Peripheral fatigue: new mechanistic insights from recent technologies

Effects of Muscular Fatigue on the Performance of Handgrip Tasks During Force Generation and Relaxation

This study investigated the effects of muscular fatigue on the accuracy of force control in the respective generation and relaxation phases while performing an isometric handgrip force-tracking task. Participants were instructed to track a target line moving upward and downward, corresponding to 0 to 30% of maximum voluntary contraction (MVC) at a constant for 7 s. Eight sets of 25 continuous trials each were conducted. The force-tracking accuracy and electromyography (EMG) of extensor carpi radialis (ECR) and flexor carpi ulnaris (FCU) were evaluated. The force-tracking accuracy was compared between the phase (upward: generation and downward: relaxation), set (first and eighth), and within-set periods (early: 1-5, middle: 10-15, and late: 20-25 trials). The force-tracking accuracy at the middle and late periods significantly declined compared to the early period in the relaxation phase. Integrated EMG of ECR and FCU was significantly larger in the generation than in the relaxation phase. The integrated EMG of FCU in the generation phase was significantly different between the periods. Furthermore, the median frequency of FCU was significantly different between the phases and periods. These findings suggest that the effects of muscular fatigue on force control varied between generation and relaxation due to the amount and frequency band of muscle activity.

Verification of fingertip blood lactate measurement as an index of muscle fatigue recovery

[Purpose] To evaluate the validity of measurement sites for assessing blood lactic acid levels following unilateral upper-limb exercises. [Participants and Methods] Blood lactic acid levels were measured at the fingertips of both hands in 40 healthy young men (mean age 20.9 ± 2.5 years). Measurements were taken before, immediately after, and at 90-s intervals following unilateral upper-limb exercises involving palmar flexion and dorsiflexion of the non-dominant hand. Exercise load was determined through maximum voluntary contraction testing. [Results] No significant differences were found in the average blood lactic acid levels between the fingers at any measurement time point. The peak blood lactic acid level occurred approximately 90 s earlier in the motor limb than in the non-motor limb. [Conclusion] This study found no significant difference in blood lactic acid levels between the motor and non-motor limbs when using fingertip measurements during unilateral upper-limb exercises. Therefore, either fingertip may be a suitable measurement site. However, because the peak lactic acid level in the non-motor limb was delayed by around 90 s, the timing of peak muscle fatigue evaluation in the non-motor limb should be considered.

Stimulation-Induced Muscle Deformation Measured with A-Mode Ultrasound Correlates with Muscle Fatigue

Muscle fatigue is a common physiological phenomenon whose onset can impair physical performance and increase the risk of injury. Traditional assessments of muscle fatigue are primarily constrained by their dependence on maximum voluntary contractions (MVCs), which not only rely heavily on participant motivation, reducing measurement accuracy, but also require large, stationary equipment such as isokinetic dynamometers, limiting their application to discrete assessments in lab-based environments. In this work, we introduce a wearable muscle fatigue tracking strategy that employs low-profile single-element ultrasound and electrical stimulation. This integrated approach demonstrates that muscle deformation from electrically-induced muscle contractions correlates with muscle fatigue, thus circumventing the need for bulky hardware and eliminating the variability associated with human volition. We define a deformation index, which fuses stimulation-induced changes in muscle thickness with baseline muscle swelling to track muscle fatigue. Our results demonstrate that the deformation index reliably tracks muscle fatigue (r = 0.85 ± 0.15), under specific conditions, namely extended joint angles and increased stimulation, as measured by changes in knee extension torque during a series of dynamic, volitional fatiguing contractions on 8 subjects on an isokinetic dynamometer. This approach has the potential to enable real-time, semi-continuous muscle fatigue monitoring in unconstrained environments.

Fatigue Alleviation by Low-Level Laser Preexposure in Ischemic Neuromuscular Electrical Stimulation

PURPOSE

Despite its susceptibility to muscle fatigue, combined neuromuscular electrical stimulation (NMES) and blood flow restriction (BFR) are effective regimens for managing muscle atrophy when traditional resistance exercises are not feasible. This study investigated the potential of low-level laser therapy (LLLT) in reducing muscle fatigue after the application of combined NMES and BFR.

METHODS

Thirty-six healthy adults were divided into control and LLLT groups. The LLLT group received 60 J of 850-nm wavelength LLLT before a training program of combined NMES and BFR of the nondominant extensor carpi radialis longus (ECRL). The control group followed the same protocol but received sham laser therapy. Assessments included maximal voluntary contraction, ECRL mechanical properties, and isometric force tracking for wrist extension.

RESULTS

The LLLT group exhibited a smaller normalized difference in maximal voluntary contraction decrement (-4.01 ± 4.88%) than the control group (-23.85 ± 7.12%) ( P < 0.001). The LLLT group demonstrated a smaller decrease in muscle stiffness of the ECRL compared with the control group, characterized by the smaller normalized changes in frequency ( P = 0.002), stiffness ( P = 0.002), and relaxation measures ( P = 0.011) of mechanical oscillation waves. Unlike the control group, the LLLT group exhibited a smaller posttest increase in force fluctuations during force tracking ( P = 0.014), linked to the predominant recruitment of low-threshold MU ( P < 0.001) without fatigue-related increases in the discharge variability of high-threshold MU ( P > 0.05).

CONCLUSIONS

LLLT preexposure reduces fatigue after combined NMES and BFR, preserving force generation, muscle stiffness, and force scaling. The functional benefits are achieved through fatigue-resistant activation strategies of motor unit recruitment and rate coding.

Is the disappearance of the cervical flexion-relaxation phenomenon associated with cervical degeneration in healthy people?

PURPOSE

This study aims to explore the differences in cervical degeneration between healthy people with and without cervical flexion-relaxation phenomenon (FRP) and to identify whether the disappearance of cervical FRP is related to cervical degeneration.

