127 A role of "the stretching of the chest" in the kinetic chain observed in baseball pitching

Abstract

In baseball pitching, pitcher's trunk has a kinetic role for conveying the kinetic energy from the lower limb to the upper limb. In such context, a term "the stretching of the chest", one of the upper-trunk motions, is often used in coaching. Although several qualitative reports on the term have been published, its biomechanical features and functions are still unclear, due to the lack of quantitative studies. Accordingly, the term is used ambiguously in both coaching and clinic. In order to eliminate such ambiguity, we have been trying to develop a quantitative analysis of the stretching of the chest. First, Fukushima (2011) developed parameter S defined from the relative positions of the reflective markers secured to the skin surface of the thoracic vertebrae for evaluating the curvature of the vertebrae. His preliminary studies using basic motions of the upper limb revealed that the S-value is independent from most of the motion parameters of the upper limb, except its horizontal abduction and adduction affecting the stretching of the chest. In this paper, we applied the parameter to the analysis of baseball pitching in order to clarify 1) time series of S1-value, 2) the relationship between S and the motion parameters of the upper limb, 3) kinetic chain during pitching. Experiments using 12 subjects revealed that parameter S could be a variable capable of evaluating the stretching of the chest, providing us with new kinetic information during pitching and the stretching of the chest could be considered a factor of the kinetic chain.