Comparison of acute responses in spinal excitability between older and young people after neuromuscular electrical stimulation.

PURPOSE: This study aims at comparing acute responses in spinal excitability, as measured by H-reflex, between older and young individuals, following a single session of NMES superimposed onto voluntary isometric contractions of the ankle plantar-flexor muscles (NMES+), with respect to passive NMES (pNMES) and voluntary isometric contractions only (ISO). METHODS: Thirty-two volunteers, 16 older (OLDER) and 16 young (YOUNG), were asked to sustain a constant force at 20% of maximal voluntary isometric contraction (MVIC) of the ankle plantar-flexor muscles in the dominant limb during each of the 3 conditions (NMES+ , pNMES and ISO). Fifteen repetitions of 6 s were performed, with a resting interval of 6 s between repetitions. Before and after each condition, soleus H-reflexes were elicited by percutaneous electrical stimulation of the posterior tibial nerve and H-reflex amplitudes recorded by surface EMG. RESULTS: In OLDER, H-reflex amplitude did not change following any experimental condition (ISO: p = 0.203; pNMES: p = 0.542; NMES+: p = 0.431) compared to baseline. On the contrary, in YOUNG, H-reflex amplitudes significantly increased (p < 0.000) and decreased (p = 0.001) following NMES+ and pNMES, respectively, while there was no significant change in reflex responses following ISO (p = 0.772). CONCLUSION: The lack of change in H-reflex responses following either NMES+ or pNMES might reflect a reduced ability of older people in modulating spinal excitability after the conditions. Specifically, an age-related alteration in controlling mechanisms at presynaptic level was suggested.

Acute effects of neuromuscular electrical stimulation on cortical dynamics and reflex activation.

Superimposing neuromuscular electrical stimulation (NMES) on voluntary muscle contractions has shown the potential to improve motor performance even more than voluntary exercise alone. Nevertheless, the neurophysiological and neurocognitive mechanisms underlying this technique are still unclear. The aim of this study was to investigate the acute responses in spinal excitability and brain activity following three conditions: NMES superimposed on isometric contractions (NMES + ISO), passive NMES, and voluntary isometric contractions (ISO). Each condition involved 15 intermittent ankle plantar-flexions at submaximal level. Before and after each condition, tibial nerve stimulation was used to elicit H-reflexes, which represent a measure of spinal excitability, and somatosensory evoked potentials (SEPs), which index the activity of subcortical and cortical somatosensory areas. H-reflex amplitudes increased after NMES + ISO and decreased after passive NMES compared with baseline values, whereas they remained unaltered after ISO. Subcortical lemniscal activity remained unaltered after the three conditions. Activity in both primary and secondary somatosensory cortices (S1 and S2) increased after NMES + ISO and decreased after the ISO condition, whereas no differences emerged after NMES. At later stages of S2 processing, ISO induced no changes in cortical activity, which, conversely, increased after NMES and NMES + ISO. These findings indicate that the beneficial effects of NMES may be mediated by potentiation of the reflex pathways at the spinal level. At the brain level, peripheral input representation in the brain stem was not influenced by the experimental conditions, which, conversely, altered cortical activity by affecting synaptic efficiency through the somatosensory pathway.NEW & NOTEWORTHY Neuromuscular electrical stimulation superimposed on voluntary contractions (NMES+) is effective to improve motor performance in several populations. Here, we investigated the changes in cortical activation and reflex response following three acute conditions, including NMES+. Our results show that NMES+ has a greater excitatory effect at both spinal and cortical levels compared with passive stimulation and voluntary exercise alone. These results open up original perspectives for the implementation of NMES+ in neurorehabilitation and training environments.

NMES superimposed on movement is equally effective as heavy slow resistance training in patellar tendinopathy.