METHODS

According to the flexion relaxation ratio (FRR), healthy subjects were divided into the normal FRP group and the abnormal FRP group. Besides, MRI was used to evaluate the degeneration of the passive subsystem (vertebral body, intervertebral disc, cervical sagittal balance, etc.) and the active subsystem (deep flexors [DEs], deep extensors [DFs], and superficial extensors [SEs]). In addition, the correlation of the FRR with the cervical degeneration score, C2-7Cobb, Borden method, relative total cross-sectional area (rTCSA), relative functional cross-sectional area (rFCSA), and fatty infiltration ratio (FIR) was analyzed.

RESULTS

A total of 128 healthy subjects were divided into the normal FRP group (n=52, 40.63%) and the abnormal FRP group (n=76, 59.38%). There were significant differences between the normal FRP group and the abnormal FRP group in the cervical degeneration score (z=-6.819, P<0.001), C2-7Cobb (t=2.994, P=0.004), Borden method (t=2.811, P=0.006), and FIR of DEs (t=-4.322, P<0.001). The FRR was significantly correlated with the cervical degeneration score (r=-0.457, P<0.001), C2-7Cobb (r=0.228, P=0.010), Borden method (r=0.197, P=0.026), and FIR of DEs (r=-0.253, P=0.004).

CONCLUSION

The disappearance of cervical FRP is related to cervical degeneration. A new hypothesis mechanism for FRP is proposed. The cervical FRP test is an effective and noninvasive examination for the differential diagnosis of healthy people, people with potential NSNP, and patients with NSNP.

Shoulder Tensiomyography and Isometric Strength in Swimmers Before and After a Fatiguing Protocol

CONTEXT

Shoulder muscles are active during front-crawl swimming to provide propulsion and stabilize the glenohumeral and scapulothoracic joints. Researchers have proposed that fatigue might contribute to altered activation of these muscles and represent a risk factor for injuries. Tensiomyography (TMG) might function as a noninvasive tool to detect changes in contractile measures of the skeletal muscles due to exercise-induced neuromuscular fatigue, though it has not yet been used in the shoulder muscles of swimmers.

OBJECTIVE

To assess the effects of a fatiguing swimming protocol on shoulder muscle TMG measures and isometric strength in competitive swimmers.

DESIGN

Cross-sectional study.

SETTING

Swimming pool facility.

PATIENTS OR OTHER PARTICIPANTS

A total of 14 young front-crawl competitive swimmers (11 males and 3 females; age = 21 ± 3 years [range, 17-26 years], height = 1.78 ± 0.06 m, mass = 73.1 ± 9.2 kg).

MAIN OUTCOME MEASURE(S)

Participants completed TMG and isometric strength assessments before and after 30-minute, high-intensity swim training. The TMG assessment was performed on 7 muscles of the shoulder according to front-crawl biomechanics and the applicability of the technique to obtain data, such as time to contraction and muscle-belly radial displacement. Isometric strength was assessed using a digital handheld dynamometer during shoulder flexion, extension, external rotation, and internal rotation.

RESULTS

Fatigue induced a smaller radial displacement, mostly observable in latissimus dorsi (-1.0 mm; 95% CI = -1.7, -0.3 mm; P = .007) and pectoralis major muscles (-1.4 mm; 95% CI = -2.4, -0.4 mm; P = .007). Only shoulder extension showed an isometric strength reduction after the fatiguing protocol (-0.03 N/kg; 95% CI = -0.05, -0.01 N/kg; F1,13 = 4.936; P = .045; ηp2 = 0.275).

CONCLUSIONS

This study provides preliminary evidence for the usefulness of TMG to detect fatigue-induced changes in contractile properties of the shoulder muscles in swimmers, in particular the latissimus dorsi and pectoralis major.

Identification of Peripheral Fatigue through Exercise-Induced Changes in Muscle Contractility

The aim of this study was to assess whether tensiomyography is a tool sensitive enough to detect peripheral fatigue. Twenty-six strength-trained men were split into two groups: 1) a fatigued group (FG), who performed a full-squat (SQ) standardized warm-up plus 3 x 8 SQs with 75% 1RM with a 5-min rest interval, and 2) a non-fatigued group (NFG), who only did the SQ standardized warm-up. The countermovement jump (CMJ), maximal isometric force (MIF) in the SQ at 90º knee flexion, and TMG in vastus medialis (VM) and vastus lateralis (VL) muscles were assessed pre- and post-protocols. Data were analyzed through mixed ANOVA, logistic regression analysis, and receiver-operating curves. There were significant group x time interactions (p < 0.01) for CMJ height, MIF, maximal radial displacement (Dm), and radial displacement velocity (Vrd90) since the FG acutely decreased in these variables, while no significant changes were observed for the NFG. The logistic regression showed a significant model for detecting fatigue, whether it used the CMJ or MIF, with only the relative change in VL-Vrd90 as a fatigue predictor. The determination of the area under the curve showed that Dm and Vrd90 had good to excellent discriminative ability. Dm and Vrd90 are sensitive to detect fatigue in VL and VM muscles in resistance training contexts.

Exploration and Application of a Muscle Fatigue Assessment Model Based on NMF for Multi-Muscle Synergistic Movements

Muscle fatigue significantly impacts coordination, stability, and speed in daily activities. Accurate assessment of muscle fatigue is vital for effective exercise programs, injury prevention, and sports performance enhancement. Current methods mostly focus on individual muscles and strength evaluation, overlooking overall fatigue in multi-muscle movements. This study introduces a comprehensive muscle fatigue model using non-negative matrix factorization (NMF) weighting. NMF is employed to analyze the duration multi-muscle weight coefficient matrix (DMWCM) during synergistic movements, and four electromyographic (EMG) signal features in time, frequency, and complexity domains are selected. Particle Swarm Optimization (PSO) optimizes feature weights. The DMWCM and weighted features combine to calculate the Comprehensive Muscle Fatigue Index (CMFI) for multi-muscle synergistic movements. Experimental results show that CMFI correlates with perceived exertion (RPE) and Speed Dynamic Score (SDS), confirming its accuracy and real-time tracking in assessing multi-muscle synergistic movements. This model offers a more comprehensive approach to muscle fatigue assessment, with potential benefits for exercise training, injury prevention, and sports medicine.

