Flexion Task Research Articles

Relationship between the flexion relaxation phenomenon and kinematics of the multi-segmental spine in nonspecific chronic low back pain patients

The flexion relaxation phenomenon (FRP) is characterized by the reduction of paraspinal muscle activity at maximum trunk flexion. FRP is reported to be altered (persistence of spinal muscle activity) in more than half of nonspecific chronic low back pain (NSCLBP) patients. Little is known about how the multi-segmental spine affects FRP. The aim of this observational study was to investigate the relationship between FRP and kinematic parameters of the multi-segmental spine in NSCLBP patients. Forty NSCLBP patients and thirty-five asymptomatic participants performed a standing maximal trunk flexion task. Surface electromyography was recorded along the erector spinae longissimus. The kinematics of the spine were assessed using a 3D motion analysis system. The investigated spinal segments were upper thoracic, lower thoracic, thoracolumbar, upper lumbar, lower lumbar, and lumbopelvic. Upper lumbar ROM, anterior sagittal inclination of the upper lumbar relative to the lower lumbar in the upright position, and ROM of the upper lumbar relative to the lower lumbar during full trunk flexion were significantly correlated with the flexion relaxation ratio (Rho 0.42 to 0.58, p < 0.006). The relative position and movement of the upper lumbar segment seem to play an important role in the presence or absence of FRP in NSCLBP patients.

Neck muscle stiffness during craniocervical flexion under functional upper extremity conditions in patients with chronic non-specific neck pain: A shear-wave elastography study

ObjectiveThe primary objective of the present study was to examine the differences and patterns of change in the neck extensor (NE) muscle stiffness during the Cranio-cervical flexion (CCF) task under different functional conditions of the upper extremity between CNSNP participants and asymptomatic controls. MethodsIn the current case-control study, 25 participants with CNSNP and 25 asymptomatic controls were recruited. The stiffness of the superficial (i.e., upper trapezius, splenius capitis, and semispinalis capitis) and deep (i.e., semispinalis cervicis and multifidus) NE muscles was measured at prone resting, sitting resting, and during the CCF task in different functional conditions of upper extremity using shear wave elastography. ResultsThe findings showed that there was a significant main effect of condition in all NE muscles (P < 0.05), as well as a significant main effect of the group on the stiffness of superficial neck muscles (P < 0.05), indicating higher muscle stiffness in participants with CNSNP compared to the control group. There was no significant group-by-condition interaction effect on the stiffness of NE muscles (P > 0.05), except for the upper trapezius muscle (P = 0.00), indicating a different pattern of stiffness changes compared to the other muscles. ConclusionsIndividuals with CNSNP exhibited significantly greater stiffness in the superficial neck extensor muscles compared to healthy controls. Furthermore, the CCF task performed under functional upper extremity conditions, which includes bilateral shoulder scaption and shoulder abduction-external rotation while seated, resulted in increased stiffness of the NE muscles. Additionally, participants with CNSNP exhibited a higher magnitude of stiffness changes in the upper trapezius muscle across different conditions.

Firefighter helmets and cervical intervertebral Kinematics: An OpenSim-Based biomechanical study

The assessment of cervical intervertebral kinematics can serve as the basis for understanding any degenerative changes in the cervical spine due to the prolonged wear of a heavyweight, imbalanced firefighting helmet. Therefore, this study aimed to analyze cervical intervertebral kinematics using the OpenSim musculoskeletal modeling platform in order to provide much-needed insights into how the inertial properties of firefighter helmets affect cervical spinal mobility. A total of 36 firefighters (18 males and 18 females) were recruited to perform static and dynamic neck flexion, extension, and left and right lateral bending tasks for three conditions: 1) no-helmet, 2) US-style helmet with a comparatively superior center of mass (COM), and 3) European-style helmet with relatively higher mass but an inferior COM. Three custom-made OpenSim head-neck models were created to calculate cervical intervertebral kinematics for each helmet condition. Results showed that helmet use significantly (p < 0.001) affects neck and cervical spinal kinematics. Despite its lighter weight, the superior COM placement in the US-style helmet caused more pronounced angular changes and higher velocity of peak flexion and extension angles compared to the European-style helmet across all cervical joints. Moreover, results revealed discrepancies between OpenSim-derived neck and cervical range-of-motion and those reported in previous in-vivo studies. In conclusion, the present study underscores the importance of designing firefighter helmets with a lower profile (less superior COM) to enhance neck range of motion and minimize potential neck injuries.

