Individual Muscle Activation Research Articles

Development of EMG-based criteria to evaluate the difficulty of realization of sign language: A potential contribution for understanding the negative hand paintings

The sign language uses a combination of complex finger and wrist configurations. The frequency of use of a particular sign is highly dependent on its physiological difficulty. However, no method allows to quantify accurately this difficulty. In the context of paleolithic negative hand paintings this absence of methods is problematic since the hand signs which are painted may be related to a primitive hand sign language. The objective of this study was to develop and validate a method based on electromyography recordings for quantifying sign language difficulty. Electromyography of the six main hand muscles were recorded and analyzed to determine individual muscle activity, summed muscle activity and muscle coactivation. Those results were correlated to subjective scales of difficulties to determine the electromyographic variables and/or the combinations of them which are good candidates for determining hand sign difficulties. Among all variables the summed muscle activities and the thumb muscle coactivation presented the most promising criterion. On the top of that, those criterions presented encouraging correlation with the frequence of occurrence of ten hand paintings of the Gargas Cave which open further studies for analyzing the origin of negative hand paintings.

Effects of a plank-based strength training programme on muscle activation in patients with long COVID: a case series.

This study aimed to analyse the effects of a plank-based strength training programme on muscle activation in patients with long COVID. Case series study that included patients with long COVID who participated in a 12-week trunk and pelvic muscle strength training programme. Clinical variables and the modified fatigue impact scale (MFIS) were used to assess fatigue levels. Percentage of muscle activation during a core muscle plank was measured via surface electromyography. Pre- and post-intervention results were compared using the Wilcoxon signed-rank test and evaluated with Cohen's D effect size (ES). Twenty-one subjects participated in the study; 81% female, mean age 47.5 years (range: 28-55 years), and median duration of symptoms 21 months (range: 5-24 months); 90.5% of the participants experienced fatigue (MFIS score = 38). Muscle activation during plank exercises improved across all muscles after the intervention, with significant increases in the left (p = 0.011, medium ES) and right external oblique (p =0.039, small ES) muscles and the right latissimus dorsi muscle (p = 0.039, small ES). Additionally, significant reductions in fatigue were observed in the total MFIS score (p = 0.004, medium ES) and in the physical (p < 0.001, large ES) and psychosocial subscales (p = 0.033, small ES). Results suggest that a plank-based strength training programme may be effective in enhancing trunk and pelvic muscle activation in individuals with long COVID.

COMPARING THE EFFECTS OF CYCLE TIME PROTOCOLS OF AUSSIE CURRENT NEUROMUSCULAR ELECTROSTIMULATION ON MULTIFIDUS MUSCLE ACTIVITY IN CHRONIC LOW BACK PAIN: A RANDOMIZED CLINICAL TRIAL

Introduction: Neuromuscular electrostimulation (NMES) represents a therapeutic approach for addressing chronic low back pain (CLBP); however, the influence of NMES dose on muscle activity remains subject to debate. Objective: To compare the impact of two distinct NMES protocols employing Aussie current, characterized by varying dosages emphasizing time cycle alterations, on electromyographic activity within the multifidus muscles in individuals afflicted with CLBP. Methods: A randomized clinical trial encompassed 18 volunteers diagnosed with mechanical CLBP. These volunteers were randomly assigned to two NMES intervention groups with dissimilar dosages: 15 repetitions ([Formula: see text][Formula: see text]) and 30 repetitions ([Formula: see text][Formula: see text] ). In both interventions, the current amplitude was tailored to individual perception and documented at the culmination of each session. Over the course of four weeks, two sessions took place per week. Electromyographic activity of the multifidus muscles was evaluated using surface electromyography before and after the intervention. The assessment focuses on both time-domain analysis using Root Mean Square (RMS) and frequency-domain analysis involving mean activation frequency (FREQ). Results: There are no interactions between the time and intervention, but there is the time effect on RMS, indicating that post-intervention muscle activity exceeded pre-intervention values in both groups. FREQ values did not exhibit statistically significant discrepancies. Conclusions: This study showed that NMES using the Aussie current is effective in increasing muscle activity in individuals with CLBP, and the results were not influenced by the different cycle times with equal volumes.

