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Abstract: Mathematical models of small animals that predict in vivo forces acting on the lower extremities are critical for studies of musculoskeletal biomechanics and diseases. Rabbits are advantageous in this regard because they remodel their cortical bone similar to humans. Here, we enhance a recent mathematical model of the rabbit knee joint to include the loading behavior of individual muscles, ligaments, and joint contact at the knee and ankle during the stance phase of hopping. Geometric data from the hindlimbs of three adult New Zealand white rabbits, combined with previously reported intersegmental forces and moments, were used as inputs to the model. Muscle, ligament, and joint contact forces were computed using optimization techniques assuming that muscle endurance is maximized and ligament strain energy resists tibial shear force along an inclined plateau. Peak forces developed by the quadriceps and gastrocnemius muscle groups and by compressive knee contact were within the range of theoretical and in vivo predictions. Although a minimal force was carried by the anterior cruciate and medial collateral ligaments, force patterns in the posterior cruciate ligament were consistent with in vivo tibial displacement patterns during hopping in rabbits. Overall, our predictions compare favorably with theoretical estimates and in vivo measurements in rabbits, and enhance previous models by providing individual muscle, ligament, and joint contact information to predict in vivo forces acting on the lower extremities in rabbits. [Copyright &y& Elsevier]

Abstract: Background: The incidence of shoulder complaints in wheelchair users is high and the etiology is poorly understood. The goal of this study was to examine the effect of lesion level and isolated triceps muscle paresis on the internal load on the shoulder by simulation. Methods: Kinematic and kinetic profiles from four able-bodied subjects and four subjects with tetraplegia were used as input for an inverse dynamics biomechanical model. The model was modified to simulate lesion level and triceps muscle paresis. Findings: The simulations resulted in a significantly higher (+56%) glenohumeral contact force (P =0.037) for tetraplegic profiles than for able-bodied profiles. The model modifications to simulate lesion level only had a minor effect (+7%) on the calculated glenohumeral contact force. More simulations were successful at lower triceps force levels for tetraplegic profiles compared to able-bodied profiles (P =0.012). The muscle forces at the simulated T1 lesion were not significantly higher in tetraplegic profiles compared to able-bodied profiles. Interpretation: The glenohumeral contact force for the tetraplegic profiles is mainly higher due to different task performance. Model modifications only have a minor effect on the calculated glenohumeral contact force. For able-bodied profiles the triceps force seems to be an important factor. The high internal load at the shoulder recommends new techniques of weight relief lifting and proper training of the arm–shoulder muscles in rehabilitation. [Copyright &y& Elsevier]

Objective. To assess the mechanical load on the glenohumeral joint and on shoulder muscles during wheelchair propulsion at everyday intensities.Design. Model simulations based on experimental input dataBackground. Virtually nothing is known about the mechanical load on the upper extremity during wheelchair propulsion. Hand rim wheelchair propulsion is a significant risk factor for shoulder pain and injury among wheelchair users. A musculoskeletal model of the upper extremity during wheelchair propulsion will quantify the stresses placed on anatomic structures and may provide insight into the source of symptoms and injuries.Methods. Three experienced wheelchair users underwent wheelchair exercise tests at combinations of two load levels (10 and 20 W) and two velocities (0.83 and 1.39 m·s−1) during which input data were collected for a musculoskeletal model of the upper extremity. The model was then used for the estimation of the glenohumeral contact force, as well as individual muscle forces.Results. Peak glenohumeral contact forces were between 800 and 1400 N (100–165% body weight) and differed significantly between load levels. Averaged over the push phase, these forces were 500–850 N. In absolute terms the m. deltoideus and rotator cuff muscles were highly active (>100 N). In relative terms the load on the m. supraspinatus was high, with peak values of over 50% of its maximum attainable force.Conclusions. Low intensity wheelchair propulsion does not appear to lead to high contact forces. The muscle forces in the rotator cuff and especially in the m. supraspinatus are high. This might indicate a risk for muscle damage and the subsequent development of shoulder complaints, such as rotator cuff tears.Relevance Within the wheelchair user population, there is a high prevalence of upper extremity complaints. Not much is known about the causes of those complaints. Wheelchair propulsion is likely to be a major risk factor. If the (nature of this) mechanical load can be identified, specific exercise programs and/or design changes can be better tuned to prevent overuse injuries. [Copyright &y& Elsevier]

Rotator cuff tear is a common musculoskeletal injury, negatively affecting shoulder function. Rotator cuff tear severity ranges from small to massive tears, but it is unclear how tear severity affects glenohumeral joint loading and how changes contribute to secondary injury. This study's objective was to determine how glenohumeral joint contact force changes with tear severity during functional task performance using computational models. Eight models of increasing tear severity were developed, ranging from no rotator cuff tear to massive three-tendon tears. Simulations were performed using models representing increasing tear severity and kinematics for five functional tasks (n = 720 simulations). For each task, magnitude and orientation of peak resultant joint contact force for each tear severity was identified, then compared to the no rotator cuff tear model. For all tasks, compared to the no rotator cuff tear model, joint contact force magnitude decreased 9.5% on average with infraspinatus involvement, then plateaued at 20.8% average decrease with subscapularis involvement. Projected orientation of peak joint contact force vector was located more superior in the glenoid with increased tear severity, with significant changes (p < 0.0003) for all tasks with infraspinatus involvement. Decreased magnitude and superior orientation of joint contact force suggest fewer intact muscles contribute to force distribution across the joint, although more work is needed characterizing associated compensation strategies. All force vectors remained oriented within the glenoid rim for all tasks and models, suggesting the system prioritizes joint stability. This work identifies how joint contact force changes with rotator cuff tear severity. • Joint contact force magnitude decreases for increased rotator cuff tear severity. • Similar trends of change in force are seen with all tasks. • Increased tear severity results in a superiorly oriented joint contact force. • All force vectors remain oriented within the glenoid for all tasks and severities. [ABSTRACT FROM AUTHOR]

