Interleaved neuromuscular electrical stimulation: Motor unit recruitment overlap

Introduction: In this study, we quantified the “overlap” between motor units recruited by single pulses of neuromuscular electrical stimulation (NMES) delivered over the tibialis anterior muscle (mNMES) and the common peroneal nerve (nNMES). We then quantified the torque produced when pulses were alternated between the mNMES and nNMES sites at 40 Hz (“interleaved” NMES; iNMES). Methods: Overlap was assessed by comparing torque produced by twitches evoked by mNMES, nNMES, and both delivered together, over a range of stimulus intensities. Trains of iNMES were delivered at the intensity that produced the lowest overlap. Results: Overlap was lowest (5%) when twitches evoked by both mNMES and nNMES produced 10% peak twitch torque. iNMES delivered at this intensity generated 25% of maximal voluntary dorsiflexion torque (11 Nm). Discussion: Low intensity iNMES leads to low overlap and produces torque that is functionally relevant to evoke dorsiflexion during walking. Muscle Nerve 55 : 490–499, 2017     

Muscle weakness,fatigue, and torque variability: Effects of age and mobility status

Introduction: Whereas deficits in muscle function, particularly power production, develop in old age and are risk factors for mobility impairment, a complete understanding of muscle fatigue during dynamic contractions is lacking. We tested hypotheses related to torque‐producing capacity, fatigue resistance, and variability of torque production during repeated maximal contractions in healthy older, mobility‐impaired older, and young women. Methods: Knee extensor fatigue (decline in torque) was measured during 4 min of dynamic contractions. Torque variability was characterized using a novel 4‐component logistic regression model. Results: Young women produced more torque at baseline and during the protocol than older women (P < 0.001). Although fatigue did not differ between groups (P = 0.53), torque variability differed by group (P = 0.022) and was greater in older impaired compared with young women (P = 0.010). Conclusions: These results suggest that increased torque variability may combine with baseline muscle weakness to limit function, particularly in older adults with mobility impairments. Muscle Nerve 49 : 209–217, 2014     

A novel modulation strategy to increase stimulation duration in neuromuscular electrical stimulation

Introduction: Neuromuscular electrical stimulation (NMES) has been shown to be an effective treatment for muscular dysfunction. Yet, a fundamental barrier to NMES treatments is the rapid onset of muscle fatigue. The purpose of this study is to examine the effect of feedback‐based frequency modulation on the closed‐loop performance of the quadriceps during repeated dynamic contractions. Methods: In the first experiment, subjects completed four different frequency modulation NMES protocols utilizing the same amplitude modulation control to compare the successful run times (SRTs). A second experiment was performed to determine the change in muscle response to high‐ and low‐frequency stimulation. Results: Compared with constant‐frequency stimulation, results indicate that using an error‐driven strategy to vary the stimulation frequency during amplitude modulation increases the number of successful contractions during non‐isometric conditions. Conclusion: Simultaneous frequency and amplitude modulation increases the SRT during closed‐loop NMES control. Muscle Nerve 44: 382–387, 2011     

Influence of joint position on synergistic muscle activity after fatigue of a single muscle head

Introduction: We investigated synergistic muscle activity after fatigue of a single muscle in different joint positions. Methods: Two experimental groups (n = 12 each) performed maximal voluntary contractions (MVCs) before and after fatiguing the gastrocnemius lateralis (GL), using neuromuscular electrical stimulation (NMES). Neuromuscular tests, including muscle activity during MVC, H‐reflex, and twitch interpolation, were performed. One group completed the experiment in a knee‐extended position with the second group in a knee‐flexed position. Results: In the knee‐flexed position, the muscle activity increased in non‐stimulated synergistic muscles. In contrast, in the knee‐extended position, muscle activity of the synergistic muscles remained unaltered. The MVC force remained unaltered in the flexed position and decreased in the extended position. Conclusions: Synergistic muscles compensate for the fatigued muscle in the flexed position but not in the extended position. Compensation mechanisms seem to depend on joint position. Muscle Nerve 51 : 259–267, 2015     