Muscular Activation Levels of Subjects with Low Medial Longitudinal Arch During Single-Leg Squat and Single-Leg Balancing Exercises on Surfaces of Different Hardness.

The primary aim of this study is to examine the effect of the stiffness of rehabilitation material on muscle activation in individuals with low medial longitudinal arch. The secondary aim is to compare the effects of single-leg balancing and single-leg squat exercises on the activation of the muscles around the ankle in the same individuals. Twelve individuals with low medial longitudinal arch and 18 healthy control participants were included. Participants were asked to perform the single-leg balance and single-leg squat exercises on Theraband stability trainers of three different stiffness levels. Activation of the peroneus longus, peroneus brevis, tibialis anterior, medial and lateral gastrocnemius muscles during exercises was evaluated by surface electromyography. The single-leg balancing exercise performed on soft and very soft ground was more effective than single-leg squat in increasing the activation of the lateral gastrocnemius muscle in participants with a low medial longitudinal arch. It was determined that performing single-leg squat exercise on a very soft surface instead of hard ground caused more muscular activation in the peroneus brevis, tibialis anterior, and medial gastrocnemius in the same individuals. The exercise progression to be applied on different surfaces to increase the activation of the muscles around the ankle in individuals with a low medial longitudinal arch and in healthy individuals without a low medial longitudinal arch was different.

Novel comprehensive analysis of skilled reaching and grasping behavior in adult rats

BackgroundReaching and grasping (R&G) in rats is commonly used as an outcome measure to investigate the effectiveness of rehabilitation or treatment strategies to recover forelimb function post spinal cord injury. Kinematic analysis has been limited to the wrist and digit movements. Kinematic profiles of the more proximal body segments that play an equally crucial role in successfully executing the task remain unexplored. Additionally, understanding of different forelimb muscle activity, their interactions, and their correlation with the kinematics of R&G movement is scarce. New methodIn this work, novel methodologies to comprehensively assess and quantify the 3D kinematics of the proximal and distal forelimb joints along with associated muscle activity during R&G movements in adult rats are developed and discussed. ResultsOur data show that different phases of R&G identified using the novel kinematic and EMG-based approach correlate with the well-established descriptors of R&G stages derived from the Whishaw scoring system. Additionally, the developed methodology allows describing the temporal activity of individual muscles and associated mechanical and physiological properties during different phases of the motor task. Comparison with existing method(s)R&G phases and their sub-components are identified and quantified using the developed kinematic and EMG-based approach. Importantly, the identified R&G phases closely match the well-established qualitative descriptors of the R&G task proposed by Whishaw and colleagues. ConclusionsThe present work provides an in-depth objective analysis of kinematics and EMG activity of R&G behavior, paving the way to a standardized approach to assessing this critical rodent motor function in future studies.

Altered muscle synergy structure in patients with poststroke stiff knee gait

Stiff knee gait (SKG) occurrence after a stroke is associated with various abnormal muscle activities; however, the interactions among these muscles are unclear. This study aimed to elucidate the muscle synergy characteristics during walking in patients with SKG after a stroke. This cross-sectional study included 20 patients with poststroke SKG (SKG group), 16 patients without poststroke SKG (non-SKG group), and 15 healthy adults (control group). Participants walked a 10-m distance at a comfortable speed, and electromyographic data were recorded from six lower-limb muscles. Non-negative matrix factorization was employed to derive time-varying activity (C), muscle weights (W), and the percentage of total variance accounted for (tVAF) for muscle synergies. The SKG group showed a higher tVAF than the control group. The initial stance module (including knee extensors) showed increased activity during the swing phase. The initial swing module (including hip flexors and ankle dorsiflexors) exhibited a higher activity during the single-support phase but a lower activity during the swing phase. The synergy structure in patients with SKG after stroke was simplified, with specific abnormalities in synergy activities. SKG may arise from several synergy alterations involving multiple muscles, indicating that approaches focused on controlling individual muscle activities are unsuitable.

