The S.A.I.D principle (Specific Adaptation to Imposed Demand) refers to the idea that your body adapts to the specific type of stress put upon it. So, when an endurance runner pushes to finish that 20-miler in a specific time frame, his/her body is adapting to that specific type of stress being put upon it.
The problem is many athletes (aerobic or anaerobic) don't give their bodies time to adapt before imposing more stress on their bodies. So they never make it to the gain threshold. They're constantly stuck in the recovery period or worse, they become injured. This is called overtraining.
Overtraining can also be due to repetitive exercise. If you don't vary your workouts and you're constantly subjecting your body to the same stress over and over, those muscles can become overtrained. A good rule of thumb is to wait at least 48 hours before working the same muscle groups again. So for example, if your do a chest/triceps workout one day, you should wait at least 2 days before working those muscles again. Professional bodybuilders will often workout a muscle group so hard in one workout, that they'll wait an entire week before working that muscle group again.
In running you should think more in terms of hard/easy. Hard workouts can include speed workouts such as intervals, repeats, tempo runs, hill-work, or long runs. Easy workouts can include short or mid-distance runs that are run at an easy to moderate intensity (60-75% of your VO2Max). So, if you do a hard run one day you should wait at least two days before running your next hard run.
Some common signs of over training include:
- persistent achiness, stiffness, or pain in the muscles and/or joints (beyond the typical delayed onset muscle soreness felt after a workout)
- waking up with an elevated pulse (good idea to take your waking resting pulse frequently to give you a base from which to compare)
- lack of energy
- fatigued and/or achy muscles
- frequent headaches
- feeling lethargic or sluggish
- drop in athletic performance
- not able to complete your normal workout
- depressed, moody, unmotivated
- lack of sleep and/or appetite
- weight loss
- lowered immune system
An elevated pulse is a good indicator of possible overtraining or even sickness such as a respiratory infection. If your waking resting pulse is elevated more than a few beats, you could have an infection or be suffering from overtraining. In either case, taking a day off may be the best thing. Rest is the best thing for overcoming overtraining. If rest doesn't do the trick, schedule an appointment with your doctor.