Peripheral fatigue regulation during knee extensor exercise in type 1 diabetes and consequences on the force-duration relationship.

PURPOSE: This study aimed to examine if peripheral fatigue is adjusted during knee extensor (KE) exercise in order not to surpass a critical threshold patient with type 1 diabetes (T1D) and the consequences of this mechanism on the force-duration relationship. METHODS: Eleven T1D individuals randomly performed two different sessions in which they performed 60 maximum voluntary contractions (MVC; 3 s contraction, 2 s relaxation). One trial was performed in the non-fatigued state (CTRL) and another after fatiguing neuromuscular stimulation of the KE (FNMES). Peripheral and central fatigue were quantified by the difference between pre and post exercise in quadriceps voluntary activation (ΔVA) and potentiated twitch (ΔPtw). Critical torque (CT) was determined as the average force of the last 12 contractions, whereas W' was calculated as the area above the CT. RESULTS: Although FNMES led to a significant decrease in potentiated twitch (Ptw) before performing the 60-MVCs protocol (p < 0.05), ΔVA (∼ -7.5%), ΔPtw (∼ -39%), and CT (∼816 N) post-MVCs were similar between the two conditions. The difference in W' between CTRL and FNMES was correlated with the level of pre-fatigue induced in FNMES (r2 = 0.60). In addition, W' was correlated with ΔPtw (r2 = 0.62) in the CTRL session. CONCLUSION: Correlative results in the present study indicate that regulating peripheral fatigue mechanisms at a critical threshold limit W'. Additionally, peripheral fatigue during KE exercise is limited to an individual threshold in T1D patients.

Feasibility Testing of Wearable Device for Musculoskeletal Monitoring during Aquatic Therapy and Rehabilitation.

The objectives of this study were to test the feasibility of the developed waterproof wearable device with a Surface Electromyography (sEMG) sensor and Inertial Measurement Unit (IMU) sensor by (1) comparing the onset duration of sEMG recordings from maximal voluntary contractions (MVC), (2) comparing the acceleration of arm movement from IMU, and (3) observing the reproducibility of onset duration and acceleration from the developed device for bicep brachii (BB) muscle between on dry-land, and in aquatic environments. Five healthy males participated in two experimental protocols with the activity of BB muscle of the left and right arms. Using the sEMG of BB muscle, the intra-class correlation coefficient (ICC) and typical error (CV%) were calculated to determine the reproducibility and precision of onset duration and acceleration, respectively. In case of onset duration, no significant differences were observed between land and aquatic condition (p = 0.9-0.98), and high reliability (ICC = 0.93-0.98) and precision (CV% = 2.7-6.4%) were observed. In addition, acceleration data shows no significant differences between land and aquatic condition (p = 0.89-0.93), and high reliability (ICC = 0.9-0.97) and precision (CV% = 7.9-9.2%). These comparable sEMG and acceleration values in both dry-land and aquatic environment supports the suitability of the proposed wearable device for musculoskeletal monitoring during aquatic therapy and rehabilitation as the integrity of the sEMG and acceleration recordings maintained during aquatic activities.Clinical Relevance-This study and relevant experiment demonstrate the feasibility of the developed wearable device to support clinicians and therapists for musculoskeletal monitoring during aquatic therapy and rehabilitation.

Surface electromyography analysis of core stabilizing muscles during isometric shoulder contractions in athletes with low back pain.

