A deload is a planned reduction in volume and/or intensity, with the express aim of allowing the body to recover from fatigue caused by weeks of hard training. Why is this necessary?
Feeling exhausted in your training? A deload could be in order
Why you should be doing them
Whilst training leads to positive improvements in things like strength and muscle mass, they come at a cost: fatigue. Fatigue is why we cannot just keep increasing the load on the bar forever without taking a rest. At a certain point, the fatigue generated by an overload event will outweigh your body’s ability to recover and supercompensate. This is sometimes accompanied with an increase in joint niggles, as the connective tissues of the joints take longer to strengthen than muscle. When this happens, it is generally advised that you take a deload/light week in order to let your body fully recover.
NOTE: This only applies to people who are training hard consistently. If you are someone who regularly misses gym sessions or is not trying to increase their strength, then you do NOT need to deload as you aren't fatiguing your body enough to warrant one.
The fitness-fatigue model of training
How often should you do them
General guidelines on how often one should deload tend to range from every 4 - 6 weeks. Personally, I think that taking this as gospel is a BIG mistake (not least because there is little research to back this up). I have found that people’s deload frequency is highly individual. For me personally, I massively short-changed my progress in my first few years of training by deloading every 4th/5th week. It was only in the past couple years that I realised that i could train productively for over 8 weeks without having to deload.
My advice for people is that if a deload isn't something you're really looking forward to, then you probably don't need one as you're not training hard enough to warrant one. Some red flags that you need a deload can include one or more of the following: a reduction in training performance, worsened mood/sleep/appetite, a loss of energy/motivations to train, increase in niggles/joint pains.
If you've been training for quite a few weeks and are experiencing an onset of joint pains, it can be a good indicator that a deload week is needed.
How to deload
Like I described above, we need to reduce the stress of training to give your body room to recover. This does NOT mean doing nothing for a week. We still need to train to maintain our technique and a base level of strength and fitness. As for how much you reduce, I think this is also highly individual. Some people do great by cutting their volume and intensity of sessions in half. For me, I lose a lot of strength that way; so much so that when I tried that approach I would start a new training block right where I left off, or sometimes even slightly weaker. I’ve since learned that I do best by lowering the volume slightly (about 1 set less and half the reps) and keeping the weights equal to what I used on week 1 of my last training block. This stil signifies a major reduction in workload, while not going so light that I lose lots of strength.
Note how the deloads are roughly equivalent in loading to the first week of each previous training block.
A side note
One thing I have observed is that people who are better natural athletes seem to be more suited to the traditional deload paradigm where every 4 - 6 weeks a deload is taken, and loads are reduced significantly. While this is just a hypothesis: I think it seems logical that people who naturally gain strength faster don’t need to overload for as long, and similarly they can afford to go much lighter for a week without having their strength decay. Whereas people with average genetics may need more constant reminders for their body to maintain strength.
Deloads are a necessary part of making continued progress in resistance training. The details of how often and how large a reduction in workload will likely be individual to you. Experiment to find how light you need to go to feel fully recovered. But also don’t just sit on your ass for a week or you will most definitely lose gains.