The Central Nervous System and Recovery
There are many factors that go into the equation regarding strength and conditioning, along with a physical therapy plan of care. The key to solving this equation is your central nervous system (CNS) and understanding your own unique recovery needs. Throughout this post, I will discuss some of the science behind CNS fatigue and overtraining, along with a number of factors behind understanding your own nervous system.
Many of the patients I treat as a physical therapist are involved in high intensity training like CrossFit or are firefighters and police men and women who put their lives on the line each and every day. These realms are so diverse. With increased diversity comes greater demand of the CNS since there is greater adaptation required. These individuals are similar in many ways including the constant change of job and environment related stressors like training programs, inconsistent sleep patterns, poor diet, and psychological challenges. However, each group still requires their own unique recovery needs.
These types of individuals are usually Type A or highly active most days of the week (either due to training schedule, job requirements, or both). What I try to convey to these patients is the understanding that unless you are properly recovered, training over and over again only slows down the whole strength gaining process. People come to me complaining, or boasting sometimes, about training heavy complex movement patterns day after day with some of these training sessions occurring less than 12 hours a part. They wonder why they are getting hurt or not making any progress. The answer resides within a complex interaction between our brain and body.
Many of us also forget that there are other stress influences in our lives other than exercise (money, kids, lack of sleep, poor diet, work, and relationships) that tax our bodies both mentally and physically. Continuous attention is needed in all aspects of our lives. Let’s start with the most obvious: the training session.
Train Smarter, Not Harder
Most people believe in the mentality that more is better, but there is a cost-to-gain ratio that controls progress in the gym. We balance this ratio by learning and understanding the pace of our CNS. Adding volume to a workout by increasing frequency or load in order to build a fatigue buffer often leads to burn out, lethargy, and increased risk for injury.
It is harder for your brain to recover from an intense training session than it is for your muscles (up to 5-6 times longer). To give you an idea, it could take about a month for your CNS to recover from a heavy squat day at maximum effort, whereas your legs might feel ready to go after only 3-4 days.
Why should you care about this? Specifically, your CNS is responsible for coordinating muscle activity, recruiting motor units, and controlling the firing rate of muscles. If you use up the battery of your body (the nervous system), your performance in the gym and in life will suffer severely. So, how do you avoid overtraining? The key is to listen to your body – a form of auto-regulation. Our body sends us signals every day that tells us what physiologic state we are in at any given time. It personally took me years to finally understand this concept. I realized how much I was actually setting myself back, both mentally and physically, by not recovering properly.
Anytime you move your body, your brain interacts with your muscles using signals to initiate muscular contractions. Likewise, the brain reacts with nerve impulses generated by chemical activity. This is termed “central drive.” According to neurologist Simon Gandevia, intense physical activity decreases central drive even when muscles haven’t yet fatigued. This drop in central drive also results in a reduction in strength levels, power, speed, fine motor control, and movement efficiency. Consequently, highly technical movements or exercises will become disordered and messy. Imagine a patient like a policeman needs to perform highly stressful and particular movements after a waiting period. For example, chasing a suspect down a busy street could involve transitioning from sitting in a car to a dead sprint, jumping over fences, or dodging unpredictable objects. Quick reaction times and high level cognitive processing are also required when entering a burning building to save lives. You can’t afford to be running on a drained battery in these situations. The same reasoning holds true if you plan on achieving a personal best in a maximal clean and jerk during a workout.
Overall, this process of CNS fatigue is explained by the central governor hypothesis developed by Tim Noakes, a sports scientist at the University of Cape Town. The central governor is a protective mechanism that integrates and modulates various signals form the body, like feelings of fatigue (muscular, cardiogenic, signals if inflammation), to make sure we rest ourselves before too much damage can take place and keep the body within safe boundaries to minimize injury. We generally measure this through rate of perceived exertion (RPE) scores that tell us perceived level of difficulty during exercise.
High levels of inflammation trigger a type of “sickness behavior” in the CNS, as described by Lucile Smith’s cytokine hypothesis of overtraining. She notes that signals from the body create a neurological response that correlates with feelings of fatigue. These feedback loops between physical and mental stress are self-sustaining and are seen in cases of chronic muscle pain and chronic fatigue. Those involved in chronic exercise or who have very physically demanding jobs become used to pain and exhaustion, sensitizing the neural pathways and further exacerbating the condition.
How Do We Break the Cycle?
There are many ways break the nasty feedback loop of CNS fatigue, but first we need to become aware of certain biomarkers that indicate how fatigued our CNS really is at any given time.
Here are a few of the most common and easy to track:
- Heart Rate and Heart Rate Variability (HRV)
- Record your heart rate first thing in the morning while lying in bed upon waking. Do this for one week without training to establish a baseline.
- A higher than normal resting heart rate first thing in the morning (a 10-20 beats per minute increase) is an indicator your CNS may not be ready for high intensity.
- You can also purchase a relatively cheap heart rate monitor and use an app like Recover Faster to track your stress levels and readiness to train.
- For example, my resting heart rate is usually around 45-50 beats per minute. If I wake up to a heart rate in the 60-70bpm range my training for that day will be very light in terms of volume and intensity. I will, at most, foam roll, stretch, or enjoy a hike with my dogs.
- Jump Tests (vertical and broad)
- Test yourself for several days to establish a baseline.
- A drop in your score of as little as 2cm before a training session is an indicator of fatigue due to decreased power output
- This measure is not as specific as your heart rate to distinguish between muscular and CNS fatigue.
- Grip Strength
- You can use a handgrip dynamometer or a simple grip strength device.
- Once again, establish a morning baseline.
- Scores that decrease by 2 kilos or not being able to grip the device together means you are not fully recovered from a previous workout or workday.
Ultimately, the CNS is most influenced by sleep quality, which also affects hormonal function, including sex-binding hormones such as cortisol, testosterone, estrogen, melatonin, and serotonin production. Melatonin and serotonin are crucial for restful sleep. Chronic sleep deprivation affects the others by throwing off the balance between these vital hormones, leading to decreased libido, impaired athletic performance, insomnia, stubborn fat deposition around the mid-section despite diet and exercise, and mood imbalances. General lethargy and lack of interest in things you once loved are great indicators of chronic CNS fatigue.
Moral of the story is this: sleep is vitally important for CNS recovery. If you are limiting high quality sleep to squeeze in an extra workout, you may be setting yourself up for a milieu of problems.
You need to listen to your own body to understand what you need to make progress in the gym, be effective at your job, and to stay healthy and injury free. Those needs are as unique as you are. Your CNS is, in my opinion, one of the most important factors in how you train and how you live your life. You need to learn how to use it appropriately.