Effects of exhaustive whole-body exercise and caffeine ingestion on muscle contractile properties in healthy men

BACKGROUND

The influence of exhaustive whole-body exercise and caffeine ingestion on electromechanical delay (EMD) has been underexplored. This study investigated the effect of exhaustive cycling exercise on EMD and other parameters of muscle contractile properties and the potential ability of caffeine to attenuate the exercise-induced impairments in EMD and muscle contractile properties.

METHODS

Ten healthy men cycled until exhaustion (88±2% of V̇O2max) on two separate days after ingesting caffeine (5 mg.kg-1 of body mass) or cellulose (placebo). Parameters of muscle contractile properties of the quadriceps muscles were assessed via volitional and electrically evoked isometric contractions, performed before and 50 minutes after ingestion of the capsules, and after exercise. Muscle recruitment during volitional contractions was determined via surface electromyography.

RESULTS

Exhaustive cycling exercise did not affect volitional and relaxation EMD (P>0.05) but increased evoked EMD. In addition, the exhaustive cycling exercise also increased muscle recruitment at the beginning of volitional isometric muscle contraction (P<0.05). The peak twitch force, maximal rate of twitch force development, and twitch contraction time were all compromised after exhaustive cycling exercise (P<0.05). Acute caffeine ingestion had no effect on muscle contractile properties (P>0.05), except that caffeine increased twitch contraction time at postexercise (P<0.05).

CONCLUSIONS

Exercise-induced decline in peripheral components of the EMD might be compensated by an increase in the muscle recruitment. In addition, acute caffeine ingestion had minimal influence on exercise-induced changes in muscle contractile proprieties.

Fatigue task-dependent effect on spatial distribution of lumbar muscles activity

This study aims to identify how spatial distribution of lumbar muscle activity is modulated by different fatigue tasks. Twenty healthy adults performed two different isometric trunk extension endurance tasks (the modified Sorensen test and the inverted modified Sorensen test) until exhaustion. During these tasks, bilateral superficial lumbar muscle activity was recorded using high-density electromyography. The spatial distribution of activation within these muscles was obtained using the centroid coordinates in the medio-lateral and cranio-caudal directions. The effects of task and endurance time (left and right sides) were investigated using repeated measures ANOVA. Results revealed a significant lateral shift of the centroid throughout the fatigue tasks on both sides and no difference between tasks. Significant task × time interaction effects were found for the cranio-caudal direction on both sides showing a significantly more caudal location of the centroid in the modified Sorensen test compared to the inverted test at the beginning of the tasks. Our findings suggest that spatial distribution of lumbar muscle activity is task-dependent in a pre-fatigue stage while an alternative but similar muscle recruitment strategy is used in both tasks to maintain performance in the later stages of muscle fatigue.

Quantifying muscle contraction with a conductive electroactive polymer sensor: introduction to a novel surface mechanomyography device

Clinicians seek an accurate method to assess muscle contractility during activities to better guide treatment. We investigated application of a conductive electroactive polymer sensor as a novel wearable surface mechanomyography (sMMG) sensor for quantifying muscle contractility. The radial displacement of a muscle during a contraction is detected by the physically stretched dielectric elastomer component of the sMMG sensor which quantifies the changes in capacitance. The duration of muscle activation times for quadriceps, hamstrings, and gastrocnemius muscles demonstrated strong correlation between sMMG and EMG during a parallel squat activity and isometric contractions. A moderate to strong correlation was demonstrated between the sMMG isometric muscle activation times and force output times from a dynamometer. The potential wearable application of an electroactive polymer sensor to measure muscle contraction time is supported.

Effects of muscle fatigue on exercise-induced hamstring muscle damage: a three-armed randomized controlled trial

PURPOSE

Hamstring injuries in soccer reportedly increase towards the end of the matches' halves as well as with increased match frequency in combination with short rest periods, possibly due to acute or residual fatigue. Therefore, this study aimed to investigate the effects of acute and residual muscle fatigue on exercise-induced hamstring muscle damage.

METHODS

A three-armed randomized-controlled trial, including 24 resistance-trained males, was performed allocating subjects to either a training group with acute muscle fatigue + eccentric exercise (AF/ECC); residual muscle fatigue + eccentric exercise (RF/ECC) or a control group with only eccentric exercise (ECC). Muscle stiffness, thickness, contractility, peak torque, range of motion, pain perception, and creatine kinase were assessed as muscle damage markers pre, post, 1 h post, and on the consecutive three days.

RESULTS

Significant group × time interactions were revealed for muscle thickness (p = 0.02) and muscle contractility parameters radial displacement (Dm) and contraction velocity (Vc) (both p = 0.01), with larger changes in the ECC group (partial η2 = 0.4). Peak torque dropped by an average of 22% in all groups; stiffness only changed in the RF/ECC group (p = 0.04). Muscle work during the damage protocol was lower for AF/ECC than for ECC and RF/ECC (p = 0.005).

CONCLUSION

Hamstring muscle damage was comparable between the three groups. However, the AF/ECC group resulted in the same amount of muscle damage while accumulating significantly less muscle work during the protocol of the damage exercise.

TRIAL REGISTRATION

This study was preregistered in the international trial registration platform (WHO; registration number: DRKS00025243).

Muscular and Kinematic Responses to Unexpected Translational Balance Perturbation: A Pilot Study in Healthy Young Adults

Falls and fall-related injuries are significant public health problems in older adults. While balance-controlling strategies have been extensively researched, there is still a lack of understanding regarding how fast the lower-limb muscles contract and coordinate in response to a sudden loss of standing balance. Therefore, this pilot study aims to investigate the speed and timing patterns of multiple joint/muscles' activities among the different challenges in standing balance. Twelve healthy young subjects were recruited, and they received unexpected translational balance perturbations with randomized intensities and directions. Electromyographical (EMG) and mechanomyographical (MMG) signals of eight dominant-leg's muscles, dominant-leg's three-dimensional (3D) hip/knee/ankle joint angles, and 3D postural sways were concurrently collected. Two-way ANOVAs were used to examine the difference in timing and speed of the collected signals among muscles/joint motions and among perturbation intensities. This study has found that (1) agonist muscles resisting the induced postural sway tended to activate more rapidly than the antagonist muscles, and ankle muscles contributed the most with the fastest rate of response; (2) voluntary corrective lower-limb joint motions and postural sways could occur as early as the perturbation-induced passive ones; (3) muscles reacted more rapidly under a larger perturbation intensity, while the joint motions or postural sways did not. These findings expand the current knowledge on standing-balance-controlling mechanisms and may potentially provide more insights for developing future fall-prevention strategies in daily life.

