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What Happens When We Work Out?

Have you ever found yourself asking that question? Hopefully, if you have spent time training consistently, you will have seen improvements in performance. Faster times, farther distances with greater ease, less fatigue for the same distance, or a lower heart rate at a given pace are all potential signals that you are trending in the right direction. But what actually occurs within our bodies when we work out? What enables us to raise the bar, to hit new PRs?

The above graphic is an illustration of what is known as the stress-adaptation curve. One key concept to understand is that our bodies are essentially just machines that attempt to maintain balance - or homeostasis. When we apply training stimulus via a century ride or an interval session at the track, we are purposely taking the body out of its comfort zone to force change to occur. This is represented by the red line in the graphic. Our body, through the signals received during the workout and the breakdown of tissue that occurs, (because remember, any workout we do is simply causing damage to the tissues in order to force the body to make repairs) starts to repair itself immediately following the workout to return to homeostasis.

It’s logical, and this is why you wouldn’t want to do a highly taxing workout the day before a race because the recovery and subsequent adaptations take time. This recovery period is represented by the orange line on the graphic. If the training stimulus was adequate, that is, it was sufficient stress on the system to precipitate adaptation but not so great as to cause injury, we should see adaptation, represented by the green line in the graphic. Ultimately, this supercompensation is what we all seek from training. This is what translates to us getting faster, stronger, or having more endurance.

The last section of the graphic, represented by the blue line, is what is referred to as adaptive dissipation, also known as detraining. This is what happens when we take long periods off from training due to any number of factors, such as injury or following a hard effort such as a race. Unfortunately, fitness is not static and if it’s not increasing, it’s decreasing. Fitness, for the same reasons as described above, is also not linear. You must induce stress on the system, adequately recover, and then induce stress again, also known in fitness as the principle of progressive overload. For this reason, an ideal ramp-up in fitness might look something like the graph below.

You’ll notice that a gradual increase in fitness requires repeated stress and recovery, gradually leading to greater and greater physiological adaptation. This is why injury prevention is so critical to peak performance; any significant amount of time off from training gets things moving in the opposite direction because the more highly trained a body is, the more metabolically demanding it is, and in an effort to maintain homeostasis, the body seeks to be as efficient with its use of energy as possible.

What are the adaptations that occur on a physiological level?

You may be asking, what is it exactly that changes inside of me when I do cardiovascular exercise on a consistent, ongoing basis? Several adaptations that occur to different parts of the body when you are able to string together weeks and months of consistent training:

  1. Cardiac Hypertrophy: Hypertrophy, for those who are unfamiliar, is just an increased size of the muscle, specifically the sarcomeres, the basic contractile units of muscle tissue. This increases the strength potential of cardiac contractions.

  2. Increase in Resting & Exercising Stroke Volume: Stroke Volume (SV) is the amount of blood pumped from the left ventricle of the heart with each contraction. This of course goes hand-in-hand with cardiac hypertrophy. Stronger cardiac contractions = greater stroke volume.

  3. Decrease in Resting Heart Rate (RHR): RHR, typically measured in BPM (beats per minute) while often used as a proxy to determine overall recovery, can also be used to gauge general fitness. This is because as our hearts grow stronger from training and pump more blood with each stroke, they need to pump a reduced number of times in order to circulate the same quantity of blood around the body.

  4.  Capillarization of Skeletal Muscle Tissue and Alveoli: Capillarization, or the increase in density of capillaries, are the smallest blood vessels connecting arterioles with venules and forming networks throughout the body. These are the small tubes that feed your muscles oxygenated blood from your lungs. In your lungs, the alveoli are the structures that allow for the release of carbon dioxide and the uptake of oxygen. Increased capillary density in these structures translates to more oxygen being delivered to the muscles with each breath, a huge advantage when it comes to athletic performance.

  5. Reduction in Resting Blood Pressure: With consistent training, particularly cardiovascular training, another effect that we see in the body is a reduction of average resting blood pressure. This is particularly important as it relates to long-term health as there is evidence to suggest that a higher average resting blood pressure is related to worse health outcomes.

  6. Decreased Heart Rate Recovery Time: What this means is the fitter you are, the sooner your heart rate will return to baseline after a hard effort. So not only will a more highly-trained athlete be able to perform at a higher level at a lower average heart rate, but they will be able to return to baseline more quickly, resulting in faster recovery times.

  7. Increased Blood Volume: Over time, blood volume increases as the result of consistent training, which allows for greater heat dissipation and thermoregulatory stability as well as larger vascular volume for greater cardiac filling and the aforementioned stroke volume.

Other physiological adaptations occur in the body with more specific types of training. Running is considered a “high-impact” activity when compared to activities such as biking and swimming, so it puts unique stresses onto the bones and connective tissue of the skeletal system that prompt increases in bone density and ligament strength in ways that other, lower-impact activities do not. Therefore, for the sake of staying healthy, a balanced training approach that considers the specific stresses of each type of training will ultimately be the most effective approach for maximizing adaptations while minimizing downtime, resulting in the greatest possible improvement in the shortest amount of time. However, regardless of how efficient you make your training, and we can all do better, the types of physiological adaptations we can hope to enjoy from effective training require patience and determination. 

Coach Ethan

Health threats from high blood pressure. American Heart Association. Accessed Dec. 13, 2023

Convertino VA. Blood volume response to physical activity and inactivity. Am J Med Sci. 2007 Jul;334(1):72-9. doi: 10.1097/MAJ.0b013e318063c6e4. PMID: 17630597.

Romero SA, Minson CT, Halliwill JR. The cardiovascular system after exercise. J Appl Physiol (1985). 2017 Apr 1;122(4):925-932. doi: 10.1152/japplphysiol.00802.2016. Epub 2017 Feb 2. PMID: 28153943; PMCID: PMC5407206.

Reimers AK, Knapp G, Reimers CD. Effects of Exercise on the Resting Heart Rate: A Systematic Review and Meta-Analysis of Interventional Studies. J Clin Med. 2018 Dec 1;7(12):503. doi: 10.3390/jcm7120503. PMID: 30513777; PMCID: PMC6306777.

Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. J Physiol. 2008 Jan 1;586(1):35-44. doi: 10.1113/jphysiol.2007.143834. Epub 2007 Sep 27. PMID: 17901124; PMCID: PMC2375555.

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