Muscle Activation Levels Research Articles

Comparative Study of Infraspinatus and Posterior Deltoid Muscle Activation According to Angle of External Rotation of Glenohumeral Joint

Background: Based on the understanding of the muscle activation relationship between the infraspinatus and posterior deltoid muscles to according to the angle of motion during external rotation on glenohumeral joint, effective shoulder joint strengthening exercise for the prevention and rehabilitation of shoulder injury due to muscle strength imbalance can be performed by achieving the ideal muscle activity ratio during exercise. Objectives: To compare and analyze the muscle activation changes and activity ratio of the infraspinatus and posterior deltoid muscles according to the glenohumeral external rotation angle. Design: Quasi-randomized trial. Methods: The study included 48 healthy male and female adults who provided informed consent for participation in the study. All the subjects performed isometric glenohumeral external rotation by setting the angle of motion to 30°, 45°, and 60° using a 5 kg resistance weight pulley. On surface electromyography, the differences in muscle activation and activity ratio between the infraspinatus and posterior deltoid muscles were investigated. Results: A significant difference in muscle activation was found in the comparison between the infraspinatus and posterior deltoid muscles according to the glenohumeral external rotation angle (P<.05). The muscle activation levels of the infraspinatus and posterior deltoid muscles were highest at the external rotation angles of 30° and 60°, respectively. The muscle activity ratio between the infraspinatus and posterior deltoid muscles also showed a significant difference (P<.05) and was highest at the shoulder external rotation angle of 30°. Conclusion: The findings of this study suggest that muscle activity is the highest at the shoulder external rotation angle of 30° in healthy individuals.

Modulation of finger muscle activation patterns across postures is coordinated across all muscle groups.

Successful grasp requires that grip forces be properly directed between the fingertips and the held object. Changes in digit posture significantly affect the mapping between muscle force and fingertip force. Joint torques must subsequently be altered to maintain the desired force direction at the fingertips. Our current understanding of the roles of hand muscles in force production remains incomplete, as past studies focused on a limited set of postures or force directions. To thoroughly examine how hand muscles adapt to changing external (force direction) and internal (posture) conditions, activation patterns of six index finger muscles were examined with intramuscular electrodes in 10 healthy subjects. Participants produced submaximal isometric forces in each of six orthogonal directions at nine different finger postures. Across force directions, participants significantly altered activation patterns to accommodate postural changes in the interphalangeal joint angles but not changes in the metacarpophalangeal joint angles. Modulation of activation levels of the extrinsic hand muscles, particularly the extensors, were as great as those of intrinsic muscles, suggesting that both extrinsic and intrinsic muscles were involved in creating the desired forces. Despite considerable between-subject variation in the absolute activation patterns, principal component analysis revealed that participants used similar strategies to accommodate the postural changes. The changes in muscle coordination also helped increase joint impedance in order to stabilize the end-point force direction. This effect counteracts the increased signal-dependent motor noise that arises with greater magnitude of muscle activation as interphalangeal joint flexion is increased. These results highlight the role of the extrinsic muscles in controlling fingertip force direction across finger postures.NEW & NOTEWORTHY We examined how hand muscles adapt to changing external (force direction) and internal (posture) conditions. Muscle activations, particularly of the extrinsic extensors, were significantly affected by postural changes of the interphalangeal, but not metacarpophalangeal, joints. Joint impedance was modulated so that the effects of the signal-dependent motor noise on the force output were reduced. Comparisons with theoretical solutions showed that the chosen activation patterns occupied a small portion of the possible solution space, minimizing the maximum activation of any one muscle.