Assessing Quadriceps Muscle Contraction Using a Novel Surface Mechanomyography Sensor during Two Neuromuscular Control Screening Tasks

Electromyography (EMG) is the clinical standard for capturing muscle activation data to gain insight into neuromuscular control, yet challenges surrounding data analysis limit its use during dynamic tasks. Surface mechanomyography (sMMG) sensors are novel wearable devices that measure the physical output of muscle excursion during contraction, which may offer potential easy application to assess neuromuscular control. This study aimed to investigate sMMG detection of the timing patterns of muscle contraction compared to EMG. Fifteen healthy participants (mean age = 31.7 ± 9.1 y; eight males and seven females) were donned with EMG and sMMG sensors on their right quadriceps for simultaneous data capture during bilateral deep squats, and a subset performed three sets of repeated unilateral partial squats. No significant difference in the total duration of contraction was detected by EMG and sMMG during bilateral (p = 0.822) and partial (p = 0.246) squats. sMMG and EMG timing did not differ significantly for eccentric (p = 0.414) and concentric (p = 0.462) phases of muscle contraction during bilateral squats. The sMMG magnitude of quadriceps excursion demonstrated excellent intra-session retest reliability for bilateral (ICC_3,1 = 0.962 mm) and partial (ICC_3,1 = 0.936 mm, n = 10) squats. The sMMG sensors accurately and consistently provided key quadriceps muscle performance metrics during two physical activities commonly used to assess neuromuscular control for injury prevention, rehabilitation, and exercise training.

Diagnostic accuracy of Tensiomyography parameters for monitoring peripheral neuromuscular fatigue

The diagnostic accuracy of tensiomyography (TMG) parameters compared to the gold standard in neuromuscular fatigue evaluation using voluntary and electrically induced muscle activation is unclear. This study aimed to investigate the diagnostic accuracy of TMG parameters to detect individual changes after interventions that were designed to induce central or peripheral fatigue. Nineteen males (age: 32.2 ± 9.3 years) performed two interventions, consisting of maintaining 25% of maximal voluntary contraction (MViC_25%) and a 30 s all-out cycling test (Wingate), respectively. TMG parameters, maximum voluntary contraction (PtMViC), voluntary activation (VA%) and electrically elicited double twitches (Dtw) were assessed on the knee extensors before (PRE), one minute (POST) and seven minutes after (POST7) the intervention. The diagnostic accuracy (AUC) of TMG parameters were evaluated in comparison to two criteria measures (PtMViC and Dtw). RM ANOVA revealed a significant interaction between the effects of intervention and time on VA% (p = 0.001) and Dtw (p < 0.001) but not for PtMViC (p = 0.420). AUC showed that TMG parameters had a good ability in detecting muscular fatigue assessed by Dtw but not by PtMViC. The results of the current study suggest that TMG parameters can be used to monitor peripheral neuromuscular fatigue.

Is the Interpolated-Twitch Technique-Derived Voluntary Activation Just Neural? Novel Perspectives from Mechanomyographic Data

PURPOSE

Voluntary activation (VA) determined by interpolation-twitch technique could be affected by the characteristics of the in-series elastic components. To overcome this possible bias, a novel approach based on the mechanomyographic (MMG) signal to detect voluntary activation (VA MMG ) has been proposed. We examined the changes in VA and VA MMG after passive stretching to check the influence of neural and mechanical factors in the force output.

METHODS

Twenty-six healthy men underwent VA assessment using the interpolated-twitch technique before and after unilateral passive stretching of the plantarflexors (five 45-s on + 15-s off). In addition to the force signal, the MMG signal was detected on gastrocnemius medialis, gastrocnemius lateralis, and soleus. From the force and MMG signal analysis, VA and VA MMG were calculated in the stretched and contralateral nonstretched limbs. Joint passive stiffness was also defined.

RESULTS

In the stretched limb, passive stretching increased dorsiflexion range (mean ± SD = +18% ± 10%, P < 0.001, ES = 1.54) but reduced joint passive stiffness (-22% ± 8%, P < 0.001, ES = -1.75), maximum voluntary contraction (-15% ± 7%, P < 0.001, ES = -0.87), VA (-7% ± 3%, P < 0.001, ES = -2.32), and VA MMG (~-5% ± 2%, P < 0.001, ES = -1.26/-1.14). In the contralateral nonstretched limb, passive stretching increased dorsiflexion range (+10% ± 6%, P < 0.001, ES = 0.80) but reduced joint passive stiffness (-3% ± 2%, P = 0.041, ES = -0.27), maximum voluntary contraction (-4% ± 3%, P = 0.035, ES = -0.24), VA (-4% ± 2%, P < 0.001, ES = -1.77), and VA MMG (~- 2% ± 1%, P < 0.05, ES = -0.54/-0.46). The stretch-induced changes in VA correlated with VA MMG ( R ranging from 0.447 to 0.583 considering all muscles) and with joint passive stiffness (stretched limb: R = 0.503; contralateral nonstretched limb: R = 0.530).

CONCLUSIONS

VA output is overall influenced by both neural and mechanical factors, not distinguishable using the interpolated-twitch technique. VA MMG is a complementary index to assess the changes in VA not influenced by mechanical factors and to examine synergistic muscles.

