Does increasing the number of channels during neuromuscular electrical stimulation reduce fatigability and produce larger contractions with less discomfort?

PURPOSE: Neuromuscular electrical stimulation (NMES) is often delivered at frequencies that recruit motor units (MUs) at unphysiologically high rates, leading to contraction fatigability. Rotating NMES pulses between multiple electrodes recruits subpopulations of MUs from each site, reducing MU firing rates and fatigability. This study was designed to determine whether rotating pulses between an increasing number of stimulation channels (cathodes) reduces contraction fatigability and increases the ability to generate torque during NMES. A secondary outcome was perceived discomfort. METHODS: Fifteen neurologically intact volunteers completed four sessions. NMES was delivered over the quadriceps through 1 (NMES1), 2 (NMES2), 4 (NMES4) or 8 (NMES8) channels. Fatigability was assessed over 100 contractions (1-s on/1-s off) at an initial contraction amplitude that was 20% of a maximal voluntary contraction. Torque-frequency relationships were characterized over six frequencies from 20 to 120 Hz. RESULTS: NMES4 and NMES8 resulted in less decline in peak torque (42 and 41%) over the 100 contractions than NMES1 and NMES2 (53 and 50% decline). Increasing frequency from 20 to 120 Hz increased torque by 7, 13, 21 and 24% MVC, for NMES1, NMES2, NMES4 and NMES8, respectively. Perceived discomfort was highest during NMES8. CONCLUSION: NMES4 and NMES8 reduced contraction fatigability and generated larger contractions across a range of frequencies than NMES1 and NMES2. NMES8 produced the most discomfort, likely due to small electrodes and high current density. During NMES, more is not better and rotating pulses between four channels may be optimal to reduce contraction fatigability and produce larger contractions with minimal discomfort compared to conventional NMES configurations.

Enhancing Adaptations to Neuromuscular Electrical Stimulation Training Interventions.

Neuromuscular electrical stimulation (NMES) applied to skeletal muscles is an effective rehabilitation and exercise training modality. However, the relatively low muscle force and rapid muscle fatigue induced by NMES limit the stimulus provided to the neuromuscular system and subsequent adaptations. We hypothesize that adaptations to NMES will be enhanced by the use of specific stimulation protocols and adjuvant interventions.

Indirect Vibration of the Upper Limbs Alters Transmission Along Spinal but Not Corticospinal Pathways.

The use of upper limb vibration (ULV) during exercise and rehabilitation continues to gain popularity as a modality to improve function and performance. Currently, a lack of knowledge of the pathways being altered during ULV limits its effective implementation. Therefore, the aim of this study was to investigate whether indirect ULV modulates transmission along spinal and corticospinal pathways that control the human forearm. All measures were assessed under CONTROL (no vibration) and ULV (30 Hz; 0.4 mm displacement) conditions while participants maintained a small contraction of the right flexor carpi radialis (FCR) muscle. To assess spinal pathways, Hoffmann reflexes (H-reflexes) elicited by stimulation of the median nerve were recorded from FCR with motor response (M-wave) amplitudes matched between conditions. An H-reflex conditioning paradigm was also used to assess changes in presynaptic inhibition by stimulating the superficial radial (SR) nerve (5 pulses at 300Hz) 37 ms prior to median nerve stimulation. Cutaneous reflexes in FCR elicited by stimulation of the SR nerve at the wrist were also recorded. To assess corticospinal pathways, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation of the contralateral motor cortex were recorded from the right FCR and biceps brachii (BB). ULV significantly reduced H-reflex amplitude by 15.7% for both conditioned and unconditioned reflexes (24.0 ± 15.7 vs. 18.4 ± 11.2% M max ; p < 0.05). Middle latency cutaneous reflexes were also significantly reduced by 20.0% from CONTROL (-1.50 ± 2.1% Mmax) to ULV (-1.73 ± 2.2% Mmax; p < 0.05). There was no significant effect of ULV on MEP amplitude (p > 0.05). Therefore, ULV inhibits cutaneous and H-reflex transmission without influencing corticospinal excitability of the forearm flexors suggesting increased presynaptic inhibition of afferent transmission as a likely mechanism. A general increase in inhibition of spinal pathways with ULV may have important implications for improving rehabilitation for individuals with spasticity (SCI, stroke, MS, etc.).