BACKGROUND: The study was carried out in the athletes with and without Low Back Pain (LBP) to determine the surface electromyography activity of core stabilizing muscles while performing isometric shoulder and trunk contractions. STUDY DESIGN: Cross-sectional study. METHODS: This study enlisted the participation of 40 athletes. Group A included 20 athletes (18 males and 2 females) without LBP, and Group B included 20 athletes (12 males and 8 females) with LBP. Athletes with LBP were assessed using the Modified Oswestry Disability Questionnaire (MODQ) and Visual Analog Scale (VAS) to determine their level of disability and pain severity, respectively. EMG activity of the rectus abdominis, external oblique, longissimus, and multifidus was recorded in both groups as they performed bilateral isometric shoulder and trunk contractions. RESULTS: In the LBP group, EMG activity of the rectus abdominis and external oblique muscles was significantly lower (P < 0.05). The LBP group had significantly more multifidus activity (P = 0.03) than the NLBP group. Among all the exercises, bilateral isometric shoulder extension contraction activated the rectus abdominis, right external oblique, and longissimus group of muscles significantly more (P < 0.05) in both groups. In both groups, bilateral isometric shoulder flexion contraction resulted in significantly higher multifidus muscle activation (P = 0.002). CONCLUSION: The activation of core stabilizing muscles was altered in athletes with LBP. When athletes are unable to contract and activate trunk muscles owing to pain, upper extremity exercises can be used to activate these muscles.

Influence of hip flexion angle on strength and gluteal muscle activities in the clinical pilates clamshell exercise.

The study aimed to investigate the presence of directional preference using the Clinical Pilates method of clamshell exercise among relatively healthy young adults, and the influence of hip flexion angles on maximum isometric strength (handheld dynamometer) and muscle activities (electromyography). The clamshell assessment involves testing in varying angles of hip flexion (0°, 30°, 60° and 90°) and 21 participants were tested. Directional preferences in the transverse and anteroposterior axes of movements were present among the participants. Gluteus medius activation was highest in the non-problem side across all hip flexion angles. In contrast, the problem side showed dysfunction of the gluteus medius, especially at the 60° hip flexion angle, 0.89 unit (ratio of maximal voluntary isometric contraction) reduction per 1 N force exerted, 95% CI -1.69 to -0.09, p = 0.031. Comparison against the non-problem side found gluteus medius activity lower on the problem side, mean difference (SD) was 26.2 (56.1), p = 0.045. Therefore, directional preference is not limited to people with injuries and can exist in healthy individuals or people who have recovered from injuries. The gluteus medius was identified to be dysfunctional on the problem side and Clinical Pilates clamshell assessment is potentially useful in assessing for motor coordination impairment.

Comparison of muscle activity of hamstrings as knee flexors and hip extensors and effect of tibial and hip rotation on the contribution of hamstrings.

BACKGROUND: Limited studies have compared the muscle activity of the medial and lateral hamstrings as knee flexors with tibial internal and external rotation and hip extensors with hip internal and external rotation. In particular, hamstring activity during hip extension with hip rotation has rarely been investigated. OBJECTIVE: This study aimed to compare the muscle activity of the medial and lateral hamstrings as knee flexors and hip extensors and to compare the activity of these muscles according to tibial rotation during isometric knee flexion and hip rotation during isometric hip extension. METHODS: A total of 23 healthy adults participated in the study. The electromyographic (EMG) activity of the hamstrings was measured during maximal isometric knee flexion and maximal isometric hip extension. In addition, tibial rotation was applied actively during maximal isometric knee flexion, whereas hip rotation was applied actively during maximal isometric hip extension. RESULTS: EMG activity during maximal isometric knee flexion with tibial internal and external rotation was significantly higher than that during maximal isometric hip extension with hip internal and external rotation, respectively. For EMG activity according to tibial and hip rotation, there was no significant difference between tibial internal and external rotation during maximal isometric knee flexion, whereas there was a significant difference between hip internal and external rotation during maximal isometric hip extension. CONCLUSION: Hamstring activity was higher for knee flexors than for hip extensors. However, hip rotation during maximal isometric hip extension is an effective intervention for selective muscle activation of the medial and lateral hamstrings.

Age-related differences in the neuromuscular performance of fatigue-provoking exercise under severe whole-body hyperthermia conditions.

