Maximal Voluntary Contraction Research Articles

Immediate effect of local vibration on motor unit firing behavior and muscle strength in healthy young adult males.

The aim of this study was to examine the effect of vibration on motor unit (MU) firing behavior and physical performance of antagonist muscles in healthy young adult males. Fourteen males (age = 24.3 ± 3.6years) were included in this study. There were two conditions, one in which participants received 80Hz vibration in the distal tendon of the hamstring for 30s and the control condition (no vibration). High-density surface electromyography (HD-SEMG) signals and maximal voluntary contraction (MVC) of knee extensor muscles were evaluated before and after the respective conditions and recorded from the vastus lateralis muscle during submaximal ramp-up and sustained contractions at 30% MVC. Convolution blind source separation was used to decompose the HD-SEMG signals into individual MU firing behaviors. In total, 739 MUs were detected (control; 360 MUs and vibration; 379 MUs), and a total of 312 matched MUs were identified across both submaximal contraction conditions (control: 150 MUs; vibration: 162 MUs). Vibration significantly increased the discharge rate (p = 0.047) and decreased the recruitment threshold before and after intervention (p = 0.001) but not in the control condition. Furthermore, the recruitment threshold is a factor that influences discharge rate. Significant correlations were observed between the recruitment threshold and both the ∆ discharge rate and the ∆ recruitment threshold under the vibration condition (p < 0.001). Vibration increased in the discharge rate and decreased the recruitment threshold of the antagonist muscle. These findings suggested that vibration contributes to immediate changes in the neural control of antagonist muscles.

Comparative Electromyographic Activity of Hamstrings During Sprinting Versus Strengthening Exercises: Implications for Injury Prevention.

Hamstring injuries are a major problem in sports involving sprinting, such as soccer, rugby, and track and field, and lead to sports stoppages and psychological, social, and financial repercussions. For several years now, these injuries have been stagnating or even increasing. Preventing these injuries is therefore a fundamental issue for at-risk athletes. The aim of this study was to compare the electromyographic(EMG) activity of the hamstrings in athletes during sprinting, Nordic hamstring exercise (NHE), and high-speed concentric exercise on an isokinetic dynamometer. A pilot study was conducted on a population of 15 sprint-exposed field athletes (22.54 ± 3.71 years, Tegner score ≥ 6) with no history of hamstring injury in the last six months. The protocol included a warm-up, followed by three repetitions of the NHE, two sets of 10 repetitions on the isokinetic dynamometer at 300°/sec, and a maximal sprint. Exercises were randomized for each subject, and muscle activity was assessed using wireless EMG sensors during each test. EMG data were normalized to the measured maximum voluntary isometric contraction (MVIC), and test results were statistically analyzed to determine which exercise resulted in maximal hamstring activation. Comparison of hamstring muscle activity between exercises showed a significant difference for most of our results (p-value < 0.05). The results show significantly higher mean hamstring activity during sprinting (0.4800 ± 0.19 μV) compared with strengthening exercises. The NHE (0.3201 ± 0.09 μV) was the second most active exercise. In the last place was the high-speed concentric exercise on the isokinetic machine, which produced less activation than the other exercises (0.2487 ± 0.07 μV). Sports involving sprinting are at risk of a hamstring injury but it appears that its use in rehabilitation and prevention of hamstring injury is relevant, as it would allow high-intensity muscle activation to prepare the hamstring for this type of loading. However, it is also fundamental to integrate strengthening exercises such as NHE in combination with sprinting in our rehabilitation. Finally, the use of the isokinetic dynamometer does not constitute a first-line choice for hamstring injury management.

Comparative Analysis of Neuromuscular Activation Patterns Associated with Force between Semi-Professional Female Soccer Players with Previous Anterior Cruciate Ligament Surgery and Healthy Players in Thigh Musculature Related to Valgus Collapse

This study investigates electromyography activation and force development differences in key lower limb muscles between female football players with previous anterior cruciate ligament injuries compared with healthy players. Twenty-two semi-professional players were divided into ACL-injured (n = 11) and non-injured groups (n = 11). Participants underwent maximal voluntary isometric contractions while electromyography activation, peak and average, and peak torque of force were measured. Results indicated significant differences in electromyography activation patterns between anterior cruciate ligament players and non-injured players, particularly in biceps femoris and gluteus maximus muscles. These differences were also evident when comparing between limbs within anterior cruciate ligament players. Interestingly, both groups exhibited similar peak torque of force during maximal contractions, suggesting a compensatory neuromuscular strategy that supports a return to sport based on kinetic and kinematic factors. However, these findings underscore persistent muscle integration imbalances potentially contributing to the high rate of anterior cruciate ligament reinjury. In conclusion, this study highlights the importance of evaluating electromyography activation alongside force development in understanding neuromuscular adaptations post anterior cruciate ligament injury. These insights emphasize the need for comprehensive rehabilitation strategies that address muscle imbalance to mitigate the risk of recurrent anterior cruciate ligament injuries in female football players.