Decreased excitability of motor axons contributes substantially to contraction fatigability during neuromuscular electrical stimulation.

The present study was designed to (i) determine the time course of changes in motor axon excitability during and after neuromuscular electrical stimulation (NMES); and (ii) characterize the relationship between contraction fatigability, NMES frequency, and changes at the axon, neuromuscular junction, and muscle. Eight neurologically intact participants attended 3 sessions. NMES was delivered over the common peroneal nerve at 20, 40, or 60 Hz for 8 min (0.3 s "on", 0.7 s "off"). Threshold tracking was used to measure changes in axonal excitability. Supramaximal stimuli were used to assess neuromuscular transmission and force-generating capacity of the tibialis anterior muscle. Torque decreased by 49% and 62% during 8 min of 40 and 60 Hz NMES, respectively. Maximal twitch torque decreased only during 60 Hz NMES. Motor axon excitability decreased by 14%, 27%, and 35% during 20, 40, and 60 Hz NMES, respectively. Excitability recovered to baseline immediately (20 Hz) and at 2 min (40 Hz) and 4 min (60 Hz) following NMES. Overall, decreases in axonal excitability best predicted how torque declined over 8 min of NMES. During NMES, motor axons become less excitable and motor units "drop out" of the contraction, contributing substantially to contraction fatigability and its dependence on NMES frequency. Novelty: The excitability of motor axons decreased during NMES in a frequency-dependent manner. As excitability decreased, axons failed to reach threshold and motor units dropped out of the contraction. Overall, decreased excitability best predicted how torque declined and thus is a key contributor to fatigability during NMES.

Neuromuscular determinants of explosive torque: Differences among strength-trained and untrained young and older men.

This study compared the differences in neural and muscular mechanisms related to explosive torque in chronically strength-trained young and older men (>5 years). Fifty-four participants were allocated into four groups according to age and strength training level: older untrained (n = 14; 65.6 ± 2.9 years), older trained (n = 12; 63.6 ± 3.8 years), young untrained (n = 14; 26.2 ± 3.7 years), and young trained (n = 14; 26.7 ± 3.4 years). Knee extension isometric voluntary explosive torque (absolute and normalized as a percentage of maximal voluntary torque) was assessed at the beginning of the contraction (ie, 50, 100, and 150 ms-T50, T100, and T150, respectively), and surface electromyogram (sEMG) amplitude (normalized as a percentage of sEMG recorded during maximal voluntary isometric contraction) at 0-50, 50-100, and 100-150 time windows. Supramaximal electrically evoked T50 was assessed with octet trains delivered to the femoral nerve (8 pulses at 300 Hz). Voluntary T50, T100, and T150 were higher for trained than untrained in absolute (P < 0.001) and normalized (P < 0.030) terms, accompanied by higher sEMG at 0-50, 50-100, and 100-150 ms (P < 0.001), and voluntary T50/octet T50 ratio for trained. Greater octet T50 was observed for the young trained (P < 0.001) but not for the older trained (P = 0.273) compared to their untrained counterparts. Age effect was observed for voluntary T50, T100, and T150 (P < 0.050), but normalization removed these differences (P > 0.417). Chronically strength-trained young and older men presented a greater explosive torque than their untrained pairs. In young trained, the greater explosive performance was attributed to enhanced muscular and neural mechanisms, while in older trained to neural mechanisms only.

Performance on an Associative Memory Test Decreases 8 hr After Cardiovascular Exercise.

This study was designed to assess the effects of acute exercise on performance of a paired associate learning (PAL) test, an operationalization of hippocampal-dependent associative memory. Participants performed a PAL test and then ran on a treadmill (exercise group, n = 52) or solved Sudoku puzzles (control group, n = 54). Participants returned 2, 5, or 8 hr later to perform a second, different, PAL test. PAL scores for the control group did not change over time. Similarly, scores on tests taken 2 and 5 hr after exercise were not different from baseline or control data. Scores on tests taken 8 hr after exercise, however, fell significantly below baseline (by 8.6%) and control (by 9.8%) scores. These data demonstrate that acute exercise can negatively affect the encoding and retrieval of new information even hours after the exercise bout, which should be a consideration when designing exercise programs to enhance, and not hinder, learning.