OBJECTIVE: This study aimed at investigating the effectiveness of an 8-week training protocol, based on neuromuscular electrical stimulation of the quadriceps, which was superimposed onto voluntary exercise (NMES+), in comparison to a traditional heavy slow resistance training (HSRT), in individuals with patellar tendinopathy. METHODS: Thirty-two physically active participants, aged: 33.6±10.2 years, were divided into two groups: NMES+ or HSRT. Maximal voluntary isometric contraction (MVIC) of knee extensor and flexor muscles, power during a countermovement jump (CMJ), and VISA-p questionnaire scores were recorded at the start(T0), 2-weeks(T1), 4-weeks(T2), 6-weeks(T3), 8-weeks(T4) and 4-months post-training (T5). Knee pain and rate of perceived exertion (RPE) were recorded at each training session with a 0-10 scale. RESULTS: Knee pain was significantly lower in NMES+ compared to HSRT during all training sessions. No significant between-group differences were found for VISA-p scores and forces recorded during MVICs at T0,T1,T2,T3,T4 and T5. A significant increase of VISA-p and peak forces during MVIC was recorded across-time in both groups. No significant between-group or across-time differences were found for RPE and CMJ parameters. CONCLUSIONS: NMES+ and HSRT were equally effective in decreasing tendinopathy symptoms and increasing strength, with NMES+ having the advantage to be a pain-free resistance training modality.

Early Superimposed NMES Training is Effective to Improve Strength and Function Following ACL Reconstruction with Hamstring Graft regardless of Tendon Regeneration.

The study aimed at investigating the effects of neuromuscular electrical stimulation superimposed on functional exercises (NMES+) early after anterior cruciate ligament reconstruction (ACLr) with hamstring graft, on muscle strength, knee function, and morphology of thigh muscles and harvested tendons. Thirty-four participants were randomly allocated to either NMES+ group, who received standard rehabilitation with additional NMES of knee flexor and extensor muscles, superimposed on functional movements, or to a control group, who received no additional training (NAT) to traditional rehabilitation. Participants were assessed 15 (T1), 30 (T2), 60 (T3), 90 (T4) and at a mean of 380 days (T5) after ACLr. Knee strength of flexors and extensors was measured at T3, T4 and T5. Lower limb loading asymmetry was measured during a sit-to-stand-to-sit movement at T1, T2, T3, T4 and T5, and a countermovement-jump at T4 and T5. An MRI was performed at T5 to assess morphology of thigh muscles and regeneration of the harvested tendons. NMES+ showed higher muscle strength for the hamstrings (T4, T5) and the quadriceps (T3, T4, T5), higher loading symmetry during stand-to-sit (T2, T3, T4, T5), sit-to-stand (T3, T4) and countermovement-jump (T5) than NAT. No differences were found between-groups for morphology of muscles and tendons, nor in regeneration of harvested tendons. NMES+ early after ACLr with hamstring graft improves muscle strength and knee function in the short- and long-term after surgery, regardless of tendon regeneration.

Deficit in knee extension strength following anterior cruciate ligament reconstruction is explained by a reduced neural drive to the vasti muscles.

The persistence of quadriceps weakness represents a major concern following anterior cruciate ligament reconstruction (ACLR). The underlying adaptations occurring in the activity of spinal motoneurons are still unexplored. This study examined the discharge patterns of large populations of motor units (MUs) in the vastus lateralis (VL) and vastus medialis muscles following ACLR. Nine ACLR individuals and 10 controls performed unilateral trapezoidal contractions of the knee extensor muscles at 35%, 50% and 70% of the maximal voluntary isometric force (MVIF). High-density surface electromyography (HDsEMG) was used to record the myoelectrical activity of the vasti muscles in both limbs. HDsEMG signals were decomposed with a convolutive blind source separation method and MU properties were extracted and compared between sides and groups. The ACLR group showed a lower MVIF on the reconstructed side compared to the contralateral side (28.1%; P < 0.001). This force deficit was accompanied by reduced MU discharge rates (∼21%; P < 0.05), lower absolute MU recruitment and derecruitment thresholds (∼22% and ∼22.5%, respectively; P < 0.05) and lower input-output gain of motoneurons (27.3%; P = 0.009). Deficits in MU discharge rates of the VL and in absolute recruitment and derecruitment thresholds of both vasti MUs were associated with deficits in MVIF (P < 0.05). A strong between-side correlation was found for MU discharge rates of the VL of ACLR individuals (P < 0.01). There were no significant between-group differences (P > 0.05). These results indicate that mid- to long-term strength deficits following ACLR may be attributable to a reduced neural drive to vasti muscles, with potential changes in excitatory and inhibitory synaptic inputs. KEY POINTS: Impaired expression and control of knee extension forces is common after anterior cruciate ligament reconstruction and is related to high risk of a second injury. To provide novel insights into the neural basis of this impairment, the discharge patterns of motor units in the vastus lateralis and vastus medialis were investigated during voluntary force contractions. There was lower knee extensor strength on the reconstructed side with respect to the contralateral side, which was explained by deficits in motor unit discharge rate and an altered motoneuronal input-output gain. Insufficient excitatory inputs to motoneurons and increased inhibitory afferent signals potentially contributed to these alterations. These results further our understanding of the neural underpinnings of quadriceps weakness following anterior cruciate ligament reconstruction and can help to develop effective rehabilitation protocols to regain muscle strength and reduce the risk of a second injury.

