Acute fatigue is characterized by a short term disruption of homeostasis from training, often resulting in slight improvements in performance. However, if acute fatigue beings to accumulate from increasing training intensity or volume without adequate recovery, or without periods of rest, chronic fatigue can occur.

Chronic fatigue can be divided into 2 categories: overreaching and overtraining, where overreaching can be referred to as functional, or non-functional.

Functional overreaching can be defined as a temporary decrease in performance lasting no more than a few days to weeks, while non-functional overreaching is defined by a decrease or no change in performance over weeks to months. If overreaching symptoms persist for prolong periods and performance begins to drop significantly, it is defined as overtraining, sometimes called overtraining syndrome.

I'm sure you've heard of the saying "there is no such thing as overtraining, only under recovery". While this is technically true, there are training volumes and intensities that an athlete simply can't adequately recover from without being in a drug-induced supra-physiological state. But unless you're an elite level Olympic athlete pushing the boundaries and closing in on a world record, or a beginner trainee attempting to perform an advanced training program, it's not likely you'll find yourself in that situation. Instead, overtraining syndrome develops when recovery is neglected whether it be due to laziness, or compromised due to financial situations, work schedules and unplanned life events.

Training and adaptation is a multi-factorial equation, taking into account the inputs (types of movement, intensity, volume, frequency), outputs (recovery, fatigue management, sleep, nutrition) and everything in between (training history, individual response to exercise, genetics).

Overtraining is very real and can occur when the balance between training and recovery is not met.