Contraction fatigability during interleaved neuromuscular electrical stimulation of the ankle dorsiflexors does not depend on contraction amplitude.

Interleaved neuromuscular electrical stimulation (iNMES) involves alternating stimulus pulses between the tibialis anterior muscle and common peroneal nerve. The current investigation aimed to characterize the relationship between contraction amplitude, motor unit (MU) "overlap", and contraction fatigability during iNMES. It was hypothesized that as iNMES generates progressively larger contractions, more MUs would be recruited from both sites (i.e., more MU overlap), resulting in more fatigability for larger than smaller contractions. Fourteen participants completed 3 sessions. Fatigability was assessed as the decline in torque over 180 contractions (0.3 s "on", 0.7 s "off") when iNMES was delivered to produce initial contractions of ∼5%, 15%, or 30% of a maximal voluntary contraction. Although MU overlap increased significantly with contraction amplitude, the relative (percent) decline in torque was not different between the contraction amplitudes and torque declined on average by 23%. Contraction fatigability was not significantly correlated with either MU overlap or initial contraction amplitude. In conclusion, iNMES can produce fatigue-resistant contractions across a functionally-meaningful range of contraction amplitudes for rehabilitation. Novelty Interleaved neuromuscular electrical stimulation progressively recruits MUs as contraction amplitude increases. However, the relative amount of fatigability of recruited MUs was not different as contraction amplitude increased. This suggests iNMES can be used effectively to produce fatigue-resistant and functionally meaningful contractions.

A Warm-Up Routine That Incorporates a Plyometric Protocol Potentiates the Force-Generating Capacity of the Quadriceps Muscles.

Johnson, M, Baudin, P, Ley, AL, and Collins, DF. A warm-up routine that incorporates a plyometric protocol potentiates the force-generating capacity of the quadriceps muscles. J Strength Cond Res 33(2): 380-389, 2019-This study was designed to investigate whether a warm-up routine that incorporates drop jumps, induces post-activation potentiation (PAP), and if so, assess the magnitude and time course of the induced PAP. Participants performed a standard warm-up that incorporated either drop jumps (plyometric protocol) or a low-paced walk (control protocol). Post-activation potentiation was assessed by changes in electrically evoked isometric muscle twitches recorded throughout both protocols. The plyometric protocol increased peak twitch torque (PTT), rate of torque development (RTD), and impulse significantly (by 23, 39, and 46%, respectively) with no change in the amplitude of simultaneously evoked M-waves, indicating that the augmented torque was due to PAP. These increases returned to baseline within 6 minutes, and PTT and RTD fell below baseline values at 11-16 minutes after the drop jumps. Peak twitch torque, RTD, and impulse decreased significantly after the standard warm-up. These results provide evidence that drop jumps induce PAP, markedly enhancing the force-generating capacity of the muscle. By contrast, the standard warm-up did not potentiate, but rather reduced, the force-generating capacity of the muscle. We suggest that drop jumps be incorporated into warm-up routines directly before athletic performance to maximize the force-generating capacity of muscle.

Utilizing Physiological Principles of Motor Unit Recruitment to Reduce Fatigability of Electrically-Evoked Contractions: A Narrative Review.

Neuromuscular electrical stimulation (NMES) is used to produce contractions to restore movement and reduce secondary complications for individuals experiencing motor impairment. NMES is conventionally delivered through a single pair of electrodes over a muscle belly or nerve trunk using short pulse durations and frequencies between 20 and 40Hz (conventional NMES). Unfortunately, the benefits and widespread use of conventional NMES are limited by contraction fatigability, which is in large part because of the nonphysiological way that contractions are generated. This review provides a summary of approaches designed to reduce fatigability during NMES, by using physiological principles that help minimize fatigability of voluntary contractions. First, relevant principles of the recruitment and discharge of motor units (MUs) inherent to voluntary contractions and conventional NMES are introduced, and the main mechanisms of fatigability for each contraction type are briefly discussed. A variety of NMES approaches are then described that were designed to reduce fatigability by generating contractions that more closely mimic voluntary contractions. These approaches include altering stimulation parameters, to recruit MUs in their physiological order, and stimulating through multiple electrodes, to reduce MU discharge rates. Although each approach has unique advantages and disadvantages, approaches that minimize MU discharge rates hold the most promise for imminent translation into rehabilitation practice. The way that NMES is currently delivered limits its utility as a rehabilitative tool. Reducing fatigability by delivering NMES in ways that better mimic voluntary contractions holds promise for optimizing the benefits and widespread use of NMES-based programs.