Activation of skeletal muscle mechanoreceptors and nociceptors reduces the exercise performance of the contralateral homologous muscles.

Increasing evidence suggests that activation of muscle nerve afferents may inhibit central motor drive, affecting contractile performance of remote exercising muscles. Although these effects are well documented for metaboreceptors, very little is known about the activation of mechano- and mechanonociceptive afferents on performance fatigability. Therefore, the purpose of the present study was to examine the influence of mechanoreceptors and nociceptors on performance fatigability. Eight healthy young males undertook four randomized experimental sessions on separate occasions in which the experimental knee extensors were the following: 1) resting (CTRL), 2) passively stretched (ST), 3) resting with delayed onset muscle soreness (DOMS), or 4) passively stretched with DOMS (DOMS+ST), whereas the contralateral leg performed an isometric time to task failure (TTF). Changes in maximal voluntary contraction (ΔMVC), potentiated twitch force (ΔQtw,pot), and voluntary muscle activation (ΔVA) were also assessed. TTF was reduced in DOMS+ST (-43%) and ST (-29%) compared with CTRL. DOMS+ST also showed a greater reduction of VA (-25% vs. -8%, respectively) and MVC compared with CTRL (-28% vs. -45%, respectively). Rate of perceived exertion (RPE) was significantly increased at the initial stages (20-40-60%) of the TTF in DOMS+ST compared with all conditions. These findings indicate that activation of mechanosensitive and mechanonociceptive afferents of a muscle with DOMS reduces TTF of the contralateral homologous exercising limb, in part, by reducing VA, thereby accelerating mechanisms of central fatigue.NEW & NOTEWORTHY We found that activation of mechanosensitive and nociceptive nerve afferents of a rested muscle group experiencing delayed onset muscle soreness was associated with reduced exercise performance of the homologous exercising muscles of the contralateral limb. This occurred with lower muscle voluntary activation of the exercising muscle at the point of task failure.

Enhancing lower limb and core muscle activation with blood flow restriction training: a randomized crossover study on high-intensity squat exercises.

The primary objective of this study was to assess the impact of high-intensity deep squat training integrated with various blood flow restriction (BFR) modalities on the activation of lower limb and core muscles. A randomized, self-controlled crossover experimental design was employed with 12 participants. The exercise protocol consisted of squat training at 75% of one-repetition maximum (1RM), performed in 3 sets of 8 repetitions with a 2-min inter-set rest period. This was conducted under four distinct BFR conditions: continuous low BFR (T1), intermittent medium BFR (T2), intermittent high BFR (T3), and a non-restricted control (C). Surface electromyography (EMG) was utilized to collect EMG signals from the target muscles during the BFR and squat training sessions. The root mean square (RMS) amplitude standard values were calculated for each squat set to quantify muscle activation levels, with these values expressed as a percentage of the maximum voluntary contraction (%MVC). Rating of Perceived Exertion was evaluated after each squat set, and leg circumference measurements were taken. 1) During the first two sets of deep squats, the %MVC of the vastus lateralis and vastus medialis in all compression groups was significantly higher than that in the control group (p < 0.05). Furthermore, in the first set, the %MVC of the vastus lateralis in Group T3 was significantly higher than in Group T2 (p < 0.05). In the third set, the %MVC of the vastus medialis in Groups T1 and T3 was significantly lower than in the first two sets (p < 0.05). 2) Group T1 showed an increased activation of the biceps femoris and semitendinosus muscles in the second and third sets, with %MVC values significantly greater than in the first set (p < 0.05). Group T2 only showed an increase in biceps femoris activation in the third set (p < 0.05). Group T3 significantly increased the activation of the biceps femoris and semitendinosus muscles only in the first set (p < 0.05). 3) No significant differences were observed in the changes of rectus abdominis %MVC among the groups (p > 0.05). In the first set, Group T3's erector spinae %MVC was significantly higher than the control group's; in the second set, it was significantly higher than both Group T2 and the control group's (p < 0.05). 4) After training, a significant increase in thigh circumference was observed in all groups compared to before training (p < 0.05). 5) For RPE values, Group T2's post-squat values were significantly higher than the control group's after all three sets (p < 0.05). Group T1's RPE values were also significantly higher than the control group's after the third set (p < 0.05). Groups T1, T2, and C all had significantly higher RPE values in the second and third sets compared to the first set (p < 0.05). All BFR modalities significantly enhanced the activation level of the anterior thigh muscles, with the continuous low BFR mode demonstrating a more stable effect. No significant differences were found in the activation level of the rectus abdominis among the groups. However, the intermittent high BFR mode was the most effective in increasing the activation level of the erector spinae muscles. While BFR did not further augment leg circumference changes, it did elevate subjective fatigue levels. The RPE was lowest during squatting under the intermittent high BFR condition.