Antioxidant, anti-inflammatory and immunomodulatory effects of spirulina in exercise and sport: A systematic review

Arthrospira platensis, also known as spirulina, is currently one of the most well-known algae supplements, mainly due to its high content of bioactive compounds that may promote human health. Some authors have hypothesized that spirulina consumption could protect subjects from exercise-induced oxidative stress, accelerate recovery by reducing muscle damage, and stimulate the immune system. Based on this, the main goal of this review was to critically analyze the effects of spirulina on oxidative stress, immune system, inflammation and performance in athletes and people undergoing exercise interventions. Of the 981 articles found, 428 studies were considered eligible and 13 met the established criteria and were included in this systematic review. Most recently spirulina supplementation has demonstrated ergogenic potential during submaximal exercise, increasing oxygen uptake and improving exercise tolerance. Nevertheless, spirulina supplementation does not seem to enhance physical performance in power athletes. Considering that data supporting benefits to the immune system from spirulina supplementation is still lacking, overall evidence regarding the benefit of spirulina supplementation in healthy people engaged in physical exercise is scarce and not consistent. Currently, spirulina supplementation might be considered in athletes who do not meet the recommended dietary intake of antioxidants. Further high-quality research is needed to evaluate the effects of spirulina consumption on performance, the immune system and recovery in athletes and active people.

Systematic review registration

[https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=262896], identifier [CRD42021262896].

Rehabilitation of motor function after stroke: A bibliometric analysis of global research from 2004 to 2022

BACKGROUND AND AIMS

The mortality rate of stroke has been increasing worldwide. Poststroke somatic dysfunctions are common. Motor function rehabilitation of patients with such somatic dysfunctions enhances the quality of life and has long been the primary practice to achieve functional recovery. In this regard, we aimed to delineate the new trends and frontiers in stroke motor function rehabilitation literature published from 2004 to 2022 using a bibliometric software.

METHODS

All documents related to stroke rehabilitation and published from 2004 to 2022 were retrieved from the Web of Science Core Collection. Publication output, research categories, countries/institutions, authors/cocited authors, journals/cocited journals, cocited references, and keywords were assessed using VOSviewer v.1.6.15.0 and CiteSpace version 5.8. The cocitation map was plotted according to the analysis results to intuitively observe the research hotspots.

RESULTS

Overall, 3,302 articles were retrieved from 78 countries or regions and 564 institutions. Over time, the publication outputs increased annually. In terms of national contribution, the United States published the most papers, followed by China, Japan, South Korea, and Canada. Yeungnam University had the most articles among all institutions, followed by Emory University, Fudan University, and National Taiwan University. Jang Sung Ho and Wolf S.L. were the most productive (56 published articles) and influential (cited 1,121 times) authors, respectively. "Effect of constraint-induced movement therapy on upper extremity function 3-9 months after stroke: the Extremity Constraint Induced Therapy Evaluation randomized clinical trial" was the most frequently cited reference. Analysis of keywords showed that upper limbs, Fugl-Meyer assessment, electromyography, virtual reality, telerehabilitation, exoskeleton, and brain-computer interface were the research development trends and focus areas for this topic.

CONCLUSION

Publications regarding motor function rehabilitation following stroke are likely to continuously increase. Research on virtual reality, telemedicine, electroacupuncture, the brain-computer interface, and rehabilitation robots has attracted increasing attention, with these topics becoming the hotspots of present research and the trends of future research.

How Does Lower Limb Respond to Unexpected Balance Perturbations? New Insights from Synchronized Human Kinetics, Kinematics, Muscle Electromyography (EMG) and Mechanomyography (MMG) Data

Making rapid and proper compensatory postural adjustments is vital to prevent falls and fall-related injuries. This study aimed to investigate how, especially how rapidly, the multiple lower-limb muscles and joints would respond to the unexpected standing balance perturbations. Unexpected waist-pull perturbations with small, medium and large magnitudes were delivered to twelve healthy young adults from the anterior, posterior, medial and lateral directions. Electromyographical (EMG) and mechanomyographical (MMG) responses of eight dominant-leg muscles (i.e., hip abductor/adductors, hip flexor/extensor, knee flexor/extensor, and ankle dorsiflexor/plantarflexors) together with the lower-limb joint angle, moment, and power data were recorded. The onset latencies, time to peak, peak values, and/or rate of change of these signals were analyzed. Statistical analysis revealed that: (1) agonist muscles resisting the delivered perturbation had faster activation than the antagonist muscles; (2) ankle muscles showed the largest rate of activation among eight muscles following both anteroposterior and mediolateral perturbations; (3) lower-limb joint moments that complied with the perturbation had faster increase; and (4) larger perturbation magnitude tended to evoke a faster response in muscle activities, but not necessarily in joint kinetics/kinematics. These findings provided insights regarding the underlying mechanism and lower-limb muscle activities to maintain reactive standing balance in healthy young adults.

Determining voluntary activation in synergistic muscles: a novel mechanomyographic approach

PURPOSE

Drawing on correlations between the mechanomyographic (MMG) and the force signal, we devised a novel approach based on MMG signal analysis to detect voluntary activation (VA) of the synergistic superficial heads of the quadriceps muscle. We hypothesized that, after a fatiguing exercise, the changes in the evoked MMG signal of each quadriceps head would correlate with the changes in the level of VA in the whole quadriceps.

METHODS

Twenty-five men underwent a unilateral single-leg quadriceps exercise to failure. Before and after exercise, VA was assessed by interpolated-twitch-technique via nerve stimulation during and after maximum voluntary contraction (MVC). The force and MMG signal were recorded from vastus lateralis, vastus medialis, and rectus femoris. The MMG peak-to-peak was calculated and the voluntary activation index (VA_MMG), defined as the superimposed/potentiated MMG peak-to-peak ratio, was determined from the MMG signal for each head.

RESULTS

VA_MMG presented a very high intraclass correlation coefficient (0.981-0.998) and sensitivity (MDC_95%: 0.42-6.97%). MVC and VA were decreased after exercise in both the exercising [MVC:-17(5)%, ES -0.92; VA: -7(3)%, ES -1.90] and the contralateral limb [MVC: -9(4)%, ES -0.48; VA: -4(1)%, ES -1.51]. VA_MMG was decreased in both the exercising [~ -9(6)%, ES -1.77] and contralateral limb [~ -3(2)%, ES -0.57], with a greater decrease in VA_MMG noted only in the vastus medialis of the exercising limb. Moderate-to-very high correlations were found between VA_MMG and VA (R-range: 0.503-0.886) before and after exercise.