PURPOSE: The purpose of this study was to determine if aging would lead to greater decline in neuromuscular function during a fatiguing task under severe whole-body hyperthermia conditions. METHODS: Twelve young (aged 19-21 years) and 11 older (aged 65-80 years) males were enrolled in the study, which comprised a randomized control trial under a thermoneutral condition at an ambient temperature of 23°C (CON) and an experimental trial with passive lower body heating in 43°C water (HWI-43°C). Changes in neuromuscular function and fatigability, and physical performance-influencing factors such as psychological, thermoregulatory, neuroendocrine, and immune responses to whole-body hyperthermia were measured. RESULTS: A slower increase in rectal temperature, and a lower heart rate, thermal sensation, and sweating rate were observed in older males than young males in response to HWI-43°C trial (p < 0.05). Nevertheless, prolactin increased more in response to hyperthermia in young males, while interleukin-6 and cortisol levels increased more in older males (p < 0.05). Peripheral dopamine levels decreased in older males and increased in young males in response to hyperthermia (p < 0.05). Surprisingly, older males demonstrated greater neuromuscular fatigability resistance and faster maximal voluntary contraction (MVC) torque recovery after a 2-min sustained isometric MVC task under thermoneutral and severe hyperthermic conditions (p < 0.05). CONCLUSION: Neuromuscular performance during fatigue-provoking sustained isometric exercise under severe whole-body hyperthermia conditions appears to decline in both age groups, but a lower relative decline in torque production for older males may relate to lower psychological and thermophysiological strain along with a diminished dopamine response and prolactin release.

Low-Level Laser Therapy Facilitates Postcontraction Recovery with Ischemic Preconditioning.

PURPOSE: Despite early development of muscle fatigue, ischemic preconditioning is gaining popularity for strength training combined with low-load resistance exercise. This study investigated the effect of low-level laser (LLL) on postcontraction recovery with ischemic preconditioning. METHODS: Forty healthy adults (22.9 ± 3.5 yr) were allocated into sham (11 men, 9 women) and LLL (11 men, 9 women) groups. With ischemic preconditioning, they were trained with three bouts of intermittent wrist extension of 40% maximal voluntary contraction (MVC). During the recovery period, the LLL group received LLL (wavelength of 808 nm, 60 J) on the working muscle, whereas the sham group received no sham therapy. MVC, force fluctuations, and discharge variables of motor units (MU) for a trapezoidal contraction were compared between groups at baseline (T0), postcontraction (T1), and after-recovery (T2). RESULTS: At T2, the LLL group exhibited a higher normalized MVC (T2/T0; 86.22% ± 12.59%) than that of the sham group (71.70% ± 13.56%; P = 0.001). The LLL group had smaller normalized force fluctuations (LLL, 94.76% ± 21.95%; sham, 121.37% ± 29.02%; P = 0.002) with greater normalized electromyography amplitude (LLL, 94.33% ± 14.69%; sham, 73.57% ± 14.94%; P < 0.001) during trapezoidal contraction. In the LLL group, the smaller force fluctuations were associated with lower coefficients of variation of interspike intervals of MUs (LLL, 0.202 ± 0.053; sham, 0.208 ± 0.048; P = 0.004) with higher recruitment thresholds (LLL, 11.61 ± 12.68 %MVC; sham, 10.27 ± 12.73 %MVC; P = 0.003). CONCLUSIONS: LLL expedites postcontraction recovery with ischemic preconditioning, manifesting as superior force generation capacity and force precision control for activation of MU with a higher recruitment threshold and lower discharge variability.

Voluntary torque production is unaffected by changes in local thermal sensation during normothermia and hyperthermia.