Is there a relationship between knee crepitus with quadriceps muscle thickness and strength in individuals with patellofemoral pain? A cross-sectional study

ObjectiveTo explore the relationship between knee crepitus, quadriceps muscle thickness and isometric strength in individuals with patellofemoral pain (PFP). DesignCross-sectional. ParticipantsIndividuals with PFP. Main outcome measuresParticipants with PFP underwent assessments for presence, frequency and severity of knee crepitus. Real-time ultrasound images of the quadriceps muscles (rectus femoris, vastus medialis and lateralis) at rest and during contraction were obtained, muscle thickness was measured in both conditions. Maximal voluntary isometric contraction tests were performed to measure knee extensor strength. The relationship between knee crepitus and quadriceps muscle thickness and knee extensor strength was explored using logistic and linear regressions. ResultsSixty individuals with PFP were included (age: 24; 60% women; 38% with crepitus). Knee crepitus severity was related to rectus femoris and vastus medialis thickness during rest (R2 = 0.19 and 0.09, respectively) and contraction (R2 = 0.16 and 0.07, respectively) and with vastus lateralis during contraction (R2 = 0.08). Isometric knee extensor strength was not related to knee crepitus presence, frequency, or severity. ConclusionHigher severity of knee crepitus is related to lower quadriceps muscle thickness in individuals with PFP. There is no relationship between the presence and frequency of knee crepitus with quadriceps muscle thickness or knee extensor strength.

Comparative Analysis of Muscle Activity and Circulatory Dynamics: A Crossover Study Using Leg Exercise Apparatus and Ergometer.

Background and Objectives: Bedridden patients are at a high risk of venous thromboembolism (VTE). Passive devices such as elastic compression stockings and intermittent pneumatic compression are common. Leg exercise apparatus (LEX) is an active device designed to prevent VTE by effectively contracting the soleus muscle and is therefore expected to be effective in preventing disuse of the lower limbs. However, few studies have been conducted on the kinematic properties of LEX. Therefore, this study aimed to compare the exercise characteristics of LEX with those of an ergometer, which is commonly used as a lower-limb exercise device, and examine its effect on the two domains of muscle activity and circulatory dynamics. Materials and Methods: This study used a crossover design in which each participant performed both exercises to evaluate the exercise characteristics of each device. Fifteen healthy adults performed exercises with LEX and an ergometer (Terasu Erugo, SDG Co., Ltd., Tokyo, Japan) for 5 min each and rested for 10 min after each exercise. Muscle activity was measured using surface electromyography (Clinical DTS, Noraxon, Scottsdale, AZ, USA), and circulatory dynamics were recorded using a non-invasive impedance cardiac output meter (Physioflow Enduro, Manatec Biomedical, Paris, France). The primary outcome was the mean percentage of maximum voluntary contraction (%MVC) of the soleus muscle during exercise. Results: The mean %MVC of the soleus muscle was significantly higher in the LEX group, whereas no significant differences were observed across the periods and sequences. Heart rate, stroke volume, and cardiac output increased during exercise and decreased thereafter; however, the differences between the devices were not significant. Conclusions: LEX may not only have a higher thromboprophylaxis effect, but also a higher effect on preventing muscle atrophy as a lower-extremity exercise device. Additionally, LEX could potentially be used safely in patients who need to be monitored for changes in circulatory dynamics.