Age differences in anticipatory and executory mechanisms of gait initiation following unexpected balance perturbations.

PURPOSE: An age-related decline in anticipatory postural mechanisms has been reported during gait initiation; however, it is unclear whether such decline may jeopardize whole-body stability following unexpected balance perturbations. This study aimed to compare young and older individuals' ability to generate postural responses and preserve stability in response to external waist perturbations delivered within gait initiation. METHODS: Ten young and ten older participants performed 10 gait initiation trials followed by 48 unperturbed and 12 perturbed trials in a random order. A stereophotogrammetric system and three force platforms were used to quantify mechanical parameters from the preparatory phase (e.g., timing and amplitude of postural adjustments) and from the stepping phase (e.g., step characteristics and dynamic stability). Activation patterns of lower leg muscles were determined by surface electromyography. RESULTS: Older participants responded to perturbation with lower increase in both magnitude (p < 0.001; η2p = 0.62) and duration (p = 0.001; η2p = 0.39) of preparatory parameters and soleus muscle activity (p < 0.001; η2p = 0.55), causing shorter (p < 0.001; η2p = 0.59) and lower (p < 0.001; η2p = 0.43) stepping, compared to young participants. Interestingly, young participants showed greater correlations between preparatory phase parameters and dynamic stability of the first step than older participants (average r of - 0.40 and - 0.06, respectively). CONCLUSION: The results suggest that young participants took more time than older to adjust the anticipatory biomechanical response to perturbation attempting to preserve balance during stepping. In contrast, older adults were unable to modify their anticipatory adjustments in response to perturbation and mainly relied on compensatory mechanisms attempting to preserve stability via a more cautious stepping strategy.

Neuromechanical response of the upper body to unexpected perturbations during gait initiation in young and older adults.

BACKGROUND: Control of upper body motion deteriorates with ageing leading to impaired ability to preserve balance during gait, but little is known on the contribution of the upper body to preserve balance in response to unexpected perturbations during locomotor transitions, such as gait initiation. AIM: To investigate differences between young and older adults in the ability to modify the trunk kinematics and muscle activity following unexpected waist lateral perturbations during gait initiation. METHODS: Ten young (25 ± 2 years) and ten older adults (73 ± 5 years) initiated locomotion from stance while a lateral pull was randomly applied to the pelvis. Two force plates were used to define the feet centre-of-pressure displacement. Angular displacement of the trunk in the frontal plane was obtained through motion analysis. Surface electromyography of cervical and thoracic erector spinae muscles was recorded bilaterally. RESULTS: A lower trunk lateral bending towards the stance leg side in the preparatory phase of gait initiation was observed in older participants following perturbation. Right thoracic muscle activity was increased in response to the perturbation during the initial phase of gait initiation in young (+ 68%) but not in older participants (+ 7%). CONCLUSIONS: The age-related reduction in trunk movement could indicate a more rigid behaviour of the upper body employed by older compared to young individuals in response to unexpected perturbations preceding the initiation of stepping. Older adults' delayed activation of thoracic muscles could suggest impaired reactive mechanisms that may potentially lead to a fall in the early stages of the gait initiation.

Return to sport decisions after an acute lateral ankle sprain injury: introducing the PAASS framework-an international multidisciplinary consensus.

