Muscle coactivation: definitions, mechanisms, and functions

Useful and Useless Misnomers in Motor Control

This article addresses the issue of using terms and concepts in motor control that are ill-defined, undefined, and/or imported from nonbiological fields. In many of such cases, the discourse turns nonscientific and unproductive. Some of such terms are potentially useful but need to be properly and exactly defined. Other terms seem to be misleading and nonfixable. There is also an intermediate group with terms that may or may not be useful if defined properly. The paper presents three examples per group: "reflex," "synergy," and "posture" versus "motor program," "efference copy," and "internal model" versus "muscle tone," "stiffness and impedance," and "redundancy." These terms are analyzed assuming that motor control is a branch of natural science, which must be analyzed using laws of nature, not a subfield of the control theory. In the discussion, we also accept the framework of the theory of movement control with spatial referent coordinates as the only example built on laws of nature with clearly formulated physical and physiological nature of the control parameters.

Force drifts and matching errors in the lower extremities: implications for the control and perception of foot force

Accurate control of force on the environment is mechanically necessary for many tasks involving the lower extremities. We investigated drifts in the horizontal (shear) active force produced by right-footed seated subjects and the effects of force matching by the other foot. Subjects generated constant shear force at 15% and 30% of maximal voluntary contraction (MVC) using one foot. Visual feedback of shear force magnitude was provided for the first 5s, then turned off for 30s. During the 30% MVC task, we observed parallel drops in active shear and vertical force magnitudes leading to consistent drifts in the resultant force magnitude, not in its direction. Force matching by the other foot resulted in significantly lower forces when feedback was available throughout the trial. No feedback was provided for the matching foot. When the matching foot began exerting force, the task foot experienced a notable drop in all force components, with a change in force direction only for the task foot. After this initial drop, the downward drift in the task foot stopped or reversed. Subjects were unaware of these drifts and errors. Our findings suggest that shear force production involves setting a referent coordinate vector, which shows drifts and matching errors, while its direction remains stable. Involvement of the matching foot appears to perturb the neural commands to the task foot, with minor differences observed between feet. The discrepancy between the consistent force drifts and lack of awareness of the drifts indicates a difference between force perception-to-act and perception-to-report.

A neuronal least-action principle for real-time learning in cortical circuits

One of the most fundamental laws of physics is the principle of least action. Motivated by its predictive power, we introduce a neuronal least-action principle for cortical processing of sensory streams to produce appropriate behavioral outputs in real time. The principle postulates that the voltage dynamics of cortical pyramidal neurons prospectively minimizes the local somato-dendritic mismatch error within individual neurons. For output neurons, the principle implies minimizing an instantaneous behavioral error. For deep network neurons, it implies the prospective firing to overcome integration delays and correct for possible output errors right in time. The neuron-specific errors are extracted in the apical dendrites of pyramidal neurons through a cortical microcircuit that tries to explain away the feedback from the periphery, and correct the trajectory on the fly. Any motor output is in a moving equilibrium with the sensory input and the motor feedback during the ongoing sensory-motor transform. Online synaptic plasticity reduces the somatodendritic mismatch error within each cortical neuron and performs gradient descent on the output cost at any moment in time. The neuronal least-action principle offers an axiomatic framework to derive local neuronal and synaptic laws for global real-time computation and learning in the brain.

Neuromechanical Circuits of the Spinal Motor Apparatus

The evolution of mechanisms for terrestrial locomotion has resulted in multi-segmented limbs that allow navigation on irregular terrains, changing of direction, manipulation of external objects, and control over the mechanical properties of limbs important for interaction with the environment, with corresponding changes in neural pathways in the spinal cord. This article is focused on the organization of these pathways, their interactions with the musculoskeletal system, and the integration of these neuromechanical circuits with supraspinal mechanisms to control limb impedance. It is argued that neural pathways from muscle spindles and Golgi tendon organs form a distributive impedance controller in the spinal cord that controls limb impedance and coordination during responses to external disturbances. These pathways include both monosynaptic and polysynaptic components. Autogenic, monosynaptic pathways serve to control the spring-like properties of muscles preserving the nonlinear relationship between stiffness and force. Intermuscular monosynaptic pathways compensate for inertial disparities between the inertial properties of limb segments and help to control inertial coupling between joints and axes of rotation. Reciprocal inhibition controls joint stiffness in conjunction with feedforward cocontraction commands. Excitatory force feedback becomes operational during locomotion and increases muscular stiffness to accommodate the higher inertial loads. Inhibitory force feedback is widely distributed among muscles. It is integrated with excitatory pathways from muscle spindles and Golgi tendon organs to determine limb stiffness and interjoint coordination during interactions with the environment. The intermuscular distribution of force feedback is variable and serves to modulate limb stiffness to meet the physical demands of different motor tasks. © 2024 American Physiological Society. Compr Physiol 14:5789-5838, 2024.

The effect of a contralateral foot touch on stability of one-leg stance in young adults: an exploratory study

PURPOSE

Previous studies demonstrated that providing light finger touch to a stationary object leads to reduced body sway. The focus of the current exploratory study was on the investigation of postural sway during one-leg stance when light touch is provided by the contralateral foot.

METHODS

Eleven healthy young adults participated in the study. They stood on the top of the force platform with eyes open and on their dominant leg with no touch and with a touch from the contralateral foot applied to the stance leg. Medial arch, heel, and big toe of the contralateral foot were used to touch the medial malleolus or mid shank of the stance leg. The excursion, velocity, and sway area of the centre of pressure were obtained and analysed.

RESULTS

Standing with light touch from the contralateral foot to the medial malleolus resulted in significantly smaller postural sway as compared to standing with no touch (p < 0.05). There was no difference in the study outcomes between conditions of standing with a touch applied by the medial arch, heel, or big toe to the stance leg.

CONCLUSIONS

The results of the study suggest that the application of light touch provided by a contralateral foot could be an effective strategy for enhancing body stability when no external support is available. The study outcome provides a foundation for future studies exploring ways to enhance balance control during one-leg stance.

A Potential Mechanism Involved in the Regularity of Center-of-Pressure Displacements During Achieving Unipedal Equilibrium on Stable and Unstable Surfaces

Sample entropy (SampEn) is a widely used measure for estimating the regularity of center-of-pressure (COP) trajectories, with high COP regularity (low SampEn) commonly interpreted as ineffective postural control. The current study aimed to investigate one possible source of COP regularity-agonist-antagonist coactivation-resulting in joint stiffness or action stability. A cross-correlation analysis was conducted to determine the similarity (coactivation) level of 5 pairs of lower limb agonist-antagonist electromyographic signals. SampEn was applied to anteroposterior (ap) and mediolateral (ml) COP displacements. Electromyographic and COP signals were simultaneously derived from 25 active young adults (25.6 ± 4.0 years) during unipedal balancing on stable and multiaxial-unstable surfaces. Then, the correlation between individual agonist-antagonist cross-correlation coefficients and SampEn-COP was explored. The results show that only the tibialis anterior-peroneus longus (TA-PL) coactivations are negatively correlated with SampEn-COPap/ml (P ≤ .030) in both surface conditions, with greater TA-PL coactivation appearing with lower SampEn-COPap/ml. In addition, sex effects as one factor that might influence postural control were also tested. Women exhibited greater TA-PL coactivation (P = .017) and lower SampEn-COPap (P = .005) than men. Together, TA-PL coactivation could be one possible origin of COP regularity, but its effects might be detrimental to unipedal postural control, especially for women.

Reproducibility of Anterior Scalene Stiffness Measurement with Shear Wave Elastography: An Inter-Examiner Reliability Study

PURPOSE

Although previous studies have highlighted the clinical relevance of the anterior scalene muscle (AS) in patients with neck pain or nerve compressive syndromes, evidence reporting the diagnostic accuracy of shear wave elastography (SWE) for assessing the AS stiffness properties is lacking. This study aimed to analyze the SWE inter-examiner reliability for calculating the Young's modulus and shear wave speed of the AS muscle in asymptomatic subjects.

MATERIALS AND METHODS

Using a linear transducer, ultrasound images of the antero-lateral neck region at the C7 level were acquired in 35 healthy volunteers by one experienced examiner and one novice examiner. After codifying the images to blind the participants' identity, the trial, and the side, Young's modulus and shear wave speed were obtained by an independent experienced rater in randomized order. Intra-class correlation coefficients (ICC), standard error of measurement (SEM), minimal detectable changes (MDC), and coefficient of variation (CV%) were calculated.

RESULTS

The assessed AS metrics showed no side-to-side differences (p>0.05). Sex differences were found for muscle size (p=0.002), but muscle brightness and stiffness were similar (p>0.05). Inter-examiner reliability was good for determining the AS muscle stiffness (ICC = 0.881 for Young's modulus and 0.850 for shear wave speed).

