ArticlePDF Available

Influence of vibration on delayed onset of muscle soreness following eccentric exercise * COMMENTARY

Authors:
  • Semnan University of Medical Sciences
  • Neuromuscular Rehabilitation Research Center, Semnan University of Medical Sciences, Semnan, Iran

Abstract and Figures

Delayed onset muscle soreness (DOMS), which may occur after eccentric exercise, may cause some reduction in ability in sport activities. For this reason, several studies have been designed on preventing and controlling DOMS. As vibration training (VT) may improve muscle performance, we designed this study to investigate the effect of VT on controlling and preventing DOMS after eccentric exercise. Fifty healthy non-athletic volunteers were assigned randomly into two experimental, VT (n = 25) and non-VT (n = 25) groups. A vibrator was used to apply 50 Hz vibration on the left and right quadriceps, hamstring and calf muscles for 1 min in the VT group, while no vibration was applied in the non-VT group. Then, both groups walked downhill on a 10 degrees declined treadmill at a speed of 4 km/hour. The measurements included the isometric maximum voluntary contraction force (IMVC) of left and right quadriceps muscles, pressure pain threshold (PPT) 5, 10 and 15 cm above the patella and mid-line of the calf muscles of both lower limbs before and the day after treadmill walking. After 24 hours, the serum levels of creatine-kinase (CK), and DOMS level by visual analogue scale were measured. The results showed decreased IMVC force (P = 0.006), reduced PPT (P = 0.0001) and significantly increased mean of DOMS and CK levels in the non-VT group, compared to the VT group (P = 0.001). A comparison by experimental groups indicates that VT before eccentric exercise may prevent and control DOMS. Further studies should be undertaken to ascertain the stability and effectiveness of VT in athletics.
Content may be subject to copyright.
A preview of the PDF is not available
... Sixteen articles were finally included in the meta-analysis based on inclusion criteria. The literature screening process and results are shown in Figure 1 and Table 2. [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] ...
... Twelve of the included studies [25][26][27][28][29][32][33][34][35][36][37][38]40,41] evaluated the effect of VT on subjective pain in DOMS using the pain VAS score in a total of 317 subjects. Statistical heterogeneity was detected (χ 2 = 310.77, ...
... Three published systematic reviews [19][20][21] were analyzed, as shown in Table 3. To address previous shortcomings, [38,49,50] we include 16 studies written in Chinese or English literature for analysis. Our findings revealed that after performing VT intervention in DOMS, VAS and CK and LDH concentrations decreased significantly relative to controls, and PPT increased significantly, but there was no significant improvement in knee mobility. ...
Article
Full-text available
Background: Delayed onset muscle soreness (DOMS) is caused by unaccustomed exercise, especially eccentric exercise, and is highly likely to cause skeletal muscle injury. It mainly manifests as ultrastructural changes in skeletal muscle, as well as decreased muscle strength, muscle soreness, swelling, and elevated levels of creatine kinase (CK). Vibration training (VT) has been attracting increasing attention as a new type of rehabilitation therapy. It can effectively minimize the occurrence and relieve the symptoms of DOMS, reduce muscle stiffness and soreness, and reduce serum concentrations of CK and lactate dehydrogenase (LDH). This article systematically assessed the impact of VT on the mitigation of DOMS through a meta-analysis to provide updated evidence-based information. Methods: Electronic databases such as China Knowledge Network, VIP Electronics, PubMed, EBSCO, and Web of Science were searched to identify randomized controlled trials of VT on DOMS. Searches were performed from database creation to November 2021. The quality of the literature was assessed using the Cochrane Manual for the Systematic Review of Interventions, and meta-analyses were performed using RevMan 5.4 software. Results: VT intervention in DOMS was shown to effectively reduce subjective pain, improve pain tolerance, and accelerate the reduction of serum CK and LDH concentrations. Subgroup analysis of different test time periods showed that subjective pain decreased more significantly after 48 hours than after the other 2 time periods, and pain tolerance increased more significantly after 72 hours than the other 2 time periods; serum CK was significantly increased after 24 and 48 hours of intervention, but showed no significant change compared with the control group after 72 hours. Serum LDH decreased significantly after 24 hours of intervention, but there was no significant difference compared with the control group after 48 hours or 72 hours. Conclusion: VT effectively reduced the subjective pain sensation after DOMS, increased the pain threshold, reduced serum LDH and CK concentrations, and accelerated muscle damage repair compared with control interventions. However, the effect of improving the range of motion of the joints is not clear and should be studied further. Registration: number: INPLASY2021120115.
