You know the feeling. You’re doing a high-intensity workout and after 15 burpees in a row, you’re wiped out. You feel like plopping down on the floor and massaging your sweaty forehead with a wet towel- and then calling it a day. Exercise fatigue is very real and it’s what limits the performance of professional and amateur athletes alike.
The Enigma of Exercise Fatigue
Believe it or not, scientists don’t fully understand what causes exercise fatigue – but it seems to come in two varieties: peripheral fatigue and central fatigue. Peripheral fatigue is at the level of the muscles. It’s the heavy-legged feeling you get when you’ve run a long distance and the burning you get in your quads and hamstrings when you squat to near failure.
Peripheral fatigue is brought on by depletion of glycogen stores in the muscles and liver. Also, during intense exercise, lactic acid builds up in the muscle and the pH of the muscle drops. This interferes with the ability of calcium to interact with the muscle fiber myofilaments, actin and myosin, a necessary step for muscle contraction. During repeated contractions of a muscle, other waste products build up, including phosphate, chloride, and reactive oxygen species. These products, too, can interfere with calcium and make it harder for a muscle to contract.
Peripheral fatigue is uncomfortable but it isn’t always what limits us. In studies where researchers encouraged rodents to run to exhaustion, the animals still had adequate stores of ATP in their muscles to fuel further exercise, yet they stop running prematurely, behaving as if their fuel stores were completely spent. So, peripheral fatigue is probably not what forces us to stop exercising.
Beyond peripheral fatigue, what is likely a greater limiting factor to performance is central fatigue. Central fatigue refers to the role your brain plays in sustaining exercise. It’s your brain that sends messages to your muscles to contract and without its input, nothing would get done. The message travels down a pathway from the brain where it reaches alpha motor neurons in the spinal cord. These neurons innervate skeletal muscle fibers and signal them to contract via an action potential. An alpha motor neuron and the muscles it innervates is called a motor unit. During exercise, signals are continuously coming from your brain, down your spinal cord, to the muscle telling the muscle to contract.
With the brain ultimately being at the helm of muscle contractions, it makes sense that it would try to protect you from overexertion and from completely depleting your available fuel sources. It would also want to tell you to stop before you do harm to yourself. That’s what researchers think initially limits exercise performance – not complete depletion of ATP or the build-up of waste products, although these are a factor, but a protective mechanism initiated by the brain.
Can You Reprogram Your Brain?
If the brain is what truly limits exercise performance, it might be possible