CONCLUSION

The obtained results suggest that assessing the AS stiffness properties in asymptomatic subjects is a reliable procedure. Further studies should verify the SWE capacity for discriminating healthy and clinical populations and identify potential factors contributing to the variance of measurement errors.

Training in Roller Speed Skating: Proposal of Surface Electromyography and Kinematics Data for Educational Purposes in Junior and Senior Athletes

Introduction: Roller skating shares biomechanical similarities with other sports, but specific studies on speed skaters are limited. Injuries, particularly to the groin, are frequent and related to acute and chronic muscle stress. Technology, particularly surface electromyography, can now be used to monitor performance and prevent injuries, especially those caused by muscular asymmetries. Such studies can be used to enhance training and for educational purposes. Materials and Methods: This pilot study was conducted on three subjects: two cadet-athletes and a novice, compared with the performance model of an elite athlete. Surface electromyography and kinematic analysis monitored the lower limb muscles during the propulsion and recovery phases of skating. Electrodes were placed on specific muscles, and triaxial accelerometers were used to detect kinematic differences and asymmetries. The results: Cadet 1 was closest to the elite athlete's performance model compared to Cadet 2, especially in kinematics and muscle efficiency. However, both cadets showed electromyographic differences compared to the elite athlete, with uneven muscle co-activations. The novice exhibited more oscillations and earlier propulsion compared to the elite athlete. Discussion: Using electromyography and kinematic analysis made it possible to identify differences between elite athletes, cadets, and novices. These observations provide useful data for developing personalized training and educational plans and preventing injuries related to muscle overload.

Passive shoulder occupational exoskeleton reduces shoulder muscle coactivation in repetitive arm movements

Humans naturally employ muscle coactivation to facilitate a broad range of movements, enhancing joint stability and movement accuracy. However, excessive muscle coactivation can become unfavorable or even detrimental. This phenomenon is often observed in industrial workers who endure repetitive or prolonged joint stress, particularly in areas such as the shoulders. Prolonged stress can result in soft tissue damage and the onset of work-related musculoskeletal disorders (MSDs). In recent years, there have been efforts to mitigate the emergence of work-related MSDs among industrial workers through the implementation of upper-limb occupational exoskeletons (OEs). While previous research has demonstrated their effectiveness in reducing shoulder muscle activation, particularly in static and overhead work activities, there has been a lack of studies examining the impact of upper-limb OEs on muscle coactivation during repetitive arm movements. To bridge this gap in knowledge, our study systematically assesses the influence of a passive exoskeleton's anti-gravitational support on shoulder muscle coactivation during repetitive arm movements. Results show that peak and mean coactivation levels linearly decrease with the increase of the amount of anti-gravitational support provided by the upper-limb OE, reaching approximately 51% and 54%, respectively. Conversely, the percentage of the movement cycle corresponding to the coactivation peak appears unaffected by the level of assistance. This study marks the first instance in which a passive upper-limb OE has been shown to reduce shoulder muscle coactivations, potentially paving the way for a novel methodology in their evaluation.

Lower limb muscle activation patterns in ice-hockey skating and associations with skating speed

In this study, we aimed to describe lower limb kinematic and muscle activation patterns and then to examine the potential associations between those variables and skating speed in highly trained ice-hockey players. Twelve players (age 18.4-22.0 years) performed five maximal 30-metre forward skating sprints. Skating speeds, muscle activities from eight lower limb muscles (gluteus maximus, gluteus medius, adductor magnus, rectus femoris, vastus lateralis, biceps femoris, tibialis anterior and soleus), and sagittal plane joint angles from the hip and knee joint were measured. A lower activity of the gluteus maximus (r = -0.651, p = 0.022, β = -0.08) and a reduced gluteus maximus to rectus femoris coactivity (r = -0.786, p = 0.002, β = -3.26) during the recovery phase were found to be associated with faster skating speed. No significant associations were observed between sagittal plane hip and knee kinematics and skating speed. This study provides evidence that muscle activities during the recovery phase of skating may have an important role in skating performance.

Understanding corticomotor mechanisms for activation of non-target muscles during unilateral isometric contractions of leg muscles after stroke

PURPOSE

Muscle activation often occurs in muscles ipsilateral to a voluntarily activated muscle and to a greater extent after stroke. In this study, we measured muscle activation in non-target, ipsilateral leg muscles and used transcranial magnetic stimulation (TMS) to provide insight into whether corticomotor pathways contribute to involuntary activation.

MATERIALS AND METHODS

Individuals with stroke performed unilateral isometric ankle dorsiflexion, ankle plantarflexion, knee extension, and knee flexion. To quantify involuntary muscle activation in non-target muscles, muscle activation was measured during contractions from the ipsilateral tibialis anterior (TA), medial gastrocnemius (MG), rectus femoris (RF), and biceps femoris (BF) and normalized to resting muscle activity. To provide insight into mechanisms of involuntary non-target muscle activation, TMS was applied to the contralateral hemisphere, and motor evoked potentials (MEPs) were recorded.

RESULTS

We found significant muscle activation in nearly every non-target muscle during isometric unilateral contractions. MEPs were frequently observed in non-target muscles, but greater non-target MEP amplitude was not associated with greater non-target muscle activation.

CONCLUSIONS

Our results suggest that non-target muscle activation occurs frequently in individuals with chronic stroke. The lack of association between non-target TMS responses and non-target muscle activation suggests that non-target muscle activation may have a subcortical or spinal origin. Non-target muscle activation has important clinical implications because it may impair torque production, out-of-synergy movement, and muscle activation timing.

Co-contraction embodies uncertainty: An optimal feedforward strategy for robust motor control

Despite our environment often being uncertain, we generally manage to generate stable motor behaviors. While reactive control plays a major role in this achievement, proactive control is critical to cope with the substantial noise and delays that affect neuromusculoskeletal systems. In particular, muscle co-contraction is exploited to robustify feedforward motor commands against internal sensorimotor noise as was revealed by stochastic optimal open-loop control modeling. Here, we extend this framework to neuromusculoskeletal systems subjected to random disturbances originating from the environment. The analytical derivation and numerical simulations predict a characteristic relationship between the degree of uncertainty in the task at hand and the optimal level of anticipatory co-contraction. This prediction is confirmed through a single-joint pointing task experiment where an external torque is applied to the wrist near the end of the reaching movement with varying probabilities across blocks of trials. We conclude that uncertainty calls for impedance control via proactive muscle co-contraction to stabilize behaviors when reactive control is insufficient for task success.

Analysis of Wavelet Coherence in Calf Agonist-Antagonist Muscles during Dynamic Fatigue

Dynamic muscle fatigue during repetitive movements can lead to changes in communication between the central nervous system and peripheral muscles. This study investigated these changes by examining electromyogram (EMG) characteristics from agonist and antagonist muscles during a fatiguing task. Twenty-two healthy male university students (age: 22.92 ± 2.19 years) performed heel raises until fatigue. EMG signals from lateral gastrocnemius (GL) and tibialis anterior (TA) muscles were processed using synchrosqueezed wavelet transform (SST). Root mean square (RMS), mean frequency (MF), power across frequency ranges, wavelet coherence, and co-activation ratio were computed. During the initial 80% of the task, RMS and EMG power increased for both muscles, while MF declined. In the final 20%, GL parameters stabilized, but TA showed significant decreases. Beta and gamma intermuscular coherence increased upon reaching 60% of the task. Alpha coherence and co-activation ratio remained constant. Results suggest that the central nervous system adopts a differentiated control strategy for agonist and antagonist muscles during fatigue progression. Initially, a coordinated "common drive" mechanism enhances both muscle groups' activity. Later, despite continued increases in muscle activity, neural-muscular coupling remains stable. This asynchronous, differentiated control mechanism enhances our understanding of neuromuscular adaptations during fatigue, potentially contributing to the development of more targeted fatigue assessment and management strategies.

Sure Steps: Key Strategies for Protecting Basketball Players from Injuries-A Systematic Review

Background: Basketball is a high-intensity sport, which includes actions such as jumping, changes of direction, accelerations, and decelerations, which generates fatigue situations that may increase the risk of injury. Specifically, the joints at greatest risk are the ankle and knee, with ankle sprains and anterior cruciate ligament (ACL) tears being the most prevalent injuries. There are several strategies aimed at reducing the incidence, based on training methods or other prophylactic measures. Therefore, the purpose of the study is to perform a systematic review of the different injury prevention strategies in competitive-level basketball players with respect to general injuries, ankle sprains, and ACL injuries. Methods: For this purpose, the PRISMA methodology was applied, performing a search in three databases (PubMed, SPORTDiscus, and Cochrane) between 25 September 2023 and 8 October 2023. Results: A total of 964 articles were identified, out of which 283 were duplicates and 644 were discarded. Out of the remaining 37, 23 were excluded because they did not meet the inclusion criteria; therefore, 14 articles were finally included. With respect to general injuries, 8 out of 14 studies reviewed them. Concerning ankle sprains, 7 studies specifically analyzed them. Finally, 3 studies focused on ACL injuries. Conclusions: Training programs that combine different contents, known as neuromuscular training, including strength work, stabilization or core, mobility, and agility are the most effective for both general injuries and ACL injuries. For ankle sprains, the most effective measures are training programs based on analytical ankle stability exercises and the use of ankle braces. Adherence to prevention programs is essential, so they can be included as part of the warm-up. Other strategies such as training load control, functional assessment, or rule modification are not used in the included articles, so their effectiveness as prophylactic methods could not be justified.