CONCLUSION

VA_MMG may be implemented to assess VA and provide further information when multiple synergistic muscle heads are involved in fatiguing exercises.

Trade-Off Between Maximal Power Output and Fatigue Resistance of the Knee Extensors for Older Men

This study investigated associations of fatigue resistance determined by an exercise-induced decrease in neuromuscular power with prefatigue neuromuscular strength and power of the knee extensors in 31 older men (65-88 years). A fatigue task consisted of 50 consecutive maximal effort isotonic knee extensions (resistance: 20% of prefatigue isometric maximal voluntary contraction torque) over a 70° range of motion. The average of the peak power values calculated from the 46th to 50th contractions during the fatigue task was normalized to the prefatigue peak power value, which was defined as neuromuscular fatigue resistance. Neuromuscular fatigue resistance was negatively associated with prefatigue maximal power output (r = -.530) but not with prefatigue maximal voluntary contraction torque (r = -.252). This result highlights a trade-off between prefatigue maximal power output and neuromuscular fatigue resistance, implying that an improvement in maximal power output might have a negative impact on neuromuscular fatigue resistance.

Acute Effects of Different Intensities during Bench Press Exercise on the Mechanical Properties of Triceps Brachii Long Head

This study aimed to analyze acute changes in the muscle mechanical properties of the triceps brachii long head after bench press exercise performed at different external loads and with different intensities of effort along with power performance. Ten resistance-trained males (age: 27.7 ± 3.7 yr, body mass: 90.1 ± 17.1 kg, height: 184 ± 4 cm; experience in resistance training: 5.8 ± 2.6 yr, relative one-repetition maximum (1RM) in the bench press: 1.23 ± 0.22 kg/body mass) performed two different testing conditions in a randomized order. During the experimental session, participants performed four successive sets of two repetitions of the bench press exercise at: 50, 70, and 90% 1RM, respectively, followed by a set at 70% 1RM performed until failure, with a 4 min rest interval between each set. Immediately before and after each set, muscle mechanical properties of the dominant limb triceps brachii long head were assessed via a Myoton device. To determine fatigue, peak and average barbell velocity were measured at 70% 1RM and at 70% 1RM until failure (only first and second repetition). In the control condition, only muscle mechanical properties at the same time points after the warm-up were assessed. The intraclass correlation coefficients indicated “poor” to “excellent” reliability for decrement, relaxation time, and creep. Therefore, these variables were excluded from further analysis. Three-way ANOVAs (2 groups × 2 times × 4 loads) indicated a statistically significant group × time interaction for muscle tone (p = 0.008). Post hoc tests revealed a statistically significant increase in muscle tone after 70% 1RM (p = 0.034; ES = 0.32) and 90% 1RM (p = 0.011; ES = 0.56). No significant changes were found for stiffness. The t-tests indicated a significant decrease in peak (p = 0.001; ES = 1.02) and average barbell velocity (p = 0.008; ES = 0.8) during the first two repetitions of a set at 70% 1RM until failure in comparison to the set at 70% 1RM. The results indicate that low-volume, high-load resistance exercise immediately increases muscle tone but not stiffness. Despite no significant changes in the mechanical properties of the muscle being registered simultaneously with a decrease in barbell velocity, there was a trend of increased muscle tone. Therefore, further studies with larger samples are required to verify whether muscle tone could be a sensitive marker to detect acute muscle fatigue.

Effect of sex and fatigue on quiet standing and dynamic balance and lower extremity muscle stiffness

PURPOSE

The purpose of the present study was to determine whether there are sex differences in fatigue-induced changes in quiet standing and dynamic balance and establish whether changes in muscle torque and resting stiffness may explain the potential sex differences in balance responses.

METHODS

Sixteen recreationally active men (age; 24.8 ± 5.0 years, height; 178.2 ± 5.6 cm, mass; 77.8 ± 13.2 kg) and 10 women (age; 21.0 ± 1.6 years, height; 167 ± 5.3 cm, mass; 61.3 ± 8.9 kg) were assessed for postural sway, Y balance test performance, isokinetic and isometric knee extensor torque and resting stiffness of the vastus lateralis (VL), gastrocnemius lateralis (GL) and Achilles tendon (AT) before and immediately after fatiguing exercise. The fatigue protocol consisted of five sets of 20-drop jumps.

RESULTS

The fatiguing exercise elicited similar magnitude (effects size; ES) reductions in muscle torque (men; ES = 0.45-0.80, women; ES = 0.46-0.52), dynamic balance (men; ES = 0.45-0.74, women; ES = 0.47-0.79) and resting VL stiffness (men; ES = 0.46, women; ES = 0.36) in men and women (all p < 0.05). For quiet standing balance, fatigue induced an increase in postural sway metrics (ES = 0.64-1.28) and reduction in resting GL stiffness (ES = 0.40) in men (both p < 0.001) but not women (p > 0.05).

CONCLUSION

Fatiguing exercise, when producing a similar level of force reduction, induces similar magnitude reductions in dynamic postural control and resting VL stiffness in men and women. Distinct deteriorations in quiet standing balance in men but not women were accompanied by modifications in calf muscle stiffness following exercise-induced muscle fatigue.

Bipolar versus high-density surface electromyography for evaluating risk in fatiguing frequency-dependent lifting activities

Workers often develop low back pain due to manually lifting heavy loads. Instrumental-based assessment tools are used to quantitatively assess the biomechanical risk in lifting activities. This study aims to verify the hypothesis that high-density surface electromyography (HDsEMG) allows an optimized discrimination of risk levels associated with different fatiguing lifting conditions compared to traditional bipolar sEMG. 15 participants performed three lifting tasks with a progressively increasing lifting index (LI) each lasting 15 min. Erector spinae (ES) activity was recorded using both bipolar and HDsEMG systems. The amplitude of both bipolar and HDsEMG can significantly discriminate each pair of LI. HDsEMG data could discriminate across the different LIs starting from the fourth minute of the task while bipolar sEMG could only do so towards the end. The higher discriminative power of HDsEMG data across the lifting tasks makes such methodology a valuable tool to be used to monitor fatigue while lifting and could extend the possibilities offered by currently available instrumental-based tools.

