AbstractThe 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 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.