... According to Lu et al. (2019), 'Vibration treatment is becoming more popular in sports, to enhance skeletal musculature performance and injury recovery' . Despite these promising findings, conflicting evidence has been reported regarding its effectiveness for DOMS management (Aminian-Far et al., 2011;Bakhtiary et al., 2007;Dabbs et al., 2015;Timon et al., 2016). According to the study done by Aminian-Far et al. (2011), applying vibration before eccentric exercise may decrease DOMS symptoms. ...
... After completing 35 min of walk, they were asked to continue walking for another 5 min on the treadmill on a flat surface at 2.5 km/h to cool down. This protocol's reliability for DOMS inducement was approved in previous studies (Aminianfar et al., 2019;Bakhtiary et al., 2007). ...
... This study demonstrated that the WBV group experienced less pain 96 h post-DOMS than the control group. Previous research supports our findings (Aminian-Far et al., 2011;Bakhtiary et al., 2007;Rhea et al., 2009;Timon et al., 2016). Additionally, comparing WBV to conventional exercises in providing pain relief for DOMS, Wheeler and Jacobson concluded that WBV is as effective as light exercise (Wheeler and Jacobson, 2013), so it can be inferred that WBV training can be used in adjunction to exercise to save time and energy. ...
Article
Delayed onset muscle soreness (DOMS) is a condition that happens following eccentric or intensive exercises. Whole-body vibration (WBV) is a potential treatment for DOMS; however, there is a lack of studies assessing its effectiveness in the untrained population. Our study objective was to test the efficacy of WBV compared with no treatment on pain and knee strength in healthy adults with DOMS. We randomly allocated 52 participants, 12 men and 40 women, aged 18 to 28 years, into WBV (n=26) and control (n=26) groups. The eligibility criteria included no history of injury, strengthening or aerobic exercises within the past six months. The participants walked on a treadmill to introduce DOMS. 24 h later, the WBV group received one session of WBV treatment, with their knees bent to 60° for 1 min (frequency, 30 Hz; amplitude 2-5 mm). We assessed pressure pain threshold (PPT), visual analogue scale (VAS), and knee maximum isometric force (MIF) at four time points. We did a one-way repeated measures ANOVA of each outcome measure, followed by a t-test and Bonferroni post-hoc test. The between-group differences were not significant at the baseline and 24 h post-DOMS (P>0.05). Statistical analyses revealed significant differences between the two groups for all variables at 96 h post-DOMS inducement (P<0.05), with mean differences for PPT, MIF and VAS being 1.19 kilo Pascals (95%CI = 0.78-1.32), 42.87 Newtons (95%CI = 28.53-56.98), and -2.39 (95%CI = -3.13- -1.98), respectively. Moreover, differences between the two groups were statistically significant for MIF 168 h post-DOMS (P<0.05). WBV can effectively improve pain and muscle strength; therefore, beneficial treatment for recovery of DOMS symptoms. However, determining the exact dose, frequency, and best time of application is pending future research. Iranian Registry of Clinical Trials registration number: IRCT2016092429958N1
... The overall picture is encouraging, with meta-analysis finding vibration effective for prevention or treatment of DOMS [29,30]. Therapies studied include those provided before [31][32][33][34][35][36] and after [9,20,35,[37][38][39][40][41][42][43][44][45][46][47][48][49] exercise; in athletes [36,49], untrained [9, 31-35, 37, 38, 40, 42, 45, 47, 48] and recreationally active [20,39,41,43,44,46] participants; and using local vibration [20,31,32,34,35,37,38,40,45,48,49] and WBV [9, 33, 36, 39, 41-44, 46, 47]. However, the specific case of WBV used post-exercise in elite athletes has not previously been examined. ...
... The overall picture is encouraging, with meta-analysis finding vibration effective for prevention or treatment of DOMS [29,30]. Therapies studied include those provided before [31][32][33][34][35][36] and after [9,20,35,[37][38][39][40][41][42][43][44][45][46][47][48][49] exercise; in athletes [36,49], untrained [9, 31-35, 37, 38, 40, 42, 45, 47, 48] and recreationally active [20,39,41,43,44,46] participants; and using local vibration [20,31,32,34,35,37,38,40,45,48,49] and WBV [9, 33, 36, 39, 41-44, 46, 47]. However, the specific case of WBV used post-exercise in elite athletes has not previously been examined. ...