Interlimb Coordination and Auditory-Motor Synchronization in Children with Developmental Coordination Disorder: Examining Antiphase Knee Movements with Auditory Metronomes While Seated.

Background: Children with developmental coordination disorder (DCD) exhibit reduced interlimb coordination compared to typically developing children (TDC) during complex tasks like running, which requires dynamic postural control. However, the extent of interlimb coordination difficulties in DCD during tasks that demand minimal dynamic balance, such as self-paced and externally auditory-paced tasks, remains unclear. This study aimed to compare interlimb coordination and auditory-motor synchronization between children with DCD and TDC during a seated antiphase coordination task of the lower limbs, which has minimal postural control requirements. Methods: Twenty-one children with DCD and 22 TDC performed an antiphase knee flexion and extension task while seated, in three conditions (baseline silence, metronome discrete, and metronome continuous), for three minutes. The interlimb coordination, synchronization, and spatiotemporal movement parameters were analyzed using a mixed model analysis; Results: Children with DCD displayed less coordinated interlimb movements compared to TDC (p = 0.0140), which was the result of the greater variability in coordinating antiphase knee flexion-extension movements (p < 0.0001). No group differences in spatiotemporal movement parameters were observed. Children with DCD, compared to TDC, had a lower synchronization consistency to metronomes (p = 0.0155). Discrete metronomes enhanced interlimb coordination compared to the baseline silence condition (p = 0.0046); Conclusions: The study highlights an inferior interlimb coordination and auditory-motor synchronization in children with DCD compared to TDC. Implementing metronomes with a discrete temporal structure improved the interlimb coordination of both groups during the used fundamental seated interlimb coordination task, supporting theorical frameworks of event-based timing.

How Effective Are Forecasting Models in Predicting Effects of Exoskeletons on Fatigue Progression?

Forecasting can be utilized to predict future trends in physiological demands, which can be beneficial for developing effective interventions. This study implemented forecasting models to predict fatigue level progression when performing exoskeleton (EXO)-assisted tasks. Specifically, perceived and muscle activity data were utilized from nine recruited participants who performed 45° trunk flexion tasks intermittently with and without assistance until they reached medium-high exertion in the low-back region. Two forecasting algorithms, Autoregressive Integrated Moving Average (ARIMA) and Facebook Prophet, were implemented using perceived fatigue levels alone, and with external features of low-back muscle activity. Findings showed that univariate models without external features performed better with the Prophet model having the lowest mean (SD) of root mean squared error (RMSE) across participants of 0.62 (0.24) and 0.67 (0.29) with and without EXO-assisted tasks, respectively. Temporal effects of BSIE on delaying fatigue progression were then evaluated by forecasting back fatigue up to 20 trials. The slope of fatigue progression for 20 trials without assistance was ~48-52% higher vs. with assistance. Median benefits of 54% and 43% were observed for ARIMA (with external features) and Prophet algorithms, respectively. This study demonstrates some potential applications for forecasting models for workforce health monitoring, intervention assessment, and injury prevention.

Kinematic patterns in performing trunk flexion tasks influenced by various mechanical optimization targets: A simulation study

BackgroundLow back pain is the most prevalent and disabling condition worldwide, with a high recurrence rate in the general adult population. MethodsA set of open-sourced trunk musculoskeletal models was used to investigate trunk flexion kinematics under different motor control strategies, including minimizing shearing or compressive loads at the L4/L5 or L5/S1 level. FindingsA control strategy that minimizes the load on the lower lumbar intervertebral disc can result in two kinematic patterns—the “restricted lumbar spine” and the “overflexed lumbar spine”—in performing the trunk flexion task. The “restricted” pattern can reduce the overall load on the lower lumbar levels, whereas the “overflexed” pattern can reduce the shearing force only at the L4/L5 level and increase the compressive and shearing forces at the L5/S1 level and the compressive force at the L4/L5 level. InterpretationThis study investigated the relationships between specific trunk kinematics in patients with low back pain and lumbar intervertebral loading via musculoskeletal modelling and simulation. The results provide insight into individualized treatment for patients with low back pain.

Age-dependent flexion relaxation phenomenon in chronic low back pain patients.

