Day 37 is high intensity interval training: 20 seconds of split jacks, 20 seconds of knee-ups and 20 seconds of high knees, back to back for one 60-second set. Rest 60 seconds, then 3, 5 or 7 sets by level. All three are standing; the new one is in the middle.
Knee-ups are drawn with the arms reaching straight up overhead, then one knee driving up as both arms pull down to meet it, and back up. It is a high knee with the whole upper body joining in, so the abs and shoulders work as hard as the leg. Split jacks before it get the legs switching front and back, and high knees after it turn the knee drive into a run. The three intervals share one idea, knees up, and change the arms around it.
Land the split jacks softly with the switch happening in the air. On knee-ups, reach fully at the top and pull the arms down with force, alternating knees; keep the chest up rather than crunching forward. High knees with the arms pumping and the feet light. Aim for a pace you can keep for the full 20 seconds of each interval.
Three intervals, three ways to lift a knee, one minute to get through them.
Make it Harder: Add a fourth 20-second interval of split jacks to every set before the rest.
Make it Easier: Keep the feet on the floor for the split jacks and march the high knees at the same tempo.
