During a typical baseball pitch, the player's trunk performs several functions that convey motion energy from the lower- to the upper-limb. In this context, the phrase “stretching the chest” is used, most often by the coach. But, its biomechanical essence is still unclear, being used in the qualitative sense. To clarify this term in coaching contexts, we proposed a novel evaluation parameter denoted by S, using the curvature of the thoracic spine, which is measurable by an ordinary marker-based motion capture system. In the first step of its validation, we analyzed MR images of the thoracic spine, while placing the chest under normal stress, in order to verify the relationship between the stretching-process and the curvature of the thoracic spine. We then conducted motion capture-based analysis of the relationship between parameter S and basic motions of the upper arm and trunk. Consequently, parameter S was determined to be independent of other motion parameters, except horizontal adduction and abduction of the upper arm. Finally, we applied parameter S to the analysis of actual baseball pitches, and came to understand its natural flexibility over time, and independence from horizontal adduction and abduction of the upper arm and retroflexion of the trunk.

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.

In baseball, a special type of offensive technique, called bunt, is used to ground the ball with the intention of advancing a baserunner and/or scoring a runner from third base. Unlike in the normal batting, the primary goal of a bunt is to minimize the speed of the batted ball. The purpose of this study was to describe the impact characteristics of bunt for minimizing the batted ball speed in baseball bunt. Thirty-one healthy men having experience of baseball for five years or longer participated in the study. Each subject performed eight to thirty-two trials of bunt, aiming at grounding the ball toward the pitcher. Behavior of ball impact was recorded with two high-speed cameras (2500 fps) for three-dimensional analysis. Stepwise multiple regression analysis was conducted to predict the batted ball speed (Y) from impact location of the bat relative to the sweet spot (short axis: X₁ & X₁² long axis: X₃ & X₃²), bat speed immediately before impact (X₂) and distance between the position of the top-hand and bottom-hand (X₄). The batted ball speed was associated with the four variables (Y=17.5+0.0069X₁² +1.12X₂-241.5X₃²-11.72X_3, R²=0.95), in particular, the squared value of the impact location of short axis of the bat (X₁²) was found to make the largest contribution to the prediction of the batted ball speed. These results indicate that making the ball impact in the middle of the bat width is the more important skill, than either moving the bat in the direction of a catcher before impact or making the ball impact at the distal end of the bat, for minimizing the batted ball speed in bunt.