Torque, Current, and Discomfort During 3 Types of Neuromuscular Electrical Stimulation of Tibialis Anterior.

BACKGROUND: The benefits of neuromuscular electrical stimulation (NMES) for rehabilitation depend on the capacity to generate functionally relevant torque with minimal fatigability and discomfort. Traditionally, NMES is delivered either over a muscle belly (mNMES) or a nerve trunk (nNMES). Recently, a technique that minimizes contraction fatigability by alternating pulses between the mNMES and nNMES sites, termed "interleaved" NMES (iNMES), was developed. However, discomfort and the ability to generate large torque during iNMES have not been explored adequately. OBJECTIVE: The study objective was to compare discomfort and maximal torque between mNMES, nNMES, and iNMES. METHODS: Stimulation trains (12 pulses at 40 Hz) were delivered to produce dorsiflexion torque using mNMES, nNMES, and iNMES. Discomfort was assessed using a visual analogue scale for contractions that generated 5-30% of a maximal voluntary isometric contraction (MVIC), and for the maximal tolerable torque. RESULTS: Discomfort scores were not different between NMES types when torque was ≤20% MVIC. At 30% MVIC, mNMES produced more discomfort than nNMES and iNMES. nNMES produced the most torque (65% MVIC), followed by iNMES (49% MVIC) and mNMES (33% MVIC); in these trials, mNMES produced more discomfort than nNMES, but not iNMES. LIMITATIONS: The present results may be limited to individuals with no history of neuromusculoskeletal impairment. CONCLUSIONS: In terms of discomfort, there were no differences between mNMES, nNMES, or iNMES for contractions between 5-20% MVIC. However, mNMES produced more discomfort than nNMES and iNMES for contractions of 30% MVIC, while for larger contractions, mNMES only produced more discomfort than nNMES. The advantages and disadvantages of each NMES type should be considered prior to implementation in rehabilitation programs.

Interleaved neuromuscular electrical stimulation after spinal cord injury.

INTRODUCTION: Neuromuscular electrical stimulation (NMES) over a muscle belly (mNMES) recruits superficial motor units (MUs) preferentially, whereas NMES over a nerve trunk (nNMES) recruits MUs evenly throughout the muscle. We performed tests to determine whether "interleaving" pulses between the mNMES and nNMES sites (iNMES) reduces the fatigability of contractions for people experiencing paralysis because of chronic spinal cord injury. METHODS: Plantar flexion torque and soleus electromyography (M-waves) were recorded from 8 participants. A fatigue protocol (75 contractions; 2 s on/2 s off for 5 min) was delivered by iNMES. The results were compared with previously published data collected with mNMES and nNMES in the same 8 participants. RESULTS: Torque declined ∼40% more during mNMES than during nNMES or iNMES. M-waves declined during mNMES but not during nNMES or iNMES. DISCUSSION: To reduce fatigability of electrically evoked contractions of paralyzed plantar flexors, iNMES is equivalent to nNMES, and both are superior to mNMES. Muscle Nerve 56: 989-993, 2017.

Interleaved neuromuscular electrical stimulation reduces muscle fatigue.

INTRODUCTION: Neuromuscular electrical stimulation (NMES) can be delivered over a muscle belly (mNMES) or nerve trunk (nNMES). Both methods generate contractions that fatigue rapidly due, in part, to non-physiologically high motor unit (MU) discharge frequencies. In this study we introduce interleaved NMES (iNMES), whereby stimulus pulses are alternated between mNMES and nNMES. iNMES was developed to recruit different MU populations with every other stimulus pulse, with a goal of reducing discharge frequencies and muscle fatigue. METHODS: Torque and electromyography were recorded during fatigue protocols (12 min, 240 contractions) delivered using mNMES, nNMES, and iNMES. RESULTS: Torque declined significantly 3 min into iNMES and 1 min into both mNMES and nNMES. Torque decreased by 39% during iNMES and by 67% and 58% during mNMES and nNMES, respectively. CONCLUSIONS: iNMES resulted in less muscle fatigue than mNMES and nNMES. Delivering NMES in ways that reduce MU discharge frequencies holds promise for reducing muscle fatigue during NMES-based rehabilitation. Muscle Nerve, 2016 Muscle Nerve 55: 179-189, 2017.