NEW FINDINGS: What is the central question of this study? Hyperthermia reduces the human capacity to produce muscular force, which is associated with decreased neural drive: does mitigating a reduction in neural drive by altering localised thermal sensation help to preserve voluntary force output? What is the main finding and its importance? Altering thermal sensation by cooling and heating the head independent of core temperature did not change neural drive or benefit voluntary force production. Head cooling did slow the rate of rise in core temperature during heating, which may have practical applications in passive settings. ABSTRACT: This study investigated altered local head and neck thermal sensation on maximal and rapid torque production during voluntary contractions. Nine participants completed four visits in two environmental conditions: at rectal temperatures ∼39.5°C in hot (HOT; ∼50°C, ∼39% relative humidity) and ∼37°C in thermoneutral (NEU; ∼22°C, ∼46% relative humidity) conditions. Local thermal sensation was manipulated by heating in thermoneutral conditions and cooling in hot conditions. Evoked twitches and octets were delivered at rest. Maximum voluntary torque (MVT), normalised surface electromyography (EMG) and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions of the knee extensors. Rate of torque development (RTD) and EMG were measured during rapid voluntary contractions. MVT (P = 0.463) and RTD (P = 0.061) were similar between environmental conditions despite reduced VA (-6%; P = 0.047) and EMG at MVT (-31%; P = 0.019). EMG in the rapid voluntary contractions was also lower in HOT versus NEU during the initial 100 ms (-24%; P = 0.035) and 150 ms (-26%; P = 0.035). Evoked twitch (+70%; P < 0.001) and octet (+27%; P < 0.001) RTD during the initial 50 ms were greater in the HOT compared to NEU conditions, in addition to a faster relaxation rate of the muscle (-33%; P < 0.001). In conclusion, hyperthermia reduced neural drive without affecting voluntary torque, likely due to the compensatory effects of improved intrinsic contractile function and faster contraction and relaxation rates of the knee extensors. Changes in local thermal perception of the head and neck whilst hyperthermic or normothermic did not affect voluntary torque.

Heat acclimation reduces the effects of whole-body hyperthermia on knee-extensor relaxation rate, but does not affect voluntary torque production.

PURPOSE: This study investigated the effects of acute hyperthermia and heat acclimation (HA) on maximal and rapid voluntary torque production, and their neuromuscular determinants. METHODS: Ten participants completed 10 days of isothermic HA (50 °C, 50% rh) and had their knee-extensor neuromuscular function assessed in normothermic and hyperthermic conditions, pre-, after 5 and after 10 days of HA. Electrically evoked twitch and octet (300 Hz) contractions were delivered at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary contractions. RESULTS: Acute hyperthermia reduced neural drive (EMG at MVT and during rapid voluntary contractions; P < 0.05), increased evoked torques (P < 0.05), and shortened contraction and relaxation rates (P < 0.05). HA lowered resting rectal temperature and heart rate after 10 days (P < 0.05), and increased sweating rate after 5 and 10 days (P < 0.05), no differences were observed between 5 and 10 days. The hyperthermia-induced reduction in twitch half-relaxation was attenuated after 5 and 10 days of HA, but there were no other effects on neuromuscular function either in normothermic or hyperthermic conditions. CONCLUSION: HA-induced favourable adaptations to the heat after 5 and 10 days of exposure, but there was no measurable benefit on voluntary neuromuscular function in normothermic or hyperthermic conditions. HA did reduce the hyperthermic-induced reduction in twitch half-relaxation time, which may benefit twitch force summation and thus help preserve voluntary torque in hot environmental conditions.

The contralateral effects of foam rolling on range of motion and muscle performance.

A single bout of foam rolling (FR) can acutely increase joint range of motion (ROM) without detrimental effects on subsequent muscle performance. Similarly, long-term FR training can increase ROM, while muscle performance seems to be unaffected. Although the acute and long-term effects of FR on the treated muscle are understood, the impact of FR on the contralateral side is not well known. Therefore, this scoping review aims to summarize the current evidence on the acute and long-term effect of FR on the ipsilateral limb on ROM and muscle performance (i.e., maximum force, rate of force development, jump height) for the contralateral (non-treated) limb. Potential explanatory mechanisms are also discussed. There is evidence that a single bout of FR on the ipsilateral limb increases ROM of the contralateral limb; however, evidence is limited for long-term effects. The most likely mechanism for contralateral ROM increases is a reduced perception of pain. With regard to isolated muscle contractions, no changes in muscle performance (i.e., maximum voluntary isometric contraction, maximum voluntary dynamic contraction) were found in the contralateral limb after a single bout of FR on the ipsilateral limb. Notably, only one study reported large impairments in rate of force development of the contralateral limb following FR on the ipsilateral leg, possibly due to decreased motor unit recruitment. Furthermore, to date there are only two studies examining the long-term FR training of the ipsilateral limb on performance (i.e., maximal strength and jump performance) which reported moderate improvements. Although, trivial to very large changes on a variety of parameters were found in this study, the functional and practical relevance of our findings should be interpreted with caution.

