Ever been nearing the end of an intense practice and you find your muscles seem to be quitting on you? You begin to question whether or not you will be able to physically finish that last lap, sprint or squat. As an elite athlete, you find a way to finish the practice as best you can—through any means. It is not uncommon for athletes to experience this feeling in a practice or race on a semi-regular basis. But what is happening to make your muscles feel like they are rebelling against you? It’s the process of peripheral fatigue.
What is peripheral fatigue?
In the world of sports, fatigue can be defined as the inability to maintain a given exercise intensity. This can mean falling off your pace if you are running, rowing, or swimming, or struggling to complete those last few repetitions in your strength workout. Peripheral fatigue is an athlete’s inability to maintain their expected level of exercise intensity. Several factors play into peripheral fatigue, often acting in combination.
General causes of fatigue
There are many causes of fatigue that range from impairment within the muscles to psychological reasons for the decreased intensity. Any one or a combination of these can affect your peripheral fatigue.
- Metabolite Depletion: ATP (adenosine triphosphate) and CP (creatine phosphate) levels both decline with the onset of exercise. To review, ATP is the main energy source for the majority of cellular functions in your body and CP works to help maintain ATP levels by donating a phosphate group to ADP to make ATP. As these metabolites deplete (especially CP), muscle fatigue will set in as there is less energy to fuel muscle contractions.
- Metabolite Accumulation: The accumulation of dihydrogen phosphate (H2PO4-) and Lactic Acid (H=) cause muscle contractions to decrease in force. An increase in phosphate levels in the muscle causes a decrease in the force of muscle contraction. Lactic Acid accumulation also decreases muscle power by displacing Calcium ions that are essential to the process of muscle contraction.
- Disturbances in Homeostasis: This can refer to many things including dehydration, altitude, heat and cold. When it is very hot outside, blood is diverted from the working muscles to the skin to release the excess heat. Contrary, when it is very cold, blood is also rerouted from the working muscles to the body’s core to keep more vital organs warm. Both these processes limit oxygen delivery and can contribute to muscle fatigue. For additional information on homeostatic imbalance, please review how dehydration and altitude can contribute to the onset of fatigue from our previous building blocks posts.
- Central/Psychological Factors: Recent research published by Professor of Exercise and Sport Science, Tim Noakes, stresses the importance of the brain in fatigue. Because every muscle movement begins with signaling in the brain, research now shows that there can be muscle fatigue even when the muscle itself is not damaged. The interrelationship between central (the brain) and peripheral (muscles, motor nerves etc.) systems are important in the onset of fatigue. This study shows how local muscle fatigue is influenced by central nervous factors.
Peripheral fatigue is a part of elite athletics. Energy sources deplete and harmful metabolites accumulate, both contributing to the inability to maintain exercise intensity. Not only do cellular changes within the muscle contribute to fatigue, but there is also increasing evidence that the brain has a major effect on an athlete’s perception of fatigue. The human body is a complex network and the connections between the nervous and muscular systems are essential to proper function and high-level performance. Fatigue is not preventable; however, proper nutrition, hydration and the right mindset going into a hard workout or race can all help delay fatigue onset and help you reach your goals.
- Brooks, George et al. Exercise Physiology: Human Bioenergetics and Its Applications. McGraw Hill: New York. 2005.
- Asmussen E and B. Mazin. Recuperation after muscular fatigue by "diverting activities". Eur J Appl Physiol Occup Physiol. 1978 Feb 21;38(1):1-7.