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.

The pulse duration of electrical stimulation influences H-reflexes but not corticospinal excitability for tibialis anterior.

The afferent volley generated by neuromuscular electrical stimulation (NMES) influences corticospinal (CS) excitability and frequent NMES sessions can strengthen CS pathways, resulting in long-term improvements in function. This afferent volley can be altered by manipulating NMES parameters. Presently, we manipulated one such parameter, pulse duration, during NMES over the common peroneal nerve and assessed the influence on H-reflexes and CS excitability. We hypothesized that compared with shorter pulse durations, longer pulses would (i) shift the H-reflex recruitment curve to the left, relative to the M-wave curve; and (ii) increase CS excitability more. Using 3 pulse durations (50, 200, 1000 µs), M-wave and H-reflex recruitment curves were collected and, in separate experiments, CS excitability was assessed by comparing motor evoked potentials elicited before and after 30 min of NMES. Despite finding a leftward shift in the H-reflex recruitment curve when using the 1000 µs pulse duration, consistent with a larger afferent volley for a given efferent volley, the increases in CS excitability were not influenced by pulse duration. Hence, although manipulating pulse duration can alter the relative recruitment of afferents and efferents in the common peroneal nerve, under the present experimental conditions it is ineffective for maximizing CS excitability for rehabilitation.

H-reflexes reduce fatigue of evoked contractions after spinal cord injury.

INTRODUCTION: Neuromuscular electrical stimulation (NMES) over a muscle belly (mNMES) generates contractions predominantly through M-waves, while NMES over a nerve trunk (nNMES) can generate contractions through H-reflexes in people who are neurologically intact. We tested whether the differences between mNMES and nNMES are present in people with chronic motor-complete spinal cord injury and, if so, whether they influence contraction fatigue. METHODS: Plantar-flexion torque and soleus electromyography were recorded from 8 participants. Fatigue protocols were delivered using mNMES and nNMES on separate days. RESULTS: nNMES generated contractions that fatigued less than mNMES. Torque decreased the least when nNMES generated contractions, at least partly through H-reflexes (n = 4 participants; 39% decrease), and torque decreased the most when contractions were generated through M-waves, regardless of NMES site (nNMES 71% decrease, n = 4; mNMES, 73% decrease, n = 8). CONCLUSIONS: nNMES generates contractions that fatigue less than mNMES, but only when H-reflexes contribute to the evoked contractions.

Asynchronous recruitment of low-threshold motor units during repetitive, low-current stimulation of the human tibial nerve.

Motoneurons receive a barrage of inputs from descending and reflex pathways. Much of our understanding about how these inputs are transformed into motor output in humans has come from recordings of single motor units during voluntary contractions. This approach, however, is limited because the input is ill-defined. Herein, we quantify the discharge of soleus motor units in response to well-defined trains of afferent input delivered at physiologically-relevant frequencies. Constant frequency stimulation of the tibial nerve (10-100 Hz for 30 s), below threshold for eliciting M-waves or H-reflexes with a single pulse, recruited motor units in 7/9 subjects. All 25 motor units recruited during stimulation were also recruited during weak (<10% MVC) voluntary contractions. Higher frequencies recruited more units (n = 3/25 at 10 Hz; n = 25/25 at 100 Hz) at shorter latencies (19.4 ± 9.4 s at 10 Hz; 4.1 ± 4.0 s at 100 Hz) than lower frequencies. When a second unit was recruited, the discharge of the already active unit did not change, suggesting that recruitment was not due to increased synaptic drive. After recruitment, mean discharge rate during stimulation at 20 Hz (7.8 Hz) was lower than during 30 Hz (8.6 Hz) and 40 Hz (8.4 Hz) stimulation. Discharge was largely asynchronous from the stimulus pulses with "time-locked" discharge occurring at an H-reflex latency with only a 24% probability. Motor units continued to discharge after cessation of the stimulation in 89% of trials, although at a lower rate (5.8 Hz) than during the stimulation (7.9 Hz). This work supports the idea that the afferent volley evoked by repetitive stimulation recruits motor units through the integration of synaptic drive and intrinsic properties of motoneurons, resulting in "physiological" recruitment which adheres to Henneman's size principle and results in relatively low discharge rates and asynchronous firing.