Research study on the effects of multi-component exercise on older adults with chronic conditions.

BACKGROUND: Multicomponent exercises involving eccentric contraction (ECC) exercise, chair yoga (CY) and tai chi (TC) are practiced extensively to treat older adults with chronic conditions. The study investigated the effect of CY and TC performance after various intensity of ECC exercise induced muscle pain, delayed onset of muscle sores (DOMS), and strength in sedentary older adults. METHODS: Nine sedentary older adults with chronic condition(s) aged between 60 and 90 performed CY and TC after ECC exercise protocol for 9 weeks. The medical preprotocol, ECC exercise, CY, and TC performance, subjective (health activities of daily living [ADL] difficulty scale; Visual Analog Scale [VAS], and DOMS), objective (pressure pain threshold [PPT]), and medical postprotocol assessments were carried out and compared the physical status before and after the protocol. RESULTS: Our results show that functional limitations, DOMS, VAS, and PPT levels were improved in the CY and TC performed weeks 3, 4, 7, and 8 compared to ECC exercise weeks 2 and 6. The manual muscle testing (MMT), active range of motion (AROM), and maximal voluntary isometric contraction (MVIC) were improved and showed significant differences on the weak muscle groups. The mean Tinetti balance and gait score, and general endurance showed significant difference. CONCLUSIONS: Our findings have important implications for clinical exercise prescription as low perceived CY and TC, and ECC exercise might suit well for older adults with chronic conditions. Performing multicomponent exercises can help to reduce the risk of injury from falls and prevent age-related mobility, balance, and physical function decline in older adults.

Feasibility of a Novel Video Game-Based Electromyography Biofeedback System in Patients With Knee Osteoarthritis.

CONTEXT: A novel virtual game system Knee Biofeedback Rehabilitation Interface for game-based home therapy (KneeBright) was developed for strength training using integrated electromyography biofeedback of the quadriceps muscle to control the game. The study aimed to compare the KneeBright and electromyography biofeedback interface among patients with knee osteoarthritis. DESIGN: Controlled before and after design. METHODS: Nineteen patients with knee osteoarthritis took part in this laboratory-based study. Exercise sessions took place on 2 separate days. During session 1, participants used a conventional electromyography biofeedback system while performing 3 sets of lower body exercises with emphasis on maximal muscle activation, endurance, and precision. During session 2, participants used the KneeBright game to match the exercise sets in the first session. For both sessions, knee extension torque during the isometric muscle activation exercises and time to voluntary additional exercise were recorded. Patient engagement was assessed using the technology acceptance model and System Usability Score questionnaires. RESULTS: The peak knee extension torque produced during the control exercise session and the KneeBright exercise session were positively correlated. Knee extension torque generated during KneeBright game exercise sessions was increased by an average of 25% compared to the control sessions (2.14 vs 1.77 N·m/kg, P = .02). The mean technology acceptance model score for the KneeBright system was 3.4/5 and the mean System Usability Score was 79, both indicating positive patient engagement. CONCLUSIONS: Patients using the KneeBright game produced greater knee torque than patients using the conventional system, had positive levels of engagement, and exercised longer with the KneeBright game.

Evaluation of the Efficacy of Supplementation with Planox® Lemon Verbena Extract in Improving Oxidative Stress and Muscle Damage: A Randomized Double-Blind Controlled Trial.