Biomechanical assessment of two back-support exoskeletons in symmetric and asymmetric repetitive lifting with moderate postural demands

Two passive back-support exoskeleton (BSE) designs were assessed in terms of muscular activity, energy expenditure, joint kinematics, and subjective responses. Eighteen participants (gender-balanced) completed repetitive lifting tasks in nine different conditions, involving symmetric and asymmetric postures and using two BSEs (along with no BSE as a control condition). Wearing both BSEs significantly reduced peak levels of trunk extensor muscle activity (by ~9–20%) and reduced energy expenditure (by ~8–14%). Such reductions, though, were more pronounced in the symmetric conditions and differed between the two BSEs tested. Participants reported lower perceived exertion using either BSE yet raised concerns regarding localized discomfort. Minimal changes in lifting behaviors were evident when using either BSE, and use of both BSEs led to generally positive usability ratings. While these results are promising regarding the occupational use of BSEs, future work is recommended to consider inter-individual differences to accommodate diverse user needs and preferences.

Characterizing forearm muscle activity in university-aged males during dynamic radial-ulnar deviation of the wrist using a wrist robot

Functioning as wrist stabilizers, the wrist extensor muscles exhibit higher levels of muscle activity than the flexors in most distal upper-limb tasks. However, this finding has been derived mostly from isometric or wrist flexion-extension protocols, with little consideration for wrist dynamics or radial-ulnar wrist deviations. The purpose of this study was to assess forearm muscle activity during the execution of dynamic wrist radial-ulnar deviation in various forearm orientations (pronation/supination). In 12 healthy university-aged males, surface electromyography (EMG) was recorded from eight muscles of the dominant arm: flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), extensor digitorum (ED), biceps brachii (BB) and triceps brachii (TB). While grasping a handle, participants performed dynamic radial-ulnar deviation using a three-degrees-of-freedom wrist manipulandum. The robotic device applied torque to the handle, in either a radial or ulnar direction, and in one of three forearm postures (30° supinated/neutral/30° pronated). Results indicated that forearm posture influenced the muscles acting upon the hand (FDS/ED), whereas movement phase (concentric-eccentric) and torque direction influenced nearly every muscle. The ECR demonstrated the greatest task-dependency of all forearm muscles, which is possibly reflective of forearm muscle lines of action. Co-contraction ratios were much higher in radial trials than ulnar (Radial: 1.20 ± 0.78, Ulnar: 0.28 ± 0.18, P < 0.05), suggesting greater FCU and ECU contribution to wrist joint stability in radial-ulnar movement. These findings highlight a greater complexity of wrist extensor function than has previously been reported in isometric work.

Electromyographic analysis of selected shoulder muscles during a series of exercises commonly used in patients with symptomatic degenerative rotator cuff tears

In the nonoperative treatment of degenerative rotator cuff (RC) tears, exercise therapy is advocated. Exercises focusing on strengthening the anterior deltoid (AD) and the scapular muscles are proposed to compensate for RC dysfunction. However, the amount of electromyographic (EMG) activity in these muscles during these exercises remains unclear. Moreover, it is unknown whether muscle activity levels during these exercises alter with increasing age. Therefore, the purpose of this study was to evaluate EMG activity in the deltoid and scapular muscles during 2 series of commonly used shoulder rehabilitation exercisesand assess possible age-related changes in muscle activity. Fifty-five healthy participants (aged 18-60 years) participated in this study. Surface EMG activity was measured in 8 shoulder girdle muscles during a progression of a closed chain elevationprogram (bench and wall slides) and during a progression of previously published AD exercises. In addition, muscle activity was compared between 3 age categories (18-32 years, 33-46 years, and 47-60 years). The proposed progressions exhibited increasing activity from <10% of maximal voluntary isometric contractionto >20% of maximal voluntary isometric contraction for the AD for both exercise programsand for the middle deltoid, upper trapezius, and middle trapezius during the closed chain elevation exercises. Activity levels in the other muscles remained <20% throughout the progression. Age-related analysis revealed increased activity in the AD, infraspinatus, and middle trapeziusand decreased lower trapezius activity during the bench and wall slides. No age-related changes were noted for the AD exercises. These findings may assist the clinician in prescribing appropriate progressive exercise programs for patients with symptomatic RC tears.

