Part of my treating patients with neck pain or low back pain often entails giving them specific stretches and exercises to help with those conditions. When it comes to the stretches, I recommend they be done daily or in some cases up to three times per day. With exercises, however, the point is to strengthen the muscles and build their endurance. As a result, these exercises have a better effect on the body when they are performed less often.
It is a common misconception with exercise that more is better. The idea being that if inactivity is bad and exercise is good then a whole lot of exercise must be really good. This line of thinking ignores the metabolic effects of exercise on the body. To put it very simply, exercise breaks down the body. It has a catabolic (breaking down) effect. It is really the period of rest between exercise sessions that allows the body to rebuild stronger than previously, thus providing an anabolic (building up) effect.
Problems occur when we do not sufficiently rest between exercise sessions. The body does not recuperate from the exercise stress and emerge stronger. Instead, there is a progressive breakdown in which the body does not have ample opportunity to recover.
To this end, when I recommend core exercises for my patients, I instruct them to perform one set to exhaustion every other day. Performing that one set provides the stimulus to the body to rebuild the core muscles stronger. A day of rest gives the body sufficient time to do so.
The same principle applies to cardio exercise. Research is showing that short bursts of extremely intense activity (high intensity interval training) are superior to medium intensity even when that medium intensity is performed for longer periods of time. High intensity interval training is usually performed as short bursts of exercise (biking, swimming, sprinting, etc.) for a period of 30-45 seconds followed by 90 seconds of medium intensity. These intervals can be done 4-8 times with a three minute low intensity warm-up and cool down at the beginning and end. Without getting too deep into the physiology, high-intensity exercise provides a much greater effect and consequent stimulus for the body to recover than the medium intensity exercise.
The bottom line, so often the case with the human body, is that balance is the key. Intense exercise followed by ample rest will produce the best health benefits.