... This trial investigated post-exercise vibration therapy. Vibration therapy has been most often studied as a recovery modality, but favourable results have been reported by multiple studies of its pre-exercise use [32,33,36]. In a trial which compared the use of local vibration before or after exercise with a third control group, pre-exercise therapy produced greater reductions in pain and laboratory measures of muscle damage than post-exercise therapy [35]. ...
Article
Full-text available
Background Delayed onset muscle soreness (DOMS) is a common non-structural muscle injury which can disrupt training and impair performance in elite athletes. Vibration therapy reduces inflammation and improves neuromuscular efficiency, leading to reductions in pain and stiffness, and may be effective for the prevention or treatment of DOMS. However, the effect of whole-body vibration (WBV) used after sport in elite athletes has not been reported. Methods A randomised, controlled trial was performed. Participants were elite (national or international level) hockey players and underwent an eccentric exercise protocol previously shown to produce clinical DOMS. After exercise, one group underwent static stretching with WBV therapy, and the other performed stretching only. Baseline and serial post-exercise pain scores and measurements of quadriceps tightness were obtained. Results Eleven participants were recruited into each study arm. There were no significant differences in baseline group characteristics. Participants receiving WBV had significant reductions in both pain (p = 0.04) and quadriceps tightness (p = 0.02) compared with stretching only. Conclusions Post-exercise WBV is effective in elite hockey players to reduce DOMS after eccentric exercise. Elite athletes in multi-sprint sports are at risk of DOMS during training and competition, and its reduction could contribute to reduced injury risk and improved performance. This treatment modality is favourable because it can be incorporated with minimal disruption into the recovery section of existing training regimes. These findings may also be extrapolated to other multi-sprint sports.
... In previous studies, VT (50 Hz, 5 min) on the elbow in ordinary young women shows that the MIVC increases at 72 h (Koh et al., 2013). VT (50 Hz, 30 min) on common male/female femoral quadriceps reveals a significant increase in MIVC for 24 h (Bakhtiary et al., 2007). The above study partly supports the present study's conclusions. ...
... The analysis may be related to the activation of increased motor units (DOMS sites) in increased muscle tension. Muscle spindle stimulation by vibration may increase the afferent activities of muscle spindles, which may increase background tension in the vibrated muscles (Bakhtiary et al., 2007;Broadbent et al., 2010;Koh et al., 2013). Furthermore, given the reduced muscle pain, the subject is believed to have reduced muscle strength caused by pain (afraid to force) during the isometric muscle force test. ...
Article
Full-text available
Objective: Kinesio taping (KT) and vibration treatment (VT) can alleviate delayed-onset muscle soreness (DOMS) to some extent. However, the literature reports on the difference between the two treatments, and whether a joint intervention (JI) works better than single treatments remains unknown. This study compares the effects of KT, VT and JI on DOMS in college students. Methods: A total of 88 college students were randomly divided into the KT (KTG, n = 21), VT (VTG, n = 22), JI (JIG, n = 23) and control (CG, n = 22) groups. All subjects underwent DOMS moulding. The baseline; immediate and 24, 48 and 72 h visual analogue scale (VAS) scores and knee extensor maximum isometric voluntary contraction (MIVC) were determined. Results: The intergroup comparison showed the following results. 1) The VAS scores of the four groups peaked at 24 h and gradually decreased. The immediate, 24 h and 48 h VAS scores followed the order: JIG > KTG and VTG > CG. The 72 h VAS score followed the order: KTG < VTG < JIG < CG. 2) The knee extension MIVC in the four groups was lowest at 24 h and then gradually increased. JIG had larger immediate MIVC than CG. KTG, VTG and JIG had larger 24 h MIVC than CG. JIG had larger 48 h MIVC than KTG and CG. Conclusion: KT and VT can reduce muscle pain and strength loss caused by DOMS to varying degrees. VT is better than KT in improving pain. The combined intervention worked better than single interventions.