Comparison of muscle activation levels and knee valgus between individuals with medial patellofemoral ligament reconstruction and healthy individuals during fatiguing step down task

BackgroundMedial patellofemoral ligament reconstruction becomes first-choice surgical procedure for patients with a history of lateral patellar dislocations but there is limited knowledge about neuromuscular activation patterns of individuals with a history of patellar dislocation who underwent medial patellofemoral ligament reconstruction. ObjectiveThe aim of this study was to compare muscle activation levels and knee valgus during step down performance test between individuals with a history of medial patellofemoral ligament reconstruction and healthy individuals. MethodsFifteen individuals with medial patellofemoral ligament reconstruction and 15 healthy individuals were included. Vastus medialis obliquus, vastus lateralis and gluteus medius muscle activation levels and knee valgus were measured during 60-s- step down performance test. Two-way repeated-measures of analysis of covariance was used for statistical analysis. FindingsCompared to the healthy individuals, individuals with medial patellofemoral ligament reconstruction showed lower vastus medialis obliquus (p = .04) and gluteus medius (p = .005) activation levels, and higher knee valgus (p = .002) in last period of the step down performance test. InterpretationSince the significant results were only observed in the fatiguing section of the test, endurance tests may provide more information about neuromuscular control of the individuals with history of medial patellofemoral ligament reconstruction. Future studies should investigate whether endurance exercises that target to improve vastus medialis obliquus and gluteus medius activity result in better clinical outcomes than conventional programs for individuals with medial patellofemoral ligament reconstruction.

Effects of stretching on muscle activation in gas cylinder handling.

Previously, a stretching regimen was designed for manual material handling (MMH) of gas cylinders as a potential ergonomic solution for reducing occupational injury. No studies have made use of objective process measures, such as muscle activation levels, for evaluation of effects of stretching programs. Examine acute effects of stretching on muscle activation levels and driver perceived level of exertion in gas cylinder handling during simulated delivery operations. A within-subject experiment was conducted with eight male participants being subjected randomly to two conditions over a two-day period: stretching before delivery trials and no stretching. Surface electromyography and the Borg CR-10 scale for perceived exertion were used. Generally, results were variable among muscle responses. The extensor muscle bundle in the forearm was found to show a significant decrease (p = 0.0464) in activation level because of stretching. The anterior deltoid and trapezius significantly increased (p < .0001) the EMG activation level with stretching. Also counter to expectations, participants rated perceived exertion significantly higher (p = 0.0423) for trials preceded by stretching. This research indicates a muscle stretching regimen in advance of MMH activities has mixed effects on activation levels across muscles. It is possible that effects are attributable to body posture positions, or manner of muscle use, during actual work activities. Findings indicate that stretching prior to work activity does have an impact on specific muscle activation.

Effects of Basketball Shoe Midsole Hardness on Lower Extremity Biomechanics and Perception during Drop Jumping from Different Heights

This study investigated how midsole hardness of basketball footwear affects lower extremity biomechanics and impacts perception in drop vertical jumps. Eighteen male basketball players performed drop vertical jumps from three heights (31 cm, 46 cm, 61 cm) in basketball shoes of different midsole hardness (50, 60 Asker C). Biomechanical variables of the lower extremity and subjective perception were measured. This study found a significant drop height effect on the lower extremity biomechanics (p &lt; 0.05), with greater ground reaction forces, joint kinetics, and prelanding muscle activation levels observed at higher drop heights. Basketball shoes with a softer midsole led to higher forefoot peak force (p = 0.028) amid lower rearfoot peak force (p = 0.046), lower peak flexion moments at the ankle (p = 0.024) and hip joints (p = 0.029), and greater prelanding muscle activation in the rectus femoris (p = 0.042) and tibialis anterior (p = 0.043). It is concluded that changing midsole hardness within a commercially relevant range triggered a different prelanding muscle activation strategy and hence altered the magnitudes of ground reaction forces and joint loadings during landing. Subjectively, participants perceived higher landing impacts with greater drop heights, though the strength of the associations were weak.