Excessive exercise load can cause muscle soreness and fatigue, as well as inflammation and oxidative stress. Lemon verbena (Aloysia triphylla; Lippia citriodora) is often used as a spice in tea or beverages. Its leaves are rich in polyphenols, which have antioxidant and anti-inflammatory bioactivities. In the present study, we investigated whether supplementation with Planox® lemon verbena extract (LVE) could improve muscle damage and biochemical indicators after exhaustive exercise challenge. All subjects (30 males and 30 females) underwent a double-blind trial and were randomly divided into a placebo group (0 mg/human/day) and an LVE supplement group (400 mg/human/day), with gender-equal distribution. All subjects started supplementation 10 days before exhaustive exercise and continued it until all tests were completed. Before the intervention, after the exhaustive exercise, and on the following 3 days, the participants underwent 12-minute Cooper running/walking; blood collection; assessments of pain, muscle stiffness, maximum jump heights, and isometric maximum muscle strength. The results showed that supplementation with LVE effectively increased GPx and reduced CK, IL-6, 8-OHdG and muscle pain after the exhaustive exercise, but it had significant effect on strength recovery. In summary, LVE is a safe and edible natural plant extract that can reduce muscle damage and soreness after exercise. This trial was registered at clinicaltrials.gov as NCT04742244.

Does partial activation of the neuromuscular system induce cross-education training effect? Case of a pilot study on motor imagery and neuromuscular electrical stimulation.

INTRODUCTION: Cross education defines the gains observed in the contralateral limb following unilateral strength training of the other limb. The present study questioned the neural mechanisms associated with cross education following training by motor imagery (MI) or submaximal neuromuscular electrical stimulation (NMES), both representing a partial activation of the motor system as compared to conventional strength training. METHODS: Twenty-seven participants were distributed in three groups: MI, NMES and control. Training groups underwent a training program of ten sessions in two weeks targeting plantar flexor muscles of one limb. In both legs, neuromuscular plasticity was assessed through maximal voluntary isometric contraction (MViC) and triceps surae electrophysiological responses evoked by electrical nerve stimulation (H-reflexes and V-waves). RESULTS: NMES and MI training improved MViC torque of the trained limb by 11.3% (P < 0.001) and 13.8% (P < 0.001), respectively. MViC of the untrained limb increased by 10.3% (P < 0.003) in the MI group only, accompanied with increases in V-waves on both sides. In the NMES group, V-waves only increased in the trained limb. In the MI group, rest H-reflexes increased in both the trained and the untrained triceps suraes. CONCLUSION: MI seems to be effective to induce cross education, probably because of the activation of cortical motor regions that impact the corticospinal neural drive of both trained and untrained sides. Conversely, submaximal NMES did not lead to cross education. The present results emphasize that cross education does not necessarily require muscle activity of the trained limb.

A novel oral nutritional supplement improves gait speed and mitochondrial functioning compared to standard care in older adults with (or at risk of) undernutrition: results from a randomized controlled trial.

Undernutrition in older adults is mainly addressed by oral nutritional supplements, which do not affect physical functioning. In this study, we tested a novel oral nutritional supplement that included whey and casein protein, ursolic acid, free branch-chained amino acids and vitamin D against a standard supplement. We included older adults (>65y) with (or at risk of) undernutrition (n=82) and randomized them to 12 weeks of novel or standard supplement. Both groups showed significant increases in body mass. No within or between-group differences in lean body mass were observed. Fat mass increased significantly more in the standard than the novel supplement group (time*treatment effect P=0.045). The novel supplement group showed a larger improvement in walking performance on distances of 4m (treatment x time interaction P=0.048) and 400m (treatment x time interaction P=0.038) than the standard treatment group. Gene sets related to mitochondrial functioning and oxidative phosphorylation were upregulated in the novel supplement group and downregulated in the standard supplement group. We conclude that a 12-week intervention with the novel supplement improved walking performance both during short and long distance as compared to a standard supplement, which can largely be explained by increased mitochondrial functioning in the group receiving the novel supplement.

