什么是蹲踞式起跑---**Textual Information:** “预备”时，逐渐抬起臀部，使身体重心向前上方移动，此时身体重量落在两臂和前腿之间，其重心投射点距离起跑线15-20厘米。臀部抬起稍高于肩，使两小腿趋于平行。“预备”姿势时，不过分地把身体重量移向两手是非常重要的，因为这能减少两手推地的困难，加快两手推离地面的速度。此时，膝关节角度状态有重要意义，适当增大膝角，有利于蹬伸。大小腿之间最佳的角度：前腿膝角约为90-100度，后腿膝角约为110-130度。 **Chart/Diagram Description:** * **Type:** Illustration of a figure in a starting stance (likely a sprint start). * **Main Elements:** * **Figure:** A simplified human figure in a crouched position, with hands on the ground and knees bent. The figure is colored with a red top, yellow shorts, and blue limbs/outline. * **Ground Line:** A horizontal blue line representing the ground or track. * **Starting Line:** A vertical blue line appears to represent a starting line, though not explicitly labeled as such. The figure's hands are positioned slightly behind this line, and the front foot is also behind it. * **Limbs:** The figure's arms are extended, hands on the ground. The left leg is forward, with the knee bent. The right leg is back, also with the knee bent. * **Circle:** A white circle with a blue outline is drawn around the bent knee of the front leg. This seems to highlight the knee joint area. * **Lines:** Several horizontal blue lines are present. One passes through the figure's shoulders, indicating the relative height of the shoulders. Another is the ground line. A short dashed line extends horizontally from the figure's rear foot towards the left, with an arrow pointing left, suggesting backward push or force direction. A horizontal blue line also marks the starting line. * **Relative Position:** The figure is positioned on the ground line, facing right. Hands are near a vertical line (starting line). Knees are bent, and the hips appear raised above the shoulders. The weight distribution is implied by the text to be between the arms and the front leg.