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An in-depth look into why we fatigue - Part 1

Why do we actually have to stop. Why is it that our muscles simply refuse to work, why can’t we push out that extra few reps or finish that extra round for time… Let’s explore a little further.

Where does energy come from?

ATP is the energy currency used to fuel exercise and muscle contraction. How this ATP is produced is dependent on the workout, the intensity, recovery periods, fitness of the individual and duration of exercise. As competitive fitness workouts vary so much from strength to purely aerobic workouts (paddle boarding!) then they can rely on all three energy systems (ATP-PC, Anaerobic glycolysis and the aerobic system) for energy.

Short explosive workouts like Olympic lifting, deadlifts or short sprints rely on ATP availability (how much in the muscle), high intensity workouts (typical WOD’s) rely on anaerobic glycolysis where the energy comes from the breakdown of glycogen (stored carbs) and something called lactate, whereas the longer more endurance based workouts will depend on carbohydrates and fats.

So why do we actually fatigue?

Although fatigue is a multifaceted complex process that can't be attributed to one thing, two things we should consider are substrate depletion (fuel) and metabolic bi-products.

1. Substrate depletion (running out of fuel)

For more intense exercise sessions where ATP-PCr is the predominant substate, a reduction in ATP availability will result in fatigue of maximal activity. And for workouts involving multiple rounds where recovery is 30s or less, there will be a considerable drop in replenishment. In fact type II fibres will take far more time to recover than type I (Morton. (Casey et. al. 1996). (Greenhaff et al. 1994)

This is why increased PCr availability can help prolong this time to fatigue or supplementing with creatine (casey et. al 1996) and increase speed of recovery between bouts (Bishop 2010).

This is one of the reasons why creatine is such an effective ergogenic aid. Creatine supplementation increases intramuscular creatine and PCr availability. This enables quicker reproduction of the energy we require for exercise, ATP. This is turn, means more reps, more sprints and the ability to go for longer leading to a host of developments in speed, strength, power and lean muscle mass. From diet alone, we can only saturate our creatine stores up to around 60-80%, where supplementation can increase stores by a further 20% (Kreider et al., 2017).

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As well as the drop in fuel to produce ATP, a reduction in carbohydrates can also affect the skill components of competitive fitness. ‘Carbohydrate intake has been associated with significantly better maintenance of whole-body motor skills and mood state, and reduced perception of exertion, fatigue and force production’ (Phillips, Sproule and Turner, 2011). And with glucose being the preferred fuel for the brain, this drop can lead to ‘mental fatigue’ which is not ideal for something like Olympic lifting.

So it goes without saying that for longer and intense workouts adequate carbohydrate availability is a must!!

Final Thought

It is fairly well accepted that carbohydrates availability is one of the main limiting factor for fatigue during high intensity exercise, so if you're competing in a sport that is high intensity in nature then for pity sake, EAT YOUR CARBS!

Need more help?

Check out our FREE 5 day Nutrition email course and learn what, when and how much you need to be eating: http://freecourse.boxnutrition.co.uk


- Bishop, D. (2010). Dietary Supplements and Team-Sport Performance. Sports Medicine, 40(12), pp.995-1017.

- Casey, A., Constantin-Teodosiu, D., Huntman, E. and Greenhaff, P. (1996). Metabolic response of type I and II muscle fibers during repeated bouts of maximal exercise in humans. Journal of applied phsiology, 271(1).

- Gatorade Sports Science Institute. (2017). SSE #155 Metabolic Factors In Fatigue. [online] Available at: http://www.gssiweb.org/en/Article/sse-155-metabolic-factors-in-fatigue [Accessed 18 Jan. 2017].

- Greenhaff, P., Nevill, M., Soderlund, K., Bodin, K., Boobis, L., Williams, C. and Hultman, E. (1994). The metabolic responses of human type I and II muscle fibres during maximal treadmill sprinting. The Journal of Physiology, 478(1), pp.149-155.

- Kreider, R., Kalman, D., Antonio, J., Ziegenfuss, T., Wildman, R., Collins, R., Candow, D., Kleiner, S., Almada, A. and Lopez, H. (2017). International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. Journal of the International Society of Sports Nutrition, 14(1).

- Lancha Junior, A., de Salles Painelli, V., Saunders, B. and Artioli, G. (2015). Nutritional Strategies to Modulate Intracellular and Extracellular Buffering Capacity During High-Intensity Exercise. Sports Medicine, 45(S1), pp.71-81.

- MacLaren, D. and Morton, J. (2012). Biochemistry for sport and exercise metabolism. 1st ed. Chichester, West Sussex: Wiley-Blackwell.

- Phillips, S., Sproule, J. and Turner, A. (2011). Carbohydrate Ingestion during Team Games Exercise. Sports Medicine, 41(7), pp.559-585.

#sportsnutrition #competitivefitness #sportsdiet #sportsnutritionist #exercisediet #exercisenutrition

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