... After WBV, lower pain ratings and lower quadriceps tension were indicated. It has also been shown that WBV can be used as a warm-up to reduce the risk of DOMS before exercise [6,24]. ...
Article
Full-text available
The aim of this project was to indicate the optimal parameters such as frequency, duration of a single vibrotherapy, and body position, which will be used as a form of recovery modality after physical exercise. Sixteen healthy male volunteers were involved in this study. The aerobic and anaerobic capacity of participants was assessed. Each of the subjects performed a set of intensive physical exercises and then underwent vibrotherapy treatment. In random order, each of the men tested the effectiveness of eight of the combinations of frequency, duration, and body position. The effect of the procedure accelerating recovery was assessed 24 h after physical exercise with the Wingate test. Changes in oxygen saturation and biochemical markers (interleukins: Il-1β, Il-6, and creatine kinase: CK), hemoglobin (Hb), and hematocrit (Hct) were assessed 1 h and 24 h after the physical effort. Lactate concentrations were measured 3, 15, 30, and 60 min after the end of the vibration. It was indicated that the optimal treatment should be based on lower ranges of frequency values (2–52 Hz). The procedure with raised feet is also more beneficial than the flat, supine position. To improve the overall work, and a number of biochemical markers (CK and Il-1β), a 45 min treatment will be more efficient, because significantly lower CK activity was indicated for the 45 min treatment. For this duration, higher values of Il-1β were indicated in the measurement carried out for samples collected 60 min after the treatment and lower in the measurement carried out 24 h after the treatment.
... Some interventions like cryotherapy [7] had some positive effects on muscle soreness or other DOMS symptoms while some other methods like stretching [8,9] , demonstrated no effect on the alleviation of DOMS. Massage [10] , vibration [11] ultrasound and electrical current modalities [4,12] have shown controversial effects. In addition, exercises are among the most effective means to alleviate pain in DOMS; however, the analgesic effect is temporary [13] . ...
... A study concerning the effect of vibrotherapy on muscle soreness and CK activity after eccentric exercise was conducted by Bakhitiary et al. [38]. The main difference between the protocols used by Bakhitiary et al. and us was in the timing of when the vibration was applied. ...
Article
Full-text available
Prolonged exercise can lead to muscle damage, with soreness, swelling, and ultimately reduced strength as a consequence. It has been shown that whole-body vibration (WBV) improves recovery by reducing the levels of stress hormones and the activities of creatine kinase (CK) and lactate dehydrogenase (LDH). The aim of the study was to demonstrate the effect of local vibration treatment applied after exercise on the level of selected markers of muscle fiber damage. The study involved 12 untrained men, aged 21.7 ± 1.05 years, with a VO2peak of 46.12 ± 3.67 mL. kg −1. min −1. A maximal intensity test to volitional exhaustion was performed to determine VO2peak and individual exercise loads for prolonged exercise. The subjects were to perform 180 min of physical effort with an intensity of 50 ± 2% VO2peak. After exercise, they underwent a 60 min vibration treatment or placebo therapy using a mattress. Blood samples were taken before, immediately after the recovery procedure, and 24 h after the end of the exercise test. Myoglobin (Mb) levels as well as the activities of CK and LDH were recorded. Immediately after the hour-long recovery procedure (vibra-tion or placebo), the mean concentrations of the determined indices were significantly different from baseline values. In the vibration group, significantly lower values of Mb (p = 0.005), CK (p = 0.030), and LDH (p = 0.005) were seen. Differences were also present 24 h after the end of the exercise test. The results of the vibration group compared to the control group differed in respect to Mb (p = 0.002), CK (p = 0.029), and LDH (p = 0.014). After prolonged physical effort, topical vibration improved post-workout recovery manifested by lower CK and LDH activity and lower Mb concentration compared to a control group.
... Aminian-Far et al. reported knee extensor isometric torque loss to be attenuated with just one minute of WBV used prior to the EIMD protocol [17]. In another study, Bakhtiary et al. used one minute of localized vibration of the quadriceps prior to an eccentric protocol of decline treadmill walking and reported less isometric strength loss at 24 h compared to the no vibration control [53]. Both studies utilized WBV prior to their respective EIMD protocols, which appear to be less intense than many other EIMD protocols for similar muscle groups. ...
