Hollow body knee raises

On the floor, face up.

Contract your abs so that your upper back lifts off the ground.

Bend your legs, bringing your knees to your chest.

Extend them again to complete one repetition.

With respect to technique, extend your arms while protracting your shoulder blades, so that your upper back is rounded.

For perfect technique, check that you have no lumbar curvature and that your scapulae are protracted, while simultaneously engaging your back muscles to depress them.