Day 13 runs the running-in-place family in a new pairing. Four 15-second intervals - butt kicks, high knees, butt kicks, high knees - one minute of rest, three to seven sets. Same footwork, opposite direction of travel.
High knees drive the thigh up in front of you; butt kicks flick the heel up behind you toward the glute. Front of the leg and back of the leg, alternating every fifteen seconds. That is not just variety - it means the hamstrings get a fast, light, high-repetition interval sandwiched between two hip flexor efforts, and the two muscle groups end up sharing the session rather than one carrying it. It also keeps your cadence honest: butt kicks are almost impossible to do slowly without them turning into a jog.
On butt kicks, keep your knees pointing at the floor rather than swinging forward, stay tall, and let your heels come all the way up to make contact. On high knees, drive the knees high and the arms hard. In both, quick light contact with the floor beats heavy stomping - your calves and knees will thank you around set four.
Heels up, knees up, and no interval long enough to let either one become comfortable.
Make it Harder: Bring your hands behind you at hip height and touch your heels to your palms on every butt kick.
Make it Easier: Take both intervals at a jog cadence and let the range be whatever your legs will give you today.