The flexion relaxation phenomenon (FRP) is characterized by suddenly reduced paraspinal muscle activity during full flexion. Previous studies showed significant differences in FRP and flexion angles in chronic low back pain (cLBP) patients compared to individuals without back pain (no-BP). However, the relationship between FRP and flexion angles remains insufficiently understood in older populations. Thus, this study investigated the relationship between FRP and flexion angles concerning to the age and presence of cLBP. Forty no-BP subjects (20m/20f; mean age 41.5years) and thirty-eight cLBP patients (19m/19f; mean age 43.52years) performed maximum full upper body flexion task. Electromyographic (EMG) measurements were conducted to assess the activity of lumbar erector spinae (ESL), thoracic erector spinae (EST), and multifidus (MF). Lumbar, thoracic, and pelvic angles at the onset (OnsetL/T/P) and offset of the FRP (OffsetL/T/P) and maximum trunk inclination (MaxL/T/P) were calculated. The FRP was evaluated using a flexion relaxation ratio (FRR). cLBP patients showed smaller FRR in MF and right ESL compared to no-BP individuals (p < 0.05), while no differences were found in flexion angles between two groups. Subjects over 40 showed smaller FRR in MF and ESL, and smaller flexion angles on OffsetL and MaxL (p < 0.05). Age-related analysis in the cLBP group revealed that patients over 40, compared to younger ones, had smaller FRR in MF and ESL, and smaller values in all thoracic and lumbar flexion angles (p < 0.05). While in no-BP group, significant larger flexion angles in OnsetL and OffsetT (p < 0.05) were observed in participants over 40. Pain-related analysis in the older group revealed that the cLBP patients, compared to no-BP individuals, had smaller FRR in right MF and right ESL, and smaller values in all lumbar and thoracic flexion angles (p < 0.05), while in younger group, there were no significant pain-related differences in FRR, with larger values in all lumbar flexion angles (p < 0.05). Our findings indicate a reduction or absence of FRP in cLBP patients compared to no-BP individuals, with age being a significant factor as those over 40 showed smaller FRP and flexion angles compared to younger individuals.

Predicting Perceived Back Fatigue During Exoskeleton Supported Trunk Bending Tasks using Machine Learning

Repetitive trunk flexion tasks performed over long durations can increase low-back injury risk, where Back Support Industrial Exoskeletons (BSIEs) can be beneficial. While BSIEs have shown effectiveness in lab assessments, real-world outcomes have shown variation based on task complexity, necessitating monitoring of physical demands. Fourteen participants performed repetitive trunk BSIE-assisted forward bending and return, without fatigue and then at medium-high fatigue. We recorded muscle activity in low-back and thigh muscles using Electromyography (EMG) and whole-body stability using force plates. Classification algorithms, namely, Support Vector Machine (SVM), Random Forest (RF), and XGBoost (XGB) were utilized to predict perceived medium-high back fatigue using sensor data. Highest performance was observed with XGB algorithm using data from a single low-back EMG sensor (Accuracy: 86.1%, Recall: 86%), and force plate (93.5, 94.1%). Outcomes of our study can be helpful in developing novel fatigue detection products, benefiting ergonomists in properly implementing BSIEs in industrial scenarios.

A Holistic Focus of Attention Increases Torque Production and Electromyography Activity in an Isokinetic Elbow Flexion

Recent research suggests that both a holistic focus of attention (i.e., focusing on the general feeling of a movement) and an external focus of attention improve motor performance relative to an internal focus of attention. The purpose of this study was to determine how a holistic, internal, and external focus impacts torque production and electromyography (EMG) activity in the biceps brachii during an isokinetic elbow flexion task. Twenty-four young adults completed five repetitions of an isokinetic elbow flexion task in internal, external, and holistic focus conditions. Peak torque, integrated torque, peak EMG amplitude, integrated EMG, and neuromuscular efficiency were averaged across trials in each condition. Peak torque, integrated torque, peak EMG amplitude, and integrated EMG were all significantly higher with a holistic focus than an internal or external focus. Internal and external focus conditions did not differ from each other in any variable. No differences due to focus were observed for neuromuscular efficiency. The present results suggest that using a holistic focus of attention can be a useful strategy for motor tasks requiring peak force production. However, it does not appear that this benefit is related to increased neuromuscular efficiency, as we hypothesized.

