In a sit-up, how should the athlete lie?

In a sit-up, having the athlete lie in a supine position with the knees bent at 90 degrees is the correct approach because this position effectively engages the core muscles while providing stability and reducing strain on the lower back. By bending the knees, the feet remain flat on the ground, which helps to secure the lower body and allows for a more controlled and effective movement as the athlete lifts their torso towards the thighs. This position also allows for optimal contraction of the abdominal muscles, leading to better strength development.

Other lying positions, such as being supine with legs straight or prone with arms extended, would not only hinder the effectiveness of the exercise but could also place unnecessary strain on the lower back and make it more difficult to target the abdominal muscles properly. Similarly, sitting upright with the back straight does not utilize the sit-up mechanics and does not allow for the primary engagement of the core that the sit-up is designed to promote. Thus, the bent-knee position is the safest and most effective for executing sit-ups properly.