Kevin B. Shelburne

The aim of this study was to determine the relative contributions of the deltoid and rotator cuff muscles to glenohumeral joint stability during arm abduction. A three-dimensional model of the upper limb was used to calculate the muscle and joint-contact forces at the shoulder for abduction in the scapular plane. The joints of the shoulder girdle—sternoclavicular joint, acromioclavicular joint, and glenohumeral joint—were each represented as an ideal three degree-of-freedom ball-and-socket joint. The articulation between the scapula and thorax was modeled using two kinematic constraints. Eighteen muscle bundles were used to represent the lines of action of 11 muscle groups spanning the glenohumeral joint. The three-dimensional positions of the clavicle, scapula, and humerus during abduction were measured using intracortical bone pins implanted into one subject. The measured bone positions were inputted into the model, and an optimization problem was solved to calculate the forces developed by the shoulder muscles for abduction in the scapular plane. The model calculations showed that the rotator cuff muscles (specifically, supraspinatus, subscapularis, and infraspinatus) by virtue of their lines of action are perfectly positioned to apply compressive load across the glenohumeral joint, and that these muscles contribute most significantly to shoulder joint stability during abduction. The middle deltoid provides most of the compressive force acting between the humeral head and the glenoid, but this muscle also creates most of the shear, and so its contribution to joint stability is less than that of any of the rotator cuff muscles.

The aim of this study was twofold: first, to determine which muscles and ligaments resist the adduction moment at the knee during normal walking; and second, to describe and explain the contributions of muscles, ligaments, and the ground reaction force to medial and lateral compartment loading. Muscle forces, ground reaction forces, and joint motions obtained from a dynamic optimization solution for normal walking were used as input to a three‐dimensional model of the lower limb. A static equilibrium problem was solved at each instant of the gait cycle to determine tibiofemoral joint loading at the knee. Medial compartment loading was determined mainly by the orientation of the ground reaction force. Because this force vector passed medial to the knee, it applied an adduction moment about the joint during stance. In contrast, all of the force transmitted by the lateral compartment was due to muscle and ligament action. The muscles that contributed most to support and forward propulsion during normal walking (quadriceps and gastrocnemius) also contributed most to knee stability in the frontal plane. The knee ligaments, particularly those of the posterior lateral corner, provided stability to the knee at certain periods of the stance phase, when activity of the important stabilizing muscles was low. © 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:1983–1990, 2006

Mensional model of the lower limb was used to calculate anterior tibial translation in the intact and ACLd knee during gait. The model was then used to predict the amount of quadriceps and hamstrings force needed to restore anterior tibial translation (ATT) in the ACLd knee to an intact or maximum allowable level. Results: It was possible to reduce ATT in the ACLd knee to the level calculated for the intact knee by increasing the magnitude of hamstrings force (a hamstrings facilitation pattern). Although this strategy decreased the knee extensor moment calculated for walking, the effect was much less than that obtained when quadriceps force was reduced. Reducing quadriceps force to restore normal ATT resulted in complete elimination of the knee extensor moment (a quadriceps avoidance pattern); however, this strategy was insufficient to restore ATT to the level calculated for the intact knee over portions of the gait cycle. Conclusion: The model simulations showed that increased hamstrings force was sufficient to stabilize the ACLd knee during gait. Reduced quadriceps force was insufficient to restore normal ATT for portions of the gait cycle....