Why do muscle gains slow down over time and how can we speed it back up?

When it comes to training, your body is a remarkable adaptive machine, which is a good and a bad thing at the same time.
It is great because we survived the ice age due to our ability to adapt. It is bad because your bodies do not want us to pack on unlimited muscle mass (insert sad Rubio face here)

Adaptation means that  proteins are constantly breaking down and rearranging in preparation to being able to handle a new stimulus
Resistance training is an example of such a stimulus to the body. Sufficient amounts of intense muscular work ( meaning picking stuff up and putting it back down) causes microtrauma in muscle and connective tissue, which leads to inflammation and the ignition of muscle growth, or muscle protein synthesis aka GAINS.
Muscle growth is only one of the ways that your body adapts to the repeated, progressive stress of a properly executed training regimen. The other component that is improving is your recovery ability.

In untrained lifters, research has shown that elevated rates of muscle protein synthesis (growth) can last as long as 48 hours after a workout session . This is the reason for the typical “rest a day in between training a muscle again” recommendation, and this is why inexperienced trainees can grow quickly with a low volume and low frequency approach; say performing 3 sets of chest once a week, for example.
However, as said trainee continues lifting week after week, his body starts adapting to training and becomes less responsive. We do know that muscle protein synthesis rates can rise and drop back to baseline in as little as 12 hours in advanced lifters
Clearly, a low volume, low frequency approach to resistance training is not going to be sufficient to maximize progress for advanced trainees. And no, the answer is NOT to start doing 20 sets of chest on Monday, to “extend the anabolic window” trust me I have tried.

Instead , once you are past the newbie gains stage, you will need to increase volume AND frequency in order to maximize your rate of muscle growth. This basically means you ll have to train every muscle group at least twice a week with sufficient volume, which is basically what my personal training clients in NYC are doing ( whether they like it or not!)
Arnold-style twice a day double split, anyone?
Training during the “recovery” period does not hurt progress, but actually leads to more results. Have you ever wondered while elite cyclists have massive quadriceps? It’s because they work those muscles to exhaustion on an almost daily basis, while also including intense lower body resistance training.
I will leave you with this: the fastest ever recorded (rate of muscle gain in human subjects occurred during quadriceps training performed to exhaustion 24 times in two weeks. How can it be overtraining if you are gaining muscle and getting stronger?
So if you have been training longer than 18 months, it is probably time to crank up the volume!