A few physiologists who study the effects of exercise, suggest the old-fashioned burpee, the squat or walking. Unless you can do H.I.T.s or high intensity interval training. An example from the article:
“The approach seems promising, since most of us have minimal time to exercise each week. Gibala last month published a new study of H.I.T., requiring only a stationary bicycle and some degree of grit. In this modified version, you sprint for 60 seconds at a pace that feels unpleasant but sustainable, followed by 60 seconds of pedaling easily, then another 60-second sprint and recovery, 10 times in all. ‘There’s no particular reason why’ H.I.T. shouldn’t be adaptable to almost any sport, Gibala said, as long as you adequately push yourself….Of course, to be effective, H.I.T. must hurt.”
Sounds klnda like lead climbing or even bouldering, doesn’t it?