A coactivation strategy in anticipatory postural adjustments during voluntary unilateral arm movement while standing in individuals with bilateral spastic cerebral palsy

Individuals with bilateral spastic cerebral palsy (BSCP) reportedly has problems with anticipatory postural adjustments (APAs) while standing. However, the use of coactivation strategy in APAs in individuals with BSCP has conflicting evidence. Hence, this study aimed to investigate postural muscle activities in BSCP during unilateral arm flexion task in which postural perturbations occur in the sagittal, frontal, and horizontal planes. We included 10 individuals with BSCP with level II on the Gross Motor Function Classification System (BSCP group) and 10 individuals without disability (control group). The participants stood on a force platform and rapidly flexed a shoulder from 0° to 90° at their own timing. Surface electromyograms were recorded from the rectus femoris, medial hamstring, tibialis anterior, and medial gastrocnemius. The control group showed a mixture of anticipatory activation and inhibition of postural muscles, whereas the BSCP group predominantly exhibited anticipatory activation with slight anticipatory inhibition. Compared with the control group, the BSCP group tended to activate the ipsilateral and contralateral postural muscles and the agonist–antagonist muscle pairs. The BSCP group had a larger disturbance in postural equilibrium, quantified by the peak displacement of center of pressure during the unilateral arm flexion, than those without disability. Individuals with BSCP may use coactivation strategy, mainly the anticipatory activation of postural muscle activity, during a task that requires a selective postural muscle activity to maintain stable posture.

Normal values of scapular muscle activity ratio during arm elevation and the relationship to muscle strength and sex difference.

The normal value of the scapular muscle activity ratio during arm elevation is not clear. To obtain normal values of the scapular muscle activity ratio during arm elevation in healthy individuals. This cross-sectional study enrolled 47 healthy people. The participants performed shoulder flexion and lowered task with 90-degree movements every 2 seconds. Muscle activities of scapular muscles were measured. For normalization, the maximum isometric contraction of the shoulder flexion was measured at 90∘ (reference contraction). The integrated electromyographic value (IEMG) obtained during the task was normalized by IEMG during the reference contraction and the relative IEMG value was calculated. Then, the scapular muscle activity ratio was computed. The relationship between muscle strength and other factors was also investigated. The median values for upper trapezius/serratus anterior and upper trapezius/lower trapezius were often approximately 1, and that for upper trapezius/middle trapezius was often between 1 and 2. The shoulder flexion isometric strength and scapular muscle activity ratio showed significant negative correlations in multiple phases. Normal values for upper trapezius/serratus anterior and upper trapezius/lower trapezius during arm elevation and lowering were generally 1. Low muscle strength may contribute to an abnormal scapular muscle activity balance.

Assessing the Short-Term Effects of Dual Back-Support Exoskeleton within Logistics Operations

Logistics activities involve significant risk factors for the development of work-related musculoskeletal disorders (WMSD), particularly low back pain. Exoskeletons have emerged as potential solutions to mitigate these risks. This study assesses the short-term effects of dual passive back-support exoskeletons (Auxivo and Htrius) on WMSD risk factors in logistics operations. Two workstations were evaluated using self-report ratings, postural assessment, and surface electromyography (EMG). The results indicate that both exoskeletons provided relief and support during tasks, with Htrius showing a slight advantage. Exoskeletons reduced perceived exertion, especially during trunk flexion tasks, and improved posture, particularly in tasks involving manual lifting loads at lower height levels. While variations in muscular activity were observed, the Htrius exoskeleton showed a trend of reducing lumbar muscle activity. Overall, Htrius demonstrated promise in improving workers’ comfort, safety, and efficiency, potentially reducing WMSD risk and muscular fatigue. However, individual preferences and workplace-specific characteristics should be considered when selecting exoskeleton models. Future research should explore the effects on different loads, genders, and EMG of different muscles to further enhance the understanding and application of exoskeletons in occupational contexts.

The association between unilateral and bilateral performance-related measures in elite female soccer players: a multifaceted investigation.