To our knowledge, no study has directly measured the loads in the trapeziometacarpal joint during an isometric key pinch. The aim of this study was to measure the load acting on the trapeziometacarpal joint for increasingly greater key pinch forces (0.5 kg-1.5 kg). We performed a cadaver study using 10 fresh-frozen, unembalmed adult forearms and hands (5 right and 5 left). Thumb pinch was simulated by loading the main actuator tendons involved in the key pinch grip (i.e., adductor pollicis, flexor pollicis longus, extensor pollicis longus, extensor pollicis brevis and abductor pollicis longus tendons). Measurements were made inside the joint using a force-sensing resistor sensor (Tekscan® FlexiForce™ force sensor). All specimens were tested twice in a row in the same condition. The median load values recorded in the trapeziometacarpal joint were 1.9 kg (IQR 2.2-1.5), 3 kg (IQR 3.4-2.7) and 4.1 kg (IQR 4.4-3.9) during 0.5 kg, 1 kg, and 1.5 kg key pinch, respectively. For each specimen, similar load values were observed during both loading trials. Our findings indicate that the loads measured directly in the trapeziometacarpal joint during a simple key pinch are materially lower than those estimated in biomechanical models of the thumb (generally greater than 10 kg for 1 kg of applied force) probably due to intersubject variability. This pilot study will serve as a basis for further studies, for example, comparing biomechanical thumb models and experimental measurements under the same set-up conditions., (Copyright © 2021 SFCM. Published by Elsevier Masson SAS. All rights reserved.)

Background and Purpose: Gait modification is often used to reduce the external knee adduction moment (KAM) in human walking, but the relationship between KAM reduction and changes in medial knee joint contact force (JCF) is not well established. Our purpose was to examine the limiting case of KAM-based gait modification: reducing the KAM as much as possible, and the resulting effects on JCF., Methods: We used musculoskeletal modeling to perform three optimal control simulations: normal walking, a modified gait that reduced the KAM as much as theoretically possible (Min(KAM) simulation), and a second modified gait that minimized the KAM plus the metabolic cost of transport (Min(KAM+CoT) simulation)., Results: The two modified gaits both reduced the peak KAM from normal walking (-82% for Min(KAM) simulation, -74% for Min(KAM+CoT) simulation) by increasing trunk lean, toe-out, and step width, and reducing knee flexion. Even though the Min(KAM+CoT) simulation had the larger KAM, it had a greater reduction in peak medial JCF (-27%) than the Min(KAM) simulation (-15%) because it reduced the KAM using less knee muscle activity. These results were qualitatively robust to a sensitivity analysis of the knee joint model, but the magnitude of changes varied by an order of magnitude., Conclusions: The results suggest that (i) gait modification can benefit from considering whole-body motion rather than single adjustments, (ii) accurate interpretation of KAM effects on medial JCF requires consideration of muscle forces, and (iii) subject-specific knee models are needed to accurately determine the magnitude of KAM reduction effects on JCF., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Musculo-tendon forces and joint reaction forces are typically estimated using a two-step method, computing first the musculo-tendon forces by a static optimization procedure and then deducing the joint reaction forces from the force equilibrium. However, this method does not allow studying the interactions between musculo-tendon forces and joint reaction forces in establishing this equilibrium and the joint reaction forces are usually overestimated. This study introduces a new 3D lower limb musculoskeletal model based on a one-step static optimization procedure allowing simultaneous musculo-tendon, joint contact, ligament and bone forces estimation during gait. It is postulated that this approach, by giving access to the forces transmitted by these musculoskeletal structures at hip, tibiofemoral, patellofemoral and ankle joints, modeled using anatomically consistent kinematic models, should ease the validation of the model using joint contact forces measured with instrumented prostheses. A blinded validation based on four datasets was made under two different minimization conditions (i.e., C1 - only musculo-tendon forces are minimized, and C2 - musculo-tendon, joint contact, ligament and bone forces are minimized while focusing more specifically on tibiofemoral joint contacts). The results show that the model is able to estimate in most cases the correct timing of musculo-tendon forces during normal gait (i.e., the mean coefficient of active/inactive state concordance between estimated musculo-tendon force and measured EMG envelopes was C1: 65.87% and C2: 60.46%). The results also showed that the model is potentially able to well estimate joint contact, ligament and bone forces and more specifically medial (i.e., the mean RMSE between estimated joint contact force and in vivo measurement was C1: 1.14BW and C2: 0.39BW) and lateral (i.e., C1: 0.65BW and C2: 0.28BW) tibiofemoral contact forces during normal gait. However, the results remain highly influenced by the optimization weights that can bring to somewhat aphysiological musculo-tendon forces., (© 2013 Published by Elsevier Ltd.)