Musculoskeletal modelling of the shoulder during baseball pitching

Abstract

A musculoskeletal model of the shoulder region, which uses kinematic data as an input an estimates muscle forces and joint loads as an output, can give valuable information on the pitching motion. This helps us to get more insight into the pitching motion and its biomechanical interactions and may also help in reducing the injury risk and increasing pitching velocity. Currently, there are three problems that impede proper simulations: the lack of proper kinematic recordings, the fact that maximum force of the model could be too limited and the extreme character of the motion.
An experimental study was performed to create a dataset including upper limb kinematics and PCSA scaling factors to scale maximum force in the DSEM. An acromion cluster was used to track the scapula. PCSA scaling factors ranged from 1.11 to 2.02.
Following the experimental study, a case study was performed simulating this dataset in the DSEM. During simulation, the main problem arose in the optimization of the clavicle and scapula angles relative to the thorax, because the optimized angles contained large jumps, which are not realistic. This impeded proper simulation, because the jumps in muscle length caused both unsolved frames in the kinematics as well as in the dynamic model. Using a soft constraint instead of a hard constraint reduced these jumps and allowed for a complete solution in the kinematic model and an increase in the number of frames solved by the dynamic model. PCSA scaling also increased the number of frames solved by the dynamic model, however still unsolved frames were present, even after extreme scaling. Because of a change in range of motion as reported for pitchers, optimum muscle length might be different. This has a large impact in the model. If this would be the case, scaling optimum muscle length is recommended. In addition, segment scaling used in combination with using the soft constraint is recommended to improve the match between input angles and optimized angles, while still being compatible to the model.
To study the motion, the kinematic model of the DSEM was used to estimate muscle length and velocity for all muscles during the pitching motion. Comparing these values to the force-velocity and the active force-length relationship showed whether muscles were limited by one or both of these relationships to produce force. This was the case for the teres minor, triceps (all three heads), infraspinatus, anconeus and serratus anterior. The triceps showed a ‘stretch effect’, meaning shortening in the acceleration phase preceded by lengthening in the cocking phase. This means that there is a possibility for elastic energy to be stored for this muscle.