The present study aimed to investigate a) the associations between bilateral performance utilizing countermovement jump (CMJ), squat jump (SJ), speed and unilateral CMJ, isokinetic peak torque in knee extension and flexion with angular velocities of 60°/s and 180°/s and tensiomyography (TMG) parameters; b) whether the asymmetries derived from unilateral tests are associated with bilateral CMJ, SJ and speed in elite female soccer players. Thirty-five elite female soccer players (average age: 20 ± 5years) completed CMJ, SJ, speed, isokinetic muscle strength and TMG tests. Compared to the non-dominant leg, the dominant leg demonstrated greater peak torque output in both knee flexion (7.4%) and knee extension (5.6%) isokinetic tasks, as well as m. vastus medialis contraction time (7.6%), and soccer-specific agility test (4.1%). Conversely, the hamstring to quadriceps peak torque ratio at 180°/s (8.5%) was significantly greater in the non-dominant leg. The associations between CMJ, SJ and speed performance were positive and ranged from weak (r = 0.350) to high (r = 0.710). For speed and TMG-derived variables, correlations were negative and ranged from weak (r = -0.345, p = 0.042, for vastus medialis contraction time) to moderate (r = -0.530, p = 0.001, for biceps femoris contraction time). Furthermore, both bilateral CMJ and SJ negatively correlated with TMG-derived variables, ranging from weak (r = -0.350, p = 0.039, for vastus lateralis contraction time) to moderate (r = -0.537, p = 0.003, for rectus femoris contraction time). The overall significant, albeit inconsistent, correlations between the diverse performance scores obtained highlight the necessity for a multifaceted and thorough diagnostic strategy in female soccer players.

Connecting the dots: Sensory cueing enhances functional connectivity between pre-motor and supplementary motor areas in Parkinson's disease.

People with Parkinson's disease often exhibit improvements in motor tasks when exposed to external sensory cues. While the effects of different types of sensory cues on motor functions in Parkinson's disease have been widely studied, the underlying neural mechanism of these effects and the potential of sensory cues to alter the motor cortical activity patterns and functional connectivity of cortical motor areas are still unclear. This study aims to compare changes in oxygenated haemoglobin, deoxygenated haemoglobin and correlations among different cortical regions of interest during wrist movement under different external stimulus conditions between people with Parkinson's disease and controls. Ten Parkinson's disease patients and 10 age- and sex-matched neurologically healthy individuals participated, performing repetitive wrist flexion and extension tasks under auditory and visual cues. Changes in oxygenated and deoxygenated haemoglobin in motor areas were measured using functional near-infrared spectroscopy, along with electromyograms from wrist muscles and wrist movement kinematics. The functional near-infrared spectroscopy data revealed significantly higher neural activity changes in the Parkinson's disease group's pre-motor area compared to controls (p = 0.006), and functional connectivity between the supplementary motor area and pre-motor area was also significantly higher in the Parkinson's disease group when external sensory cues were present (p = 0.016). These results indicate that external sensory cues' beneficial effects on motor tasks are linked to changes in the functional connectivity between motor areas responsible for planning and preparation of movements.

Age-related changes in reticulospinal contributions to anticipatory postural adjustments between back extensors and abdominal muscles.

Anticipatory postural adjustments (APAs) give feedforward postural control of the trunk, but they are delayed with ageing, affecting balance and mobility in older individuals. The reticulospinal tract contributes to postural control of the trunk; however, the extent to which age-related changes affect the reticulospinal contributions to APAs of the trunk remains unknown in humans. Here, we tested the hypothesis that a startling acoustic sound, which activates the reticulospinal tract, improves delayed APAs in older individuals. Twenty-two old (75±6years) and 20 healthy young adults (21±4years) performed a self-initiated fast bilateral shoulder flexion or shoulder extension task in response to visual, visual and auditory (80dB), or visual and startling (115dB) cues. Electromyography (EMG) was recorded from bilateral anterior deltoid (AD) and erector spinae (ES) during shoulder flexion and from bilateral posterior deltoid (PD) and rectus abdominis (RA) during shoulder extension. EMG onset of all muscles shortened during the startling cue in both age groups, suggesting a non-specific modulation of the reticulospinal tract on prime movers (AD or PD) and non-prime movers (ES or RA). Interestingly, APAs of the ES were accelerated in older participants to a similar degree as in younger participants during the startling cue. Conversely, APAs of the RA were not influenced by the startling cue in older participants. Our results suggest differential effects of ageing on functional contributions of the reticulospinal tract to APAs between back extensors and abdominal muscles.

Are performance and perceived fatigability dependent on the anchor scheme of fatiguing isometric tasks in men?

