A mathematical modelling of pitching arm for calculating valgus stress on elbow during baseball pitching with a single inertia sensor

Abstract

The purpose of this study was to develop a single rigid-body-model to represent the ball, hand and forearm of a given pitching arm for an accurate determination of the valgus stress. In a past study, a single rigid-body-model was applied to determine the valgus stress and found that the determined valgus stress was well-correlated to, but substantially overestimated, the corresponding values determined with a three-segment model (consisted of a combined segment of baseball & hand, forearm and upper arm). A major reason for the overestimation is presumably that the ball, hand and forearm were assumed to be aligned straight and the inertia properties of the single-rigid-body was defined as the sum of the inertia properties of the three segments in this aligned position. To solve this problem, we computed the inertia properties of the single rigid body for various virtual positions of hand and ball to find out the best position with which the valgus stress can be determined accurately. Fourteen university students and 20 junior high school students served as subjects. Each subject was asked to throw 3~5 pitches of fastball and another 1~5 pitches of breaking-balls. The motion of each segments was captured with a motion capture system (VICON, Oxford Metrics) and an inverse dynamic procedure was applied to determine the valgus stress. The results showed that the best virtual position of the hand and ball was at 43% of the distance from the wrist toward the 3rd knuckle. This virtual position resulted in the valgus stress of 37.0 ± 12.0 Nm, for the corresponding value of 37.2±11.4 Nm with a three-segment model. This model may be combined with a single inertia sensor, so that the risk of elbow injury can be evaluated accurately and easily.