Written by Zak Roser
Most of us who have been doing CrossFit or lifting weights for any amount of time get excited when it’s time to test our new 1RM on any lift. Some of us even sneak in PR attempts in the middle of a cycle because we just can’t wait to check on our gains.
Many lifting programs, including the Invictus Performance Programming, work on cycles that begin and end with testing a 1RM for that lift and likely include sub-maximal lifting efforts to failure as well – like when we work up to a heavy Back Squat for the day then ask you to drop the weight down to 80% and squat to for max reps (i.e. to failure).
It’s true, people see many gains in strength and stamina from these types of programs but some do not, or if they do, those gains are taken from other areas of their lives as there is only so much adrenaline to go around and central fatigue occurs. What does this mean?
Lifting heavy is a GREAT stimulus…but not for everyone. Let’s examine this more, what it means, and who should consider a training program with goals other than PR’ing and finding 1RMs on the regular.
What Does it Mean to Have a Fatigued Central Nervous System?
Central fatigue (the actual scientific term) is defined as a reduction in the output of the motor-control regions in the brain, which causes a reduction in performance. Any time you move, the brain lights up with nerve impulses generated by chemical activity. A stream of these impulses flows from the brain to the working muscles, causing them to contract. We call this flow of nerve impulses ‘central drive’. After some kinds of intense training, we see that central drive is reduced, causing a kind of fatigue even if the muscles are fine.
Neurologists have demonstrated that maximum attempts, in the form of sustained maximum contractions (isometrics) and sub-maximal sets-to-failure – such as required by the Invictus Performance Programming – both cause a brief drop in central drive. Training near or at maximum levels of output generates fatigue in the central nervous system, which temporarily reduces central drive and thus strength levels. This also affects speed, power, and fine motor control, which leads to sloppiness in highly technical exercises or movements.  Think about when you can’t hit that Snatch weight or number of double-unders you can usually crush.
The Effects of Max Lifts on the Central Nervous System
Performing one heavy or close to maximal lift (90%+) even a couple times per week can tax your nervous system significantly. If done repeatedly it’s affects can show up in your daily life.
At a basic neuromuscular level, when the nervous system is tired, it is slower and less accurate. Research on central nervous fatigue has shown a direct correlation between serotonin (5-HT), dopamine and acetylcholine and the onset of CNS fatigue. Acetylcholine is released in the peripheral nervous systems to activate muscle fibers; thus, decreased levels of acetylcholine leads to noticeable muscle fatigue and a decrease in muscle strength. An imbalance in the level of serotonin and dopamine, specifically a spike in serotonin and a drop in dopamine, is associated with the onset of central nervous system fatigue. 
Everyone has felt themselves fatigue in a workout – your form starts to break down even though your brain is telling your muscles to contract in the same manner as the first repetition. When the nervous system is fatigued, this is an example of what can occur at both chronic and acute levels. So what does this actually feel like?
Signs that Your CNS is Fried
You find yourself having trouble falling or staying asleep. You are constantly getting sick, meaning your immune system is suppressed and not working properly. You have dips in energy in the day. Increased energy in the evening vs AM. Feeling like death during metcons. Low libido. Lack of motivation. Constant, persistent muscle soreness.
Your CNS and parasympathetic nervous system (PNS) need time to recover after stimulus. This recovery comes largely from sleep and nutrition and it can be a vicious cycle if one of these is off track.
Lifting Heavy is NOT Good for These Types of Athletes
The reaction of the CNS is in constant fluctuation depending on all kinds of lifestyle factors. People who struggle with high cortisol levels, have a high stress job/lifestyle or struggle with sleep (No Sleep is No Joke), this could be something to consider. As you might imagine, these factors elevate your dopamine and serotonin levels. Lifting heavy or exercising at maximal intensities will only exacerbate the issues. Again, it is important that every athlete is in tune with their body’s response to training, food, sleep and lifestyle factors.
What to do if Lifting 1RMs isn’t for You
Don’t worry, you don’t have to give up lifting altogether – just try a different version of your program! Our favorite one, of course, is the Invictus Fitness Program which will tax your aerobic and anaerobic metabolic systems in a functional way that still allows for strength building.
If you push your limits with the weights and reps prescribed, not only is it extremely challenging both mentally and physically – you are actually doing much more in each session than you would be in our Performance Program as far as number of exercises, reps and longer time under tension. Your breathing and heart rate will be elevated for the entire hour but your CNS won’t be taxed as hard as you won’t typically be asked to go over 70-75% of your 1RM capabilities.
It is important to find a balance in the stimulus of your central nervous system, obviously some heavy lifting is beneficial but, as always, you are encouraged to experiment with a wide range of stimuli. This is where the Fitness program comes in, it gives individuals the opportunity to surprise the body with a new level of stimuli that requires a different response. This fluctuation is key for having a healthy and efficient nervous system response.
Be in Tune – Be Conscious
It is paramount that you are in tune with how your body is feeling and responding to all these stimuli that your constantly confronted with. No one person will respond the same or take the same amount of time to recover. Try slowly implementing variables that you find work best for you and each week, adding in a new recovery option to see how that helps or impedes the healing process.
Being conscious of how you feel on a daily basis and why can give you a clue on what steps you may need to take. Play with recovery. Everyone is different, finding a system of recovering that works well for you is crucial for longevity.
References & Resources:
 Squat Every Day: Thoughts on Overtraining and Recovery in Strength Training by Mat Perryman
 Cleveland Clinic: The immune system and its role in chronic fatigue syndrome (CFS)
A Brief But Scientific Look at the “F” Word: Fatigue by Alan Cocchetto. The National Forum 2004.