Mountain climbers

Assume a pushup position with the legs straight, elbows extended, and head in a neutral position looking at the floor. The starting position of the push-up is basically a plank position, so all the rules apply here – your head, neck, spine, hips, knees, ankles should be in a straight line. Shoulders, elbows and wrists are also aligned.

Keeping your core as still as possible and butt as lower as possible, bring your knee towards your chest.

Switch legs.
Perform for the recommended number of repetitions.