Gastrocnemius electrical stimulation increases ankle dorsiflexion strength in patients with post-acute sequelae of SARS-COV-2 (PASC): a double-blind randomized controlled trial

Post-Acute sequelae of SARS-CoV-2 (PASC) is a multisystem disorder causing persistent musculoskeletal deconditioning and reduced lower extremity strength. Electrical stimulation (E-Stim) to the gastrocnemius muscle can enhance strength outcomes by increasing the frequency of muscle fiber activation. We investigated its effect on individuals with PASC. Participants were randomized into intervention (IG) or control (CG) groups. The IG self-administered daily one-hour E-Stim to both their gastrocnemius muscles using a functional device over 4-week, while the CG used a sham device. Primary outcomes were ankle dorsiflexion strength assessed via dynamometry during maximum voluntary contractions, and gastrocnemius voluntary activation (GVA) via surface electromyography. The secondary outcome assessed activities of daily living (ADL), instrumental ADL, and mobility queries. Percentage improvement was calculated. Eighteen patients were analyzed (IG = 10; CG = 8). After 4 week, the IG showed a significantly higher improvement in ankle dorsiflexion strength (222.64%) compared to the CG (51.27%, p = 0.002). Additionally, the IG’s ankle dorsiflexion strength improvement significantly correlated with GVA improvement (rho = 0.782) at 4 week. The secondary outcomes did not reveal significant changes in neither group. Self-administered gastrocnemius E-Stim improves ankle dorsiflexion strength in individuals with PASC. However, larger sample sizes and longer interventions are needed to validate these findings.

Gaussianity Evaluation of HD-sEMG Signals with Aging and Sex During Low and Moderate Isometric Contractions of the Biceps Brachii

IntroductionAging is associated with muscle decline, which alters both functional and anatomical properties of the neuromuscular system. These modifications can be reflected in high-density surface electromyography (HD-sEMG) signals. This study examines how age and sex impact the shape of the amplitude Probability Density Function (PDF) of HD-sEMG signals. Materials and MethodsMonopolar HD-sEMG signals were collected from the Biceps Brachii in a cohort of 17 individuals: 10 women (mean age: 22.9 ± 3.6 years) and 7 men (mean age: 24.4 ± 2.5 years) in the younger group, and 10 women (mean age: 69.8 ± 4.8 years) and 7 men (mean age: 72.8 ± 2.7 years) in the elderly group. The recordings were conducted during an elbow flexion at both 20% and 40% maximum voluntary contraction. The signal amplitude was evaluated using root means square amplitude (RMSA) and the PDF shape of each HD-sEMG signal was assessed through skewness, excess Kurtosis, and robust functional statistics. These shape distance metrics evaluate the departure from Gaussianity related to muscle aging. a) We conducted a comparison study of the HD-sEMG PDF shapes between younger and elderly individuals. b) Evaluating differences between men and women. c) Considering monopolar and Laplacian electrode configurations that are sensitive to different muscle regions. ResultsA) The HD-sEMG PDFs of elderly subjects demonstrated a lower departure from Gaussianity than their younger counterparts. B) Women exhibited lower RMSA values than men, and, on average, a lower departure from Gaussianity whatever the age and contraction level C) Trends of departure from Gaussianity with contraction level, seems to be influenced by the electrode configuration. In fact, a decrease in Gaussianity departure is observed with monopolar recordings where an increase is observed with Laplacian one, clearly indicating different muscle region assessment. DiscussionThe findings highlight the influence of factors such aging, sex, contraction level and electrode montage on the shape of the HD-sEMG PDF, emphasizing the significance of using this descriptor for monitoring and better assessment of muscle aging.

Acute neuromuscular, cardiovascular, and muscle oxygenation responses to low-intensity aerobic interval exercises with blood flow restriction.

We investigated the influence of short- and long-interval cycling exercise with blood flow restriction (BFR) on neuromuscular fatigue, shear stress and muscle oxygenation, potent stimuli to BFR-training adaptations. During separate sessions, eight individuals performed short- (24×60s/30s; SI) or long-interval (12×120s/60s; LI) trials on a cycle ergometer, matched for total work. One leg exercised with (BFR-leg) and the other without (CTRL-leg) BFR. Quadriceps fatigue was quantified using pre- to post-interval changes in maximal voluntary contraction (MVC), potentiated twitch force (QT) and voluntary activation (VA). Shear rate was measured by Doppler ultrasound at cuff release post-intervals. Vastus lateralis tissue oxygenation was measured by near-infrared spectroscopy during exercise. Following the initial interval, significant (P<0.05) declines in MVC and QT were found in both SI and LI, which were more pronounced in the BFR-leg, and accounted for approximately two-thirds of the total reduction at exercise termination. In the BFR-leg, reductions in MVC (-28±15%), QT (-42±17%), and VA (-15±17%) were maximal at exercise termination and persisted up to 8min post-exercise. Exercise-induced muscle deoxygenation was greater (P<0.001) in the BFR-leg than CTRL-leg and perceived pain was more in LI than SI (P<0.014). Cuff release triggered a significant (P<0.001) shear rate increase which was consistent across trials. Exercise-induced neuromuscular fatigue in the BFR-leg exceeded that in the CTRL-leg and was predominantly of peripheral origin. BFR also resulted in diminished muscle oxygenation and elevated shear stress. Finally, short-interval trials resulted in comparable neuromuscular and haemodynamic responses with reduced perceived pain compared to long-intervals.

