Résumé·s
Purpose: The rotator cuff is prone to injury, remarkably so for manual wheelchair users.
To understand its pathomechanisms, finite element models incorporating threedimensional activated muscles are needed to predict soft tissue strains during given
tasks. This study aimed to develop such a model to understand pathomechanisms
associated with wheelchair propulsion. Methods: We developed an active muscle model
associating a passive fiber-reinforced isotropic matrix with an activation law linking
calcium ion concentration to tissue tension. This model was first evaluated against known
physiological muscle behavior; then used to activate the rotator cuff during a wheelchair
propulsion cycle. Here, experimental kinematics and electromyography data was used to
drive a shoulder finite element model. Finally, we evaluated the importance of muscle
activation by comparing the results of activated and non-activated rotator cuff muscles
during both propulsion and isometric contractions. Results: Qualitatively, the muscle
constitutive law reasonably reproduced the classical Hill model force-length curve and the
behavior of a transversally loaded muscle. During wheelchair propulsion, the deformation
and fiber stretch of the supraspinatus muscle-tendon unit pointed towards the possibility
for this tendon to develop tendinosis due to the multiaxial loading imposed by the
kinematics of propulsion. Finally, differences in local stretch and positions of the lines of
action between activated and non-activated models were only observed at activation
levels higher than 30%. Conclusion: Our novel finite element model with active muscles
is a promising tool for understanding the pathomechanisms of the rotator cuff for various
dynamic tasks, especially those with high muscle activation levels