Adaptive Adjustments in Lower Limb Muscle Coordination during Single-Leg Landing Tasks in Latin Dancers.

Previous research has primarily focused on evaluating the activity of individual muscles in dancers, often neglecting their synergistic interactions. Investigating the differences in lower limb muscle synergy during landing between dancers and healthy controls will contribute to a comprehensive understanding of their neuromuscular control patterns. This study enrolled 22 Latin dancers and 22 healthy participants, who performed a task involving landing from a 30 cm high platform. The data were collected using Vicon systems, force plates, and electromyography (EMG). The processed EMG data were subjected to non-negative matrix factorization (NNMF) for decomposition, followed by classification using K-means clustering algorithm and Pearson correlation coefficients. Three synergies were extracted for both Latin dancers and healthy participants. Synergy 1 showed increased contributions from the tibialis anterior (p < 0.001) and medial gastrocnemius (p = 0.024) in Latin dancers compared to healthy participants. Synergy 3 highlighted significantly greater contributions from the vastus lateralis in healthy participants compared to Latin dancers (p = 0.039). This study demonstrates that Latin dancers exhibit muscle synergies similar to those observed in healthy controls, revealing specific adjustments in the tibialis anterior and medial gastrocnemius muscles among dancers. This research illustrates how dancers optimize control strategies during landing tasks, offering a novel perspective for comprehensively understanding dancers' neuromuscular control patterns.

Anatomical considerations of medial eye wrinkles: Guidelines for botulinum neurotoxin injections.

Crow's feet lines in the lateral canthal region are a common concern among aging patients, initially appearing as dynamic wrinkles during facial expressions and becoming more pronounced with age. Botulinum neurotoxin temporarily paralyzes muscles by inhibiting acetylcholine release, smoothing wrinkles and enhancing skin's youthful appearance. Effective treatment requires tailored approaches considering individual anatomy and muscle activity. Recent cadaveric studies identified the tear trough muscle, emphasizing its role in infraorbital support and aging. Clinically, patients often present medial eye wrinkles after BoNT treatment for crow's feet, prompting exploration of underlying mechanisms and management strategies. Three cases demonstrated that medial BoNT injections in the orbicularis oculi muscle significantly improve medial eye wrinkles and tear trough appearance. The study underscores the importance of understanding muscle hyperactivity and anatomical variations for precise treatment. Enhanced injection techniques targeting specific areas can achieve better outcomes and minimize complications, particularly in culturally sensitive regions where facial expressions are valued. This research highlights the necessity for comprehensive anatomical knowledge and patient-specific treatment strategies to address medial eye wrinkles effectively.

Effects of combined anodal transcranial direct current stimulation and motor control exercise on cortical topography and muscle activation in individuals with chronic low back pain: A randomized controlled study.

