Fatigue (whether neurological, metabolic or neuroendocrine) will inevitably occur with any type of physical training.
The key to athletic success is being able to manage the accumulation of fatigue whilst also receiving sufficient training stimulus to make progress.
So how do we manage fatigue?
Many weight training programs call for a deload after a specified time frame of training, however these typically fail to account for variation between individuals in regards to work capacity and they certainly fail to account for “life” variations such as increased stress, workloads, holidays etc.
Another commonly suggested form of fatigue management is taking a deload when you “feel” like you need one. There are a couple of flaws with this method. First and foremost, often what you “feel” doesn’t line up at all with objective physical performance (I’ve certainly had countless training sessions where I felt like shit but still managed a PB). Secondly, but perhaps most importantly, many of the symptoms of excessive fatigue lag behind the point where you should have taken that week off. That is, by the time these symptoms (desire to train, decreased appetite, mood changes, sleep disturbances, illness) get bad enough you notice them you’ve already been in an “excessively fatigued” state for a period of time.
The good news is that there are a number of objective fatigue measures which are usable with minimal equipment, and importantly will change either before or during periods of excessive fatigue accumulation.
A really easy way to gauge your current fatigue level is to track your sets/reps at a particular weight on a number of exercises* (using a number will allow for variation in localized muscular fatigue vs global fatigue). Whilst we all have “off days”, if your performances are significantly below what you would expect to achieve over a number of sessions then odds are you are in a state of excessive fatigue and should take a deload. This is going to be the easiest measure for most people to use as you don’t need to do anything different than what you normally would in your training program, nor any additional equipment.
*continued in comments*