Fitness experts suggest that it’s a bad idea to do weights and cardio back to back. If you want to build muscle, keep weights and cardio sessions separate. Doing cardio just before weights will lead to fatigue, which will result in poor lifting performance. On the other hand, doing cardio straight after a weights session can undo the muscle-building effects of a workout. The best thing is to do them on separate days, which gives your muscles time to recover between sessions.