The second cardio day of the cycle keeps the shape and changes the engine. Four 15-second intervals - march steps, high knees, march steps, high knees - one minute of rest, three to seven sets. Both exercises are running in place; the only variable that changes between them is how high the knee gets and how fast it gets there.
That single variable is doing all the work. March steps lift the knee to roughly hip height at a steady walking cadence, arms swinging in opposition. High knees take the same pattern and turn the dial up: the knees drive higher and faster, and the exercise leaves the ground. Running the two in alternation gives you a genuine two-speed session where the slower interval never becomes a rest - your feet keep moving, your heart rate keeps its altitude, and the next fifteen seconds arrives with nowhere to hide.
March with intent rather than shuffling, and use your arms - a driven arm swing lifts the knee for you. On high knees, stay tall through the spine instead of leaning back, land on the balls of your feet, and aim for quick contact with the floor rather than height for its own sake. If your knees start dropping in the last five seconds, that is the interval doing its job.
Same two feet, two entirely different gears - and 60 seconds of work that feels considerably longer.
Make it Harder: Bring your hands to shoulder height and slap your palms with your knees on every high knee rep.
Make it Easier: Run the high knee intervals as a light jog in place with lower knees and keep the marching brisk.
