What exercise does not require the knees to be raised high and should face straight down?

The exercise that does not require the knees to be raised high and should face straight down is the double knee lifts. In this exercise, the movement primarily focuses on engaging the core and working the hip flexors and lower abdominal muscles. The knees are lifted together in a controlled manner, typically while lying on your back or in a seated position, maintaining a direct upward motion rather than high energy or uncoordinated movements.

This allows for a smoother and more targeted engagement of the muscle groups involved. Unlike other options where the execution demands higher knee elevation—such as bunny hops or single leg curls—a double knee lift is characterized by a lower, steadier motion that aligns directly with the body's vertical position.

In contrast, bunny hops involve significant knee lift and explosive movement, which doesn't correlate with the question's requirements, while single leg curls and leg swings also incorporate elevated leg positions. Therefore, double knee lifts meet the criteria effectively.