Quadriceps low‐frequency fatigue and muscle pain are contraction‐type‐dependent

Eccentric contractions are thought to induce greater low‐frequency fatigue (LFF) and delayed‐onset muscle soreness (DOMS) than concentric contractions. In this study we induced a similar amount of eccentric quadriceps muscle fatigue during either a concentric or eccentric fatigue task to compare LFF and DOMS. Subjects (n = 22) performed concentric or eccentric fatigue tasks using 75% of the pre‐fatigue maximal voluntary contraction (MVC) torque, and both tasks ended when the MVC eccentric torque decreased by 25% pre‐fatigue. When subjects reached the failure criterion during the eccentric and concentric tasks, the concentric MVC was 78 ± 9.8% and 64 ± 8.4% of initial, respectively. LFF was greater after the concentric than the eccentric protocols (22 ± 12.4% and 15 ± 7.6% increase, respectively; P < 0.01). DOMS was over 100% greater for the eccentric protocol. These results indicate that DOMS is not dependent on the events that contribute to LFF. Muscle Nerve, 2010     

Neuromuscular fatigue after low‐ and medium‐frequency electrical stimulation in healthy adults

Introduction: In this study we investigated fatigue origins induced by low‐frequency pulsed current (PC) and medium‐frequency current (MF) neuromuscular electrical stimulation (NMES) after a clinical‐like session. Methods: Eleven healthy men randomly underwent 2 NMES sessions, PC and MF, on quadriceps muscle (15‐minute duration, 6 seconds on and 18 seconds off). Maximal voluntary contraction (MVC), central activation ratio (CAR), vastus lateralis electromyographic activity (EMG), and evoked contractile properties were determined before and after the sessions. Evoked torque and discomfort during the sessions were also measured. Results: Both currents produced decreases in MVC, EMG, and evoked contractile properties after the sessions. No difference was found between currents for all variables (P > 0.05). Evoked torque during sessions decreased (P < 0.05). No difference was observed in mean evoked torque and discomfort (P > 0.05). Discussion: Both currents induced similar neuromuscular fatigue. Clinicians can choose either PC or MF and expect similar treatment effects when the goal is to generate gains in muscle strength. Muscle Nerve 58 : 293–299, 2018     

A Novel Externally Applied Neuromuscular Stimulator for the Treatment of Stress Urinary Incontinence in Women—A Pilot Study

Objectives: Neuromuscular electrical stimulation (NMES) is commonly used to treat lower urinary tract dysfunctions. This study evaluated the efficacy of a novel externally applied stimulator in the treatment of stress urinary incontinence (SUI). Materials and Methods: Nine women were included in this study. Provocative tests included a cough and jumping jack test assessed via pad weight. Ultrasound (US) imaging assessed pelvic floor muscle (PFM) contraction. A bladder filling protocol allowed for delineation of the bladder from the pelvic floor and standardized volume. External electrodes were used during 30 min, at least four times per week treatment protocol at home for eight weeks. Participants were blinded to US and were not instructed regarding pelvic floor contractions. Results: At week 1, participants could perform PFM contractions verified with US. More importantly, an 87.43% decrease in leakage was noted. At week 8, participants reported a 97.71% decrease in leakage (p= 0.0001). Changes noted in Incontinence Impact Questionnaire and Modified Oxford scores were significant (p= 0.0001 and p= 0.0001). Conclusions: NMES is frequently used to promote muscle strength and coordination. Studies have shown NMES to be effective in decreasing symptoms associated with SUI; however, few, if any, have used it as a primary treatment modality. The novel device in this study was shown to be effective in improving muscle strength, reducing or ablating the symptoms associated with SUI, and in eliciting PFM contractions. The device is noninvasive and can be used as a home treatment.     

Knee extensor fatigue resistance of young and older men and women performing sustained and brief intermittent isometric contractions

Introduction: Susceptibility to muscle fatigue during aging could depend on muscle activation patterns. Methods: Young (mean age, 22 years) and older (mean age 70 years) men and women completed two fatigue tests of knee extensor muscles using voluntary and electrically stimulated contractions. Results: Older subjects displayed a shift to the left of the torque‐frequency relationship and held a sustained voluntary isometric contraction at 50% maximal strength for significantly longer than young (P < 0.001). Young and old showed similar fatigue during electrically induced, intermittent isometric contractions (1‐s on, 1‐s off for 2 min), but women fatigued less than men (P = 0.001). Stronger muscles fatigued more quickly, and slower contractile properties were associated with longer sustained contractions. Conclusions: The slowing and weakness of older muscle was associated with superior fatigue resistance during sustained isometric contractions. Young and old showed similar fatigue following a series of brief, intermittent contractions, but women fatigued less than men. Muscle Nerve 50 : 393–400, 2014     