Ratings of perceived exertion (RPE) can be used to regulate exercise intensity. This study examined the effect of anchor scheme on performance fatigability and neuromuscular responses following fatiguing forearm flexion tasks. Twelve men (age 20.9±2.2 years; height 179.8±5.3 cm; body mass 80.2±9.9 kg) performed sustained, isometric forearm flexion tasks to failure anchored to RPE=6 (RPEFT) and the torque (TRQFT) that corresponded to RPE=6. Pre-test and post-test maximal voluntary isometric contractions (MVIC) were performed to quantify changes in the amplitude (AMP) and mean power frequency (MPF) of the electromyographic (EMG) and mechanomyographic (MMG) signals. Neuromuscular efficiency (NME) was calculated by dividing normalized torque by normalized EMG AMP. A dependent t-test was used to assess the mean difference for time to task failure (TTF). Repeated measures ANOVAs were used to compare mean differences for performance fatigability and normalized neuromuscular parameters. The RPEFT had a greater TTF than the TRQFT (P<0.001). MVIC and NME decreased from pre-test to post-test following the RPEFT and TRQFT (P<0.05) with no differences between anchor schemes. Following the TRQFT, normalized EMG MPF decreased from pre-test to post-test (P=0.004). Following the RPEFT, normalized MMG MPF increased from pre-test to post-test (P=0.021). There were no changes in normalized EMG AMP or MMG AMP (P>0.05). These findings indicated anchor scheme-specific neuromuscular responses and TTF, despite no difference in performance fatigability. Furthermore, performance fatigability was likely due to peripheral fatigue (based on normalized EMG MPF and NME) following the TRQFT, but peripheral and central fatigue (based on normalized MMG MPF and NME) following the RPEFT.

Long-latency and Volitional Responses Modulate Upper Extremity Movements During a Novel Visuomotor Tracking Task

Precise upper limb movements are essential for daily tasks and motor function. These movements are governed by feedforward and feedback mechanisms. Feedforward responses enable anticipatory movement planning, while feedback responses utilize sensory information for real-time corrections. Long-latency reflexes (LLRs) represent crucial feedback responses during unexpected perturbations. LLRs are integral in maintaining motor control and can adapt based on various factors. However, the mechanisms governing LLRs, especially in the upper extremity, remain unclear. Methods: Forty healthy participants performed a visuomotor elbow flexion task with one arm while the opposite limb remained static. Muscle activity of the biceps and triceps muscles of both upper extremities was recorded during perturbations. Participants completed a task with varying speeds and resistance levels. Electromyography data was analyzed across pre-perturbation, short-latency reflex (SLR), long-latency reflex (LLR), and voluntary response (VR) phases. Results: In response to unexpected upper extremity perturbations, participants predominantly relied on two core strategies. Inhibitory long-latency reflexes within the biceps played a central role, with participants emphasizing inhibition to maintain movement stability in a window of 50-120 ms after the disturbance. Furthermore, volitional control, primarily executed through the triceps muscles, emerged as another dominant approach, allowing participants to actively regain precision and control after the perturbation in a volitional manner starting from 120 ms. Participants’ response to perturbations reflected the task context and perceived urgency. Over repeated trials, participants refined their response strategies, becoming more precise and effcient. Variations in movement speed and resistance influenced muscle activity. Preliminary evidence suggested the involvement of cross-transcortical communication in response to perturbations, with delayed responses observed in the non-participating upper extremity. Conclusion: This study reveals that participants use both long-latency and volitional responses to counteract perturbations during an upper extremity visuomotor task, adapting their response strategies over time. Biceps inhibitory LLRs and triceps voluntary activation characterize feedback control in this context. These findings shed light on the dynamic nature of upper extremity feedback responses during unpredictable perturbations and suggest the involvement of cross-transcortical communication. Understanding these mechanisms can inform rehabilitation strategies for individuals with neurological conditions affecting motor control. Research Support: This study was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development: R01HD084645, R01HD082109. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The Effects of Anchoring a Fatiguing Forearm Flexion Task to a High vs. Low Rating of Perceived Exertion on Torque and Neuromuscular Responses.

