, difference = 59.0% ± 14.0%; P < .001; d = 5.48; dunb = 5.326).
Percent decline in torque-time integral. *First contraction with a significantly greater decline relative to contraction two (P = .004), which occurred regardless of group. Error bars represent 95% confidence intervals calculated using the equation for within-participants design that has been recommended. 55 c-NMES = conventional neuromuscular electrical stimulation; m-NMES = multipath neuromuscular electrical stimulation
T-TTI. The dependent t test revealed that the T-TTI was significantly greater during the c-NMES condition (t13 = −2.068, P = .029; d = −0.391; 95% CI for effect size: −0.783, 0.015; dunb = −0.368; Figure 6).
Total torque-time integral (T-TTI). *Significantly greater T-TTI (P = .029). Error bars indicate 95% confidence intervals calculated using a critical t value as has been recommended.51 c-NMES = conventional neuromuscular electrical stimulation; m-NMES = multipath neuromuscular electrical stimulation
Self-Reported Discomfort. The repeated-measures ANOVA revealed that there was no significant condition by time interaction (F2.7,35.7 = 0.963; P = .415; f = 0.272) or condition main effect (F1,13 = 0.419; P = .265; f = 0.179) for self-reported discomfort levels. There was a significant time main effect (F1.92,25 = 3.60; P = .022; f = 0.526; Figure 7), but post-hoc analyses did not reveal any significant differences (P > .05).
Self-reported discomfort. Error bars represent 95% confidence intervals calculated using the equation for within-participants design that has been recommended.55 c-NMES = conventional neuromuscular electrical stimulation; m-NMES = multipath neuromuscular electrical stimulation
While using similar stimulus parameters and electrode configurations, the findings of our study indicate that m-NMES was not significantly better on any of the outcome measures when compared to c-NMES; however, it did perform similarly. To the best of our knowledge, m-NMES and c-NMES have not been previously compared while implementing similar electrode configurations. Therefore, we believe these are important findings because our approach of using similar electrode configurations allowed us to better isolate the influence of the novel multipath current distribution method on outcomes related to fatigue and discomfort using similar electrode configurations.
Our findings also indicated that the outcome measures, irrespective of the stimulator, significantly changed over time. The declines in NMES-induced torque and TTI were significantly greater over time (Figures 4–5), whereas self-reported discomfort levels significantly decreased over time (Figure 7). We believe these findings are also important because they illustrate the need for additional strategies to prevent the decline in NMES-induced torque to maintain a sufficient NMES training intensity over the course of a treatment.
Studies examining NMES-induced fatigue of the quadriceps, using a variety of parameters, have reported declines in MVIC ranging from 7% to 33%.12,15,18,56–58 For the m-NMES and c-NMES conditions in our study, we observed declines of only 3.6% ± 4.4% and 5.6% ± 8.2%, respectively. It is unclear why the declines in MVIC we observed are lower than the values reported in the aforementioned studies, but it may be attributed to a variety of methodological differences (eg, total number of NMES-induced contractions, duration of contractions, rest time, and initial NMES training intensity). Although the declines in MVIC were lower than expected, our observation that the declines were not significantly different between the two NMES conditions agreed with Maffiuletti et al.12 but contradicted Morf et al.15 One possible explanation is that Maffiuletti et al. also used healthy participants but Morf et al. used individuals who had recently undergone a knee replacement.
