How much, and how fast we can grow our muscles and improve our bodies is largely dictated by how often we can get in the gym and stimulate the muscles to grow. Theoretically, if we could fully recover from training in an hour, we could go right back in and train again, multiple times per day and, pending we could eat enough, could obtain over a years’ worth of growth in one week. The limiting factor to growth is recovery. That includes recuperation of the trained muscle itself, but much more importantly, recovery to the central nervous system. Muscles quickly adapt to training, and muscle breakdown and muscle soreness decrease in duration and intensity the more one trains and the better conditioning they achieve. The central nervous system (CNS) on the other hand, is not nearly as malleable as muscle tissue, and a fatigued CNS limits not just the muscle group you trained that day, but the entire body. A fatigued central nervous system results in the inability to fully activate the muscle, overall fatigue, and a higher level of perceived exertion.
Muscle soreness, or Delayed-Onset Muscle Soreness (DOMS) is a function of micro-trauma in the muscle and surrounding tissues as a result of training. Interestingly, the eccentric (lengthening of the muscle, or negative) is nearly 100% responsible for DOMS, concentric (contracting)and isometric (static) movements have very little to do with DOMS. Breakdown of the muscle as a result of overexertion is what creates the stimulus for the muscle to then rebuild and regrow. Recent studies have shown that the actual degree of DOMS is very poorly correlated with muscle adaptation and growth. That is, the more sore you are does not determine how much you grow. Similarly, not being sore post-workout does not mean you did not stimulate and damage the muscle and induce growth. There are many other factors at play. Further, it is not necessarily a bad thing to train an already sore muscle. While the muscle may not function to its fullest capacity due to some residual damage and inflammation, there is no evidence to indicate that doing so is harmful or detrimental to further growth. Muscle soreness is overall not a big limiting factor when it comes to overtraining.
The Central Nervous System (CNS) is essentially the puppet master of all of your muscles, and it is an extremely complex system. The CNS dictates everything your muscles do through millions upon millions of nerves. It too, gets overused and over-trained. In particular, complex, whole body exercises like squats, deadlifts and other compound (using more than one joint) movements require lots of muscle cells to be activated and in turn place an enormous load on the CNS. Unlike muscle tissue, which adapts to training as you become more highly trained, the CNS does not. In fact, as we build more muscle, lift heavier weight and gain the ability to train longer and harder, a much greater load is placed on the CNS. Lacking the adaptability of muscle, the CNS gets overloaded much quicker the bigger and stronger we are. When we first begin training, we generally start off with upper body one day, lower body the next, then repeat. As we advance in our training though, this no longer works. We need more rest and switch to less body parts per day and require more off days. This is the result of CNS overload.
When the Central Nervous System gets fatigued, it loses the ability to fully and consistently recruit all of the muscle cells in a muscle group. Being that there are millions of nerves that control a muscle, if the CNS is unable to fire 100,000 of these nerves, we lose that much strength. This is true across the entire body. If you trained legs one day, back the next day and wore out your CNS system, when you go in to do chest the next day, you will not be able to fire all of your pectoral muscles and subsequently be weaker. A secondary effect of this is a much greater perceived exertion. That is, the same weight will feel heavier.
So what can we do to avoid CNS overtraining? Nothing really. It is part of pushing yourself. Maintaining proper nutrition can play a large role and allow you to get more out of your CNS, however having better nutrition itself means you can then train even harder, train that much more and find that limit where the CNS says “NO”, as if you are going to get the most of your body, you want to find that limit. Reaching CNS fatigue means you did all you could, you pushed as hard as your body would allow. The goal should never be to not be sore or not over-train. The goal should be to push yourself exactly to that point, then be intelligent enough to back off. The important thing is to be cognizant of it and understand the signals your body is giving you. If you are feeling run down, weak, lethargic or aching, that’s a good indicator that your CNS needs some recuperation.