Shoulder muscle function depends on elbow joint position: An illustration of dynamic coupling in the upper limb
Introduction
The interpretation of shoulder muscle function during upper limb movement is frequently inferred from muscle moment arm data (An et al., 1984, Kuechle et al., 1997, Ackland et al., 2008, Ackland et al., 2010). The most common function attributed to a muscle is the torque developed by the muscle to cause rotation about a joint; specifically, the torque exerted by a muscle is equal to the moment arm of the muscle multiplied by the muscle's force. Thus, a moment arm only provides information about a muscle's action about the joint(s) spanned by that muscle. More generally, a muscle has the capacity to induce motion about adjacent joints not spanned by that muscle. The phenomenon of dynamic coupling, where a torque at one joint can induce accelerations at all other joints in the system, arises due to muscle-force transmission from segment to segment via joint contact forces. This can lead to complex relations between the applied joint torques and the resultant joint accelerations. Dynamic coupling is readily observed in a multi-joint system (e.g., Hollerbach and Flash, 1982), but in principle it can also arise at a single joint with multiple degrees of freedom, such as a ball-and-socket joint, where the torque applied about one joint axis induces accelerations about all three joint axes of rotation.
The effects of dynamic coupling on muscle coordination of movement may be studied by assessing each muscle's contribution to the linear acceleration of a body segment and/or the angular acceleration of a joint. Muscle contributions to body-segmental and joint accelerations have been used to quantify muscle function in the lower limb during various motor tasks, including walking and jumping (Pandy and Zajac, 1991, Zajac, 1993, Anderson and Pandy, 2003, Arnold et al., 2005, Correa et al., 2010, Pandy et al., 2010). In the upper limb, calculations of net torque contributions to joint accelerations have been performed for single-plane arm movements (Hollerbach and Flash, 1982, Seth et al., 2003) as well as for more complex pitching motions (Hirashima et al., 2008). At present, no study has investigated the contributions of individual shoulder muscles to glenohumeral joint accelerations.
Flexion of the elbow is required during most activities of daily living (Anglin and Wyss, 2000), yet studies of shoulder muscle function typically consider movement of the upper limb with the elbow fully extended (McMahon et al., 1995, Yanagawa et al., 2008). It is generally accepted that the deltoid and supraspinatus are the primary abductors of the shoulder (Kronberg et al., 1990, Yanagawa et al., 2008, Ackland et al., 2008), with the entire rotator cuff playing a role in glenohumeral joint axial rotation (Labriola et al., 2005); however, the specific functions of these muscles when the elbow is flexed have not been assessed.
The aim of this study was twofold: first, to determine the contributions of the individual muscles of the shoulder to glenohumeral joint motion during abduction; and second, to quantify the effect of elbow joint flexion on shoulder muscle function during arm elevation. Based on results obtained previously for muscle coordination in the lower limb (Zajac, 1993, Zajac and Gordon, 1989), we hypothesized that dynamic coupling would have a substantial effect on the capacities of the individual muscles of the shoulder to accelerate the arm into abduction, axial rotation and horizontal flexion.
Section snippets
Cadaver measurements
The lines of action of the deltoid and rotator cuff muscles were previously measured in eight fresh-frozen entire upper extremities obtained from the human cadavera (4 male and 4 female; Ackland and Pandy, 2009). The mean age and mass of the subjects were 87.0±6.4 years and 58.0±13.8 kg, respectively. Muscles were identified and divided into sub-regions as described in Table 1.
A muscle's line of action was defined as the vector projection from the last tendon wrapping via-point (i.e., the point
Results
Each shoulder muscle had the capacity to accelerate the glenohumeral joint about all three axes of rotation simultaneously. When the elbow was fully extended, trends in the shoulder muscle moment arms during abduction and axial rotation closely matched those of the muscle contributions to the joint accelerations (Figs. 2 and 3) with a strong correlation observed (R²=1.00 and 0.98 during abduction and axial rotation, respectively, Fig. 4A and B). In general, trends in the horizontal flexion
Discussion
It is well accepted that shoulder muscle function is governed by the torques exerted by the muscles spanning the glenohumeral joint. However, the results of the present study indicate that shoulder muscle function not only depends on muscle torque production at the glenohumeral joint but is also substantially affected by elbow joint position. For example, we found that when the elbow is flexed, a muscle that generates only internal rotation torque at the shoulder may also accelerate the
Conflict of interest statement
The authors do not have any financial or personal relationships with other people or organizations that could inappropriately influence their work.
Acknowledgments
Supported by a Victorian Endowment for Science, Knowledge and Innovation (VESKI) Fellowship to M.G.P. Partial funding for this work was provided by a Discovery Project grant from the Australian Research Council (DP0772838).
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