Effects of Plyometric Jump Training on Electromyographic Activity and Its Relationship to Strength and Jump Performance in Healthy Trained and Untrained Populations: A Systematic Review of Randomized Controlled Trials

ABSTRACT

Ramirez-Campillo, R, Garcia-Pinillos, F, Chaabene, H, Moran, J, Behm, DG, and Granacher, U. Effects of plyometric jump training on electromyographic activity and its relationship to strength and jump performance in healthy trained and untrained populations: a systematic review of randomized controlled trials. J Strength Cond Res 35(7): 2053-2065, 2021-This systematic review analyzed the effects of plyometric jump training (PJT) on muscle activation assessed with surface electromyography during the performance of strength and jumping tasks in healthy populations across the lifespan. A systematic literature search was conducted in the electronic databases PubMed/MEDLINE, Web of Science, and SCOPUS. Only randomized controlled studies were eligible to be included in this study. Our search identified 17 studies comprising 23 experimental groups and 266 subjects aged 13-73 years, which were eligible for inclusion. The included studies achieved a median Physiotherapy Evidence Database score of 6. No injuries were reported among the included studies. Significant PJT-related improvements were reported in 7 of 10 studies and in 6 of 10 studies for measures of muscle activation during the performance of strength and jumping tasks, respectively. Moreover, a secondary correlational analysis showed significant positive relationships (r = 0.86; p = 0.012; r2 = 0.74) between changes in muscle activation and changes in jump performance. However, from the total number (n = 287) of muscle activation response variables analyzed for strength and jumping tasks, ∼80% (n = 226) were reported as nonsignificant when compared with a control condition. In conclusion, PJT may improve muscle activation during the performance of strength and jumping tasks. However, conflicting results were observed probably arising from (a) studies that incorporated a large number of outcomes with reduced sensitivity to PJT, (b) methodological limitations associated to muscle activation measurement during strength and jumping tasks, and (c) limitations associated with PJT prescription. Future studies in this field should strive to solve these methodological shortcomings.

Acute and residual neuromuscular effects of displacement in indirect vibratory stimulation

The aim of this study was to evaluate the effect of indirect vibratory stimulation on different magnitudes of displacement on acute and residual neuromuscular responses. Fifteen healthy volunteers were randomly submitted to 3 experimental sessions, with intervals of 5 to 7 days (5 maximal voluntary contractions - MVC, 12 s of duration each and 5 min of recovery) between sessions. To determine the residual responses, the volunteers performed a MVC before and after each treatment for 12 s, with a 5-minute recovery. The experimental sessions were composed of isometric actions without vibrations (CONTROL) and two sessions of isometric actions with the addition of vibrations at 20 Hz and 3 mm (Sinusoidal Vibration A) and 5 mm (Sinusoidal Vibration B). Before and after each of the experimental sessions, an isometric evaluation without vibrations was performed. For the acute effect, it was verified that the addition of vibrations induced a facilitatory effect on the explosive strength variables (p < .05), independent of the type of studied displacement in relation to the control treatment. In short, it was verified that the addition of vibration induced an acute facilitating effect on the explosive strength. However, the induced effect was not persistent (residual effect) for the explosive strength.

Capsaicinoid and Capsinoids as an Ergogenic Aid: A Systematic Review and the Potential Mechanisms Involved

CONTEXT

Capsaicinoids and capsinoids (CAP) are natural substances found primarily in chili peppers and other spicy foods that agonize the transient receptor potential vanilloid-1 in the mouth, stomach, and small intestine. Several studies have shown CAP to be a potential antiobesity agent and to exhibit an analgesic effect in both rodents and humans. However, there is no scientific consensus about the effects of CAP on physical exercise performance and its physiological mechanisms of action.

PURPOSE

This systematic review aimed to better elucidate the effects of CAP compounds as ergogenic aids and to discuss underlying mechanisms of action by which this supplement may potentially enhance endurance performance and muscular strength.

CONCLUSIONS

Among 22 studies included in the review, 14 examined the effects of capsaicinoid or capsinoid compounds on endurance and resistance exercise performance in animals, with 9 studies showing benefits on performance. In humans, 8 studies were included: 3 demonstrated significant acute endurance benefits and 2 showed acute resistance exercise performance benefits compared with a placebo condition. Therefore, while more mechanistic studies are necessary to confirm these outcomes in humans, the available scientific literature appears to suggest that these compounds could be considered an effective nutritional strategy to improve exercise performance.

A New Approach to Evaluate Neuromuscular Fatigue of Extensor Elbow Muscles

Neuromuscular fatigue evaluation is widely performed on different muscles through the conventional protocol using maximum voluntary contraction (MVC) with electrical stimuli in the analyzed muscle. In an attempt to use this protocol on elbow extensor musculature, previous studies and pilot studies showed co-contraction effects from antagonist musculature during muscular stimulations. The aim of this study was to propose a new neuromuscular fatigue protocol evaluation on elbow extensor musculature. Twenty participants preformed exercises to induce central (CenFat) and peripheral fatigue (PerFat). Neuromuscular fatigue was evaluated on knee extensor muscles by a conventional protocol that provides Twitch Superimposed (TS_K) and Twitch Potentiated (TP_K), central and peripheral parameters respectively. For elbow extensor muscles, the protocol used sustained submaximal contraction at 10, 20, 30, 40, and 50% of MVC. The neuromuscular fatigue in upper limbs was identified by Twitch Potentiated (TP_E) and multiple Twitch Superimposed (TS_E) parameters. Using the relationship between MVC (%) and evoked force, the proposed protocol used several TS_E to provide slope, y-intercept and R ². It is proposed that slope, R ², and y-intercept change may indicate peripheral fatigue and the identified relationship between y-intercept and R ² may indicate central fatigue or both peripheral and central fatigue. The results were compared using the non-parametric analyzes of Friedmann and Wilcoxon and their possible correlations were verified by the Spearmann test (significance level set at p < 0.05). After PerFat a decrease in TP_E (57.1%, p < 0.001) was found but not in any TS_E, indicating only peripheral fatigue in upper limbs. After CenFat a decrease in TP_E (21.4%, p: 0.008) and TP_K (20.9%, p < 0.001) were found but not in TS_K, indicating peripheral fatigue in upper and lower limbs but not central fatigue. A non-significant increase of 15.3% after CenFat and a statistical reduction (80.1%, p: 0.001) after PerFat were found by slope. Despite R ² showing differences after both exercises (p < 0.05), it showed a recovery behavior after CenFat (p: 0.016). Although PerFat provided only peripheral fatigue, CenFat did not provide central fatigue. Considering the procedural limitations of CenFat, parameters resulting from the proposed protocol are sensitive to neuromuscular alteration, however, further studies are required.