Moderate Exercise and the Psycho-Neural and Bio-Mechanical Nature of Chronic Low Back Pain

Low back pain (LBP) is one of the most common health problems, which in 10-15% of patients progresses to develop into a chronic low back pain (CLBP) condition, bringing enormous psychological and financial costs to the affected individuals, their families and society as a whole. There are many theories – pure bio-medical, bio-mechanical, psychological, as well as poly-modal ones, attempting to explain the pathology and aetiology of CLBP and guide its treatment. Although there is a general consensus that CLBP, as many other chronic pain conditions, is heterogeneous in nature, involving both organic (bio-mechanical and physiological) and psycho-social factors, there is still not a definite answer to the exact interactions and causative links between these factors. The present study employed a 15 minutes moderate treadmill exercise intervention in 10 CLBP sufferers and 10 healthy controls in order to further clarify the interactions between these physiological, bio-mechanical and psychological factors. The results from the present study, in accordance with previous research confirmed that CLBP participants are clearly identifiable according to their affective and cognitive profile - CLBP participants reported significantly higher levels of Pain Catastrophising (t = 3.81, p = .003), as well as Depression (t = 2.35, p = .03). Although both Pain Catastrophising and Depression were significantly correlated with the background pain (r = .755, p < .001 and r = .559, p = .010 respectively) in the CLBP group alone, contrary to previous CLBP studies, there were no significant associations between the background pain level and duration, Depression, Anxiety, Stress and even Pain Catastrophising. The Negative affective (NA) state was measured dynamically throughout the experiment and was reduced non-significantly by the exercise in both groups. Surprisingly, The NA was very low – 18 to 20 on a scale of 15 to 75, in both groups (even lower in the CLBP) and there was no significant difference between the NA of the CLBP and Control groups at any time-point of the experiment. The ES and MF back muscles activity was measured using a static standing and fully bent forward ratio, as well as flexion-relaxation ratio (FRR) pre- and post-exercise. The present study's results, although less robust than the results from previous research, confirmed that both standing and fully bent forward ES and MF measures, as well as FRR distinguished CLBP from control participants – CLBP had a higher level of static muscle activity (significant for the pre-exercise right MF in fully bent down position t = -2.48, p = . 044) and reduced FRR (significant for the pre-exercise left ES – t = 3.09, p = .013 and post-exercise left ES and right MF – t = 2.33, p = .038; t = 2.73, p = .032 respectively). Although the exercise produced non significant improvement of the ES and MF muscle activity, it also produced a divergent pain intensity (PPI) response between different CLBP participants – 5 (55.56%) had a decrease in PPI, while 4 (44.44%) had an increase. The pain decrease sub-group exhibited only a mild back muscles dysfunction, characterised by increased activity of the ES and MF in resting state (muscle tension), combined with a higher negative cognitive and affective mental set. The main characteristics of the pain increase sub-group was abnormal ES and MF flexion-relaxation, which pointed to compensatory increased activity of the superficial back muscles due to possible intrinsic spinal instability. In conclusion, the moderate treadmill exercise, utilised in the present study, was capable of identifying the existence of CLBP sub-groups, which were otherwise undistinguishable by the rest of their background pain, psychological or muscle activity characteristics. The low back pain of one of the sub-groups was associated with primary psychological top-down dysfunction of the 'active system' (muscles), while in the second sub-group the principal underlying factor identified was primary organic bio-mechanical dysfunction of the 'passive system'. The possible mechanisms and implications of this dichotomous nature of CLBP in respect to its aetiology, diagnosis, treatment and further research are discussed.

Mechanical performance, level of continuous contraction and muscle pain symptoms in home care personnel

The interrelationships between reported complaints, clinical status, mechanical performance and EMG pattern of the shoulder flexors and the knee extensors, respectively, during maximum dynamic contractions were investigated. A group of 97 randomly selected women employed in the home care service participated. Those who reported complaints had more clinical findings, such as reduced mobility, tender points and muscle tightness, than non-complainers. The level of muscle activity during the passive part of the contraction cycle was significantly higher in the shoulder flexors than in the knee extensors. Subjects without reported complaints who had been employed for more than ten years had a significantly lower activity level in the shoulder muscles than subjects without complaints who had been employed for a shorter time. We suggest that a high muscle tension level between contractions might precede muscle pain. A cluster analysis including sociodemographic, clinical and physiological variables revealed different subgroups, which points to the need to investigate the work situation in a multivariate way.