Sex-related differences in corticospinal excitability outcomemeasures of the biceps brachii during a submaximal elbow flexor contraction.

The purpose of this study was to investigate the effects of sex, muscle thickness, and subcutaneous fat thickness (SFT) on corticospinal excitability outcome measures of the biceps brachii. Eighteen participants (10 males and 8 females) completed this study. Ultrasound was used to assess biceps brachii muscle thickness and the overlying SFT. Transcranial magnetic stimulation (TMS) was used to determine corticospinal excitability by inducing motor-evoked potentials (MEPs) at eight different TMS intensities from 90% to 160% of active motor threshold (AMT) from the biceps brachii during an isometric contraction of the elbow flexors at 10% of maximum voluntary contraction (MVC). Biceps brachii maximal compound muscle action potential (Mmax) was also recorded prior to and after TMS. Males had higher (p < 0.001) biceps brachii muscle thickness and lower SFT, produced higher levels of MVC force and had, on average, higher (p < 0.001) MEP amplitudes at lower (p < 0.05) percentages of maximal stimulator output than females during the 10% elbow flexion MVC. Multiple linear regression modeling revealed that sex was not associated with any of the neurophysiological parameters examined, while SFT showed a positive association with the stimulation intensity required at AMT (p = 0.035) and a negative association with biceps brachii pre-stimulus electromyography (EMG) activity (p = 0.021). Additionally, there was a small positive association between muscle thickness and biceps brachii pre-stimulus EMG activity (p = 0.049). Overall, this study suggests that some measures of corticospinal excitability may be different between the sexes and influenced by SFT and muscle thickness.

Vitamin B12 status and skeletal muscle function among elderly: A literature review and pilot study on the effect of oral vitamin B12 supplementation in improving muscle function.

The objective of this study is to understand the role of vitamin B12 supplementation in improving skeletal muscle function among the elderly. A literature review in the Medline database was conducted to understand the association between vitamin B12 and muscle function in Section A. In Section B, 28 healthy elderly participants aged ≥60 years were recruited in a cross-sectional design for estimation of plasma vitamin B12 status and assessment of upper limb muscle strength Maximal voluntary contraction (MVC) and muscle quality (expressed as MVC/total muscle mass). Participants were grouped based on vitamin B12 status into vitamin B12-depleted (<148 pmol/L) and replete (≥148 pmol/L) groups. In a quasi-experimental study design, the vitamin B12-depleted group (n = 14) received daily oral vitamin B12 supplementation of 100 μg for 3 months. All the study measures were repeated post-supplementation. Vitamin B12 deficiency was identified to contribute adversely to muscle strength, quality, and physical performance among older people in the extensive literature review. The pilot intervention study showed significant improvement in MVC and muscle quality (p < 0.050) post-vitamin B12 supplementation, comparable to the vitamin B12-replete group. Vitamin B12 may have a crucial role in the maintenance of muscle function. 3-month oral vitamin B12 supplementation among subclinical vitamin B12 deficient elderly improved muscle strength and quality and reached levels similar to the vitamin B12 replete group.

Reliability of cervicocephalic sense of force

Cervicocephalic proprioception (CCP) is an important assessment item for people with a range of clinical conditions, where reduced CCP is associated with neck pain and imbalance. Reliability has been established for a range of positional and movements tests, but there is limited data regarding sense of force, particularly across three planes of movement. The current test–retest study assessed reliability when evaluating sense of force in healthy adults (8 males, 6 females, mean age 31.50 years [SD 10.14]) over two sessions, 4–7 days apart. A force matching protocol was used to evaluate reliability of absolute error (AE), constant error (CE), and variable error (VE) for 10 % and 25 % maximal voluntary contraction (MVC) target forces for flexion, extension, lateral flexion, and rotation. Participants were strapped to a chair to limit trunk movement and data was captured using a compressive force transducer fixed to an adjustable wall mount. Six trials were performed for each contraction-type, totaling 72 submaximal MVCs per session. ICC estimates for AE (0.15–0.77), CE (0.01–0.85), and VE (0.00–0.83) were varied and confidence intervals were mostly wide. Considering lower limits of confidence intervals, CE had best reliability values generally, but more specifically the most reliable contraction type and movement was 25 % MVC flexion (ICC 0.85, confidence interval 0.54–0.95). This study found that reliability for sense of force testing was dependent upon contraction, type of error, and target force utilized. Further reliability analysis should be performed when applying this test to measure validity outcomes in clinical populations.