Reciprocal Inhibition and Coactivation of Ankle Muscles in Low- and High-Velocity Forward and Backward Perturbations

Reciprocal inhibition and coactivation are strategies of the central nervous system used to perform various daily tasks. In automatic postural responses (APR), coactivation is widely investigated in the ankle joint muscles, however reciprocal inhibition, although clear in manipulative motor actions, has not been investigated in the context of APRs. The aim was to identify whether reciprocal inhibition can be observed as a strategy in the recruitment of gastrocnemius Medialis (GM), Soleus (So) and Tibialis Anterior (TA) muscles in low- and high-velocity forward and backward perturbations. We applied two balance perturbations with a low and a high velocity of displacement of the movable platform in forward and backward conditions and we evaluated the magnitude and latency time of TA, GM and So activation latency, measured by electromyography (EMG). In forward perturbations, coactivation of the three muscles was observed, with greater activation amplitude of the GM and lesser amplitude of the So and TA muscles. For backward, the pattern of response observed was activation of the TA muscle, a decrease in the EMG signal, which characterizes reciprocal inhibition of the GM muscle and maintenance of the basal state of the So muscle. This result indicates that backward perturbations are more challenging.

Effects of an Auditory Versus Visual Stimulus on Reaction and Response Time During Countermovement Jumps

Reacting and responding to an external stimulus is an important component of human performance, and they inform us about a participant's neurophysiological capabilities. Our purpose in this study was to determine whether reaction times (REACT), response times (RT), and countermovement jump (CMJ) performance differ when responding to an auditory (AUD) versus visual (VIS) stimulus. Participants were 17 college-aged volunteers (6 females and 11 males; M age = 23.0, SD = 3.4 years; M height = 174.57, SD = 10.37 cm; M body mass = 73.37, SD = 13.48 kg). Participants performed CMJs on force plates immediately upon receiving an AUD or a VIS stimulus. The AUD stimulus was a beep noise, while the VIS stimulus was a light on a screen in front of the participants. We determined REACT for the tibialis anterior (TA), medial gastrocnemius (GM), vastus lateralis (VL), and biceps femoris (BF) muscles to be the amount of time between stimulus onset and the initiation of the muscle's electromyographic (EMG) signal. We determined RT to be the amount of time between stimulus onset and the beginning of the participant's force production. We assessed CMJ performance via ground reaction forces during the unweighting, braking, and propulsive phases of the jump. We quantified EMG amplitude and frequency during each CMJ phase. We found RT to be faster to the AUD versus the VIS stimulus (p = .007). VL and BF muscles had faster REACT than TA and GM muscles (p ≤ .007). The AUD stimulus was associated with faster CMJ unweighting phase metrics (p ≤ .005). Thus, individuals may react and respond faster to an AUD versus VIS stimulus, with limited improvements in their subsequent physical performance.

Muscle synergies for multidirectional isometric force generation during maintenance of upright standing posture

Muscle synergies are defined as coordinated recruitment of groups of muscles with specific activation balances and time profiles aimed at generating task-specific motor commands. While muscle synergies in postural control have been investigated primarily in reactive balance conditions, the neuromechanical contribution of muscle synergies during voluntary control of upright standing is still unclear. In this study, muscle synergies were investigated during the generation of isometric force at the trunk during the maintenance of standing posture. Participants were asked to maintain the steady-state upright standing posture while pulling forces of different magnitudes were applied at the level at the waist in eight horizontal directions. Muscle synergies were extracted by nonnegative matrix factorization from sixteen lower limb and trunk muscles. An average of 5-6 muscle synergies were sufficient to account for a wide variety of EMG waveforms associated with changes in the magnitude and direction of pulling forces. A cluster analysis partitioned the muscle synergies of the participants into a large group of clusters according to their similarity, indicating the use of a subjective combination of muscles to generate a multidirectional force vector in standing. Furthermore, we found a participant-specific distribution in the values of cosine directional tuning parameters of synergy amplitude coefficients, suggesting the existence of individual neuromechanical strategies to stabilize the whole-body posture. Our findings provide a starting point for the development of novel diagnostic tools to assess muscle coordination in postural control and lay the foundation for potential applications of muscle synergies in rehabilitation.

Antiferromagnetic artificial neuron modeling of the withdrawal reflex

Replicating neural responses observed in biological systems using artificial neural networks holds significant promise in the fields of medicine and engineering. In this study, we employ ultra-fast artificial neurons based on antiferromagnetic (AFM) spin Hall oscillators to emulate the biological withdrawal reflex responsible for self-preservation against noxious stimuli, such as pain or temperature. As a result of utilizing the dynamics of AFM neurons, we are able to construct an artificial neural network that can mimic the functionality and organization of the biological neural network responsible for this reflex. The unique features of AFM neurons, such as inhibition that stems from an effective AFM inertia, allow for the creation of biologically realistic neural network components, like the interneurons in the spinal cord and antagonist motor neurons. To showcase the effectiveness of AFM neuron modeling, we conduct simulations of various scenarios that define the withdrawal reflex, including responses to both weak and strong sensory stimuli, as well as voluntary suppression of the reflex.

Force-velocity-power profiling in flywheel squats: Differences between sports and association with countermovement jump and change of direction performance

The objectives of our study were 1) to examine whether the force-velocity-power (FvP) outcomes in the concentric and eccentric phases of flywheel (FW) squats differ among sports disciplines and 2) to investigate the association between FvP outcome variables and two key sport-performance indicators: countermovement jump height (CMJ) and change of direction (CoD) time involving 90° or 180° turns. Tests were performed by 469 athletes from five different sport disciplines and physical education students. Our results showed that FvP outcomes, when measured during the eccentric phase of the FW squat, effectively differed between athletes from different sports. However, during the concentric phase of the squat, only for the slope variable differences between sports were found. Contrary to our hypothesis, there were no statistically significant correlations between FW squat FvP outcomes and CMJ or CoD test results. These results suggest that FvP outcomes derived from FW squats may lack external validity and cannot be reliably used as a predictor of athletes' jumping and agility performance. The paper discusses possible reasons for the larger differences between sports in eccentric FvP outcomes, as well as the absence of correlations between FvP outcomes and functional tests.

Referent Control of Side-to-Side Body-Weight Transfer During Standing and Stepping in Adults

Research suggests that locomotion may be primarily caused by shifting stable body balance from one location in the environment to another with subsequent rhythmical muscle activation by the central pattern generator (CPG), constituting a multi-level control system. All levels interact with environmental forces affected by proprioceptive and vestibular reflexes as well as vision. A similar multi-level control schema is likely used to shift body balance laterally when the body weight is rhythmically transferred from side-to-side. In order to do so, the system shifts a specific body posture in space. This body posture is referred to as the threshold or referent body posture, R, at which all muscles involved can be at rest but are activated depending on the deflection of the actual body posture, Q, from R. This concept has previously been investigated for forward and backward locomotion. The purpose of the present study was to verify if it was also applicable to locomotor tasks in other directions such as sidestepping. We predicted that during sidestepping, the actual and referent posture can transiently match each other bringing the activity of multiple muscles to a minimum. The existence of such minima was demonstrated in healthy adults performing three locomotor tasks involving shifts of the body weight from side-to-side thus further supporting the validity of the multi-level control scheme of locomotion.

Differences in left and right lower limb control strategies in coping with visual tracking tasks during bipedal standing

Introduction

Differences in motor control between the lower limbs may influence the risk of sports injury and recovery from rehabilitation. In this study, differences in the visual feedback ability of the left and right lower limbs were assessed using visual target tracking tasks.

Methods

Thirty-four healthy young subjects (aged 20.4 ± 1.2 years) were asked to move their bodies back and forth while tracking a visual target displayed on a monitor in front of them for 30 s. The two target motions were sinusoidal (i.e., predictable patterns) and more complex (random) patterns. To assess the ability of the lower limbs to follow visual target tracking, antero-posterior CoP (right limb, CoPap-r; left limb, CoPap-l) and medio-lateral CoP (right limb, CoPml-r; left limb, CoPml-l) data were measured using a stabilometer. Tracking ability by visual feedback ability was calculated as the difference in displacement between the target signal and the trajectories of the right and left pressure centers as trapezoidal areas, and a smaller sum of area (SoA) over the entire measurement time was defined as a greater tracking ability.