Design of Muscle Reflex Control for Upright Standing Push-Recovery Based on a Series Elastic Robot Ankle Joint.

In physical human–robot interaction environment, ankle joint muscle reflex control remains significant and promising in human bipedal stance. The reflex control mechanism contains rich information of human joint dynamic behavior, which is valuable in the application of real-time decoding motion intention. Thus, investigating the human muscle reflex mechanism is not only meaningful in human physiology study but also useful for the robotic system design in the field of human–robot physical interaction. In this paper, a specialized ankle joint muscle reflex control algorithm for human upright standing push-recovery is proposed. The proposed control algorithm is composed of a proportional-derivative (PD)-like controller and a positive force controller, which are employed to mimic the human muscle stretch reflex and muscle tendon force reflex, respectively. Reflex gains are regulated by muscle activation levels of contralateral ankle muscles. The proposed method was implemented on a self-designed series elastic robot ankle joint (SERAJ), where the series elastic actuator (SEA) has the potential to mimic human muscle–tendon unit (MTU). During the push-recovery experimental study, the surface electromyography (sEMG), ankle torque, body sway angle, and velocity of each subject were recorded in the case where the SERAJ was unilaterally kneed on each subject. The experimental results indicate that the proposed muscle reflex control method can easily realize upright standing push-recovery behavior, which is analogous to the original human behavior.

Information for anticipatory neuromotor control in catching under load uncertainty.

Humans employ anticipatory muscle activation when catching under conditions of load uncertainty. Questions addressed were (a) on what information referent do catchers base their anticipatory neuromotor control when catching balls of unknown weight?, and (b) how do catchers use this functional referent? Thirty-six participants caught visually identical balls dropped from 0.75m. Participants performed 40 trials, half with knowledge of ball weight and half without. Group L caught balls with a large weight range, while group S caught balls with a smaller range of weights. EMG integrals were computed for the ball flight period in five muscles. Anticipatory EMG integrals in the unknown weight condition were normalized to anticipatory EMG integrals for the maximum, minimum and average ball weights in the known ball weight condition. We assumed participants would base anticipatory control in the unknown weight condition on similar information, regardless of group. Therefore, differences in normalized EMG integrals between groups L and S would suggest that the specific referent tested (e.g., minimum possible ball weight) was not used to scale anticipatory muscle activation under load uncertainty. Independent sample t tests ascertained differences in normalized EMG integrals between groups L and S. The results suggested that the information referent participants used to catch balls of an unknown weight was knowledge of the maximum ball weight. Participants used this referent to generate a submaximal level of anticipatory muscle activation, i.e., about 93.2% of that used to catch the heaviest ball when ball weight was known in advance.

Effect of target distance on controllability for myocontrol

Abstract Myocontrol holds great promise because it has the potential to provide flexible and accurate prosthetic control that approaches the quality of normal movement. Speed and accuracy are important factors to consider when applying myoelectric signals to external devices. Fitts's law can be used to assess the speed-accuracy trade-off. We hypothesized that speed is affected not only by accuracy as prescribed by Fitts's law, but also by target distances independent of target size. A total of 12 healthy adult subjects were studied. Subjects controlled the vertical movement of a computer cursor by contracting their dominant first dorsal interosseus muscle to reach targets as quickly as possible. We manipulated twenty-five different target distances in order to obtain five indices of difficulty, as defined by Fitts's law, combined with five target widths. We tested the relationship between movement time and the index of difficulty as predicted by Fitts's law among different combinations of target distance and widths. Results showed a significant linear regression for all conditions, with the exception of a significantly longer movement time than predicted for targets close to the start point. Movements to these targets showed significantly higher relative variance during stabilization, higher overshoot, and lower success. Therefore, we found that with comparable index of difficulty, small distance movements had a higher variability, slower movement, and higher rates of error compared to larger distance movements. Our results are consistent with our hypothesis that low muscle activation required for short distances results in higher variability and low controllability in reaching the target as required by the task demand. Neurophysiological mechanisms underlying the violation of the Fitts's law relationship are discussed. These results have significance for myocontrol applications, and we suggest that such applications require control signals with sufficient recruitment to reduce variability at lower levels of muscle activation.

