In baseball pitching, pitcher’s trunk is known to have kinematic and kinetic functions for conveying the motion energy from the lower limb to the upper limb. In such context, a term, “the stretching of the chest”, 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. In order to eliminate such ambiguity, we have been developing a quantitative analysis of the stretching of the chest. First, Fukushima (2014) proposed parameter S defined from the positions of several reflective markers secured to the skin surface of the thoracic vertebrae. The parameter enabled us to evaluate the curvature of the vertebrae that depends primarily on the stretching of the chest in pitching. In this paper, we applied this parameter to the analysis of baseball pitching in order to clarify a role of the stretching of the chest in the kinetic chain. Experiments using 12 subjects verified that the stretching of the chest could take part in conveying the motion energy through a time-series analysis, suggesting that the stretching and stooping, motion of the chest could be one of the factors of the kinetic chain.
