The simple answer to that question is you are most likely not overtraining. True cases of overtraining are very rare. It generally occurs with individuals who are training double digit hours a week, at an extremely high intensity, and not allowing enough time for recovery. If you're an average gym-goer, training less than 6 hours a week, you are not overtraining. You are not doing enough to tax the nervous system to the point where you would develop Overtraining Syndrome (OTS).
Overtraining Syndrome (OTS) is a physiological state caused by an excess accumulation of physiological, psychological, emotional, environmental, and chemical stress that leads to a sustained decrease in physical and mental performance, and that requires a relatively long recovery period.
Overtraining IS NOT just fatigue or injury due to excess exercise. It is the cumulative effects of physiological, psychological, emotional, environmental, and chemical stress.
Some of the Symptoms of Overtraining:
Kreher, J. B., & Schwartz, J. B. (2012). Overtraining Syndrome: A Practical Guide. Sports Health, 4(2), 128–138. http://doi.org/10.1177/1941738111434406
While you may not be overtraining, that does not mean that you aren't overtaxing your body. You should allow adequate time for your muscles to rest between workouts. The length of recovery time between workouts varies depending on the volume and intensity of your workouts. The best way to avoid overtaxing is to use a periodization model. You can't just continue to increase weight or intensity endlessly. You have to cycle between loading and deloading phases. Failure to do so could result in overtaxing.
Sample Periodization Model:
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