They can’t. It sucks, but they can’t. But they can do a mean stomp!
There are two variants for this exercise, based on your modification needs.
Variant 1 – (for knee problems) Stand upright with feet shoulder width apart. Bring your hands together clasped at your midsection (belly button level). Swing one knee up to meet your clasped hands (try not to bring hands down to meet the knee). When knee has returned to starting position, bring other knee up. Count completion of both sides as one repetition.