Cell-free DNA kinetics in response to muscle-damaging exercise: A drop jump study.

A significant increase in circulating cell-free DNA (cfDNA) occurs with physical exercise, which depends on the type of exertion and the duration. The aims of this study were as follows: (1) to investigate the time course of cfDNA and conventional markers of muscle damage from immediately after to 96h after muscle-damaging exercise; and (2) to investigate the relationship between cfDNA and indicators of primary (low-frequency fatigue and maximal voluntary isometric contraction) and secondary (creatine kinase and delayed-onset muscle soreness) muscle damage in young healthy males. Fourteen participants (age, 22±2years; weight, 84.4±11.2kg; height, 184.0±7.4cm) performed 50 intermittent drop jumps at 20s intervals. We measured cfDNA and creatine kinase concentrations, maximal voluntary isometric contraction torque, low-frequency fatigue and delayed-onset muscle soreness before and at several time points up to 96h after exercise. Plasma cfDNA levels increased from immediately postexercise until 72h postexercise (P<0.01). Elevation of postexercise cfDNA was correlated with both more pronounced low-frequency fatigue (r=-0.52, P=3.4×10-11) and delayed-onset muscle soreness (r=0.32, P= 0.00019). Levels of cfDNA change in response to severe primary and secondary muscle damage after exercise. Levels of cfDNA exhibit a stronger correlation with variables related to primary muscle damage than to secondary muscle damage, suggesting that cfDNA is a more sensitive marker of acute loss of muscle function than of secondary inflammation or damaged muscle fibres.

Palmitoylethanolamide Does Not Affect Recovery from Exercise-Induced Muscle Damage in Healthy Males.

Strenuous eccentric exercise (EE) induces microstructural muscle damage, which decreases muscle performance. Palmitoylethanolamide (PEA) exerts analgesic and anti-inflammatory effects in clinical pain conditions and preclinical models of experimentally induced-inflammation. This might hold clues for improved recovery from EE. Therefore, the current study evaluates the effect of PEA supplementation on functional and molecular responses to a single EE bout. Eleven healthy male participants were included in a double-blind crossover study in which they received PEA (350 mg Levagen+) or placebo (maltodextrin) supplements, in a randomized order. In each experimental condition participants performed an acute bout of EE (24x10 eccentric contractions of the knee extensors on an isokinetic dynamometer). At baseline, 24 (D1), 48 (D2), 72 (D3) and 120 h (D5) following EE, maximal voluntary contraction and jump height were measured. Blood samples were collected at baseline and on D1-D5, and muscle biopsies were collected at baseline and on D2. Perceived muscle soreness, sleep quality and food intake were recorded daily. Muscle strength and jump height decreased following EE (up to ~40 and ~ 17% respectively; Ptime < 0.05) in both conditions. This drop was accompanied by an increase in plasma creatine kinase and perceived muscle soreness (Ptime < 0.05). Furthermore, EE, but not PEA, increased the expression of the myogenic marker Pax7 and of the catabolic markers p-FoxO1-3a, p62 and LC3BII/I (Ptime < 0.05). PEA supplementation does not improve muscle soreness, muscle strength and jump performance following a single EE bout. Additionally, PEA supplementation had no effect on local or systemic markers of muscle damage, catabolism or regeneration.

Development and External Validation of a Motor Intention–Integrated Prediction Model for Upper Extremity Motor Recovery After Intention-Driven Robotic Hand Training for Chronic Stroke

ObjectiveTo derive and validate a prediction model for minimal clinically important differences (MCIDs) in upper extremity (UE) motor function after intention-driven robotic hand training using residual voluntary electromyography (EMG) signals from affected UE. DesignA prospective longitudinal multicenter cohort study. We collected preintervention candidate predictors: demographics, clinical characteristics, Fugl-Meyer assessment of UE (FMAUE), Action Research Arm Test scores, and motor intention of flexor digitorum and extensor digitorum (ED) measured by EMG during maximal voluntary contraction (MVC). For EMG measures, recognizing challenges for stroke survivors to move paralyzed hand, peak signals were extracted from 8 time windows during MVC-EMG (0.1-5s) to identify subjects’ motor intention. Classification and regression tree algorithm was employed to predict survivors with MCID of FMAUE. Relationship between predictors and motor improvements was further investigated. SettingNine rehabilitation centers. ParticipantsChronic stroke survivors (N=131), including 87 for derivation sample, and 44 for validation sample. InterventionsAll participants underwent 20-session robotic hand training (40min/session, 3-5sessions/wk). Main Outcome MeasuresPrediction efficacies of models were assessed by area under the receiver operating characteristic curve (AUC). The best effective model was final model and validated using AUC and overall accuracy. ResultsThe best model comprised FMAUE (cutoff score, 46) and peak activity of ED from 1-second MVC-EMG (MVC-EMG 4.604 times higher than resting EMG), which demonstrated significantly higher prediction accuracy (AUC, 0.807) than other time windows or solely using clinical scores (AUC, 0.595). In external validation, this model displayed robust prediction (AUC, 0.916). Significant quadratic relationship was observed between ED-EMG and FMAUE increases. ConclusionsThis study presents a prediction model for intention-driven robotic hand training in chronic stroke survivors. It highlights significance of capturing motor intention through 1-second EMG window as a predictor for MCID improvement in UE motor function after 20-session robotic training. Survivors in 2 conditions showed high percentage of clinical motor improvement: moderate-to-high motor intention and low-to-moderate function; as well as high intention and high function.