Endurance neuromuscular electrical stimulation training improves skeletal muscle oxidative capacity in individuals with motor‐complete spinal cord injury

Introduction: Spinal cord injury (SCI) results in skeletal muscle atrophy, increases in intramuscular fat, and reductions in skeletal muscle oxidative capacity. Endurance training elicited with neuromuscular electrical stimulation (NMES) may reverse these changes and lead to improvement in muscle metabolic health. Methods: Fourteen participants with complete SCI performed 16 weeks of home‐based endurance NMES training of knee extensor muscles. Skeletal muscle oxidative capacity, muscle composition, and blood metabolic and lipid profiles were assessed pre‐ and post‐training. Results: There was an increase in number of contractions performed throughout the duration of training. The average improvement in skeletal muscle oxidative capacity was 119%, ranging from –14% to 387% (P = 0.019). There were no changes in muscle composition or blood metabolic and lipid profiles. Conclusion: Endurance training improved skeletal muscle oxidative capacity, but endurance NMES of knee extensor muscles did not change blood metabolic and lipid profiles. Muscle Nerve 55: 669–675, 2017     

Comparing the force ripple during asynchronous and conventional stimulation

Introduction: Asynchronous stimulation has been shown to reduce fatigue during electrical stimulation; however, it may also exhibit a force ripple. We quantified the ripple during asynchronous and conventional single‐channel transcutaneous stimulation across a range of stimulation frequencies. Methods: The ripple was measured during 5 asynchronous stimulation protocols, 2 conventional stimulation protocols, and 3 volitional contractions in 12 healthy individuals. Results: Conventional 40 Hz and asynchronous 16 Hz stimulation were found to induce contractions that were as smooth as volitional contractions. Asynchronous 8, 10, and 12 Hz stimulation induced contractions with significant ripple. Conclusions: Lower stimulation frequencies can reduce fatigue; however, they may also lead to increased ripple. Future efforts should study the relationship between force ripple and the smoothness of the evoked movements in addition to the relationship between stimulation frequency and NMES‐induced fatigue to elucidate an optimal stimulation frequency for asynchronous stimulation. Muscle Nerve 50: 549–555, 2014     

Comparison between the effects of 4 different electrical stimulation current waveforms on isometric knee extension torque and perceived discomfort in healthy women

Introduction: We studied the effects of different neuromuscular electrical stimulation (NMES) currents, 2 kHz‐frequency alternating currents (KACs, Russian and Aussie) and 2 pulsed currents (PCs), on isometric knee extension torque and discomfort level, both in isolation and combined, with maximum voluntary contraction (MVC). Methods: Twenty‐one women (age 21.6 ± 2.5 years) were studied. We evaluated torque evoked by NMES or NMES combined with maximum voluntary contraction of the quadriceps muscle of healthy women. Discomfort level was measured using a visual analog pain scale. Results: Despite comparable levels of discomfort, evoked torque was lower for Russian current compared with the other modalities (Russian 50.8%, Aussie 71.7%, PC500 76.9%, and PC200 70.1%; P < 0.001). There was no advantage in combining NMES with MVC compared with isolated NMES. Conclusions: The Aussie and PC approaches proved superior to Russian current for inducing isometric knee extension torque. This information is important in guiding decision making with regard to NMES protocols for muscle strengthening. Muscle Nerve 51 : 76–82, 2015     

Effects of stimulation frequency and pulse duration on fatigue and metabolic cost during a single bout of neuromuscular electrical stimulation

We have investigated the effects of stimulation frequency and pulse duration on fatigue and energy metabolism in rat gastrocnemius muscle during a single bout of neuromuscular electrical stimulation (NMES). Electrical pulses were delivered at 100 Hz (1‐ms pulse duration) and 20 Hz (5‐ms pulse duration) for the high (HF) and low (LF) frequency protocols, respectively. As a standardization procedure, the averaged stimulation intensity, the averaged total charge, the initial peak torque, the duty cycle, the contraction duration and the torque‐time integral were similar in both protocols. Fatigue was assessed using two testing trains delivered at a frequency of 100 Hz and 20 Hz before and after each protocol. Metabolic changes were investigated in vivo using ³¹P‐magnetic resonance spectroscopy (³¹P‐MRS) and in vitro in freeze‐clamped muscles. Both LF and HF NMES protocols induced the same decrease in testing trains and metabolic changes. We conclude that, under carefully controlled and comparable conditions, the use of low stimulation frequency and long pulse duration do not minimize the occurrence of muscle fatigue or affect the corresponding stimulation‐induced metabolic changes so that this combination of stimulation parameters would not be adequate in the context of rehabilitation. Muscle Nerve, 2010     