Although the decline in MVIC results in our study and those of Maffiuletti et al.12 are in direct contrast to those reported by Morf et al.,15 the mean differences observed between the NMES conditions and corresponding effect sizes are similar across all three studies. Although reporting a statistically significant difference, Morf et al. observed a mean difference between the two NMES conditions of only 3.0%, which is similar to the nonsignificant 2.6% difference observed by Maffiuletti et al. and the 2.1% nonsignificant difference we observed. Corresponding Cohen's d effect sizes for each study were considered to be small because they were 0.383, 0.311, and 0.310, respectively. Because all three studies observed similar small mean differences between the two conditions, and small corresponding effect sizes, we believe the more likely explanation for the inconsistent statistical results is the difference in sample size across the three studies (n = 20, n = 10, and n = 14, respectively). Furthermore, the comparable small effect sizes consistently observed across each of these studies suggests, irrespective of the presence or absence of statistical significance, that m-NMES does not reduce the decline in MVIC in a clinically meaningful way. However, statistically significant differences may be observed with a large enough sample.15
Others have observed declines in NMES-induced torque ranging from 8% to 61% while using a variety of treatment parameters.12,15,19,40–43,57 Our observation that NMES-induced torque significantly declined over the course of each treatment condition is in agreement with these previous studies. Likewise, our observation that the decline in NMES-induced torque was not significantly different between the two NMES conditions agrees with the two earlier studies comparing c-NMES24 and m-NMES.15 Morf et al.15 hypothesized that they did not observe a significant difference between conditions due to short contraction durations and a low target training intensity, which may have limited potential differences between the two conditions. Consequently, we chose to include a longer contraction duration (10 seconds) and higher target training intensity (30% MVIC), but we observed a similar result for the decline of NMES-induced torque so our results do not appear to support their hypothesis.
In addition to longer contraction durations, we implemented a longer rest period than earlier studies.12,15 We elected to use an on:off ratio of 10:50 rather than 5:10 because a meta-analysis addressing NMES efficacy for quadriceps strengthening revealed that a 10:50 ratio was used most often.59 Despite implementing a longer rest period between contractions, which has been shown to reduce the decline in NMES-induced torque,41,43 we observed a mean decline in NMES-induced torque of roughly 50% under each condition. This was much greater than the declines of roughly 20% to 25% observed by Maffiuletti et al. and Morf et al., respectively. The declines we observed were likely larger because we implemented a higher initial target training intensity and our NMES-induced contractions were twice the duration.
Declines in NMES-induced torque over the course of a treatment pose a significant clinical problem because this subsequently reduces the NMES training intensity, which is considered to be the primary determinant of NMES treatment efficacy.12,15 Therefore, we believe that the substantial difference in the declines in NMES-induced torque we observed versus those reported in two previous studies14,18 warrants further investigation. The smaller percent declines observed during the two earlier studies may indicate that shorter contraction durations result in smaller percent declines in NMES-induced torque, but because our target training intensity was higher and our rest intervals were longer, it is difficult to directly compare our results to those of earlier studies. To the best of our knowledge, the decline in NMES-induced torque while implementing different contraction durations has yet to be examined. We believe differences in contraction duration warrant further examination, particularly because the previously mentioned training study29 reporting improved patient outcomes while using m-NMES implemented on:off ratios of 10:20 and 5:10 for the c-NMES and m-NMES conditions, respectively. Therefore, different contraction durations may have been a confounding factor, but due to a lack of research in this area the extent to which different contraction durations influenced these results remains unclear.
Declines in NMES-induced torque are often used as a measure of muscle fatigue,12,15,19,40–43 but caution should be exercised while interpreting these results because declines in NMES-induced torque may be a combination of muscle fatigue and accommodation of the motor nerves.44,57 Alon and Smith36 defined accommodation as the transient process by which the threshold required to excite the nerve increases in response to the electrical stimulus. Accommodation has been suggested as a contributing factor to declines in NMES-induced torque output because an increased nerve threshold has the potential to result in a diminished number of recruited motor units.16,37,44 We did not directly measure accommodation in our study; thus, we are unable to confidently differentiate between muscle fatigue and accommodation. However, we did observe two interesting patterns that warrant discussion.