Reduced Neuromuscular Performance in Night Shift Orthopedic Nurses: New Insights From a Combined Electromyographic and Force Signals Approach

The effect of sleep–wake rhythm disruption on neuromuscular control and muscle fatigue has received little attention. Because nurse shift work is so varied, including overnight duty, rotating shift schedules, early awakening, and interrupted nocturnal sleep, it offers an interesting model to study this paradigm. It has been investigated so far using only subjective markers. A combined approach based on the simultaneous analysis of surface electromyographic (sEMG) and force signals can objectively detect possible deficits in neuromuscular control and muscle fatigue. With this study we investigated neuromuscular activation and muscle contraction capacity at submaximum and maximum level in nurses working two night-shift schedules and compared them to levels in nurses working entirely in day shifts. Sleep quality and activity levels were also assessed. The study sample was 71 nurses grouped by their shift work schedule: night shift for 5 days (NS₅, n = 46), night shift for 10 days (NS₁₀, n = 9), and only day/swing shift (DS, n = 16). Before and after the shift-work cycle, maximum voluntary contraction (MVC) force and muscle activation, neuromuscular control, and muscle fatigability were measured in the finger flexor muscles. Activity level and sleep quality during the shift-work cycle were recorded with a wrist actigraph. After the shift-work cycles, MVC force and muscle activation were decreased (−11 ± 3% and −33 ± 3%, p < 0.001) as was neuromuscular control (−36 ± 8%, p = 0.007), whereas muscle fatigability was increased (+ 19 ± 9%, p = 0.006) in the NS₅ and the NS₁₀ group. Sleep quality was lower in the NS₅ and the NS₁₀ group (−8 ± 1.8% and −15%3, respectively, p < 0.001), while the activity level for the three groups was similar. There was a clear reduction in neuromuscular control and an increase in muscle fatigue in the nurses working the night shift. These findings may inform of work schedule planning or recommendations for devising new recovery strategies to counteract neuromuscular alterations in night shift nurses.

Ultrasound Measurement of Skeletal Muscle Contractile Parameters Using Flexible and Wearable Single-Element Ultrasonic Sensor

Skeletal muscle is considered as a near-constant volume system, and the contractions of the muscle are related to the changes in tissue thickness. Assessment of the skeletal muscle contractile parameters such as maximum contraction thickness (Th), contraction time (Tc), contraction velocity (Vc), sustain time (Ts), and half-relaxation (Tr) provides valuable information for various medical applications. This paper presents a single-element wearable ultrasonic sensor (WUS) and a method to measure the skeletal muscle contractile parameters in A-mode ultrasonic data acquisition. The developed WUS was made of double-layer polyvinylidene fluoride (PVDF) piezoelectric polymer films with a simple and low-cost fabrication process. A flexible, lightweight, thin, and small size WUS would provide a secure attachment to the skin surface without affecting the muscle contraction dynamics of interest. The developed WUS was employed to monitor the contractions of gastrocnemius (GC) muscle of a human subject. The GC muscle contractions were evoked by the electrical muscle stimulation (EMS) at varying EMS frequencies from 2 Hz up to 30 Hz. The tissue thickness changes due to the muscle contractions were measured by utilizing a time-of-flight method in the ultrasonic through-transmission mode. The developed WUS demonstrated the capability to monitor the tissue thickness changes during the unfused and fused tetanic contractions. The tetanic progression level was quantitatively assessed using the parameter of the fusion index (FI) obtained. In addition, the contractile parameters (Th, Tc, Vc, Ts, and Tr) were successfully extracted from the measured tissue thickness changes. In addition, the unfused and fused tetanus frequencies were estimated from the obtained FI-EMS frequency curve. The WUS and ultrasonic method proposed in this study could be a valuable tool for inexpensive, non-invasive, and continuous monitoring of the skeletal muscle contractile properties.

Complexity Analysis of Surface Electromyography for Assessing the Myoelectric Manifestation of Muscle Fatigue: A Review

The surface electromyography (sEMG) records the electrical activity of muscle fibers during contraction: one of its uses is to assess changes taking place within muscles in the course of a fatiguing contraction to provide insights into our understanding of muscle fatigue in training protocols and rehabilitation medicine. Until recently, these myoelectric manifestations of muscle fatigue (MMF) have been assessed essentially by linear sEMG analyses. However, sEMG shows a complex behavior, due to many concurrent factors. Therefore, in the last years, complexity-based methods have been tentatively applied to the sEMG signal to better individuate the MMF onset during sustained contractions. In this review, after describing concisely the traditional linear methods employed to assess MMF we present the complexity methods used for sEMG analysis based on an extensive literature search. We show that some of these indices, like those derived from recurrence plots, from entropy or fractal analysis, can detect MMF efficiently. However, we also show that more work remains to be done to compare the complexity indices in terms of reliability and sensibility; to optimize the choice of embedding dimension, time delay and threshold distance in reconstructing the phase space; and to elucidate the relationship between complexity estimators and the physiologic phenomena underlying the onset of MMF in exercising muscles.