What position should the knees and hips be in for the Book Opening exercise?

For the Book Opening exercise in Pilates, the proper positioning of the knees and hips is crucial for effective movement and ensuring safety during the exercise. Specifically, having the knees bent at approximately 45 degrees allows for a more controlled rotation of the spine while promoting the mobility of the thoracic area. This position also helps in maintaining stability in the pelvis, reducing the risk of strain, and enabling the participant to engage their core effectively throughout the movement.

In contrast, having the knees straight or extended back would not support the intended mechanics of the exercise and could hinder the desired rotation. Crossing the ankles might create an unstable base and could complicate the movement pattern instead of promoting the fluidity needed for the rotation involved in the Book Opening exercise. Hence, the bent position at the knees and hips is the most beneficial for achieving the goals of the exercise while ensuring proper biomechanics.