Get in a push-up position with the hands aligned with your shoulders, and the feet about hip-width apart.

Tilt the hips into a posterior pelvic tilt (pointing the tailbone down towards the ground). If it is no longer possible to maintain the posterior pelvic tilt before the completion of the desired time, end the set.

Lift one foot off the floor and bring that knee to alignment with the hip, then return to the starting position. Alternate steps for the duration of your set.

Remember, during the entire set, tighten the core and glutes as much as possible.