Results

Regarding the SoA in the anterior-posterior CoP, the mean SoA in the sinusoidal and random tasks was significantly lower in the CoP-r data than in the CoP-l data, indicating that the right lower limb had a more remarkable ability to follow visual target tracking. Regarding the SoA in the medial-lateral direction (CoP), the mean SoA in the sinusoidal and random tasks did not significantly differ between the two legs.

Discussion

The right lower limb may have a tracking function activated by the target signal when responding to visual stimuli. Identifying the motor strategies of each lower limb in response to visual stimuli will not only help identify potential differences between each lower limb but also suggest the possibility of enhancing the role of each lower limb in balance control.

Neuromuscular control strategies of the lower limb during a typical Tai Chi brush knee and twist step in practitioners with and without knee pain: a pilot study

As a complex movement, Tai Chi (TC) could be challenging for knee control, and the compensatory changes in TC biomechanics of knee pain patients are unknown. The Brush Knee and Twist Step (BKTS) is a typical TC movement that involves basic leg motion repeated in the whole TC. This pilot study examined electromyography and retro-reflective marker trajectory data to investigate neuromuscular control strategies of the lower extremity during BKTS in TC practitioners with and without knee pain. Twelve experienced TC practitioners with (n = 6) and without knee pain (n = 6) participated. Our results revealed that knee pain practitioners presented muscle imbalance in the vastus medialis-vastus lateralis and vastus lateralis-biceps femoris, and poor alignment of the knee with the toes in TC lunge. Additionally, they adaptively developed rigid coordination strategies, showing higher levels of lower limb muscle co-contraction and activity compared to controls. Training programs for TC practitioners with knee pain should be designed to modify both abnormal muscle synergy patterns and incorrect lunge during TC, which may improve exercise safety.

Effect of scapular posterior tilting exercise on scapular muscle activities in men and women with a rounded shoulder posture

Round-shoulder posture (RSP) is a common postural condition, characterized by protraction, downward rotation, anterior tilting and internal rotation of the scapula. RSP can lead to shoulder dysfunction. Different methods have been proposed for rehabilitating and correcting the altered posture in RSP including stretching, strengthening exercises, and shoulder brace or taping. However, the findings are controversial and studies are ongoing to develop more effective method. The present study is aimed at investigating the effects of scapular posterior tilting (SPT) exercise in different support positions on scapular muscle activities in men and women with RSP. In a prospective observational clinical study, we assessed demographic, basic clinical parameters and study variables of the subjects with RSP (n = 20) (men/women = 9/11) attending Daegu University in Gyeongsan, South Korea. To do so, we compared electromyographic (EMG) activities of lower trapezius and serratus anterior muscles between men and women with RSP during SPT exercise on four different support surfaces to determine any difference in the EMG activities. The results revealed that women showed significant differences in EMG activities in the lower and left upper trapezius and serratus anterior muscles, while men showed significant differences in EMG activity only in the lower trapezius muscle during SPT exercise on four different surfaces (P < 0.05). The post-hoc analysis revealed significantly greater EMG activity values in the lower trapezius and serratus anterior muscles during SPT exercise on the upper body unstable surface and whole-body unstable surface (p < 0.05). Independent t-tests after the Bonferroni correction showed no significant differences in muscle activities between men and women on the four different surfaces (p > 0.0125).

The Components of Lumbar Motor Control Are Not Inter-related

OBJECTIVES

To verify the relationship between the indicators of components of lumbar motor control and determine the factors related to the indicators to each of these components.

METHODS

Twenty-five healthy university students were included in the study. The lumbar spine and hip kinematic parameters of posterior/anterior pelvic tilt (mobility and smoothness), ball catching (reactivity), and forward/backward rocking (adaptive stability) were measured as indicators of lumbar motor control. Lumbar proprioception, trunk muscle strength, and lower trunk muscle thickness were also measured. Kinematic parameters of the lumbar spine and hip were measured using a small accelerometer. The data verified the relevance of indicators of lumbar motor control and the relationship with relevant factors.

RESULTS

No significant correlations were found for most lumbar motor control indicators. Lumbar proprioception and rectus abdominis muscle thickness were identified as relevant indicators of lumbar motor control.

CONCLUSIONS

Each component of lumbar motor control is independent and must be evaluated for the component whose function is required. Additionally, some components of lumbar motor control are associated with lumbar proprioception and rectus abdominis muscle thickness; thus, evaluation of these components is necessary when evaluating lumbar motor control.

Changes in co-contraction magnitude during functional tasks following anterior cruciate ligament reconstruction: A systematic review

BACKGROUND

Anterior cruciate ligament reconstruction (ACLR) is a common orthopedic surgery procedure whose incidence has increased over the past few decades. Nevertheless, it is believed that neuromuscular control remains altered from the early stages after ACLR to later years. Therefore, the aim of this study was to systematically evaluate the magnitude of co-contraction during functional tasks in subjects with unilateral ACLR.

METHODS

A systematic review design was followed. The search strategy was conducted in PubMed, Scopus, EBSCO, PEDro, Cochrane Library, and Web of Science databases from inception to March 2024. The inclusion criteria involved studies using electromyography (EMG) data to calculate muscle pair activation via the co-contraction index (CCI) in ACLR individuals during functional tasks. The Preferred Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed, and study quality was evaluated using National Institutes of Health (NIH) Study Quality Assessment Tools.

RESULTS

The search strategy found a total of 792 studies, of which 15 were included in this systematic review after reviewing the eligibility criteria. The magnitude of co-contraction was assessed in a total of 433 ACLR individuals and 206 controls during functional tasks such as hop, drop-land, step-up/step-down, and gait. Overall, approximately 79.6% of individuals who had undergone ACLR exhibited increased levels of co-contraction magnitude in the ACLR limb, while 8.5% showed low co-contraction levels.

CONCLUSIONS

The findings of the review suggest that, during functional tasks, most individuals who have undergone ACLR exhibit changes of co-contraction magnitude in the involved limb.

Instrumental Evaluation of the Effects of Vertebral Consolidation Surgery on Trunk Muscle Activations and Co-Activations in Patients with Multiple Myeloma: Preliminary Results

Multiple myeloma (MM) patients complain of pain and stiffness limiting motility. To determine if patients can benefit from vertebroplasty, we assessed muscle activation and co-activation before and after surgery. Five patients with MM and five healthy controls performed sitting-to-standing and lifting tasks. Patients performed the task before and one month after surgery. Surface electromyography (sEMG) was recorded bilaterally over the erector spinae longissimus and rectus abdominis superior muscles to evaluate the trunk muscle activation and co-activation and their mean, maximum, and full width at half maximum were evaluated. Statistical analyses were performed to compare MM patients before and after the surgery, MM and healthy controls and to investigate any correlations between the muscle's parameters and the severity of pain in patients. The results reveal increased activations and co-activations after vertebroplasty as well as in comparison with healthy controls suggesting how MM patients try to control the trunk before and after vertebroplasty surgery. The findings confirm the beneficial effects of vertebral consolidation on the pain experienced by the patient, despite an overall increase in trunk muscle activation and co-activation. Therefore, it is important to provide patients with rehabilitation treatment early after surgery to facilitate the CNS to correctly stabilize the spine without overloading it with excessive co-activations.

Electromyographic analysis of the traditional and spin throwing techniques for goalball games related to ball velocity for selected upper extremity muscles

BACKGROUND

Goalball is a popular sport among visually impaired individuals, offering many physical and social benefits. Evaluating performance in Goalball, particularly understanding factors influencing ball velocity during throwing techniques, is essential for optimizing training programs and enhancing player performance. However, there is limited research on muscle activation patterns during Goalball throwing movements, needing further investigation to address this gap. Therefore, this study aims to examine muscle activity in sub-elite visually impaired Goalball players during different throwing techniques and visual conditions, focusing on its relationship with ball velocity.

METHODS

15 sub-elite Goalball players (2 female, 13 males; mean age of 20.46 ± 2.23 years) participated in the study. Muscle activity was evaluated with the Myo armband, while ball velocity was measured using two cameras and analyzed with MATLAB software. Different visual conditions were simulated using an eye band, and the effects of these conditions on muscle activation and ball velocity were examined.

RESULTS

The flexor muscles were found to be more active during the spin throw techniques with the eyes open (p = 0.011). The extensor muscles were found to be more active in the eyes-closed spin throw techniques compared to the eyes-open position (p = 0.031). Ball velocity was found related to the flexor muscles. Interestingly, no significant differences in ball velocity were observed between different throwing techniques or visual conditions (p > 0.05).