Examining the changes in strength and mechanical property of dynamic stabilizers of the medial elbow joint through repetitive pitching

The flexor-pronator muscles (FPM) and their common tendon (CT) are essential in protecting the medial ulnar collateral ligament against elbow valgus stress during pitching. This study aimed to investigate the effect of repetitive pitching on FPM strength and CT stiffness. Fifteen healthy males (mean age: 21.8 ± 1.3-years-old) with over 5 years of baseball experience performed a series of 100 full-effort fastball pitches. We measured grip and isolated digital flexion strength of the second, third, and fourth digits before and after the pitching task. The decline in muscle strength was determined using the rate of change in muscle strength after pitching relative to that before. CT stiffness was measured using a hand-held myotonometer device at rest and during grip motion at 50% maximum voluntary contraction. The increase in CT stiffness during grip motion relative to rest was calculated as the augmentation rate of CT stiffness. Statistical analyses were performed to compare the changes in grip strength, digital flexion strength, and CT stiffness due to pitching. Additionally, the reduction rate of muscle strength was compared among various strength variables. Correlation coefficients were used to evaluate the relationships between the augmentation rate of CT stiffness after pitching and the reduction rate in any muscle strength. Grip and isolated digital flexion strengths decreased significantly after pitching (P < 0.01). The decline in muscle strength was significantly higher for all isolated digital strengths than that for grip strength (P < 0.05). CT stiffness was augmented with grip motion compared to that at rest pre- and post-pitching (P < 0.001). However, no change in CT stiffness due to pitching was observed, regardless of the grip motion (P > 0.05). Additionally, a lower augmentation rate of CT stiffness after pitching was moderately associated with the greater reduction rate of the second digital flexion strength (r = 0.607, P = 0.016) without other relationships. This study found reduced grip and digital flexion strength after pitching; with no change in CT stiffness. However, given the consequences of correlation analyses, individuals with a more prominent reduction in second digital flexion strength due to pitching were impaired in CT stiffness augmentation after pitching. Digital flexion strength represents the strength of the flexor digitorum superficial; therefore, this study suggests that forearm FPM, particularly the second digit of the flexor digitorum superficial, is an important factor for enhancing CT stiffness.

Effects of acute simulated altitude on the maximal lactate steady state in humans.

We sought to determine the effects of acute simulated altitude on the maximal lactate steady state (MLSS) and physiological responses to cycling at and 10 W above the MLSS-associated power output (PO) (MLSSp and MLSSp+10, respectively). Eleven (4 females) participants (means [SD]; 28 [4] yr; V̇o2max: 54.3 [6.9] mL·kg-1·min-1) acclimatized to ∼1,100 m performed 30-min constant PO trials in simulated altitudes of 0 m sea level (SL), 1,111 m mild altitude (MILD), and 2,222 m moderate altitude (MOD). MLSSp, defined as the highest PO with stable (<1 mM change) blood lactate concentration ([BLa]) between 10 and 30 min, was significantly lower in MOD (209 [54] W) compared with SL (230 [56] W; P < 0.001) and MILD (225 [58] W; P = 0.001), but MILD and SL were not different (P = 0.12). V̇o2 and V̇co2 decreased at higher simulated altitudes due to lower POs (P < 0.05), but other end-exercise physiological responses (e.g., [BLa], ventilation [V̇e], heart rate [HR]) were not different between conditions at MLSSp or MLSSp + 10 (P > 0.05). At the same absolute intensity (MLSSp for MILD), [BLa], HR, and V̇E and all perceptual variables were exacerbated in MOD compared with SL and MILD (P < 0.05). Maximum voluntary contraction, voluntary activation, and potentiated twitch forces were exacerbated at MLSSp + 10 relative to MLSSp within conditions (P < 0.05); however, condition did not affect performance fatiguability at the same relative or absolute intensity (P > 0.05). As MLSSp decreased in hypoxia, adjustments in PO are needed to ensure the same relative intensity across altitudes, but common indices of exercise intensity may facilitate exercise prescription and monitoring in hypoxia.NEW & NOTEWORTHY This study demonstrates the power output and metabolic rate associated with the maximal lactate steady-state (MLSS) decline in response to simulated altitude; however, common indices of exercise intensity remained unchanged when cycling was performed at the work rate associated with MLSS at each simulated altitude. These results support previous studies that investigated the effects of hypoxia on alternative measures of the critical intensity of exercise and will inform exercise prescription/monitoring across altitudes.

