If you haven’t read part one of this series on Adaptation and Applied Stress I would recommend you do so by clicking the link here
During World War II General George Patton said to his troops,
“Fatigue makes cowards of us all, men in condition do not tire!”
…and he was right!
In combat sports there’s no worse feeling than gassing out and being unable to defend yourself while another person is actively striking and trying to choke you or break your limbs. If you want to see evidence in real time that the old general was correct, watch any fight sport and notice what happens when an athlete gasses…they usually shield up, stop returning fire, and regularly they collapse! Alas, being conditioned is an essential survival skill in sports and life!
So what exactly is fatigue?
In the last article we talked about central and peripheral adaptations to training. Similarly, there are both peripheral and central mechanisms of fatigue. While there is a large volume of literature on fatigue and some grey areas that are worth exploring, for the purposes of this article we are going to focus on a select few primary mechanisms of fatigue with regards to training.
- Central Nervous System Fatigue
- Calcium ion accumulation
- Supraspinal Fatigue response to Cytokines
- IL-6
- Group II/IV afferent nerves
- Increases perceived effort
- Peripheral Fatigue
- Acidosis
- Excitation Contraction Coupling Failure
- Myofibrillar Damage
Generally speaking, all kinds of fatigue produce similar experiences by reducing the amount of muscular recruitment we are capable of, reducing the velocity of contraction, reducing the force of contraction, and/or decrease our time to exhaustion. In other words, we are weaker, slower, and tire out faster when we accumulate too much fatigue.
Calcium ion accumulation is the root of most fatigue and it happens when a muscle is repeatedly activated and calcium ions build up in the cytoplasm of the cell, not only does this impair muscular contraction in the moment, it also activates proteases which cause muscle protein break down that requires time after training to recover from. In this sense calcium ion accumulation leads to both central and peripheral types of fatigue.
For a quick overview on Calcium’s role in muscular contraction, check out the video below:
By this point you may be thinking, “Ok Annie, so there’s a lot of ways to be tired and a lot of things that cause fatigue but why does any of this matter to ME?”
Because, fatigue impacts how much training you can do in a given session, how much stimulus you get from it, and how long you need before you’re able to do another stimulating session.
As someone who has come out of the powerlifting world thre’s a lot of colloquial myths about CNS fatigue that need some clearning up. You might have heard things like “deadlifts cause more CNS fatigue than squats” or that lifting particularly heavy loads causes more CNS fatigue. Surely heavy deadlifts make most of us a little tired, but to assert that they cause more CNS fatigue is not in alignment with the literature!
We know that calcium ion accumulation and the brain’s response to cytokines are a major drivers of CNS fatigue, so what causes Ca+ to accumulate?
- Volume
- The more times a muscle contracts the more moments of calcium ion release and the more that is accumulated.
- Proximity to failure
- Taking sets all the way to failure, this drives up volume but also drives up motor unit recruitment as fatigue sets in. More recruitment of type 2 fibers causes more calcium ions accumulation.
- Light weights
- Lifting lighter weights requires more repetitions and thus more contractions and more volume.
- Eccentric movements
- Eccentric actions cause more calcium ion release concentric contractions due to stretch mediated calcium ion channel opening
With this information we can ascertain that doing “junk volume” in a workout is damaging to gains as it produces so much inflammation and CNS fatigue that it requires too many days to recovery from, thus reducing the number of stimulating workouts that can be done in a week. Furthermore, using too many advanced training techniques in a given workout is counterproductive. Having overloaded eccentric movements, drop sets, and extremely high rep sets all in the same workout would be so fatiguing that the time course for recovery would be too long. Additionally, muscle protein synthesis will be redirected to repairing existing damaged tissues rather than on developing new muscular tissue.
The aim should be to “stimulate but not annihilate” with our training. In other words, we need to balance fatigue accumulation and fatigue clearance rates within our training week. What does this look like exactly? Well, different people have different capacities! The in season high school athlete will be different than the 9-5 gen-pop office working trainee. This is where monitoring comes in!
Because rate of force production and velocity of contraction are aggressively affected by CNS fatigue, using vertical jump height or maximal throw distances as pre-training tests can be useful! Even better, the training session is not going to be negatively affected by doing a few jumps or a few throws before the rest of the session, and it doesn’t take much time.
Heart rate variability can also be used to monitor fatigue though interpretations of data have to be contextualized as the readiness score from things such as WHOOP or other smart watch HRV monitoring scores can be misleading, poorly calculated by the watch’s system, or used as reasons to skip training which in the long term can actually be detrimental to progress. Unless you’re using a validated and very high quality type of HRV device I’d take the data with a grain of salt.
Using subjective metrics to monitor athletes can also be helpful but again must be taken in to context, a single bad night of sleep is not a reason to skip training, and being a little sore from the last session isn’t either. However, seeing repeat trends in sleep disturbances, stress levels, training motivation, hunger, or soreness in response to training method is something to we can use to inform our training sessions.
Now that we understand that athletes adapt to training via different mechanisms, that fatigue can impair training stimulus, and that fatigue and recovery must be managed to drive adaptation we can begin to build a system! Part 3 of this series will combine the information of parts 1 and 2 into a more comprehensive athlete testing and program design system.
Resources
- Branscheidt, M., Kassavetis, P., Anaya, M., Rogers, D., Huang, H. D., Lindquist, M. A., & Celnik, P. (2019). Fatigue induces long-lasting detrimental changes in motor-skill learning. eLife, 8, e40578.
- Eftestol, E., Egner, I. M., Lunde, I. G., Ellefsen, S., Andersen, T., Själand, C., & Bruusgaard, J.C. (2016). Increased hypertrophic response with increased mechanical load in skeletal muscles receiving identical activity patterns. American Journal of Physiology.
- Eichelberger, T. D., & Bilodeau, M. (2007). Central fatigue of the first dorsal interosseous muscle during low-force and high-force sustained submaximal contractions. Clinical Physiology and Functional Imaging, 27(5), 298-304.
- Farrow, J., Steele, J., Behm, D. G., Skivington, M., & Fisher, J. P. (2020). Lighter-Load Exercise Produces Greater Acute- and Prolonged-Fatigue in Exercised and Non-Exercised Limbs. Research Quarterly for Exercise and Sport, 1-11.
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- Jenkins, N. D. M., Miramonti, A. A., Hill, E. C., Smith, C. M., Cochrane-Snyman, K. C., Housh, T. J., & Cramer, J. T. (2017). Greater Neural Adaptations Following High-versus Low-Load Resistance Training. Frontiers in Physiology, 8, 331.
- Knuth, S. T., Dave, H., Peters, J. R., & Fitts, R. H. (2006). Low cell pH depresses peak power in rat skeletal muscle fibres at both 30 C and 15 C: implications for muscle fatigue. The Journal of Physiology, 575(3), 887-899
- Michael, S., Graham, K. S., & Davis, G. M., Oam (2017). Cardiac Autonomic Responses during Exercise and Post-exercise Recovery Using Heart Rate Variability and Systolic Time Intervals-A Review. Frontiers in physiology, 8, 301. https://doi.org/10.3389/fphys.2017.00301
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- Ramos-Campo, D., Martínez-Aranda, L. M., Caravaca, L. A., Avila-Gandi, V., & Rubio-Arias, J.A. (2020). Effects of resistance training intensity on the sleep quality and strength recovery in trained men: a randomized cross-over study. Biology of Sport, 37(1), 81-88
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