Abstract
The load scenario within the shoulder joint among its muscle–tendon complexes during fast motions is of interest, as it would allow an evaluation of critical, accident-like motions. To enhance knowledge, a modelling approach was carried out and compared to experimental data. Nine subjects were investigated while performing tasks that ranged from easy to demanding. Motions were (1) an easy lift of a small weight, (2) a push against a force measurement device, and (3) a gentle side fall against the immovable force measurement device. Extracted data were the kinematics of the right arm and the contact force on the elbow. A simple direct dynamics shoulder model actuated by Hill-type muscle models was arranged to simulate the three experimental motions. The Hatze-based activation of the muscle models was used without any further simulation of neural regulation. For fast motions, the simple shoulder model predicts well the shoulder angle or contact force values, and data fit well into the variability of the data measured experimentally. Because there was no implementation of more complex neural regulation, slow motions, as performed by the subjects, were, in part, not predicted by the shoulder model. Simple mechanisms can be described by the simple model: When activated, the larger deltoid muscle is able to protect the smaller supraspinatus muscle. Furthermore, in awkward conditions, the gentle side fall against an immovable device alone has enough momentum to damage small muscles.