Article
Full-text available
This study utilized resonant frequency vibration to the upper body to determine changes in pain, stiffness and isometric strength of the biceps brachii after eccentric damage. Thirty-one participants without recent resistance training were randomized into three groups: a Control (C) group and two eccentric exercise groups (No vibration (NV) and Vibration (V)). After muscle damage, participants in the V group received upper body vibration (UBV) therapy for 5 min on days 1–4. All participants completed a visual analog scale (VAS), maximum voluntary isometric contraction (MVIC), and shear wave elastography (SWE) of the bicep at baseline (pre-exercise), 24 h, 48 h, and 1-week post exercise. There was a significant difference between V and NV at 24 h for VAS (p = 0.0051), at 24 h and 1-week for MVIC (p = 0.0017 and p = 0.0016, respectively). There was a significant decrease in SWE for the V group from 24–48 h (p = 0.0003), while there was no significant change in the NV group (p = 0.9341). The use of UBV resonant vibration decreased MVIC decrement and reduced VAS pain ratings at 24 h post eccentric damage. SWE was strongly negatively correlated with MVIC and may function as a predictor of intrinsic muscle state in the time course of recovery of the biceps brachii.
Article
BACKGROUND: Vibration therapy (VT) causes an increase in motor unit activation tendency, an involuntary recruitment of earlier sedentary motor units, which increases the muscle fiber force generating capacity and muscle performance. OBJECTIVE: To evaluate the effect of vibratory massage therapy at 23 Hz and 35 Hz on grip strength, endurance, and forearm muscle performance (in terms of EMG activity). METHODS: Ten healthy and right-handed men participated voluntarily in this study. The experiment was characterized by the measurement of MVC (maximal voluntary contraction) grip strength and grip endurance time at 50% MVC, accompanied by the corresponding measurement of the EMG signals of the muscles viz., flexor digitorum superficialis (FDS); flexor carpi ulnaris (FCU); extensor carpi radialis brevis (ECRB); and extensor carpi ulnaris (ECU) in supine posture. RESULTS: MANCOVA results showed significant effects of VT frequency on endurance time (p <0.001); but no significant effect on the grip strength (p = 0.161) and muscle performance (in terms of EMG activities of the forearm muscles). However, VT improves the MVC grip strength and grip endurance time (better at 35 Hz). The Pearson correlation was significant between: weight, palm length, palm circumference, and forearm length with MVC grip strength; and the palm length with the endurance time. In addition, the palm length, palm circumference, and forearm circumference generally serve to better predict MVC grip strength and grip endurance time. CONCLUSIONS: Vibration therapy at 35 Hz for 10 minutes on the forearms had a significant positive effect on the neuromuscular performance to enhance muscle performance of upper extremities and can be used as the optimal range to study the effect of VT. Findings may be used to prepare guidelines for VT in rehabilitation, healthcare, sports, and medical for therapists.
Article
Full-text available
Eccentric exercise continues to receive attention as a productive means of exercise. Coupled with this has been the heightened study of the damage that occurs in early stages of exposure to eccentric exercise. This is commonly referred to as delayed onset muscle soreness (DOMS). To date, a sound and consistent treatment for DOMS has not been established. Although multiple practices exist for the treatment of DOMS, few have scientific support. Suggested treatments for DOMS are numerous and include pharmaceuticals, herbal remedies, stretching, massage, nutritional supplements, and many more. DOMS is particularly prevalent in resistance training; hence, this article may be of particular interest to the coach, trainer, or physical therapist to aid in selection of efficient treatments. First, we briefly review eccentric exercise and its characteristics and then proceed to a scientific and systematic overview and evaluation of treatments for DOMS. We have classified treatments into 3 sections, namely, pharmacological, conventional rehabilitation approaches, and a third section that collectively evaluates multiple additional practiced treatments. Literature that addresses most directly the question regarding the effectiveness of a particular treatment has been selected. The reader will note that selected treatments such as anti-inflammatory drugs and antioxidants appear to have a potential in the treatment of DOMS. Other conventional approaches, such as massage, ultrasound, and stretching appear less promising.
