As strength and conditioning coaches, it is imperative that we have a good understanding of what fatigue really is. A dictionary definition might be a decrease in energy, but most of us would say that fatigue is a decrease in force production. That might be acceptable to tell your athletes, but do you really understand what is going on? Let’s take a close look into what fatigue really is and it’s untapped potential.
There are two classes of fatigue: “peripheral” and “central”. The more commonly known and understood is peripheral fatigue. Peripheral fatigue is what is happening in the extremities, more specifically the muscles. This is usually due to action potential failure or impairment in the cross-bridge cycle. Studies have shown that there is an increase in lactic acid concentration and a decrease in pH, ATP, and creatine phosphate. Also, there is a decrease in muscle or liver glycogen stores during sub maximal exercise which is all believed to cause fatigue. These metabolic processes add up together to prevent the muscle from a forceful contraction. Essentially, you can look at peripheral fatigue as the muscle no longer capable of producing the force that it is being asked to produce. An example would be doing 1,000 bicep curls and getting the “BURN”.
Less commonly known is central fatigue. Central fatigue is a decrease in neural drive or a disruption in the efferent fibers. In more simple terms, central fatigue originates in the brain. There is limited research on this phenomenon, but studies show that during exercise there is a change in neurotransmitters, such as increase in serotonin, which can regulate muscle contraction among other things, and a decrease in dopamine and acetylcholine which play a role in voluntary movement, motivation, attention, working memory, and learning; and then opens ligand-gated sodium channels in skeletal muscle to produce muscle activation, respectively. Now, I know neurology wasn’t my favorite class either so to develop the big picture, let's just say when there is increase in neural drive, there is an increase in neurotransmitter activity which results in a decrease in brain capacity to recruit motor neurons. This is that feeling of your body just not doing what you ask it to do although your muscles aren’t on fire, that “I just don’t have it today” feeling.
The debate on central fatigue is that I mentioned it being a disruption in efferent fibers. Proponents of peripheral fatigue will argue that it is just the opposite, and that it is the afferent fibers that cause a change in the neurotransmitters. Meaning that muscles, by way of the mentioned metabolic processes, are sending sensory information to the brain that then activate the change in the neurotransmitters to stop exercise. But then studies have shown that fatigue doesn’t start in the motor cortex but even further up in cortical regions, as in the prefrontal and cingulate cortex. So what comes first, the chicken or the egg?
No studies have shown a physiological change with no change on perceived fatigue. Actually, just the opposite has been shown. Perceived fatigue has been expressed with no physiological change. Chronic fatigue syndrome is just that; patients express fatigue at rest when there is no impairment of the metabolic processes that we discussed. . Consider the fact you can produce more force during an eccentric lift than a concentric lift, which would suggest different neural drives. Studies show that at most during maximal voluntary contraction (MVC) one can only recruit 70% of motor units and some suggest that 100% recruitment would tear tendons right off the bone. On the other side perhaps central fatigue takes place as a protective mechanism. By changing neurotransmitter activity your brain will not allow 100% MVC. So what are your muscles actually capable of and what can you do about it?
Whether you believe it is the chicken or the egg that comes first, you can’t argue that they both have a role in fatigue. Metabolic processes do take place and your brain does prevent 100% MVC. So how do decrease the protective threshold? For the sake of this article, we will save the idea of over training for another time and just focus on a single training session. In order to get more out of our athletes, we need to engage their brain. Increase dopamine and acetylcholine. Studies have shown that verbal encouragement during activity will increase the duration of the contraction. Others have shown that yelling during a lift actually can increase force production. Another study showed that when subjects were asked to give maximal effort during a cycling sprint their power output decreased over the reps except it increased on their last rep, indicating a “reserve tank”. Perhaps the brain protects less knowing that it is your last rep. Perhaps this developed during the pre-historic era when our ancestors had to hunt for their food, but maintain a reserve tank of energy in case they became the hunted. Either way, we need to tap into that reserve tank. One way to achieve this is by including open looped activities. Doing 10 reps or sprinting for 1 minute is a closed loop activity because you know when it will stop allowing you to pace yourself. So including an open loop activity is one way to do short maximal effort without pacing yourself, such as having your athlete sprint for an unknown time stopping when you decide to blow the whistle. Challenge yourself to develop ways for the conscious brain to control the unconscious brain. Don’t allow your athletes to pace themselves. Training is not about surviving, it is about DESTROYING.
If a mother is able to lift a car to save her child, find a way for your athlete to lift a bus, because they can.
Keke Lyles is a DPT student at Northeastern University and has worked directly with both the Men’s and Women’s Basketball teams.
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