前言：過去許多學者都以增加投手球速為目標來研究投球機制，但研究範圍都集中在上肢而較少涉及軀幹。涉及軀幹的研究也只提出軀幹角度及角速度和球速的關係，缺乏角加速度的資料，加上過去研究中的軀幹模型同時涵蓋了骨盆與軀幹，造成實驗結果無法反應正確的軀幹轉動與球速的關係。因此本研究的目的希望使用只包含軀幹的模型，找出投球過程中軀幹的角度、角速度和角加速度的特徵，並建立這些參數和球速之間的相關，藉此釐清軀幹與球速之間的關係。 方法：12名甲組投手參與實驗，使用 10部 CCD攝影機(VICON MX13)(500Hz)，搭配 NEXUS軟體進行資料收集。受試者全力對前方 3m的目標投擲 5個直球，取球速最快的 3球進行分析。以 Visual3d軟體計算受試者球速、軀幹角度、角速度及角加速度，並以皮爾遜積差相關 (Pearson correlation)計算軀幹角度、角速度、角加速度的最大值及離手值和球速之間相關，顯著水準取α = .05。 結果：軀幹前傾速度峰值和球速相關高達 .74。軀幹前旋速度峰值和球速相關高達 .81。前傾加速度峰值和球速相關高達 .90。後旋加速度離手值和球速相關高達 .76。其餘的各參數和球速之間並沒有高度相關。 結論與建議：本研究發現軀幹前旋、前傾兩個速度的峰值、前傾加速度峰值與後旋加速度離手值等 4個參數和球速之間存在高度相關，因此在訓練上，投手應該去強化前傾及軸向旋轉的能力。若後續研究能對軀幹進一步進行模擬，則更能確認軀幹與投手球速的影響。

Purpose: Previous researches related to pitching mechanics focused on enhancing ball speed; however, these researches put emphasis on the upper limbs, rarely on the trunk rotation. These researches of trunk rotation in baseball pitching only supported the relationship of trunk angle, trunk angular velocity and ball speed, without trunk angular acceleration. And the model used by previous researches contains pelvis and trunk, so the result can’t gain a clear ideal of the relationship between trunk rotation and ball speed. The purpose of this study was to describe the kinematic characteristics of trunk with the model only contain trunk, and find the correlation between trunk angle, angular velocity, angular acceleration and ball speed to understand the relationship of trunk rotation and ball speed. Methods: Twelve college pitchers played in Chinese Taipei university baseball league division I participated in this study. Ten VICON CCD cameras (500Hz) with Nexus software were used to collected baseball pitching data. The subjects pitched five straight balls to the target; the three fastest ball speed trails were analyzed. The Visual3d software was used to compute ball speed, trunk angle, trunk angular velocity, and trunk angular acceleration. Pearson correlation was used to test the correlation between trunk parameters and ball speed. The significant level was α= .05. Results: The correlation between the peak value of trunk forward tile velocity and ball speed is.74. The correlation between the peak value of trunk forward rotation velocity and ball speed is .81. The correlation between the peak value of trunk forward tile acceleration and ball speed is .90. The correlation between the trunk backward angular acceleration at ball releasing and ball speed is .76. Between the other variables and ball speed, the correlations are not very high. Conclusions: This research found out that the peak value of trunk forward tile velocity, trunk forward rotation velocity, trunk forward tile angular acceleration, and the trunk backward angular acceleration at ball releasing are highly correlated with ball speed. So pitchers should strengthen their forward and axial rotation ability in the training session. For future study, the simulation and modeling of trunk rotation in baseball pitching is suggested in order to understand the relationship between trunk motion and ball speed.