To maintain muscle size and strength, engage in at least two resistance workouts weekly. To increase your fitness level, however, perform strength-training workouts every other day. Contrary to popular belief, you do not build muscle while lifting weights; you build it during the recovery time. In the gym, you're actually breaking down the muscle and producing microscopic tears. The muscle repairs itself during the down time, yielding size and strength gains over time. Consequently, eliminating rest days to see muscle gains sooner will only backfire.