CONCLUSIONS

Ball velocity, one of the performance indicators of the athlete, is primarily related to upper extremity flexor muscle strength rather than visual acuity. It has less visual acuity, but an athlete with more upper-extremity flexor muscle strength will have an advantage in Goalball game. The spin throw technique, which is reported to provide a biomechanical advantage for professional players in the literature, did not provide an advantage in terms of ball velocity for the sub elite players evaluated in our study. This knowledge can inform the development of targeted training programs aimed at improving technique and enhancing ball velocity in Goalball players.

Functional knee brace use for 21 h leads to a longer duration to achieve peak vertical ground reaction forces and the removal of the brace after 17.5 h results in faster loading of the knee joint

PURPOSE

To investigate the landing strategies used after discontinuing and continuing the use of a functional knee brace (FKB) while performing a drop jump.

METHODS

Following published methodology and power analysis, 23 uninjured male athletes, mean age of 19.4 ± 3.0 years, performed seven tests, during three test conditions (nonbraced, braced and removed brace or continued brace use), over 6 days of 12 testing sessions (S) for a total of 38.5 h. Each subject was provided with a custom-fitted FKB. This study focuses on the single leg drop jump kinetics during S12 when subjects were randomly selected to remove the FKB after 17.5 h or continued use of FKB. The time to peak vertical ground reaction forces (PVGRF) and PVGRF were recorded on landing in eight trials.

RESULTS

After brace removal, a significantly shorter mean time to PVGRF was recorded (9.4 ± 22.9 msec (3.9%), p = 0.005, 95% confidence interval (95% CI): -168.1, 36.1), while continued brace use required a nonsignificant (n.s.) longer mean duration to achieve PVGRF (19.4 ± 53.6 msec (8.9%), n.s., 95% CI: -49.7, 73.4). No significant mean PVGRF difference was found in brace removal (25.3 ± 65.8 N) and continued brace use (25.1 ± 23.0 N).

CONCLUSION

Removal of FKB after 17.5 h of use led to a significantly shorter time to achieve PVGRF, while continued brace use for 21 h required a longer duration to achieve PVGRF, suggesting faster and slower knee joint loading, respectively. Understanding the concerns associated with the use of FKB and the kinetics of the knee joint will assist clinicians in counselling athletes about the risks and benefits of using an FKB.

LEVEL OF EVIDENCE

Level II.

Automated analysis of pectoralis major thickness in pec-fly exercises: evolving from manual measurement to deep learning techniques

This study addresses a limitation of prior research on pectoralis major (PMaj) thickness changes during the pectoralis fly exercise using a wearable ultrasound imaging setup. Although previous studies used manual measurement and subjective evaluation, it is important to acknowledge the subsequent limitations of automating widespread applications. We then employed a deep learning model for image segmentation and automated measurement to solve the problem and study the additional quantitative supplementary information that could be provided. Our results revealed increased PMaj thickness changes in the coronal plane within the probe detection region when real-time ultrasound imaging (RUSI) visual biofeedback was incorporated, regardless of load intensity (50% or 80% of one-repetition maximum). Additionally, participants showed uniform thickness changes in the PMaj in response to enhanced RUSI biofeedback. Notably, the differences in PMaj thickness changes between load intensities were reduced by RUSI biofeedback, suggesting altered muscle activation strategies. We identified the optimal measurement location for the maximal PMaj thickness close to the rib end and emphasized the lightweight applicability of our model for fitness training and muscle assessment. Further studies can refine load intensities, investigate diverse parameters, and employ different network models to enhance accuracy. This study contributes to our understanding of the effects of muscle physiology and exercise training.

Cervicocephalic kinaesthesia reveals novel subgroups of motor control impairments in patients with neck pain

Cervical-spine sensorimotor control is associated with chronicity and recurrence of neck pain (NP). Tests used to measure sensorimotor impairments lack consistency in studied parameters. Interpretation is often based on either a handful or numerous parameters, without considering their possible interrelation. Different aspects of motor-control could be studied with different parameters, but this has not yet been addressed. The aim of this study was to determine if different parameters of cervical position (JPE) and movement (Butterfly) sense tests represent distinct components of motor-control strategies in patients with chronic NP. Principal component analysis performed on 135 patients revealed three direction-specific (repositioning from flexion, extension or rotations) and one parameter-specific (variability of repositioning) component for JPE, two difficulty-specific (easy or medium and difficult trajectory) and one movement-specific (undershooting a target) component for Butterfly test. Here we report that these components could be related to central (neck repositioning and control of cervical movement) and peripheral sensorimotor adaptations (variability of repositioning) present in NP. New technologies allow extraction of greater number of parameters of which hand-picking could lead to information loss. This study adds towards better identification of diverse groups of parameters offering potentially clinically relevant information and improved functional diagnostics for patients with NP.

Biomechanical requirements of meat cutting tasks: A pilot study quantifying tasks sustainability and assistance required at the wrist

The meat processing industry is particularly affected by distal upper limb musculoskeletal disorders. This pilot study aims at proposing a methodology able to quantify biomechanical requirements of meat cutting tasks at butchers' dominant wrist and, when necessary, at estimating the assistance needed to reach sustainability. Six professional butchers repeatedly cut pieces of pork. Joint angles were recorded using a motion capture system, cutting forces using an instrumented knife. Sustainability was computed by the maximal acceptable effort method. Assistance requirements were computed for isolated stressful exertions and for overall work cycle sustainability. Five butchers exceeded the sustainability threshold for wrist flexion. Ulnar or radial deviation torques were excessive for 2 and 3 of them, respectively. Extension torques were sustainable. The peak assistive torque for isolated exertions was at most 1.1Nm, 1.6Nm and 1.1Nm, and the percentage of assistance for overall sustainability was at most 60%, 56% and 56% for wrist flexion, ulnar and radial deviation, respectively.

The Lower Limb Muscle Co-Activation Map during Human Locomotion: From Slow Walking to Running

The central nervous system (CNS) controls movements and regulates joint stiffness with muscle co-activation, but until now, few studies have examined muscle pairs during running. This study aims to investigate differences in lower limb muscle coactivation during gait at different speeds, from walking to running. Nineteen healthy runners walked and ran at speeds ranging from 0.8 km/h to 9.3 km/h. Twelve lower limb muscles' co-activation was calculated using the time-varying multi-muscle co-activation function (TMCf) with global, flexor-extension, and rostro-caudal approaches. Spatiotemporal and kinematic parameters were also measured. We found that TMCf, spatiotemporal, and kinematic parameters were significantly affected by gait speed for all approaches. Significant differences were observed in the main parameters of each co-activation approach and in the spatiotemporal and kinematic parameters at the transition between walking and running. In particular, significant differences were observed in the global co-activation (CIglob, main effect F(1,17) = 641.04, p < 0.001; at the transition p < 0.001), the stride length (main effect F(1,17) = 253.03, p < 0.001; at the transition p < 0.001), the stride frequency (main effect F(1,17) = 714.22, p < 0.001; at the transition p < 0.001) and the Center of Mass displacement in the vertical (CoMy, main effect F(1,17) = 426.2, p < 0.001; at the transition p < 0.001) and medial-lateral (CoMz, main effect F(1,17) = 120.29 p < 0.001; at the transition p < 0.001) directions. Regarding the correlation analysis, the CoMy was positively correlated with a higher CIglob (r = 0.88, p < 0.001) and negatively correlated with Full Width at Half Maximum (FWHMglob, r = -0.83, p < 0.001), whereas the CoMz was positively correlated with the global Center of Activity (CoAglob, r = 0.97, p < 0.001). Positive and negative strong correlations were found between global co-activation parameters and center of mass displacements, as well as some spatiotemporal parameters, regardless of gait speed. Our findings suggest that walking and running have different co-activation patterns and kinematic characteristics, with the whole-limb stiffness exerted more synchronously and stably during running. The co-activation indexes and kinematic parameters could be the result of global co-activation, which is a sensory-control integration process used by the CNS to deal with more demanding and potentially unstable tasks like running.

Age-related modifications of muscle synergies during daily-living tasks: A scoping review

BACKGROUND

Aging is associated with changes in neuromuscular control that can lead to difficulties in performing daily living tasks. Muscle synergy analysis allows the assessment of neuromuscular control strategies and functional deficits. However, the age-related changes of muscle synergies during functional tasks are scattered throughout the literature. This review aimed to synthesize the existing literature on muscle synergies in elderly people during daily-living tasks and examine how they differ from those exhibited by young adults.

METHODS

The Medline, CINAHL and Web of Science databases were searched. Studies were included if they focused on muscle synergies in elderly people during walking, sit-to-stand or stair ascent, and if muscle synergies were obtained by a matrix factorization algorithm.