Aberrant movement in chronic low back pain (CLBP) is associated with a deficit in the lumbar multifidus (LM) and changes in cortical topography. Anodal transcranial direct current stimulation (a-tDCS) can be used to enhance cortical excitability by priming the neuromuscular system for motor control exercise (MCE), thereby enhancing LM activation and movement control. This study aimed to determine the effects of a 6-week MCE program combined with a-tDCS on cortical topography, LM activation, movement patterns, and clinical outcomes in individuals with CLBP. Twenty-two individuals with CLBP were randomly allocated to the a-tDCS group (a-tDCS; n=12) or sham-tDCS group (s-tDCS; n=10). Both groups received 20min of tDCS followed by 30min of MCE. The LM and erector spinae (ES) cortical topography, LM activation, movement control battery tests, and clinical outcomes (disability and quality of life) were measured pre- and post-intervention. Significant interaction (group×time; p<0.01) was found in the distance between LM and ES cortical locations. The a-tDCS group demonstrated significantly fewer discrete peaks (p<0.05) in both ES and LM and significant improvements (p<0.05) in clinical outcomes post-intervention. The s-tDCS group demonstrated a significant increase (p<0.05) in the number of discrete peaks in the LM cortical topography. No significant changes (p>0.05) in LM activation were observed in either group; however, both groups demonstrated improved movement patterns. Our findings suggest that combined a-tDCS with MCE can separate LM and ES locations over time while s-tDCS (MCE alone) reduces the distance. Our study did not find superior benefits of adding a-tDCS before MCE for LM activation, movement patterns, or clinical outcomes.

Impairments in peroneal muscle size and activation in individuals with patellofemoral pain in weight-bearing position.

Patellofemoral pain (PFP) is characterized by chronic pain in the anterior aspect of the knee during loading activities. Many studies investigating muscle morphology changes for individuals with PFP focus on the proximal joints, however, few studies have investigated muscles of the foot and ankle complex. This study aimed to explore the differences in peroneal muscle size and activation between individuals with PFP and healthy controls using ultrasound imaging in weight-bearing. A case-control study in a university lab setting was conducted. Thirty individuals with PFP (age: 20.23±3.30years, mass: 74.70±27.63kgs, height: 161.32±11.72cm) and 30 healthy individuals (age: 20.33±3.37years, mass: 64.02±11.00kgs, height: 169.31±9.30cm) participated. Cross-sectional area (CSA) images of the peroneal muscles were taken in non-weight bearing and weight-bearing positions. The functional activation ratio from lying to single-leg standing (SLS) was calculated. There was a statistically significant (p=0.041) group (PFP, healthy) by position (non-weight-bearing, weight-bearing) interaction for the peroneal muscle CSA with a Cohen's d effect size of 0.2 in non-weight-bearing position and 0.7 in weight-bearing position. The functional activation ratio for the healthy group was significantly more (p=0.01) than the PFP group. Peroneal muscles were found to be smaller in size in those with PFP compared to the healthy subjects in the weight-bearing SLS position. This study found that those with PFP have lower activation of peroneal muscles in functional position.

A surgical technique for individual control of the muscles of the rabbit lower hindlimb.

Little is known regarding the precise muscle, bone and joint actions resulting from individual and simultaneous muscle activation(s) of the lower limb. An in situ experimental approach is described herein to control the muscles of the rabbit lower hindlimb, including the medial and lateral gastrocnemius, soleus, plantaris and tibialis anterior. The muscles were stimulated using nerve-cuff electrodes placed around the innervating nerves of each muscle. Animals were fixed in a stereotactic frame with the ankle angle set at 90deg. To demonstrate the efficacy of the experimental technique, isometric plantarflexion torque was measured at the 90deg ankle joint angle at a stimulation frequency of 100, 60 and 30 Hz. Individual muscle torque and the torque produced during simultaneous activation of all plantarflexor muscles are presented for four animals. These results demonstrate that the experimental approach was reliable, with insignificant variation in torque between repeated contractions. The experimental approach described herein provides the potential for measuring a diverse array of muscle properties, which is important to improve our understanding of musculoskeletal biomechanics.

