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 to reprogram the brain so that it lets you exercise longer before putting on the brakes. Small studies show this is possible. In one study, researchers asked participants to ride an exercise bicycle at around 80% of their V02 max – until they were exhausted. After measuring a variety of parameters and asking them to rate how hard the exercise felt to them, they assigned them to two groups.
One group was asked to continue exercising over the next 2 weeks. Another group was taught how to use “self-talk” to encourage themselves to keep going when they felt fatigued. You’re probably familiar with mantras like, “I can do it” or “I’m strong. This workout is a piece of cake.” Once they had chosen their motivating and empowering phrases, the participants continued to exercise over the next 2 weeks using positive self-talk and mantras.
Then it was back to the laboratory for a final ride, similar to the first one when they pedaled at a high intensity for as long as they could. This time, the participants who learned how to talk positively to themselves fared better. They were able to pedal longer and farther than the group that didn’t use positive mantras or positive self-talk, even though they exercised at a similar intensity.
So, ultimately, it’s not a lack of fuel stores that limits you but your mind. Could repeating a mantra in your head enable YOU to exercise longer? Self-talk helps you push through partially by taking your mind off your discomfort and focusing it on something positive. Another trick, when you’re trying to push a little further or longer, such as when running a race, is to focus on shorter term goals. If you’re running a 10K, break it down in your mind into 2-mile increments and focus only on each 2-mile increment. Looking at the big picture is often daunting but it seems much more achievable when you tear it down into smaller goals and partition each one in your mind.
Studies also show that focusing laser-sharp on your goal can help you push through. For example, if you’re trying to run faster, look at an object ahead of you, like a stop sign, and fixate your gaze on it as you push yourself as hard as you can towards it. Anything that shifts your focus away from the bodily sensations that tell you you’re too tired to go on is beneficial.
The Bottom Line
Your brain is programmed to keep you from maximally exerting yourself. It wants to stop you well short of where you might experience exhaustion or injury. Plus, it’s goal is to help you conserve energy sources. So, it will tell you to stop while you still have energy reserves available. Pushing a little harder and getting more out of exercise means pushing slightly past those perceived limits but not enough to sustain injury or jeopardize health. To continue to get fitter, you sometimes HAVE to push yourself out of your comfort zone, just not every time you work out. Push hard on some days and take it easier after super tough sessions to let your body recover. There’s no glory in exhausting yourself or overtraining.
The New York Times. “Keep Telling Yourself, “This Workout Feels Goo
Biochemistry. Fifth edition. Berg JM, Tymoczko JL, Stryer L.New York: W H Freeman; 200
Applied Physiology, Nutrition, and Metabolism, 2014, 39(3): 282-291.