Characteristics associated with responsiveness to isometric handgrip training in medicated hypertensive patients: secondary data analysis.

Isometric handgrip training (IHT) has been shown to reduce blood pressure (BP) in hypertensive patients. However, factors that predict responsiveness to IHT are largely unknown. The aim of this study was to investigate the patient characteristics associated with the antihypertensive response to IHT using a recommended statistical approach for evaluating interindividual responses. Data from four randomized controlled trials were combined, totaling 81 patients undergoing IHT (48.8% women; 60 ± 11 years) and 90 control patients (45.6% women; 62 ± 12 years). IHT consisted of 4 × 2 min isometric contractions at 30% of maximal voluntary contraction, performed three times/week for 8-12 weeks. BP was measured at baseline and following IHT and control interventions. The interindividual variation was assessed by the standard deviation of the individual responses (SD ir ), and linear regression analyses were conducted to explore response predictors. IHT significantly decreased both SBP (-5.4; 95% confidence interval (CI) -9.5 to -1.3 mmHg) and DBP (-2.8; 95% CI -5.1 to -0.6 mmHg). The interindividual variation of BP change was moderate for systolic (SD ir = 5.2 mmHg, 0.30 standardized units) and low for diastolic (SD ir = 1.7 mmHg, 0.15 standardized units). Sex, age, and BMI were not associated with the antihypertensive effect of IHT. However, a higher baseline SBP ( b = -0.467, P < 0.001) and absence of dihydropyridine calcium channel blockers use ( b = 0.340, P = 0.001) were associated with greater BP reductions. IHT reduced BP in medicated hypertensive patients regardless of age, sex, and BMI. Patients with a higher baseline SBP and those not prescribed dihydropyridine calcium channel blockers were more responsive to IHT.

Influence of stimulation frequency on early and late phase rate of torque and velocity development.

The early (≤50 ms) rate of torque development (RTD) is dependent upon the speed of neuromuscular activation; however, few studies have evaluated the determinants of rate of velocity development (RVD), which may be load-dependent. The purpose here was to explore the relationship between stimulation frequency with the early and late (≥100 ms) phase isometric RTD and isotonic RVD. The knee extensors of 16 (five female) young recreationally active participants were stimulated using 14 frequencies from 1 to 100 Hz during isometric and isotonic ("unloaded" and 7.5% of the isometric maximal voluntary contraction [MVC]) contractions. Isometric RTD and isotonic RVD were evaluated for the early (0-50 ms) and late (0-100 ms) phases from torque and velocity onset, respectively. Sigmoid functions were fit and bilinear regressions were used to examine the slopes of the steep portion of the curve and the plateau frequency. RTD- and RVD-frequency relationships were well described by a sigmoid function (all r2 > 0.96). Compared with the late phase, early isometric RTD, and unloaded RVD displayed lower slopes (all P ≤ 0.001) and higher plateau frequencies (all P < 0.001). In contrast, early and late RVD of a moderately loaded isotonic contraction did not display different slopes (P = 0.055) or plateau frequencies (P = 0.690). Early isometric RTD and unloaded isotonic RVD are more dependent on changes in stimulation frequency compared with late phases. However, RVD for a moderately loaded isotonic contraction displayed similar responses for the early and late phases. Therefore, a high frequency of activation is critical for early torque and velocity generation but dependent upon the load for isotonic contractions.NEW & NOTEWORTHY We show that during an "unloaded" isotonic contraction, the early phase rate of velocity development is more dependent upon a high electrical activation frequency compared with the late phase, similar to isometric torque. However, early and late phase rates of velocity development of moderately loaded isotonic contractions display similar responses. These results indicate that the determinants of isotonic shortening function are dependent on the externally applied load, highlighting the importance of task-specificity of contraction.