Age‐related fatigue resistance in the knee extensor muscles is specific to contraction mode

The question of whether skeletal muscle fatigue is preserved or enhanced in older adults is a point of controversy. Disparate findings may be attributed to differences in subject population and study protocols, including contraction mode. The purpose of this study was to test the hypotheses that healthy older (65–80 years of age, 8 males and 8 females) adults who were matched to young adults (21–35 years of age; 8 males and 8 females) with similar physical activity levels would: (1) fatigue less during isometric knee extensor (KE) contractions, but (2) would show similar fatigue during dynamic KE contractions performed at 120° s^?1. Fatigue was induced with 4 minutes of intermittent, isometric, or dynamic maximal voluntary contractions, performed on separate days. Electrically stimulated contractions were used to evaluate central activation during both fatigue protocols. Older subjects maintained a higher percentage of baseline maximum voluntary contraction (MVC) torque than young subjects during isometric contractions (mean ± SE: 71 ± 3% and 57 ± 3%, respectively, P < 0.01). In contrast, there was no difference between age groups in torque maintenance during dynamic contractions (43 ± 3% and 44 ± 3%, respectively, P = 0.86). For both groups, changes in electrically stimulated and voluntary contractions followed similar trends, suggesting that central activation did not play a role in the age‐related differences in fatigue. Fatigue during the isometric protocol was associated with fatigue during the dynamic protocol in the young group only (r = 0.62, P = 0.01), suggesting that distinct mechanisms influence fatigue during isometric and dynamic contractions in older adults. Muscle Nerve 39: 692–702, 2009     

Effects of pulse duration on muscle fatigue during electrical stimulation inducing moderate‐level contraction

Introduction: Neuromuscular electrical stimulation (NMES) is used to prevent muscle atrophy. However, the effect of pulse duration modulation for reducing muscle fatigue and pain is unknown. Methods: Two 2‐minute stimulation protocols were applied to the knee extensors of 10 healthy individuals. In 1 session, a long pulse duration (1,000 μs) and a low current amplitude (LL), set to evoke 25% maximal voluntary contraction at 30 Hz , were applied. The other session was identical except that a short pulse duration (200 μs) and a high current amplitude (SH) were used. Results: Muscle fatigue was lower for LL than for SH (P < 0.01). Force recovery rate was higher for LL than for SH (P < 0.05). Pain scores were also lower for LL than for SH (P < 0.05). Discussion: The use of 1‐ms pulse durations reduces fatigue and pain during NMES for moderate‐level contractions compared with 200‐μs durations. Muscle Nerve 57 : 642–649, 2018     

Effects of electrical stimulation pattern on quadriceps force production and fatigue

Introduction: Mixed stimulation programs (MIX) that switch from constant frequency trains (CFT) to variable frequency trains have been proposed to offset the rapid fatigue induced by CFT during electrical stimulation. However, this has never been confirmed with long stimulation patterns, such as those used to evoke functional contractions. The purpose of this study was to test the hypothesis that MIX programs were less fatiguing than CFTs in strength training‐like conditions (6‐s contractions, 30‐min). Methods: Thirteen healthy subjects underwent 2 sessions corresponding to MIX and CFT programs. Measurements included maximal voluntary isometric torque and torque evoked by each contraction. Results: There were greater decreases of voluntary and evoked torque (P < 0.05) after CFT than MIX, and mean torque was 13 ± 1% higher during the MIX session (P < 0.05). Conclusions: These findings confirm that combining train types might be a useful strategy to offset rapid fatigue during electrical stimulation sessions with long‐duration contractions. Muscle Nerve 49 : 760–763, 2014     

Prior heat stress effects fatigue recovery of the elbow flexor muscles

Introduction: Long‐lasting alterations in hormones, neurotransmitters, and stress proteins after hyperthermia may be responsible for the impairment in motor performance during muscle fatigue. Methods: Subjects (n = 25) performed a maximal intermittent fatigue task of elbow flexion after sitting in either 73° or 26°C to examine the effects of prior heat stress on fatigue mechanisms. Results: The heat stress increased the tympanic and rectal temperatures by 2.3° and 0.82°C, respectively, but there was full recovery prior to the fatigue task. Although prior heat stress had no effects on fatigue‐related changes in volitional torque, electromyographic (EMG) activity, torque relaxation rate, motor evoked potential (MEP) size, and silent period (SP) duration, prior heat stress acutely increased the pre‐fatigue relaxation rate and chronically prevented long‐duration fatigue (P < 0.05). Conclusions: These findings indicate that prior passive heat stress alone does not alter voluntary activation during fatigue, but prior heat stress and exercise produce longer‐term protection against long‐duration fatigue. Muscle Nerve 44: 115–125, 2011     