Electrical stimulation site influences the spatial distribution of motor units recruited in tibialis anterior.

OBJECTIVE: To compare the spatial distribution of motor units recruited in tibialis anterior (TA) when electrical stimulation is applied over the TA muscle belly versus the common peroneal nerve trunk. METHODS: Electromyography (EMG) was recorded from the surface and from fine wires in superficial and deep regions of TA. Separate M-wave recruitment curves were constructed for muscle belly and nerve trunk stimulation. RESULTS: During muscle belly stimulation, significantly more current was required to generate M-waves that were 5% of the maximal M-wave (M max; M5%max), 50% M max (M 50%max) and 95% M max (M 95%max) at the deep versus the superficial recording site. In contrast, during nerve trunk stimulation, there were no differences in the current required to reach M5%max, M 50%max or M 95%max between deep and superficial recording sites. Surface EMG reflected activity in both superficial and deep muscle regions. CONCLUSIONS: Stimulation over the muscle belly recruited motor units from superficial to deep with increasing stimulation amplitude. Stimulation over the nerve trunk recruited superficial and deep motor units equally, regardless of stimulation amplitude. SIGNIFICANCE: These results support the idea that where electrical stimulation is applied markedly affects how contractions are produced and have implications for the interpretation of surface EMG data.

Changes in spinal but not cortical excitability following combined electrical stimulation of the tibial nerve and voluntary plantar-flexion.

Unilateral training involving voluntary contractions, neuromuscular electrical stimulation (NMES), or a combination of the two can increase the excitability of neural circuits bilaterally within the CNS. Many rehabilitation programs are designed to promote such "neuroplasticity" to improve voluntary movement following CNS damage. While much is known about this type of activity-dependent plasticity for the muscles that dorsi-flex the ankle, similar information is not available for the plantar-flexors. Presently, we assessed the excitability of corticospinal (CS) and spinal circuits for both soleus (SOL) muscles before and after voluntary contractions of the right plantar-flexors (VOL; 5 s on-5 s off, 40 min), NMES of the right tibial nerve (tnNMES; 5 s on-5 s off, 40 min), or both together (V + tnNMES). CS excitability for the right (rSOL) and left SOL (lSOL) muscles was assessed by quantifying motor evoked potentials elicited by transcranial magnetic stimulation. Spinal excitability was assessed using measures from the ascending limb of the M-wave versus H-reflex recruitment curve. CS excitability did not change for rSOL (the activated muscle) or lSOL following any condition. In contrast, there was a marked increase in spinal excitability for rSOL, but only following V + tnNMES; the slope of the M-wave versus H-reflex recruitment curve increased approximately twofold (pre = 7.9; post = 16.2) and H-reflexes collected when the M-wave was ~5 % of the maximal M-wave (M(max)) increased by ~1.5× (pre = 19 % M(max), post = 29 % M(max)). Spinal excitability for lSOL did not change following any condition. Thus, only voluntary contractions that were coupled with NMES increased CNS excitability, and this occurred only in the ipsilateral spinal circuitry. These results are in marked contrast to previous studies showing NMES-induced changes in CS excitability for every other muscle studied and suggest that the mechanisms that regulate activity-dependent neuroplasticity are different for SOL than other muscles. Further, while rehabilitation strategies involving voluntary training and/or NMES of the plantar-flexors may be beneficial for producing movement and reducing atrophy, a single session of low-intensity NMES and voluntary training may not be effective for strengthening CS pathways to the SOL muscle.

Neuromuscular electrical stimulation and exercise for reducing trapezius muscle dysfunction in survivors of head and neck cancer: a case-series report.