We observed a much larger decline in NMES-induced torque (roughly 50%) relative to the decline in MVIC (roughly 5%), irrespective of the NMES condition. A similar pattern was observed in recent studies attempting to examine accommodation.44,57 Matkowski et al.57 hypothesized that their observed pattern of a larger decline in NMES-induced torque relative to a much smaller decline in MVIC, which we also observed in our study, is primarily attributable to accommodation occurring during the NMES-induced contractions. In addition to declines in NMES-induced torque, we also observed a significant decline in self-reported discomfort over the course of the treatments (Figure 7). Randolph et al.56 also observed a decline in discomfort, which can be attributed to accommodation in sensory nerves, but the authors also hypothesized that accommodation affected motor nerves, therefore contributing to their observed declines in NMES-induced torque. Although we are unable to definitively determine the relative contributions of accommodation and fatigue, based on our observed pattern of a smaller decline in MVIC relative to the decline in NMES-induced torque and a corresponding decline in self-reported discomfort, we hypothesize that accommodation was the primary contributing factor to the large declines in NMES-induced torque.44,56,57
We achieved our target training intensity of 30% MVIC but, due to the large declines in NMES-induced torque, the mean NMES training intensity was only 15.2% ± 4.6% MVIC during the final contraction; which is well below the therapeutic window of 25% to 50% MVIC.36 The low NMES training intensities of the final NMES-induced contractions may offer an additional explanation for the observed minimal decreases in posttest MVIC torque. Because participants were producing little torque during the final NMES-induced contractions, their quadriceps were not heavily taxed during these contractions, which may have allowed them to fully recover from earlier, more intense contractions prior to performing posttest MVICs. This observation, and our hypothesis that accommodation was the primary contributing factor to the large declines in NMES-induced torque, highlights a need for researchers to develop clinically applicable strategies focused on combating accommodation rather than muscle fatigue. One successful strategy is to systematically increase the stimulus intensity over the course of the treatment.24
Our study is unique because we also compared TTI data between m-NMES and c-NMES. The declines we observed in TTI mirrored the declines we observed in NMES-induced torque, because the decline in the TTI was not significantly different between the two conditions but was significantly greater over time. The decline in TTI reached 55.4% ± 15.4% and 56.7% ± 12.9% during the 18th contraction for m-NMES and c-NMES, respectively. Because the declines in TTI mirrored the declines in NMES-induced torque during our study, including TTI comparisons may seem repetitive. However, we elected to include TTI comparisons because it has been suggested as a better determinant of fatigue during longer duration NMES-induced contractions like those performed during our study.45 We also believed that it was important to examine the decline in TTI because peak torque alone does not provide an adequate summary of the entire 10-second contraction. For example, it is plausible that similar peak torque values could be observed during two contractions, but if the amount of time during which the contraction is held at or near peak torque differed substantially, then the TTI of each contraction would differ.
We also examined the T-TTI, which represented the total amount of isometric work performed under each condition. Despite nonsignificant baseline differences with respect to the initial NMES training intensity or initial TTI and similar declines in NMES-induced torque and TTI during each condition, the T-TTI was significantly greater during the c-NMES condition. However, it is important to acknowledge that this difference reached statistical significance due to our use of a one-tailed test, but was contrary to our hypothesized direction. This may be due to an observed mean torque recording duration of 8.6 ± 0.2 seconds during the m-NMES condition, whereas the mean duration of the torque recording during c-NMES was 10.3 ± 0.2 seconds. Because each device used similar “on” times, this was unexpected. The isokentic dynamometer is only capable of recording torque when the contraction intensity is sufficient to overcome the force of gravity and cause the lower leg to push against the fixed lever arm. Therefore, it appears that the amount of time the NMES-induced contractions exceeded gravity during the m-NMES condition was roughly 1.7 seconds shorter than during c-NMES. This difference may be attributed to the multipath current distribution method of the m-NMES device. Research into NMES has primarily focused on NMES-induced peak torque, which influences the total amount of work performed during a particular session but is not the only determining factor of isometric work. To the best of our knowledge, there are no published studies that have examined the impact that the amount of work done during NMES sessions has on the effectiveness of the treatments, thus we believe that future research in this area is warranted.
