Suppose you were in a situation of restricted access to calories -- at or perhaps even below what you need to sustain your weight long-term -- but wanted to stay/get in the best shape possible under those circumstances. (Hypothetical example: you're a captive prisoner-of-war, and are trying to maintain shape in case you have the opportunity to escape, but not starve as a result of exercising beyond your calorie allotment.)
What's the best exercise program you can design (with or without any equipment) that will keep or improve your strength, endurance, reflexes while consuming as few calories as possible so as not to hasten starvation? Or is that basically impossible and your best bet is to conserve energy and hope for the best?