FINDINGS

Seventeen studies were included after the screening process. The muscle synergies of 295 elderly people and 182 young adults were reported, including 5 to 16 muscles per leg, or leg and trunk. Results suggest that: 1) elderly people and young adults retain similar muscle synergies' number, 2) elderly people have higher muscles weighting during walking, and 3) an increased inter and intra-subject temporal activation variability during specific tasks (i.e., walking and stair ascent, respectively) was reported in elderly people compared to young adults.

INTERPRETATION

This review gives a comprehensive understanding of age-related changes in neuromuscular control during daily living tasks. Our findings suggested that although the number of synergies remains similar, metrics such as spatial and temporal structures of synergies are more suitable to identify neuromuscular control deficits between young adults and elderly people.

The effectiveness of the STOP-X training program on the knee valgus angle and balance in female basketball players with dynamic knee valgus: a randomized controlled trial

BACKGROUND

Dynamic knee valgus (DKV) accompanied by poor balance is the cause of anterior cruciate ligament (ACL) injury in athletes, and the identification and correction of these factors are always of interest to researchers. Therefore, the purpose of this research was to investigate the effect of the STOP-X program on the knee valgus angle and static and dynamic balance in female basketball players with DKV defects.

METHODS

The present study was a quasi-experimental study. Thirty female basketball players with DKV defects were purposefully identified by the single-leg landing (SLL) test and were randomly assigned to two control (n = 15) and experimental (n = 15) groups. Static balance status was evaluated with the BASS STICK test, and dynamic balance status was evaluated with the Y-balance test (YBT). The experimental group performed the STOP-X program for 25-40 min for eight weeks (three times per week), and the control group performed their traditional warm-up program. Data were analyzed by means of 2 × 2 repeated measures ANOVA followed by post hoc comparison (Bonferroni) at the significance level of (P < 0.05) with SPSS version 26.

RESULTS

The results showed that with the use of the STOP-X program, there was a significant difference between the experimental and control groups in variables of the static balance (F = 56.45; P = 0.001; ES = 0.66, PC=↑59.64%), total dynamic balance score (F = 107.57; P = 0.001; ES=↑0.79, PC=↑19.84%), and knee valgus angle (F = 119.46; P = 0.001; ES = 0.81, PC=↓34.36%).

CONCLUSION

In addition to reducing the knee valgus angle, applying the STOP-X injury prevention program can improve static and dynamic balance in female basketball players with DKV defects. Therefore, it can be recommended that sports trainers benefit from these advantages by adding STOP-X training to routine basketball exercises.

Reduced corticospinal drive to antagonist muscles of upper and lower limbs during hands-and-knees crawling in infants with cerebral palsy: Evidence from intermuscular EMG-EMG coherence

BACKGROUND

There is growing interest in understanding the central control of hands-and-knees crawling, especially as a significant motor developmental milestone for early assessment of motor dysfunction in infants with cerebral palsy (CP) who have not yet acquired walking ability. In particular, CP is known to be associated with walking dysfunctions caused by early damage and incomplete maturation of the corticospinal tract. However, the extent of damage to the corticospinal connections during crawling in infants with CP has not been fully clarified. Therefore, this study aimed to investigate the disparities in intermuscular EMG-EMG coherence, which serve as indicators of corticospinal drives to antagonist muscles in the upper and lower limbs during crawling, between infants with and without CP.

METHODS

This study involved 15 infants diagnosed with CP and 20 typically developing (TD) infants. Surface EMG recordings were obtained from two pairs of antagonist muscles in the upper limbs (triceps brachii (TB) and biceps brachii (BB)) and lower limbs (quadriceps femoris (QF) and hamstrings (HS)), while the infants performed hands-and-knees crawling at their self-selected velocity. Intermuscular EMG-EMG coherence was computed in two frequency bands, the beta band (15-30 Hz) and gamma band (30-60 Hz), which indicate corticospinal drive. Additionally, spatiotemporal parameters, including crawling velocity, cadence, duration, and the percentage of stance phase time, were calculated for comparison. Spearman rank correlations were conducted to assess the relationship between EMG-EMG coherence and crawling spatiotemporal parameters.

RESULTS

Infants with CP exhibited significantly reduced crawling velocity, decreased cadence, longer cycle duration, and a higher percentage of stance phase time compared to TD infants. Furthermore, CP infants demonstrated decreased coherence in the beta and gamma frequency bands (indicators of corticospinal drive) in both upper and lower limb muscles. Regarding limb-related differences in the beta and gamma coherence, significant disparities were found between upper and lower limb muscles in TD infants (p < 0.05), but not in infants with CP (p > 0.05). Additionally, significant correlations between coherence metrics and crawling spatiotemporal parameters were identified in the TD group (p < 0.05), while such correlations were not evident in the CP group.

CONCLUSIONS

Our findings suggest that the corticospinal drive may functionally influence the central control of antagonist muscles in the limbs during typical infant crawling. This functional role could be impaired by neurological conditions such as cerebral palsy. The neurophysiological markers of corticospinal drive, specifically intermuscular EMG-EMG coherence during crawling in infants with cerebral palsy, could potentially serve as a tool to assess developmental response to therapy.

Combining Static and Dynamic Myofascial Dry Cupping Therapy to Improve Local and Regional Symptoms in Individuals with Low Back Pain: A Case Series

Introduction

Chronic low back pain is a common musculoskeletal healthcare presentation with an expense of over $100 billion annually. The clinical effect of myofascial cupping on pain and function is not clear, especially when different cupping techniques are combined. The purpose of this case series was to explore changes in pain and function following local static and distal dynamic myofascial dry cupping treatments in patients with chronic low back pain.

Case Descriptions

Three adults from the general population received three ten-minute treatment sessions, 48 hours between each session, of static dry cupping to the low back followed by dynamic myofascial cupping of the quadriceps and hamstring musculature. Outcome measures were taken at two different time points within one-week per participant. Subjective measures included the numeric pain rating scale and the Oswestry Disability Index, objective measures included passive straight leg raise measurements, and pressure pain threshold.

Results and Discussion

Local static combined with distal dynamic myofascial cupping reduced pain, pain sensitivity and perceived disability, and improved hamstring muscle extensibility in all three participants. These encouraging results support the initiation of a larger controlled trial aimed at investigating the efficacy of combined dry cupping interventions to treat musculoskeletal dysfunction and pain.

Level of Evidence

4 (case series).

The spinal cord facilitates cerebellar upper limb motor learning and control; inputs from neuromusculoskeletal simulation

Complex interactions between brain regions and the spinal cord (SC) govern body motion, which is ultimately driven by muscle activation. Motor planning or learning are mainly conducted at higher brain regions, whilst the SC acts as a brain-muscle gateway and as a motor control centre providing fast reflexes and muscle activity regulation. Thus, higher brain areas need to cope with the SC as an inherent and evolutionary older part of the body dynamics. Here, we address the question of how SC dynamics affects motor learning within the cerebellum; in particular, does the SC facilitate cerebellar motor learning or constitute a biological constraint? We provide an exploratory framework by integrating biologically plausible cerebellar and SC computational models in a musculoskeletal upper limb control loop. The cerebellar model, equipped with the main form of cerebellar plasticity, provides motor adaptation; whilst the SC model implements stretch reflex and reciprocal inhibition between antagonist muscles. The resulting spino-cerebellar model is tested performing a set of upper limb motor tasks, including external perturbation studies. A cerebellar model, lacking the implemented SC model and directly controlling the simulated muscles, was also tested in the same. The performances of the spino-cerebellar and cerebellar models were then compared, thus allowing directly addressing the SC influence on cerebellar motor adaptation and learning, and on handling external motor perturbations. Performance was assessed in both joint and muscle space, and compared with kinematic and EMG recordings from healthy participants. The differences in cerebellar synaptic adaptation between both models were also studied. We conclude that the SC facilitates cerebellar motor learning; when the SC circuits are in the loop, faster convergence in motor learning is achieved with simpler cerebellar synaptic weight distributions. The SC is also found to improve robustness against external perturbations, by better reproducing and modulating muscle cocontraction patterns.

Higher coactivations of lower limb muscles increase stability during walking on slippery ground in forward dynamics musculoskeletal simulation

The energy efficiency theory of human bipedal locomotion has been widely accepted as a neuro-musculoskeletal control method. However, coactivation of agonist and antagonist muscles in the lower limb has been observed during various limb movements, including walking. The emergence of this coactivation cannot be explained solely by the energy efficiency theory and remains a subject of debate. To shed light on this, we investigated the role of muscle coactivations in walking stability using a forward dynamics musculoskeletal simulation combined with neural-network-based gait controllers. Our study revealed that a gait controller with minimal muscle activations had a high probability of falls under challenging gait conditions such as slippery ground and uneven terrain. Lower limb muscle coactivations emerged in the process of gait controller training on slippery ground. Controllers with physiological coactivation levels demonstrated a significantly reduced probability of falls. Our results suggest that achieving stable walking requires muscle coactivations beyond the minimal level of muscle energy. This study implies that coactivations likely emerge to maintain gait stability under challenging conditions, and both coactivation and energy optimization of lower limb muscles should be considered when exploring the foundational control mechanisms of human walking.

