手臂圆弧动作
Also known as: 圆弧, 风车圆弧
手臂圆弧动作是指投球手臂围绕肩关节完成的完整360度旋转，这定义了快投垒球的风车式投球动作，并为出手储存弹性能量。
手臂圆弧动作是快投垒球投球区别于所有其他投掷动作的关键。手臂向后、越过肩膀摆动，绕到背后，随后向下、向前经过髋部，在此处手腕甩动完成出手。流畅、稳定的手臂圆弧动作，是每一颗投球的基础——围绕它的力学环节（蹬地、髋部旋转、手腕甩动）都与它相连。常见的问题包括圆弧动作中断或不完整，这会扰乱时机、降低球速。
Start every practice session with slow-motion arm circles at walking pace, feeling the full round shape and elbow extension before adding any velocity or stride.
Use a weighted training ball for 20 arm-circle reps at the end of each bullpen session to strengthen the shoulder stabilizers that keep the circle consistent under fatigue.
Example
一名教练用录像检查投手的手臂圆弧动作是否流畅，是否在顶点达到完全伸展，然后再向下出手。
How it shows up on video
From a side view the arm circle should appear as a smooth, round loop with the elbow fully extended through the top and the hand passing close to the ear before swinging down. This means any flattening or breaking of the circle is visible as a hitch. The top of the circle should reach full extension overhead before the arm begins to descend. An early drop from the top produces a flattened oval that reduces elastic loading. The circle's consistency frame-to-frame across pitches is what confirms mechanical reliability.
Common mistakes
- Breaking the circle by bending the elbow at the top, reducing the radius and mechanical efficiency of the delivery
- Letting the arm drift behind the body's vertical plane at the top of the circle. This creates a looping delivery path that adds time and reduces power
- Rushing the circle so it is not fully complete before the push-off, causing the arm and body to arrive out of sequence at release
- Using an overly tight, small circle in an attempt to throw faster, which actually reduces the elastic energy storage and lowers velocity
In SwingVantage Motion Lab
Motion Lab traces the full arc path of the arm through the circle and measures radius consistency, timing relative to push-off, and elbow extension at the top. This means the three key arm circle quality metrics.
Frequently asked questions
为什么手臂圆弧动作比侧臂或过肩投球更能有效产生快投垒球的球速？
完整的360度旋转能通过肩部和背部肌肉储存弹性能量，而这是更短的动作无法获取的，随后在出手时通过手腕甩动释放出来。
我怎么知道自己的手臂圆弧动作和蹬地的时机是否配合恰当？
手臂应该大致在跨步脚落地的同一瞬间到达圆弧的顶点——如果脚落地时手臂已经在向下运动，说明圆弧动作过快了。
Related terms
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