Modulation of torque evoked by wide-pulse, high-frequency neuromuscular electrical stimulation and the potential implications for rehabilitation and training.

The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque. The progressive increase in torque (extra torque) that may develop in response to low intensity wide-pulse high-frequency (WPHF) NMES holds great promise for rehabilitation as it overcomes the main limitation of NMES, namely discomfort. WPHF NMES extra torque is thought to result from reflexively recruited motor units at the spinal level. However, whether WPHF NMES evoked force can be modulated is unknown. Therefore, we examined the effect of two interventions known to change the state of spinal circuitry in opposite ways on evoked torque and motor unit recruitment by WPHF NMES. The interventions were high-frequency transcutaneous electrical nerve stimulation (TENS) and anodal transcutaneous spinal direct current stimulation (tsDCS). We show that TENS performed before a bout of WPHF NMES results in lower evoked torque (median change in torque time-integral: - 56%) indicating that WPHF NMES-evoked torque might be modulated. In contrast, the anodal tsDCS protocol used had no effect on any measured parameter. Our results demonstrate that WPHF NMES extra torque can be modulated and although the TENS intervention blunted extra torque production, the finding that central contribution to WPHF NMES-evoked torques can be modulated opens new avenues for designing interventions to enhance WPHF NMES.

A comparison of foam rolling and vibration foam rolling on the quadriceps muscle function and mechanical properties.

PURPOSE: The purpose of the study was to investigate the effects of using a vibration foam roll (VFR) or a non-vibration foam roll (NVFR) on maximum voluntary isometric contraction peak torque (MVIC), range of motion (ROM), passive resistive torque (PRT), and shear modulus. METHODS: Twenty-one male volunteers visited the laboratory on two separate days and were randomly assigned to either a VFR group or a NVFR group. Both interventions were performed for 3 × 1 min each. Before and after each intervention, passive resistive torque and maximum voluntary isometric contraction peak torque of the leg extensors were assessed with a dynamometer. Hip extension ROM was assessed using a modified Thomas test with 3D-motion caption. Muscle shear modulus of the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) was assessed with shear wave elastography (SWE). RESULTS: In both groups (VFR, NVFR) we observed an increase in MVIC peak torque (+ 14.2 Nm, + 8.6 Nm) and a decrease in shear modulus of the RF (- 7.2 kPa, - 4.7 kPa). However, an increase in hip extension ROM (3.3°) was only observed in the VFR group. There was no change in PRT and shear modulus of the VL and VM, in both the VFR group and the NVFR group. Our findings demonstrate a muscle-specific acute decrease in passive RF stiffness after VFR and NVFR, with an effect on joint flexibility found only after VFR. CONCLUSION: The findings of this study suggest that VFR might be a more efficient approach to maximize performance in sports with flexibility demands.

Alternating Current Is More Fatigable Than Pulsed Current in People Who Are Healthy: A Double-Blind, Randomized Crossover Trial.

OBJECTIVE: Tolerance level and rapid fatigue onset are limitations in the use of neuromuscular electrical stimulation (NMES) as an electrotherapeutic resource in rehabilitation and training protocols; however, it is unclear if pulsed current (PC) and alternating current (AC) produce different fatigue levels when applied at submaximal contraction level. The purpose of this study was to compare fatigue and discomfort levels between PC and AC during a submaximal contraction protocol in people who are healthy. METHODS: In this double-blind, randomized crossover trial conducted in a laboratory setting, 30 male volunteers [23.23 years of age (SD = 4.59)] performed 2 submaximal fatigue protocols (with a 7-day interval) in a randomized order: PC (pulse duration = 2 milliseconds, pulse frequency = 100 Hz) and AC (2.5 kHz, pulse duration = 0.4 milliseconds, burst frequency = 100 Hz). NMES currents were applied to the knee extensor motor point of the dominant limb. The NMES protocol consisted of 80 evoked contractions (time on:off = 5:10 seconds) and lasted 20 minutes. The current was maintained at a constant intensity throughout the NMES protocol. The primary outcome measures were maximal voluntary isometric contraction, fatigue index (evoked torque decline), fatigability (number of contractions for a 50% drop in evoked-torque from the protocol start), total evoked torque-time integral (TTI), decline in TTI, and discomfort level. RESULTS: AC at 2.5 kHz demonstrated higher maximal voluntary isometric contraction decline post-fatigue, higher fatigue index, higher fatigability (ie, fewer contractions to reach the 50% evoked torque decline from the protocol start), smaller total TTI, and higher TTI decline compared with PC. No between-currents difference was observed in discomfort level. CONCLUSION: PC is less fatigable than AC at 2.5 kHz. IMPACT: Based on this study, PC is the preferred current choice when the NMES goal is to generate higher muscle work, higher mechanical load, and smaller fatigability during training both for athletes who are healthy and for rehabilitation programs for people with disease or injury.