Article
Full-text available
High-intensity eccentric contractions induce performance decrements and delayed onset muscle soreness. The purpose of this investigation was to study the magnitude and time course of such decrements and their interrelationships in 26 young women of mean(s.d.) age 21.4(3.3) years. Subjects performed 70 maximal eccentric contractions of the elbow flexors on a pulley system, specially designed for the study. The non-exercised arm acted as the control. Measures of soreness, tenderness, swelling (SW), relaxed elbow joint angle (RANG) and isometric strength (STR) were taken before exercise, immediately after exercise (AE), analysis of variance and at 24-h intervals for 11 days. There were significant (P < 0.01, analysis of variance) changes in all factors. Peak effects were observed between 24 and 96 h AE. With the exception of STR, which remained lower (P < 0.01), all variables returned to baseline levels by day 11. A non-significant correlation between pain and STR indicated that pain was not a major factor in strength loss. Also, although no pain was evident, RANG was decreased immediately AE. There was no relationship between SW, RANG and pain. The prolonged nature of these symptoms indicates that repair to damaged soft tissue is a slow process. Strength loss is considered particularly important as it continues when protective pain and tenderness have disappeared. This has implications for the therapeutic management of patients with myopathologies and those receiving eccentric exercise for rehabilitation.
Article
Delayed onset muscle soreness (DOMS) generally occurs between 24 and 72 hours after a bout of unaccustomed exercise that involves eccentric muscle action. In this review, a variety of aerobic and anaerobic activities are described emphasizing the eccentric component. It is suggested that the experience of severe DOMS can adversely impact various aspects of performance. During endurance events there may be a decrease in economy of movement, impairment of glycogen repletion and an alteration in biomechanical execution of a movement. Reductions in strength/power are also associated with severe DOMS. It is suggested that these changes might put an athlete at increased risk of injury. Although treatment is available to alleviate or prevent DOMS, the wisdom of routinely engaging in such a practice is questioned. Finally, it is noted that one bout of unaccustomed eccentrics results in some adaptation, which has a protective effect during subsequent bouts of eccentric exercise in that DOMS and other markers of muscle trauma are significantly reduced. Recommendations are made for dealing with DOMS at the initiation of an exercise program and during a regular season. (C) 1992 National Strength and Conditioning Association
Article
It is well documented in animal and human research that unaccustomed eccentric muscle action of sufficient intensity and/or duration causes disruption of connective and/or contractile tissue. In humans, this appears to be associated with the sensation of delayed onset muscle soreness (DOMS). During the late 1970's, it was proposed that this sensation of soreness might be associated with the acute inflammatory response. However, subsequent research failed to substantiate this theory. The present article suggests that the results of much of the research concerning DOMS reflect events typically seen in acute inflammation. Similarities between the two events include: the cardinal symptoms of pain, swelling, and loss of function; evidence of cellular infiltrates, especially the macrophage; biochemical markers such as increased lysosomal activity and increased circulating levels of some of the acute phase proteins; and histological changes during the initial 72 h. In the final section of this paper, a theoretical sequence of events is proposed, based on research involving acute inflammation and DOMS.
Article
In order to investigate how vibration affects endurance during muscular contraction, knee-joint extension efforts were performed with and without superimposed vibrations. Fourteen healthy non-smoking 20-year-old males performed maximal isometric and sustained knee-joint extension efforts (angle 90°) in sitting posture three times with each leg, with or without vibration. The tests were done once with each leg in a randomly chosen order. The frequency of the vibration was 20 Hz and the acceleration 20 m/s2 RMS, applied in a horizontal sagittal direction to the ankle. The endurance was defined as the time in seconds that it took for the exerted force to decrease by 10% of the initial value. The endurance time averages 22.5 s without vibration and 15.8 s with vibration. The vibratory stress reduced endurance by 6.7±1.84 s (mean ±SEM) (P<0.005). The difference in maximal force recorded initially was 34 ± 1.9 N (P<0.1). Our conclusion is that vibration may decrease the endurance of maximal och sustained isometric muscular contraction.
Article
The purposes of this study were: (1) to compare the ease of eliciting a tonic vibration reflex (TVR) in different muscles; (2) to determine ways of altering the force of the contraction; and (3) to compare the time course and amplitude of the reflex myogram in ten patients and six normal subjects. The biceps brachii TVRs of the seven patients with hemiplegia and two patients with parkinsonism were similar to those of the normal subjects, except for an occasional twitch (phasic) response occurring with the onset of vibration. The results of this and other studies suggest that the TVR will become a useful research tool for assessing central nervous system excitability, as well as a clinical tool for facilitating voluntary muscle contraction in paretic muscles.