Effects of Exercise and Neuromuscular Electrical Stimulation on Lower Trapezius Muscle Activity in Individuals with Round Shoulder Posture

Effects of Exercise and Neuromuscular Electrical Stimulation on Lower Trapezius Muscle Activity in Individuals with Round Shoulder Posture

Forward Head Posture and Its Effect on Muscle Activation

Forward head posture (FHP), which is defined as a forward displacement of the head on the cervical spine, is a common postural disorder. It is suggested that this malalignment alters the loads on the spine, affects the length-tension relationship in muscles, and changes muscle activation. Therefore, the aim of this review is to investigate the results of studies on the examination of the changes exerted by FHP on muscle activation. Although there are many methods used to assess FHP, there is no standard clinical method for accurate measurement of this angle. Photographic measurement is the most widely used, valid, and reliable assessment method. Craniovertebral angle (CVA) is the most widely used value to assess FHP in photographic measurements. A CVA of less than 48-50° is defined as FHP, although there are differences regarding the norm value of the CVA. There are many studies on the assessment of differences in the activation of the neck and shoulder muscles by making FHP and non-FHP classifications according to the CVA to show the changes in muscle activation in individuals with FHP. Although many studies have shown increased sternocleidomastoideus and upper trapezius activation, there are also others indicating no difference. Similar conflicting results exist for the lower trapezius and serratus anterior muscles. Although there are conflicting results regarding muscle activation in studies, it seems likely that muscle activation is altered in individuals with FHP. It may be recommended that physiotherapists conduct interventions by considering these differences in muscle activation in individuals with FHP.

Comparison of two different orthoses used in the treatment of lateral epicondylitis.

Lateral epicondylitis (LE) is one of the most common causes of lateral elbow pain, and orthoses are often used in its conservative treatment. However, the number of studies examining the effectiveness of the orthoses used and their differences compared to each other is limited. To determine the effects of 2 orthoses types, wrist orthosis (WO) and counterforce brace (CB), commonly used in the treatment of lateral epicondylitis on muscle activation, grip strength, hand function, and satisfaction level in healthy individuals. Quasi-experimental study. Thirty-seven healthy individuals were recruited. Muscle activation was evaluated by surface electromyography, and maximum grip strength was evaluated with a handheld dynamometer. Hand function and satisfaction level were evaluated with Jebsen Taylor Hand Function Test and Orthotics and Prosthetics Users Survey, respectively. All evaluations were conducted for each individual in 3 conditions (no orthosis, WO, and CB). Extensor carpi radialis (ECR) muscle activation was greater when using CB compared with that when without orthosis (p = 0.036). There was no significant difference in extensor digitorum communis (EDC) muscle activation and maximum grip strength among the conditions (p > 0.05). Hand function was worse when using the WO compared with the other conditions (p < 0.001). In terms of orthosis satisfaction, individuals were found to be significantly more satisfied with the use of CB (p = 0.001). In our study highlight the use of CB in terms of EDC muscle activity, grip strength, hand function, and satisfaction, while the use of WO is prominent in terms of keeping extensor carpi radialis muscle activity low, which has been shown to be important for LE. To obtain definitive results on the effectiveness of different orthoses in the treatment of LE, more research is needed to compare the muscular activation in the extensor carpi radialis brevis (ECRB) and EDC muscles with objective methods.

Comparison of erector spinae and gluteus medius muscle thickness and activation in individuals with and without low back pain

BackgroundDysfunctional patterns of the erector spinae (ES) and gluteus medius (GM) muscles often accompany episodes of low back pain (LBP). Rehabilitative ultrasound imaging (RUSI) has been used to measure ES and GM muscle thickness, however such measurements have not been compared in individuals with and without LBP. ObjectivesTo compare ES and GM muscle thickness and change in thickness utilizing RUSI in individuals with and without LBP. DesignCross-sectional comparison. MethodsA volunteer sample of 60 adults with (n = 30) and without (n = 30) LBP was examined. Thickness measurements of the ES and GM at rest and during contraction were obtained by using RUSI during a single session. Statistical comparison was performed using ANCOVA. The demographic variable age was used as a covariate in the primary comparative analysis. ResultsMean difference for age between groups was 5.4 years (95% CI: 1.85, 8.94, p = 0.004). Average ODI score was 32.33±6.58 and pain level of 5.39±0.73 over the last 24 h in the symptomatic group. There was a statistically significant difference in the percent thickness change in both the ES, mean difference = −3.46 (95% CI: −6.71, −0.21, p = 0.039) and GM, mean difference = −1.93 (95% CI: −3.85, −0.01, p = 0.049) muscles between groups. ConclusionsIndividuals with LBP may have reduced percent thickness change of the ES and GM muscles when compared to asymptomatic individuals.