Benefits of Training the Iron Cross With Herdos Devices and External Load Added to Body Weight for Young Nonachiever Gymnasts

PURPOSE

To verify whether training the iron cross (IC) with assistive devices (herdos; HIC) and added external load (LHIC) to equate the moments of force developed on the rings could be considered an intermediate step between the nonoverloaded herdos situation (HIC) and the IC performed on the rings.

METHODS

Relative levels of surface electromyography (sEMG) activity were normalized with respect to a standing IC before comparing gymnasts who can perform the IC on the rings (achievers) and gymnast who cannot (nonachievers) in the 2 herdos conditions (HIC and LHIC). Seven muscles were chosen for sEMG analysis, namely, pectoralis major (PM), latissimus dorsi, teres major, lower trapezius, serratus anterior, biceps brachii (BB), and triceps brachii. Additionally, 3 indices were calculated to measure levels of coactivation: Elbowidx, Scapulaidx, and Shoulderidx.

RESULTS

The bigger magnitude of differences in sEMG activity among situations was found for the PM and BB (F ≥ 30.7; P < .001). When comparing the global and the PM, teres major, BB, and triceps brachii activity across groups, nonachievers activated their musculature to a greater extent than the achievers independently of the herdos situation (P ≤ .046). Achievers' Elbowidx was the only index that was significantly higher (P ≤ .005) in the IC in comparison to LHIC and HIC.

CONCLUSION

sEMG activity of PM and BB was particularly sensitive between situations, independently of the level of achievement. We recommend training the IC by adding external load in the herdos situation to increase muscle activity to levels closer to the rings situation but avoiding the potential factor of injuries.

Effects of Acute Sleep Deprivation on the Sequential Rate of Torque Development throughout the Force-Time Curve

Objective  The impact of sleep deprivation on the physiological determinants of explosive torque production remains poorly understood. We aimed at determining the acute effects of 24 hours of sleep deprivation on the sequential rate of torque development (RTD) obtained during plantar flexion through maximum voluntary isometric contraction (MVIC). Materials and Methods  The study included 14 healthy-young adults (8 men and 6 women). The participants visited the laboratory on 2 different occasions: without and with 24 hours of sleep deprivation. In each session, the subjects were tested for RTD of the plantar flexors with concomitant recordings of the electromyographic (EMG) amplitude of the soleus over the following time intervals: 0 to 30, 30 to 50, 50 to 100, and 100 to 150 ms. Results  Sleep deprivation did not affect peak RTD (without sleep deprivation: 283.3 ± 111.6 N.m.s -1 versus with sleep deprivation: 294.9 ± 99.2 N.m.s -1 ; p  > 0.05) of plantar flexion. The sequential values of RTD, as well as the normalized amplitude of the soleus EMG, remained similar between both conditions (p > 0.05). Discussion  In conclusion, we found that 24 hours of sleep deprivation do not affect muscle activation, nor explosive torque production throughout the torque-time curve. Thus, exercise performance and daily functionality in tasks involving rapid torque development might remain well preserved after 24 hours of acute sleep deprivation.

Variability analysis of muscle activation symmetry to identify indicators of individual motor strategy: a case series on elite Paralympic powerlifters

Introduction

In Paralympic powerlifting competitions, movement execution symmetry is a technical requirement influenced by individual athlete characteristics and motor strategies. Identifying the elements associated with individual motor strategies can offer valuable insight for improving sport performance. Therefore, this case series study aimed to explore muscle activation symmetry and its intra- and inter-individual variability to determine the muscles mostly related to individual motor strategies in elite Paralympic powerlifters.

Methods

Bilateral electromyographic activation of the anterior deltoid (AD), pectoralis major (PM), latissimus dorsi (LD), triceps (TRI) and external oblique (EO) muscles were analysed in five elite Paralympic powerlifters while performing four sets of one-repetition maximum of Paralympic bench press. Muscle activation symmetry indexes (SI) were obtained and transformed to consider individual-independent evaluation. The coefficient of variation (CV), variance ratio (VR), and mean deviation (MD) were computed to assess inter- and intra-individual variability in electromyographic waveforms and SI.

Results

Both transformed and non-transformed SI indicated overall symmetric activation in DA, PM, TRI, and LD. Transformed SI revealed asymmetrical muscle activation of EO when grouping data (mean bilateral difference: 10%). Athletes exhibited low intra-individual SI variability in all analysed muscles (CV < 10%) and low inter-individual variability in DA, PM, LD, and TRI (CV < 10%; VR: 4%-11%; MD: 29%-43%). In contrast, higher inter-individual variability was observed in EO (CV: 23%; VR: 23%; MD: 72%-81%).

Conclusion

The highest variability and asymmetry in abdominal muscle activation among athletes emphasize the importance of personalized training approaches for targeting these muscles due to their role in individualizing motor strategies.

A comparative study on the overlapping effects of clinically applicable therapeutic interventions in patients with central nervous system damage

This study was conducted to investigate the effects of anti-gravity treadmill (AGT) training, which provides visual feedback and Biorescue training on proprioception, muscle strength, balance, and gait, in stroke patients. A total of 45 people diagnosed with post-stroke were included as study subjects; they were randomized to an AGT training group provided with visual feedback (Group A), a Biorescue training group provided with visual feedback (Group B), and an AGT/Biorescue group that subsequently received AGT training and Biorescue training (Group C). A muscle strength-measuring device was used to evaluate muscle strength. Timed Up and Go and Bug Balance Scale assessment sheets were used to evaluate balance ability. Dartfish software was used to evaluate gait ability. The results of the study showed that Groups A and C had a significant increase in muscle strength compared with Group B; in terms of balance and gait abilities, Group C showed a significant increase in balance ability and gait speed and a significant change in knee joint angle compared with Groups A and B. In conclusion, this study suggests that including a method that applies multiple therapeutic interventions is desirable in the rehabilitation of stroke patients to improve their independence.

The History and Future of Neuromusculoskeletal Biomechanics

The Executive Council of the International Society of Biomechanics has initiated and overseen the commemorations of the Society's 50th Anniversary in 2023. This included multiple series of lectures at the ninth World Congress of Biomechanics in 2022 and XXIXth Congress of the International Society of Biomechanics in 2023, all linked to special issues of International Society of Biomechanics' affiliated journals. This special issue of the Journal of Applied Biomechanics is dedicated to the biomechanics of the neuromusculoskeletal system. The reader is encouraged to explore this special issue which comprises 6 papers exploring the current state-of the-art, and future directions and roles for neuromusculoskeletal biomechanics. This editorial presents a very brief history of the science of the neuromusculoskeletal system's 4 main components: the central nervous system, musculotendon units, the musculoskeletal system, and joints, and how they biomechanically integrate to enable an understanding of the generation and control of human movement. This also entails a quick exploration of contemporary neuromusculoskeletal biomechanics and its future with new fields of application.

Adaptations in athletic performance and muscle architecture are not meaningfully conditioned by training free-weight versus machine-based exercises: Challenging a traditional assumption using the velocity-based method

BACKGROUND

Although the superior effectiveness of free-weight over machine-based training has been a traditionally widespread assumption, longitudinal studies comparing these training modalities were scarce and heterogeneous.

OBJECTIVE

This research used the velocity-based method to compare the effects of free-weight and machine-based resistance training on athletic performance and muscle architecture.

METHODS

Thirty-four resistance-trained men participated in an 8-week resistance training program allocated into free-weight (n = 17) or machine-based (n = 17) groups. Training variables (intensity, intraset fatigue, and recovery) were identical for both groups, so they only differed in the use of a barbell or specific machines to execute the full squat, bench press, prone bench pull, and shoulder press exercises. The velocity-based method was implemented to accurately adjust the planned intensity. Analysis of covariance and effect size (ES) statistics were used to compare both training modalities on a comprehensive set of athletic and muscle architecture parameters.

RESULTS

No between-group differences were found for any athletic (p ≥ 0.146) and muscle architecture (p ≥ 0.184) variable. Both training modalities significantly and similarly improved vertical jump (Free-weight: ES ≥ 0.45, p ≤ 0.001; Machine-based: ES ≥ 0.41, p ≤ 0.001) and lower limb anaerobic capacity (Free-weight: ES ≥ 0.39, p ≤ 0.007; Machine-based: ES ≥ 0.31, p ≤ 0.003). Additionally, the machine-based group meaningfully enhanced upper limb anaerobic power (ES = 0.41, p = 0.021), whereas the free-weight group significantly improved the change of direction (ES = -0.54, p = 0.003) and 2/6 balance conditions analyzed (p ≤ 0.012). Changes in sprint capacity (ES ≥ -0.13, p ≥ 0.274), fascicle length, and pennation angle (ES ≤ 0.19, p ≥ 0.129) were not significant for either training modality.