Influence of stimulus pulse width on M-waves, H-reflexes, and torque during tetanic low-intensity neuromuscular stimulation

Neuromuscular electrical stimulation (NMES) has been shown to generate contractions that include a central recruitment of motoneurons; however, the effect of pulse width on electromyographic (EMG) and torque responses during NMES are not well documented. Soleus EMG and isometric plantarflexion torque were recorded from 14 subjects with NMES delivered to the tibial nerve using 50, 200, 500, and 1000 μs pulse widths. M-waves were significantly smaller during 20 Hz NMES compared with responses evoked by single pulses of 200, 500, and 1000 μs, but not 50 μs pulse widths. At all pulse widths, stimulation at 20 Hz depressed soleus H-reflexes compared with single pulses. Two seconds of 100 Hz NMES significantly increased H-reflexes and torque during the subsequent 20 Hz NMES with 200, 500, and 1000 μs, but not 50 μs, pulse widths. NMES delivered using wide pulses generated larger contractions with a relatively greater central contribution than narrow pulses. This may help reduce atrophy and produce fatigue-resistant contractions for rehabilitation.     

Effects of the hybrid of neuromuscular electrical stimulation and noxious thermal stimulation on upper extremity motor recovery in patients with stroke: a randomized controlled trial

Background: Neuromuscular electrical stimulation (NMES) and noxious thermal stimulation (NTS) have been developed and incorporated in stroke rehabilitation.

Objective: This study aimed to compare the effects of NMES, NTS, and the hybrid of NMES and NTS (“Hybrid”) on motor recovery of upper extremity (UE) for patients with stroke.

Methods: We conducted a prospective, single-blind randomized controlled trial with concealed allocation. Forty-three patients with chronic stroke (onset >6 months) were randomly assigned to three groups (NMES, NTS, and “Hybrid”). In addition to conventional rehabilitation, participants received 30 min of NMES or 30 min of NTS or 15 min of NTS followed by 15 min of NMES. The treatment period was 8 weeks, 3 days/week, 30 min/time. The UE subscale of Fugl-Meyer assessment (UE-FMA, the primary outcome), Motricity index, modified Ashworth scale, and Barthel index were administered by a blinded assessor at baseline, posttreatment, and one-month follow-up.

Results: Most of the participants had mild-to-moderate disability in activity of daily living. No significant differences in the outcome measures at posttreatment and one-month follow-up were found among the NMES group (n = 13), NTS group (n = 13), and the hybrid of NMES and NTS group (n = 17). However, significant score changes in UE-FMA (p < 0.025) from baseline to posttreatment and one-month follow-up were found for the “Hybrid” group.

Conclusions: This study reveals that the hybrid of NMES and NTS therapy appears to be beneficial to UE recovery after stroke but is not superior to NMES or NTS alone.     

Effect of neuromuscular electrical stimulation frequency on muscles of the tongue

Introduction: Neuromuscular electrical stimulation (NMES) for the treatment of swallowing disorders is delivered at a variety of stimulation frequencies. We examined the effects of stimulation frequency on tongue muscle plasticity in an aging rat model. Methods: Eighty‐six young, middle‐aged, and old rats were assigned to either bilateral hypoglossal nerve stimulation at 10 or 100 Hz (5 days/week, 8 weeks), sham, or no‐implantation conditions. Muscle contractile properties and myosin heavy chain (MyHC) composition were determined for hyoglossus (HG) and styloglossus (SG) muscles. Results: Eight weeks of 100‐Hz stimulation resulted in the greatest changes in muscle contractile function with significantly longer contraction and half‐decay times, the greatest reduction in fatigue, and a transition toward slowly contracting, fatigue‐resistant MyHC isoforms. Discussion: NMES at 100‐Hz induced considerable changes in contractile and phenotypic profiles of HG and SG muscles, suggesting higher frequency NMES may yield a greater therapeutic effect. Muscle Nerve, 2018