Muscle synergies inherent in simulated hypogravity running reveal flexible but not unconstrained locomotor control

With human space exploration back in the spotlight, recent studies have investigated the neuromuscular adjustments to simulated hypogravity running. They have examined the activity of individual muscles, whereas the central nervous system may rather activate groups of functionally related muscles, known as muscle synergies. To understand how locomotor control adjusts to simulated hypogravity, we examined the temporal (motor primitives) and spatial (motor modules) components of muscle synergies in participants running sequentially at 100%, 60%, and 100% body weight on a treadmill. Our results highlighted the paradoxical nature of simulated hypogravity running: The reduced mechanical constraints allowed for a more flexible locomotor control, which correlated with the degree of spatiotemporal adjustments. Yet, the increased temporal (shortened stance phase) and sensory (deteriorated proprioceptive feedback) constraints required wider motor primitives and a higher contribution of the hamstring muscles during the stance phase. These results are a first step towards improving astronaut training protocols.

Ab. No. 97 The Effect of Different Scapular Positions on Serratus Anterior and Biceps Brachii Muscle Activity During a Forward Reach Task in Healthy Individuals

Introduction: To study the effects of scapular protraction/retraction and neutral position on serratus anterior and biceps brachii muscle activity during a forward reach task in healthy individuals. Normal functioning of the scapula plays a key role in shoulder kinematics. Forward reaching is an integral activity for ADL’s and is a key neuro rehabilitation component. For example, posture that is usually adapted by people with stroke leads to inappropriate positioning of scapula that in turn can disrupt the muscle timing and activation slowing the rehabilitation process. Understanding the muscle activity in healthy individuals can give us a deeper insight to expand our knowledge for rehabilitation. Methods: 19 healthy subjects were selected under convenience sampling. Subjects were asked to perform a forward reach task. Three trials were taken for each scapular position. Surface electrodes recorded EMG activity of serratus anterior and biceps brachii muscle during the task. Average values were taken for analysis. Shapiro - Wilk test was done to analyse normality. The difference between the three positions of scapula (protraction, retraction and neutral) was established for serratus anterior (Fisher’s ANOVA) and biceps (Kruskal-Wallis) along with Post hoc analysis. Correlation between two muscles (serratus and biceps) was established using Spearman’s Correlation Coefficient. Results: Serratus muscle activity (F=0.135, p= 0.874) and biceps (F=0.29, p=0.7). did not achieve statistical significance among the three positions. Statistically significant correlation was found across the positions between biceps and serratus (r2 =0.58,p&lt;0.001). Conclusion: Mean activity of serratus anterior muscle did not change with different scapular positions. However, biceps brachii muscle activity was highest with scapula being in retraction and lowest in protraction. Moderate correlation was found between SA and biceps across positions. Implications: The starting position of scapula may have implications on muscle activation and therefore can impact rehabilitation outcomes.