Model‐Based Analysis of Lower Thoracic High‐Frequency Spinal Cord Stimulation (HF‐SCS) to Restore Effective Cough

BackgroundSpinal cord stimulation (SCS, 50 Hz) is a useful method to restore an effective cough in persons with spinal cord injury (SCI). In animal models, dorsal HF‐SCS (~500 Hz) produces similar level of expiratory muscle activation with much lower stimulus amplitudes. Currently, the mechanism of this behavior is unknown.ObjectiveThe aim of the present study was to develop a computational model of lower thoracic SCS to investigate the mechanisms of the expiratory muscle activation via HF‐SCS.MethodsWe created a computer model of an anatomically‐accurate finite element model (FEM) of the lower thoracic (T9) canine spinal cord and surrounding anatomy with an implanted SCS electrode array. This model consisted of two main components: 1) multicompartment cable models of dorsal afferent fibers and 2) ventral motor axons. Model analysis allowed calculation of the extracellular voltages generated during SCS and the corresponding activation thresholds of each fiber type.ResultsIn our simulations, we calculated the activation thresholds for several electrode combinations and stimulation frequencies (50 Hz, 500 Hz). As expected, the activation thresholds of the ventral motor axons were significantly higher than the activation thresholds of dorsal root axons. Therefore, low‐frequency SCS likely functions through the direct activation of ventral motor axons, while high‐frequency SCS operates via recruitment of myelinated afferent fibers at significantly lower stimulation amplitudes. In our simulations, the stimulation frequency did not significantly alter activation thresholds in dorsal or ventral axons.ConclusionsOur results suggest that both high‐ and low‐frequency SCS directly activate dorsal rootlets at low amplitudes (~1 mA); however, the trans‐synaptic spinal circuits responsible for eliciting cough are only effective at higher stimulation frequencies. Consequently, low‐frequency stimulation results in effective cough by bypassing this circuitry entirely to directly activate the ventral motor axons at the cost of requiring higher stimulation amplitudes. In future work, we will perform simulations directly assessing the ability of SCS to engage these spinal circuits. We will compare our model results to experimental measurements in an animal model of SCS to produce cough.DisclosureDr. DiMarco holds two U.S. patents for technology related to the content of this abstract: Method and Apparatus for Electrical Activation of the Expiratory Muscles to Restore Cough (5,999,855) and Bipolar Spinal Cord Stimulation to Activate the Expiratory Muscles to Restore Cough (8,751,004).Support or Funding InformationMerit Review Award I01‐RX‐001488 from the U.S. Department of Veterans Affairs, Rehabilitation Research and Development Service.

Electromyography (EMG) for Assessment in Low Back Pain; Erector Spinae Muscle

The purpose of electromyography (EMG) study is to identify which position of surface EMG sensor attached to erector spinae muscle related with lower back pain by squat and stoop lifting technique. This is to avoid lower back pain (LBP) occur during Manual Materials Handling (MMH). There are only one types of upper extremity muscle were chosen to be monitored in this study which is erector spinae (ES) muscle with different electrodes placement on the surface electromyography (sEMG) sensor. However, each of the lifting styles come out with the different reading of root mean square (RMS) frequency for each muscle chosen. In this study, the two subjects consist of two females with normal body mass index (BMI) range from 18.5 to 24.9 with same physical measurement, was selected in order to perform both styles of lifting which are squatting and stooping. For every session the subject will undertake 15 repetitions with 15 minutes rest in between for each movement. In furtherance of to get the analysis muscle activity, proEMG software is used. The results of study for subject female 1 showed that the squat technique had higher levels of muscle activation compared to stoop technique on left erector spinae (LES) muscle. However, the LES muscle activation for subject 2 is greater at stoop activity. On the contrary, squat technique had lower fatigue analysis compared to stoop technique for both subjects on LES muscle. Conclusion, squat technique is better than stoop technique but stoop lift is more natural and spontaneously used for MMH.

