Just because you don't feel sore doesn't mean it's time to give your muscles another pounding. However, it is OK to work the same muscles two days in a row if you're performing low-resistance exercises, such as most cardio activities. With serious weight training, though, it's a different story. Always give muscles at least a full day of recovery after performing targeted strength moves. They need their rest, and you won't perform at peak levels if you deny them recovery.
Not only is it OK to perform cardio two days in a row, but it's required to meet your weekly fill -- the U.S. Department of Health and Human Services recommends at least half an hour of moderate cardio most days of the week. If you want to lose weight or prevent fat gain, you may need an hour a day. And if you used to be overweight or obese, you could need up to 90 minutes to keep fat at bay.
Weight training doesn't just refer to dumbbells and plates -- it also includes muscle-building exercises such as pushups, pullups and lunges, in addition to yoga and cable exercises. While it's perfectly fine to perform weight training two days in a row if you use different muscles, it's not a good idea to work the same muscles without a break. Two to three days a week of strength training is enough to reap the benefits, and you can choose to work all muscles in the same day or alternate muscle groups.
If you hit the weights hard, your muscles may need more than a day of rest. The bigger the muscle, the greater the recovery time. If you shred your quads or hamstrings to the point of soreness, for example, give them a three-day break; sore abs need two days to recuperate after a shredding. No matter how much time has passed, it's a bad idea to work sore muscles. Soreness means muscles are torn, and you may have less coordination and be more prone to injury.
When you push too hard with no recovery time, you run the risk of overtraining -- even with cardio exercise. This syndrome worsens without rest, and affects you both physically and psychologically. You'll feel exhausted and make less athletic progress than you would with a reasonable routine. You'll also have higher levels of the stress hormone cortisol, and may become irritable or depressed. In chronic cases, your muscles become weaker. While everyone's exercise threshold is different, you can prevent overtraining by monitoring your heart rate each morning. If it's more than 10 percent higher than your baseline, skip the workout that day.
Nina K. is a Los Angeles-based journalist who has been published by USAToday.com, Fitday.com, Healthy Living Magazine, Organic Authority and numerous other print and web publications. She has a philosophy degree from the University of Colorado and a journalism certificate from UCLA.