CONCLUSION

Adaptations in athletic performance and muscle architecture would not be meaningfully influenced by the resistance modality trained.

Posture of Healthy Subjects Modulated by Transcutaneous Spinal Cord Stimulation

Transcutaneous electrical stimulation of the spinal cord is used to restore locomotion and body weight support in patients with severe motor disorders. We studied the effects of this non-invasive stimulation on postural control in healthy subjects. Stimulation at the L1-L2 vertebrae was performed to activate the extensor muscles of the lower limbs. Because postural regulation depends on the cognitive style, the effects of the stimulation were analyzed separately in field-dependent (FD) and field-independent (FI) participants. During the study, FD and FI participants (N = 16, 25 ± 5 years, all right dominant leg) stood on a force platform in a soundproof chamber with their eyes closed. Stimulation was applied in the midline between the L1 and L2 vertebrae or over the left or right dorsal roots of the spinal cord; under the control condition, there was no stimulation. Stimulation destabilized posture in healthy subjects, whereas patients with movement disorders usually showed an improvement in postural control. In the FD participants, left dorsal root and midline stimulation increased several postural parameters by up to 30%. Dorsal root stimulation on the side of the supporting leg reduced postural control, while stimulation on the side of the dominant leg did not. No significant changes were observed in the FI participants.

Velocity-Based Method in Free-Weight and Machine-Based Training Modalities: The Degree of Freedom Matters

ABSTRACT

Hernández-Belmonte, A, Buendía-Romero, Á, Pallares, JG, and Martínez-Cava, A. Velocity-based method in free-weight and machine-based training modalities: the degree of freedom matters. J Strength Cond Res 37(9): e500-e509, 2023-This study aimed to analyze and compare the load-velocity relationships of free-weight and machine-based modalities of 4 resistance exercises. Moreover, we examined the influence of the subject's strength level on these load-velocity relationships. Fifty men completed a loading test in the free-weight and machine-based modalities of the bench press, full squat, shoulder press, and prone bench pull exercises. General and individual relationships between relative intensity (%1RM) and velocity variables were studied through the coefficient of determination ( R2 ) and standard error of the estimate ( SEE ). Moreover, the velocity attained to each %1RM was compared between both modalities. Subjects were divided into stronger and weaker to study whether the subject's strength level influences the mean test (mean propulsive velocity [MPV Test ]) and 1RM (MPV 1RM ) velocities. For both modalities, very close relationships ( R2 ≥ 0.95) and reduced estimation errors were found when velocity was analyzed as a dependent ( SEE ≤ 0.086 m·s -1 ) and independent ( SEE ≤ 5.7% 1RM) variable concerning the %1RM. Fits were found to be higher ( R2 ≥ 0.995) for individual load-velocity relationships. Concerning the between-modality comparison, the velocity attained at each intensity (from 30 to 100% 1RM) was significantly faster for the free-weight variant. Finally, nonsignificant differences were found when comparing MPV Test (differences ≤ 0.02 m·s -1 ) and MPV 1RM (differences ≤ 0.01 m·s -1 ) between stronger and weaker subjects. These findings prove the accuracy and stability of the velocity-based method in the free-weight and machine-based variants but highlight the need to use the load-velocity relationship (preferably the individual one) specific to each training modality.

Surface-Electromyography-Based Co-Contraction Index for Monitoring Upper Limb Improvements in Post-Stroke Rehabilitation: A Pilot Randomized Controlled Trial Secondary Analysis

Persons post-stroke experience excessive muscle co-contraction, and consequently the arm functions are compromised during the activities of daily living. Therefore, identifying instrumental outcome measures able to detect the motor strategy adopted after a stroke is a primary clinical goal. Accordingly, this study aims at verifying whether the surface electromyography (sEMG)-based co-contraction index (CCI) could be a new clinically feasible approach for assessing and monitoring patients' motor performance. Thirty-four persons post-stroke underwent clinical assessment and upper extremity kinematic analysis, including sEMG recordings. The participants were randomized into two treatment groups (robot and usual care groups). Ten healthy subjects provided a normative reference (NR). Frost's CCI was used to quantify the muscle co-contraction of three different agonist/antagonist muscle pairs during an object-placing task. Persons post-stroke showed excessive muscle co-contraction (mean (95% CI): anterior/posterior deltoid CCI: 0.38 (0.34-0.41) p = 0.03; triceps/biceps CCI: 0.46 (0.41-0.50) p = 0.01) compared to NR (anterior/posterior deltoid CCI: 0.29 (0.21-0.36); triceps/biceps CCI: 0.34 (0.30-0.39)). After robot therapy, persons post-stroke exhibited a greater improvement (i.e., reduced CCI) in proximal motor control (anterior/posterior deltoid change score of CCI: -0.02 (-0.07-0.02) p = 0.05) compared to usual care therapy (0.04 (0.00-0.09)). Finally, the findings of the present study indicate that the sEMG-based CCI could be a valuable tool in clinical practice.

Body Composition and Demographic Features Do Not Affect the Diagnostic Accuracy of Shear Wave Elastography

Shear-wave elastography (SWE) is an imaging method that can be used to estimate shear wave speed and the Young's modulus based on the measured shear wave speed under certain conditions. Up to date, no research has analyzed whether body composition factors contribute to ultrasound attenuation, refraction, reflection, and, consequently, SWE measurement errors. Therefore, this study aimed to analyze the association between demographic and body composition features with SWE errors for assessing the anterior scalene stiffness (which is a key structure in patients with neck pain and nerve compressive syndromes). Demographic (sex, age, height, weight, and body mass index), body composition (water volume, fat mass, and lean mass), and anterior scalene muscle stiffness (Young's modulus and shear wave speed) data were collected from a sample of asymptomatic subjects. After calculating the absolute SWE differences between trials and the reliability estimates, a correlation matrix was generated to quantify the association among all the variables. A total of 34 asymptomatic subjects (24 males) were included in the analyses. Test-retest reliability was excellent for assessing the Young's modulus and shear wave velocity (ICC = 0.912 and 0.923, respectively). No significant associations were found between age, height, weight, body mass index, body fat, lean mass, or water volume with SWE errors (p > 0.05). However, the Young's modulus error was associated with the stiffness properties (p < 0.01), whereas shear wave speed was associated with none of them (all, p > 0.05). A detailed procedure can reliably assess the AS muscle stiffness. None of the sociodemographic or body composition features assessed were correlated with SWE errors. However, baseline stiffness seems to be associated with Young's modulus error.

The Effect of Walking in High Heels on the Activation and Deactivation of Upper Trunk Muscles

The aim of the study was to investigate how high-heeled walking affects the coordination changes of timing of upper trunk muscle activation, and the possible occurrence of health problems in this part of the body of young women. We used surface electromyography (EMG) for data collection. The research group consisted of 30 women. Statistical significance of the changes in muscle coordination was confirmed when evaluating two of the four upper trunk muscles studied. M. trapezius and m. pectoralis major are not subject to changes in gait in high heels (HH) from the point of view of timing on a statistical level, but HH increase the intensity of muscle contraction of all monitored muscles, and therefore we recommend limiting the wearing of HH in case of health problems related to these muscles.

Muscle Coactivation Index during Walking in People with Multiple Sclerosis with Mild Disability, a Cross-Sectional Study

BACKGROUND

Multiple sclerosis (MS) is a progressive neurodegenerative disease characterized by axonal degeneration and demyelination. Changes in gait, related to joint kinematics and kinetics, especially at the ankle and knee, have been observed in people with MS (pwMS). Muscle coactivation plays an important role in joint stabilization; however, excessive coactivation may interfere with gait. The aim of this study was to analyze the differences in muscle activation during gait in pwMS compared to healthy individuals.

METHODS

A cross-sectional study was conducted involving pwMS and healthy controls. Surface electromyography was used to record muscle activity during gait. The main outcome measures were the coactivation index (CI) and the area under the curve (AUC), which were calculated for several pairs of lower extremity muscles.

RESULTS

Nine pwMS and nine healthy controls were included. When comparing the MS group to the control group, the AUC was significantly higher in the lateral gastrocnemius (p = 0.023) and the CI for the lateral gastrocnemius-anterior tibialis (p = 0.022) and gluteus maximus-lateral gastrocnemius (p = 0.047).

CONCLUSION

Mildly affected pwMS have altered muscle coactivation patterns during gait, especially in the most affected limb. The results highlight the importance of muscle coactivation in pwMS and its possible role in the early detection of gait abnormalities.