背景與目的：棒球在世界上非常盛行，是一項結合打擊、投球、跑壘、守備等技術的運動，其中打擊能力的優劣是影響比賽勝負的關鍵因素。棒球打擊的過程由下肢產生的力量，透過軀幹傳遞至上肢，最終產生揮棒的速度。此動力鏈中軀幹旋轉的動作在力量的產生和傳遞方面扮演重要的角色，其中外側腹壁肌群必須提供軀幹旋轉的力量和控制。外側腹壁肌群主要由腹外斜肌、腹內斜肌以及腹橫肌所組成，腹外斜肌和腹內斜肌屬於全面肌肉系統，腹橫肌屬於局部肌肉系統。全面肌肉系統主要的功能為透過產生力矩或共同收縮的方式，平衡作用於軀幹的外力以及傳遞胸椎與骨盆之間的力量。局部肌肉系統主要的功能為透過前饋控制的方式，維持脊柱中立姿勢和弧度。當運動員進行高負荷的體育運動，必須透過全面和局部肌肉系統的共同參與以產生良好的核心控制，進而提升運動表現和功能性動作模式。棒球打擊專項的動作模式涉及單方向軀幹旋轉的動作，運動員歷經長期的訓練和比賽後，可能會造成外側腹壁肌群於休息和收縮的狀態下具有不對稱的現象。雖然不對稱性為因應運動專項需求必要的適應結果，但可能會施予脊柱兩側不正常的作用力而影響核心控制的能力，導致較差的運動表現以及功能性動作模式。過去沒有研究探討青少年棒球運動員的外側腹壁肌群是否具有不對稱的現象，且不對稱性與運動表現以及功能性動作模式之間的關係尚不清楚。因此，本研究的目的為比較高中棒球運動員軀幹兩邊的腹外斜肌、腹內斜肌以及腹橫肌，於休息狀態的厚度和收縮狀態的厚度變化率是否有顯著的差異，並探討不對稱性與運動表現以及功能性動作模式之間的關係。
方法：本研究為觀察型研究類型中的橫斷面研究，以高中棒球隊的球員為研究對象，共招募27位單側打擊的棒球非投手球員。本研究以棒球揮棒感測器及分析系統，量測揮棒的速度。以功能性動作篩檢和上肢及下肢Y平衡測試，評估功能性動作模式。以超音波影像系統記錄慣用邊和非慣用邊的外側腹壁肌群(腹外斜肌、腹內斜肌、腹橫肌)，於休息狀態(平靜自然呼吸)的厚度和收縮狀態(draw-in腹部呼吸法、腹部捲曲)的厚度變化率。以成對樣本t檢定或魏克生符號檢定，比較慣用邊和非慣用邊的外側腹壁肌群於休息狀態的厚度和收縮狀態的厚度變化率。以皮爾森積差相關係數或斯皮爾曼等級相關係數，分析慣用邊和非慣用邊的外側腹壁肌群於休息狀態厚度和收縮狀態厚度變化率的不對稱程度，與揮棒速度、功能性動作篩檢總分、上肢及下肢Y平衡測試總分之間的相關性。
結果：休息狀態時，慣用邊腹外斜肌的厚度顯著大於非慣用邊(p = 0.039)；非慣用邊腹內斜肌的厚度顯著大於慣用邊(p = 0.002)；非慣用邊腹橫肌的厚度顯著大於慣用邊(p < 0.001)。Draw-in腹部呼吸法時，慣用邊腹橫肌的厚度變化率顯著大於非慣用邊(p < 0.001)。腹外斜肌、腹內斜肌、腹橫肌於休息狀態厚度的不對稱程度，以及draw-in腹部呼吸法和腹部捲曲的狀況下厚度變化率的不對稱程度，與揮棒速度之間皆沒有顯著的相關性。腹內斜肌於休息狀態厚度的不對稱程度，與功能性動作篩檢總分之間呈現顯著的相關性(r = 0.514, p = 0.006)。腹外斜肌於腹部捲曲的狀況下厚度變化率的不對稱程度，與左側上肢Y平衡測試總分之間呈現顯著的相關性(r = 0.527, p = 0.005)。腹內斜肌於腹部捲曲的狀況下厚度變化率的不對稱程度，與右側及左側上肢Y平衡測試總分之間皆呈現顯著的相關性(r = 0.589, p = 0.001；r = 0.553, p = 0.003)。
討論與結論：高中的棒球非投手球員軀幹兩邊的腹外斜肌、腹內斜肌、腹橫肌於休息狀態的厚度，以及腹橫肌於draw-in腹部呼吸法的狀況下的厚度變化率具有不對稱的現象。外側腹壁肌群於休息和收縮狀態的不對稱性與運動表現以及功能性動作檢測之間的關係，大部分的結果皆沒有顯著的相關性。僅腹外斜肌於收縮狀態厚度變化率的不對稱程度與上肢Y平衡測試總分之間呈現顯著的正相關性，以及腹內斜肌於休息狀態厚度和收縮狀態厚度變化率的不對稱程度與功能性動作篩檢和上肢Y平衡測試總分之間呈現顯著的正相關性，表示不對稱的程度愈大功能性動作模式的效率愈好。有鑑於此關係，腹外斜肌於收縮狀態的不對稱性，以及腹內斜肌於休息和收縮狀態的不對稱性為因應運動專項需求必要的適應結果，與效率較差的功能性動作模式之間沒有關係。本研究為首篇探討高中棒球運動員軀幹兩邊的外側腹壁肌群，於休息和收縮狀態的不對稱性與運動表現以及功能性動作檢測之間的關係。由於本研究缺乏足夠的樣本數、非運動員族群的對照組、運動傷害的追蹤、運動生物力學的評估等限制因素，未來的研究仍可以針對此議題進行探討。

Repetitive and unidirectional baseball batting could result in asymmetric hypertrophy and contraction of the lateral abdominal wall muscle (LAM). LAM asymmetry seems necessary for the demands of the sport, but may influence core control and lead to inferior athletic performance and functional movement patterns. The purposes of this study were to compare the thickness and contraction ratio of the lateral abdominal wall muscle on bilateral trunk and to investigate whether lateral abdominal wall muscle asymmetry was correlated with athletic performance and functional movement screens in high school baseball players. Twenty-seven single-sided high school position players completed bat swing velocity testing, Functional Movement Screen (FMS), Upper Quarter Y-Balance Test (UQYBT), and Lower Quarter Y-Balance Test (LQYBT). Thickness and contraction ratio of the lateral abdominal wall muscle including external oblique (EO), internal oblique (IO), and transverse abdominis (TA) were recorded using the ultrasound imaging system during resting and contraction (abdominal draw-in maneuver, curl up). The results showed that the asymmetry ratio of the IO thickness was significantly correlated with the FMS composite score. The asymmetry ratio of the EO contraction ratio during curl up was significantly correlated with the left UQYBT composite score. The asymmetry ratio of the IO contraction ratio during curl up was significantly correlated with the right and left UQYBT composite score. EO and IO asymmetry are a necessary adaptation for the demands of the sport and have no relationship with less efficient functional movement patterns.

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