Hyperplasia (the breaking down of muscle in order to rebuild it) is not necessarily a proven concept however recovery of sore muscles applies to both believers and nonbelievers. The idea is that when you exercise, your muscles are broken down; hence the delayed onset of muscle soreness two days after a good workout. It is the re-building of the muscle tissue that creates strength ( and in some cases size). Essentially you should not re-work a muscle that is in pain due to exercise, it should be considered as injured and rested for 2 – 3 days. It is not necessary to wait an entire week before re-training a muscle as we used to believe. We now know that resting a muscle for over a week actually hinders results, but over working a muscle can lead to over training and injury. The trick is to find a balance between working too hard and resting for too long. You should not train so hard that your muscles are sore for days however you should train hard enough to feel the workout for around 24 hours, enabling you to go back to the gym 48 hours later.