Signal Processing for Spinal Cord Injury Monitoring with sEMG

Spinal cord injuries (SCIs) significantly impair motor and sensory functions, posing challenges to rehabilitation and patient management. Accurate monitoring of SCIs is crucial for evaluating the extent of injury and guiding therapeutic interventions. This study explores the application of surface electromyography (sEMG) as a non-invasive tool for monitoring muscle activity in individuals with SCIs. By capturing electrical signals generated by muscle fibers, sEMG provides real-time data essential for assessing neuromuscular function. To enhance the effectiveness of sEMG in spinal cord injury monitoring, advanced signal processing techniques are employed. These techniques include noise reduction, which improves signal clarity, feature extraction to identify key patterns in muscle activation, and pattern recognition algorithms to classify muscle responses. The integration of these methods enables more accurate interpretations of sEMG data, facilitating a better understanding of motor capabilities and rehabilitation progress. The findings highlight the potential of using signal processing in conjunction with sEMG to improve patient outcomes, optimize rehabilitation protocols, and provide insights into the underlying mechanisms of spinal cord injuries. This approach not only advances clinical practices but also supports ongoing research aimed at enhancing the quality of life for individuals affected by SCIs.

Electromyographic Evaluation of Masticatory Muscle Activity in Individuals with and without Cleft Lip/Palate: A Systematic Review and Meta-analysis.

This study aims to systematically review and meta-analyze the electromyographic activity of masticatory muscles in cleft palate and non-cleft patients, and identify influencing factors. Systematic Review and Meta-analysis. Patients with cleft lip/ palate. Patients without CL/P. Electrical activity of masseter and temporalis muscles at rest and during peak activation. After a comprehensive search in MEDLINE, Web of Science, EMBASE, Scopus, and Cochrane's CENTRAL up to December 2022, without language or date restrictions. Eligible trials were selected based on the PECO question and assessed for bias using Cochrane's ROBINS-E tool. Eight clinical trials with 474 participants were included in the review. Then relevant data was extracted from included studies using customized forms. A random-effects meta-analysis was performed to combine the results of the studies, meta-analyses showed that CL/P patients have elevated electrical activity in the masseter (P = .01) and temporalis (P = <.01) muscles at rest compared to non-cleft control patients. During maximum bite force, cleft patients exhibited a statistically significant decrease in electrical activity in both the masseter (P = .03) and temporalis (P = <.01) muscles. According to our meta-analysis, cleft patients exhibited increased resting muscle activity but decreased activity during maximum bite force, indicating reduced efficiency of masticatory muscles compared to non-cleft patients. These differences can be attributed to anatomical variations, compensatory mechanisms, and previous treatments.

Does enforcing glenohumeral joint stability matter? A new rapid muscle redundancy solver highlights the importance of non-superficial shoulder muscles.

The complexity of the human shoulder girdle enables the large mobility of the upper extremity, but also introduces instability of the glenohumeral (GH) joint. Shoulder movements are generated by coordinating large superficial and deeper stabilizing muscles spanning numerous degrees-of-freedom. How shoulder muscles are coordinated to stabilize the movement of the GH joint remains widely unknown. Musculoskeletal simulations are powerful tools to gain insights into the actions of individual muscles and particularly of those that are difficult to measure. In this study, we analyze how enforcement of GH joint stability in a musculoskeletal model affects the estimates of individual muscle activity during shoulder movements. To estimate both muscle activity and GH stability from recorded shoulder movements, we developed a Rapid Muscle Redundancy (RMR) solver to include constraints on joint reaction forces (JRFs) from a musculoskeletal model. The RMR solver yields muscle activations and joint forces by minimizing the weighted sum of squared-activations, while matching experimental motion. We implemented three new features: first, computed muscle forces include active and passive fiber contributions; second, muscle activation rates are enforced to be physiological, and third, JRFs are efficiently formulated as linear functions of activations. Muscle activity from the RMR solver without GH stability was not different from the computed muscle control (CMC) algorithm and electromyography of superficial muscles. The efficiency of the solver enabled us to test over 3600 trials sampled within the uncertainty of the experimental movements to test the differences in muscle activity with and without GH joint stability enforced. We found that enforcing GH stability significantly increases the estimated activity of the rotator cuff muscles but not of most superficial muscles. Therefore, a comparison of shoulder model muscle activity to EMG measurements of superficial muscles alone is insufficient to validate the activity of rotator cuff muscles estimated from musculoskeletal models.