Standardizing fatigue-resistance testing during electrical stimulation of paralysed human quadriceps muscles, a practical approach.

BACKGROUND: Rapid onset of muscular fatigue is still one of the main issues of functional electrical stimulation (FES). A promising technique, known as distributed stimulation, aims to activate sub-units of a muscle at a lower stimulation frequency to increase fatigue-resistance. Besides a general agreement on the beneficial effects, the great heterogeneity of evaluation techniques, raises the demand for a standardized method to better reflect the requirements of a practical application. METHODS: This study investigated the fatigue-development of 6 paralysed quadriceps muscles over the course of 180 dynamic contractions, evaluating different electrode-configurations (conventional and distributed stimulation). For a standardized comparison, fatigue-testing was performed at 40% of the peak-torque during a maximal evoked contraction (MEC). Further, we assessed the isometric torque for each electrode-configuration at different knee-extension-angles (70°-170°, 10° steps). RESULTS: Our results showed no significant difference in the fatigue-index for any of the tested electrode-configurations, compared to conventional-stimulation. We conjecture that the positive effects of distributed stimulation become less pronounced at higher stimulation amplitudes. The isometric torque produced at different knee-extension angles was similar for most electrode-configurations. Maximal torque-production was found at 130°-140° knee-extension-angle, which correlates with the maximal knee-flexion-angles during running. CONCLUSION: In most practical applications, FES is intended to initiate dynamic movements. Therefore, it is crucial to assess fatigue-resistance by using dynamic contractions. Reporting the relationship between produced torque and knee-extension-angle can help to observe the stability of a chosen electrode-configuration for a targeted range-of-motion. Additionally, we suggest to perform fatigue testing at higher forces (e.g. 40% of the maximal evoked torque) in pre-trained subjects with SCI to better reflect the practical demands of FES-applications.

Concurrent Validity and Reliability of a Low-Cost Dynamometer to Assess Maximal Isometric Strength in Neck Movements.

OBJECTIVE: This study aimed to evaluate the concurrent validity and reliability of a low-cost digital dynamometer to assess maximal isometric strength in neck movements. METHODS: Twelve recreationally active participants (6 women, 6 men; age: 24.1 ± 3.4 years; height: 1.71 ± 0.06 m; weight: 64.6 ± 11.7 kg) performed neck flexion, extension, and lateral flexion, and maximal isometric strength was simultaneously measured using an isokinetic dynamometer (gold standard) and a digital dynamometer to evaluate the concurrent validity of the latter. One week later, all the participants repeated 3 trials of each neck movement (3 minutes apart) registered only with the digital dynamometer. The first and second trial were guided by tester 1 and the third trial was guided by tester 2. RESULTS: The concurrent validity of the digital dynamometer showed a nearly perfect correlation between both devices (r > 0.986, P < .001), with Bland-Altman plots showing absolute agreement. Intertester and intratester reliability were nearly perfect for all neck movements (intraclass correlation coefficient > 0.86). CONCLUSION: The low-cost digital dynamometer showed valid and reliable measurements of maximal isometric strength in neck movements.