Ortega, DG, Housh, TJ, Smith, RW, Arnett, JE, Neltner, TJ, Schmidt, RJ, and Johnson, GO. The effects of anchoring a fatiguing forearm flexion task to a high versus low rating of perceived exertion on torque and neuromuscular responses. J Strength Cond Res 38(5): e219-e225, 2024-This study examined the torque and neuromuscular responses following sustained, isometric, forearm flexion tasks anchored to 2 ratings of perceived exertion (RPE). Nine men (mean ± SD: age = 21.0 ± 2.4 years; height = 179.5 ± 5.1 cm; body mass = 79.6 ± 11.4 kg) completed maximal voluntary isometric contractions (MVIC) before and after sustained, isometric, forearm flexion tasks to failure anchored to RPE = 2 and RPE = 8. The amplitude (AMP) and mean power frequency (MPF) of the electromyographic (EMG) signal were recorded from the biceps brachii. Normalized torque was divided by normalized EMG AMP to calculate neuromuscular efficiency (NME). A dependent t-test was used to assess the mean difference for time to task failure (TTF). Repeated-measures analysis of variances was used to compare mean differences for MVIC and normalized neuromuscular parameters. There was no significant difference in TTF between RPE = 2 and RPE = 8 (p = 0.713). The MVIC decreased from pretest to posttest at RPE = 2 (p = 0.009) and RPE = 8 (p = 0.003), and posttest MVIC at RPE = 8 was less than that at RPE = 2 (p < 0.001). In addition, NME decreased from pretest to posttest (p = 0.008). There was no change in normalized EMG AMP or EMG MPF (p > 0.05). The current findings indicated that torque responses were intensity specific, but TTF and neuromuscular responses were not. Furthermore, normalized EMG AMP and EMG MPF remained unchanged but NME decreased, likely due to peripheral fatigue and excitation-contraction coupling failure. Thus, this study provides information regarding the neuromuscular responses and mechanisms of fatigue associated with tasks anchored to RPE, which adds to the foundational understanding of the relationship between resistance exercise and the perception of fatigue.

Reinforcement Learning May Demystify the Limited Human Motor Learning Efficacy Due to Visual-Proprioceptive Mismatch.

Vision and proprioception have fundamental sensory mismatches in delivering locational information, and such mismatches are critical factors limiting the efficacy of motor learning. However, it is still not clear how and to what extent this mismatch limits motor learning outcomes. To further the understanding of the effect of sensory mismatch on motor learning outcomes, a reinforcement learning algorithm and the simplified biomechanical elbow joint model were employed to mimic the motor learning process in a computational environment. By applying a reinforcement learning algorithm to the motor learning of elbow joint flexion task, simulation results successfully explained how visual-proprioceptive mismatch limits motor learning outcomes in terms of motor control accuracy and task completion speed. The larger the perceived angular offset between the two sensory modalities, the lower the motor control accuracy. Also, the more similar the peak reward amplitude of the two sensory modalities, the lower the motor control accuracy. In addition, simulation results suggest that insufficient exploration rate limits task completion speed, and excessive exploration rate limits motor control accuracy. Such a speed-accuracy trade-off shows that a moderate exploration rate could serve as another important factor in motor learning.

Detecting Fatigue during Exoskeleton-Assisted Trunk Flexion Tasks: A Machine Learning Approach

Back-Support Industrial Exoskeletons (BSIEs) can be beneficial in reducing the risk of injury due to overexertion during trunk flexion tasks. Most real-world tasks include complex body movements, leading to mixed outcomes that necessitate field-based methods for detecting overall physical demands. Monitoring fatigue can be beneficial in this regard to ensure that benefits of BSIEs are translated to the real world. Our experiment included 14 participants, who performed 30 repetitions of 45° trunk-flexion while assisted by a BSIE, first without fatigue and then at medium-high back fatigue (7/10 in the Borg scale). We extracted 135 features from recorded muscle activity, trunk motion, and whole-body stability across bending, transition, and retraction portions of each trunk-flexion cycle. Four classification algorithms, namely Support Vector Machine (SVM), Logistic Regression (LR), Random Forest (RF), and XGBoost (XGB), were implemented to assess fatigue prediction. XGB (Accuracy: 86.1%, Recall: 86%, Specificity: 86.3%) was effective in classifying fatigue with data obtained from a single EMG sensor located on the lower back (erector spinae) muscle. Meanwhile, stability measures showed high predictability with both RF (92.9%, 91.9%, 94.1%) and XGB (93.5, 94.1%, 93.1%). Findings demonstrate the success of force plates, and when replaced by pressure insoles, they can facilitate real-world fatigue detection during BSIE-assisted trunk-flexion tasks.