BACKGROUND: Despite being the most commonly incurred sports injury with a high recurrence rate, there are no guidelines to inform return to sport (RTS) decisions following acute lateral ankle sprain injuries. We aimed to develop a list of assessment items to address this gap. METHODS: We used a three-round Delphi survey approach to develop consensus of opinion among 155 globally diverse health professionals working in elite field or court sports. This involved surveys that were structured in question format with both closed-response and open-response options. We asked panellists to indicate their agreement about whether or not assessment items should support the RTS decision after an acute lateral ankle sprain injury. The second and third round surveys included quantitative and qualitative feedback from the previous round. We defined a priori consensus being reached at >70% agree or disagree responses. RESULTS: Sixteen assessment items reached consensus to be included in the RTS decision after an acute lateral ankle sprain injury. They were mapped to five domains with 98% panellist agreement-PAASS: Pain (during sport participation and over the last 24 hours), Ankle impairments (range of motion; muscle strength, endurance and power), Athlete perception (perceived ankle confidence/reassurance and stability; psychological readiness), Sensorimotor control (proprioception; dynamic postural control/balance), Sport/functional performance (hopping, jumping and agility; sport-specific drills; ability to complete a full training session). CONCLUSION: Expert opinion indicated that pain severity, ankle impairments, sensorimotor control, athlete perception/readiness and sport/functional performance should be assessed to inform the RTS decision following an acute lateral ankle sprain injury. TRIAL REGISTRATION NUMBER: ACTRN12619000522112.

Muscle fiber conduction velocity in the vastus lateralis and medialis muscles of soccer players after ACL reconstruction.

The neural factors underlying the persistency of quadriceps weakness after anterior cruciate ligament reconstruction (ACLR) have been only partially explained. This study examined muscle fiber conduction velocity (MFCV) as an indirect parameter of motor unit recruitment strategies in the vastus lateralis (VL) and medialis (VM) muscles of soccer players with ACLR. High-density surface electromyography (HDsEMG) was acquired from VL and VM in nine soccer players (22.7 ± 2.9 years; BMI: 22.08 ± 1.72 kg·m-2 ; 7.7 ± 2.2 months post-surgery). Voluntary muscle force and the relative myoelectrical activity from the reconstructed and contralateral sides were recorded during linearly increasing isometric knee extension contractions up to 70% of maximal voluntary isometric force (MVIF). The relation of MFCV and force was examined by linear regression analysis at the individual subject level. The initial (intercept), peak (MFCV70 ), and rate of change (slope) of MFCV related to force were compared between limbs and muscles. The MVIF was lower in the reconstructed side than in the contralateral side (-%20.5; P < .05). MFCV intercept was similar among limbs and muscles (P > .05). MFCV70 and MFCV slope were lower in the reconstructed side compared to the contralateral for both VL (-28.5% and -10.1%, respectively; P < .001) and VM (-22.6% and -8.1%, respectively; P < .001). The slope of MFCV was lower in the VL than VM, but only in the reconstructed side (-12.4%; P < .001). These results suggest possible impairments in recruitment strategies of high-threshold motor units (HTMUs) as well as deficits in sarcolemmal excitability, fiber diameter, and discharge rate of knee extensor muscles following ACLR.

Modulation of spinal excitability following neuromuscular electrical stimulation superimposed to voluntary contraction.

PURPOSE: Neuromuscular electrical stimulation (NMES) superimposed on voluntary muscle contraction has been recently shown as an innovative training modality within sport and rehabilitation, but its effects on the neuromuscular system are still unclear. The aim of this study was to investigate acute responses in spinal excitability, as measured by the Hoffmann (H) reflex, and in maximal voluntary contraction (MVIC) following NMES superimposed to voluntary isometric contractions (NMES + ISO) compared to passive NMES only and to voluntary isometric contractions only (ISO). METHOD: Fifteen young adults were required to maintain an ankle plantar-flexor torque of 20% MVC for 20 repetitions during each experimental condition (NMES + ISO, NMES and ISO). Surface electromyography was used to record peak-to-peak H-reflex and motor waves following percutaneous stimulation of the posterior tibial nerve in the dominant limb. An isokinetic dynamometer was used to assess maximal voluntary contraction output of the ankle plantar flexor muscles. RESULTS: H-reflex amplitude was increased by 4.5% after the NMES + ISO condition (p < 0.05), while passive NMES and ISO conditions showed a decrease by 7.8% (p < 0.05) and no change in reflex responses, respectively. There was no change in amplitude of maximal motor wave and in MVIC torque during each experimental condition. CONCLUSION: The reported facilitation of spinal excitability following NMES + ISO could be due to a combination of greater motor neuronal and corticospinal excitability, thus suggesting that NMES superimposed onto isometric voluntary contractions may provide a more effective neuromuscular stimulus and, hence, training modality compared to NMES alone.