Our observation that self-reported discomfort levels were similar across conditions is contrary to the results of earlier studies.12,15 Morf et al. observed that VAS scores during c-NMES were 39% higher over the course of their treatment, and Maffiuletti et al. observed a similar 35% difference prior to the 20th contraction. We believe our results were different due to the similarly sized electrodes that we used across the two conditions. Both Morf et al. and Maffiuletti et al. acknowledged that using different electrode configurations during the NMES conditions limited their ability to attribute the improved outcomes observed during the m-NMES condition to the novel multipath current distribution method. Therefore, we used similarly sized electrodes during each condition in our study. The c-NMES electrodes covered a surface area of roughly 360 cm2, whereas the m-NMES electrodes covered an area of 427 cm2.18 Although there was a small difference in the area covered by c-NMES and m-NMES, our electrodes were more similar than the electrode configuration used during the c-NMES condition of the previous studies, which consisted of three electrodes covering only 100 cm2.12,15
Although we observed a significant time main effect indicating a decline in self-reported discomfort levels over the course of the NMES treatment conditions, we did not observe any statistically significant post-hoc pairwise comparisons. This likely occurred because of our use of the conservative Bonferroni correction to maintain family-wise error rate. The largest mean differences we observed during the pairwise comparisons were minimal, ranging from 6.4 to 7.6 mm. During a previous NMES study performed in our laboratory, we defined a 13-mm threshold for determining clinically significant differences with respect to self-reported discomfort levels.21 Therefore, we do not believe that the mean differences we observed during the pairwise comparisons represented clinically significant differences, which is in agreement with our nonsignificant findings during the post-hoc analyses.
The extent to which the findings of our study hold true with respect to females and injured populations remains unclear; thus, excluding females and using only healthy participants may be viewed as limitations of our study. We did not include females because the menstrual cycle has been shown to influence self-reported discomfort levels,60 and our study design required repeated measurements over time. In addition, during exploratory NMES studies, similar in nature to our study, it is common practice to use healthy participants.12,14,17,18,20,22,24,36,37,41,46,56–58
Despite achieving an original sample size larger than our a priori power analyses indicated, we excluded the data of 7 participants. Therefore, our sample size was smaller than desired during our t test comparisons, which may have allowed a type II error. However, due to the fact that each of our non-significant observations also had corresponding effect sizes that were below the medium to large threshold used during our a priori power analyses, we believe that our sample size was adequate for the purposes of our study, which we designed to focus on clinically meaningful differences.
A maximum comfortable stimulus intensity, which may vary across NMES conditions and individuals, should be used in clinical settings. Therefore, another limitation of our study was that we used a fixed target training intensity of 30% MVIC, which does not permit inferences regarding which NMES method allows for greater initial training intensities or regarding outcomes related to fatigue and discomfort while using a clinically applicable stimulus intensity. A maximum comfortable stimulus intensity is self-selected by each individual so baseline differences in NMES-induced torque may occur across conditions or individuals, whereas using a target training intensity is likely to prevent a baseline difference. Therefore, we standardized the stimulus intensity by using a target training intensity of 30% MVIC in an effort to enhance experimental control and subsequently facilitate our interpretation of the results. Future studies should compare the two NMES conditions while using a clinically relevant maximum comfortable stimulus intensity.
Implications for Clinical Practice
Based on our results, it does not appear that the novel multipath current distribution method positively impacts NMES-induced fatigue and discomfort in a clinically meaningful manner, but the devices did perform similarly. We believe this is also an important observation because there is a need for more patient-friendly NMES stimulators,61 which the m-NMES device accomplishes because it is portable and the neoprene sleeve containing the electrodes allows patients to easily apply the device. Feil et al.29 observed greater compliance during the m-NMES condition of their study, which they attributed to the convenience of the m-NMES device. Therefore, we agree that the m-NMES device may be clinically useful. We believe the larger electrodes integrated into the garment of the m-NMES device contribute to the improved outcomes observed during similar studies,12,15 because we did not observe any such differences while using similar electrode configurations.
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Parameters of Neuromuscular Electrical Stimulation Conditions
||Single path within two independent channels
||1 second up: 0.5 seconds down
||1 second up: 0 seconds downa
|On time/Off time
||10 seconds/50 seconds
||10 seconds/50 seconds
||mA required for 30% MVIC
||mA required for 30% MVIC
|Number of electrodes
|Total area of electrodes
Baseline Comparisons Across Conditions
|Pretest MVIC (Nm)
||221.9 ± 56.5
||217.021 ± 66.1
|Initial NMES training intensity (% MVIC)
||31.0 ± 6.0
||32.1 ± 5.0
|Initial TTI (Nm*s)
||541.2 ± 146.8
||653.5 ± 260.4