PURPOSE: Damage to the spinal accessory nerve (SAN) can result in denervation of the trapezius muscle in patients undergoing surgery for head and neck cancer. Trapezius denervation leads to muscle weakness and dysfunction that, for some patients, persists despite the return of conduction along the SAN. This prospective case series describes an intervention involving a combination of a novel type of neuromuscular electrical stimulation (NMES) with bilateral exercise. METHODS: Three survivors of head and neck cancer participated in the 6-week program. NMES was applied over the region of the SAN on the affected side while subjects performed bilateral voluntary scapular retraction and elevation exercises against resistance. The NMES was delivered using relatively wide pulse widths and high frequencies to enhance the electrically evoked sensory volley and was triggered by the onset of trapezius muscle activity on the non-affected side. Shoulder range of motion (ROM) assessments and patient-rated outcomes were administered at baseline and 6 weeks. RESULTS: All patients showed improvements in shoulder flexion and abduction ROM and reported reductions in pain and disability. CONCLUSIONS: This combination of NMES and bilateral exercise may prove to be an effective component of a comprehensive shoulder rehabilitation program for patients with persistent trapezius muscle dysfunction as a result of SAN damage.

Objectif : Des dommages au nerf spinal accessoire (NSA) peuvent donner lieu à une dénervation du muscle trapèze chez les patients qui subissent une intervention chirurgicale pour un cancer au cou ou à la tête. La dénervation du trapèze provoque une faiblesse musculaire et une dysfonction qui, chez certains patients, persistent malgré le retour d'une conduction du NSA. Cette série de cas prospectifs décrit une intervention qui met à contribution une nouvelle technique de stimulation neuromusculaire électrique (NMES), associée à des exercices bilatéraux. Méthode : Trois personnes ayant survécu à des cancers au cou ou à la tête ont pris part à un programme de six semaines. La NMES a été utilisée sur toute la région du NSA, sur le côté touché, pendant que les sujets exécutaient des mouvements de rétraction bilatérale scapulaire volontaires et des exercices d'élévation avec résistance. La NMES a été administrée en utilisant des durées d'impulsions relativement longues et des fréquences élevées pour évoquer, sous forme électrique, un élan (volée) sensoriel; la NMES était activée dès le début de l'activité musculaire du trapèze du côté non affecté. Les évaluations de l'amplitude du mouvement (ADM) et l'évaluation des résultats par les patients ont eu lieu au départ, puis six semaines plus tard. Résultats : Tous les patients ont démontré des améliorations dans l'ADM en flexion et abduction de l'épaule, et tous ont fait part de diminutions de la douleur et de l'incapacité. Conclusions : La combinaison de NMES et d'exercices bilatéraux pourrait s'avérer efficace dans le cadre d'un programme complet de réadaptation de l'épaule pour les patients aux prises avec une dysfonction persistante du muscle trapèze à la suite de dommages subis au NSA.

Postactivation depression and recovery of reflex transmission during repetitive electrical stimulation of the human tibial nerve.

H-reflexes are progressively depressed, relative to the first response, at stimulation frequencies above 0.1 Hz (postactivation depression; PAD). Presently, we investigated whether H-reflexes "recover" from this depression throughout 10-s trains of stimulation delivered at physiologically relevant frequencies (5-20 Hz) during functionally relevant tasks (sitting and standing) and contraction amplitudes [relaxed to 20% maximum voluntary contraction (MVC)]. When participants held a 10% MVC, reflex amplitudes did not change during 5-Hz stimulation. During stimulation at 10 Hz, reflexes were initially depressed by 43% but recovered completely by the end of the stimulation period. During 20-Hz stimulation, reflexes were depressed to 10% and recovered to 36% of the first response, respectively. This "postactivation depression and recovery" (PAD&R) of reflex amplitude was not different between sitting and standing. In contrast, PAD&R were strongly influenced by contraction amplitude. Reflexes were depressed to 10% of the first response during the relaxed condition (10-Hz stimulation) and showed no depression during a 20% MVC contraction. A partial recovery of reflex amplitude occurred when participants were relaxed and during contractions of 1-5% MVC. Surprisingly, reflexes could recover completely by the third pulse within a stimulation train when participants held a contraction between 5 and 10% MVC during stimulation at 10 Hz, a finding that challenges classical ideas regarding PAD mechanisms. Our results support the idea that there is an ongoing interplay between depression and facilitation when motoneurons receive trains of afferent input. This interplay depends strongly on the frequency of the afferent input and the magnitude of the background contraction but is relatively insensitive to changes in task.