Estimates of Motor Neuron Excitability During Postural Sways in Lower Leg Muscles

Motor system investigations are often performed under isometric conditions, but real‐world motor behaviors are seldom isometric. During tasks where isometric force or torque measurements are not feasible (i.e. during unconstrainedconditions), EMG is often used as a surrogate of the level of muscle activity. However, the linear relationship between EMG and force generation breaks down during non‐isometric contractions. It is not clear how this affects investigators’ ability to discriminate motor units using state‐of‐the‐art high‐density surface EMG arrays. Additionally, the paired motor unit analysis technique, which generates an estimate of the intrinsic excitability of a ‘test’ motor unit by determining discharge rate hysteresis (ΔF), has not been evaluated in unconstrained conditions. Consequently, we asked if 1) visual feedback provided by the subject’s own EMG activity allow for subjects to adequately perform triangular ramp contractions in constrained conditions and 2) if the EMG visual feedback is provided in unconstrained conditions, are ΔF values in subjects similar to those in the constrained condition? We hypothesized that during voluntary isometric ramp contractions, ΔF will be similar regardless of the feedback source. We further hypothesized that standing postural adjustments using EMG feedback in unconstrained conditions will produce similar ΔF values to those calculated during the isometric ramps. We expected that although the mean values across conditions will be similar, the inter‐subject coefficient of variation of ΔF will be greater in the less constrained condition. Accordingly, surface EMG using high‐density array electrodes was collected from five subjects under three conditions: 1) seated in a dynamometer in the isometric mode with torque feedback, 2) in the isometric dynamometer with filtered EMG feedback, and 3) during postural adjustments while standing freely with EMG feedback. In agreement with our hypothesis, we found that ΔF values across all three conditions were similar. In the constrained conditions with either torque or EMG feedback ΔF was 3.10 ± 0.70 imp/s and 3.40 ± 0.75 imp/s, respectively; during the unconstrained case with EMG feedback, ΔF was 3.08 ± 0.73 imp/s (all n = 15, mean ± SE). Contrary to our expectations, the coefficient of variation was not greater in the unconstrained standing condition. These results demonstrate that real‐time EMG feedback is sufficiently similar to torque feedback during both isometric and near‐isometric, unconstrained contractions, suggesting that detailed investigations of motor unit activity may be expanded to more functional movements and used relatively easily in more clinical settings.

The effects of stance width on muscle activation and performance during Romanian deadlift exercise

This study was conducted to determine the effect of different stance-width; i) narrow, ii) shoulder and iii) wide, on muscle activation and performance during Romanian deadlift (RDL) exercise. Thirty recreationally resistance trained men aged of 19-23 years old (22.20±1.13) were involved in this study. The participants need to perform RDL with 80% of their 1RM in three sessions with three difference stances in randomized order. To measure the muscle activation level during performing the exercise, the electromyogram (EMG) marker was placed on vastus lateralis, biceps femoris, gluteus maximus and multifidus muscles. The number of repetitions completed during each sets was recorded as indicator for performance. The mean EMG value during concentric and eccentric movement along with the number of repetitions completed were analysed using one way repeated measure analysis of variances (ANOVA). The result showed no significant differences were found on EMG reading of vastus lateralis and bicep femoris during eccentric and concentric phase of RDL when three different stances were used. However, when wide stances were used, a significant difference was observed on gluteus maximus whilst significant differences on multifidus were obviously seen when narrow stance were used. Higher number of repetition completed was significantly found when wide and shoulder width stances were used compared to narrow stance. The results of this study revealed the importance to choose correct stance width (depending on training objective) while performing RDL due to its effects on the muscle activation and performance.

Electromyography Analysis of Muscle Activation During Stand-Up Paddle Boarding: A Comparison of Paddling in Kneeling and Standing Positions

Background: This study aimed to understand individual muscle use in different paddling postures in stand-up paddle boarding (SUP). Methods: Sixteen college students were recruited in this study. Surface electromyography of 16 muscles on the dominant side was recorded. Results: In the time series, the biceps muscle exhibited a continuous activation pattern in the pull phase when kneeling, whereas when standing, the muscle contracted considerably in the exit and recovery phases, implying that it plays different roles in the two postures. The biceps also exhibited significantly higher muscle activation in the kneeling position than it did in the standing position. The maximum muscle activity levels of the external oblique abdominis and triceps were significantly higher when standing than when kneeling. In addition, an unstable SUP board activated the gastrocnemius to help paddlers maintain stability on a swaying surface. Moreover, additional power from the wrist flexor must be used in the recovery and catch phases to stabilize paddle control in the standing position. Conclusion: The knowledge that changes in SUP posture activate different muscle groups can enhance training efficiency and provide a reference for designing individualized training programs.

Spastic co-contraction is directly associated with altered cortical beta oscillations after stroke

ObjectiveSpastic co-contraction is a motor-disabling form of muscle overactivity occurring after a stroke, contributing to a limitation in active movement and a certain level of motor impairment. The cortical mechanisms underlying spastic co-contraction remain to be more fully elucidated, the present study aimed to investigate the role of the cortical beta oscillations in spastic co-contraction after a stroke. MethodWe recruited fifteen post-stroke participants and nine healthy controls. The participants were asked to perform active elbow extensions. In the study, multimodal analysis was performed to combine the evaluation of three-dimensional elbow kinematics, the elbow muscles electromyographic activations, and the cortical oscillatory activity. ResultsThe movement-related beta desynchronization was significantly decreased in post-stroke participants compared to healthy participants. We found a significant correlation between the movement-related beta desynchronization and the elbow flexors activation during the active elbow extension in post-stroke participants. When compared to healthy participants, post-stroke participants exhibited significant alterations in the elbow kinematics and greater muscle activation levels. ConclusionsCortical beta oscillation alterations may reflect an important neural mechanism underlying spastic co-contraction after a stroke. SignificanceMeasuring the cortical oscillatory activity could be useful to further characterize neuromuscular plasticity induced by recovery or therapeutic interventions.

Optimization of muscle activation schemes in a finite element neck model simulating volunteer frontal impact scenarios

Neck muscle activation is increasingly important for accurate prediction of occupant response in automotive impact scenarios and occupant excursion resulting from active safety systems such as autonomous emergency braking. Muscle activation and optimization in frontal impact scenarios using computational Human Body Models have not been investigated over the broad range of accelerations relevant to these events. This study optimized the muscle activation of a contemporary finite element model of the human head and neck for human volunteer experiments over a range of frontal impact severities (2 g to 15 g). The neck muscles were grouped as flexors and extensors, and optimization was undertaken for each group based on muscle activation level and activation time. The boundaries for optimization were defined using data from the literature and a preliminary parametric study. A linear polynomial method was used to optimize the model head kinematics to the volunteer experiments for each impact severity. The optimized models predicted muscle activation to increase with higher impact severities, and improved the average cross-correlation by 35% (0.561–0.755) relative to the Maximum Muscle Activation (MMA) scheme in the original model. Importantly, a newly proposed Cocontraction Muscle Activation (CMA) scheme for maintaining the head in a neutral posture provided a 23% on average improvement in correlation compared to the MMA scheme. In conclusion, this study identified a new scheme to obtain more accurate response kinematics across multiple impact severities in computational Human Body Models as well as contributing to the understanding of muscle influence during frontal impact scenarios.