What happens to our bodies in interval training?

It is common to talk about adaptations to training, but what about what happens during and after interval training. What is happening in our body and how do we experience this during the interval (high intensity or long duration)? Further, we know that recovery is incredibly important – but why? What happens to our bodies after these interval sessions?

Let’s first focus on what happens during intervals. When we increase metabolic stress, meaning we are increasing the need for oxygen due to increased exercise intensity and/or longer duration effort, then our body is stimulated to respond to these demands. There is increased heart rate, cardiac output, and blood pressure, which emphasizes the cardiovascular component to exercise. Additionally, our bodies are challenged in respiration to bring in enough oxygen (inhalation) and remove carbon dioxide (exhalation). When we exercise at higher intensities, meaning above the lactate threshold and tap into our anaerobic metabolism, our body generates energy without using oxygen and has by-products including carbon dioxide, blood lactate (lactic acid production with lower pH in blood), and other metabolites such as hydrogen ions. These by-products are not bad, but they hinder our ability to maintain or even increase exercise intensity from that point on. This just means that when we work harder (at or above the anaerobic threshold), we breathe harder to exhale carbon dioxide, feel a burn in our muscles as they have a build up of lactic acid in the blood, and experience several other sensations of fatigue. 

During intense exercise (high-intensity or longer duration), our muscles are under a high demand and great stress from speed of contraction, impact forces, and amount of motor units being recruited. This intensity exaggerates the needs of several ions and the inter-workings of the channels to regulate the availability and flux of sodium for depolarization (action potential sending signal to muscle to contract), calcium to come into the cell and allow contraction to take place (cross-bridge formation and sliding filament theory), and for potassium to exit the cell and allow repolarization or relaxation (outflow of potassium and re-set of ionic thresholds with support of gated channels, such as the sodium-potassium pump). When our muscles work too hard and/or for too long related to their strength and capacities, we experience loss in strength and increased fatigue. This can cause exercise-induced muscle damage related to structural damage inside the muscle fibers (sarcomere, cytoskeletal, membrane). This might be experienced with unusual soreness that persists, inability to train or compete, declines in performance or plateaus, feelings of heaviness, delayed recovery, etc. Fatigue develops gradually and in response to sustained physical activity and is characterized by a decrease in the maximal power or force that the specific muscles involved in the activity can produce.

What happens to our bodies after exercise? After the intense exercise session (high intensity exercise and/or long duration efforts), our body first experiences what is called excess post-exercise oxygen consumption (EPOC). Greater EPOC responses are typically seen in higher intensity or longer duration exercise, compared to lower intensity and shorter duration (Borsheim and Bahr, 2003). During EPOC, the body is undergoing elevated metabolic demands while working to restore homeostasis (stable inside state despite changes in the environment) through regulation of temperature, blood pressure, blood sugar, and acidity or alkalinity of blood, etc. Research has shown that high-intensity interval training (HIIT) and resistance training both allow elevated metabolic demands for at least 14 hours (Greer et al., 2021). After exercise, fluids and fuels that were lost must be replaced, along with restoration of cardiovascular function, reparation of damaged tissue, and rest. Time is of the essence in this post-exercise state to improve recovery. There are several nutritional and physical interventions which can aide in the restoration of homeostasis which include rehydration, carbohydrate and protein intake, stretching, massage, hydrotherapy, whole-body cryotherapy, compression garments, and sleep. If training is too intense and/or not accompanied by proper recovery, the body may experience under-recovery and possibly overreaching. When athletes incorporate adequate recovery into the training cycles, this should allow greater tolerance for training and positive adaptations for improved performance. 

Specifically, nutritional strategies for replacing fluids are vital in restoring cardiovascular function with research indicating recommendations of 150% of fluid lost during exercise (try to avoid caffeine, while incorporating sodium as a primary component for effective positive fluid balance). Moreover, fluid is best retained in the body when containing 3-12 % of glucose compared with water alone (Osterberg et al., 2010). 

Additionally, replacing glycogen is important – especially as a main fuel source during intense exercise (and heat exposure). Consuming plenty of carbohydrates and in a period of 24 hours is usually required for complete restoration of muscle glycogen after intense exercise (Burke et al., 2017), with the first 2 hours after exercise as the most critical. Rates of > 1.2 g/kg/hr greatly enhances glycogen resynthesis in the liver, while additional advice suggests co-ingesting 0.3-0.4 g/kg/hr of protein along with 0.5-0.8g/kg/hr of carbohydrates improves muscle glycogen resynthesis. The best is high glycemic index foods or drinks as soon as practical after exercise, and moderate amounts of carbohydrates and protein together will provide benefits to replace muscle glycogen stores while stimulating muscle protein synthesis (for muscle repair). It is best if able to consume animal-based protein sources after exercise (Peake, 2019). From my experience, not everyone can handle protein digestion during exercise, but if you can work to consume slow digestible proteins – it really aids in muscle damage and initiates recovery even before the training session is completed. 

An often overlooked, but critical element of post-exercise recovery is sleep. Quite some research has demonstrated sleep has a powerful impact on performance (physical and cognitive). We know and experience that poor sleep habits and potential barriers to regular healthy sleep routines, along with short-term disturbances (travel, jet lag, unfamiliar sleeping environments, pre-competition anxiety) interrupt our recovery. Interestingly, increasing total sleep duration for at least one week can improve performance in sleep-deprived athletes, along with reaction times, mood, and fatigue levels (Bonnar et al., 2018). Both as good advice and research indicate, including a nap of > 20 min later in the day has also been shown to assist mental preparation for the next training or competition (Bonnar et al., 2018). 

As our bodies return to homeostasis and we take adequate rest, there are physiological adaptations that occur. We adjust to a ‘new baseline’ and build a stronger foundation to allow reaching new possibilities within training. As a general perspective, we become more efficient at oxygen transport which allows us to need less oxygen to perform work (maintain pace). This is known as economy, where we have more ‘bang for the buck’ in that we can perform faster (pace – velocity – power output) for the amount of oxygen we are utilizing/consuming. This is a kind of supercompensation where a specific training outcome variable (such as heart rate, blood lactate, oxygen uptake, oxygenation, blood pressure, feeling/response to effort, etc.) has a higher performance capacity than it did prior to that training block/period/cycle.

There are several elements to consider when understanding how intervals work to improve our fitness. Increasing the amount of metabolic stress is tricky – as we need juuust the right amount, we don’t want to overreach or overtrain and require the body to take extended rest for recovery. Therefore, we should pay special attention to exercise intensity (targeting metabolic demand) and interval inter-set recovery (in both intervals and during strength training sessions). In a way, we need a particular “dose” of metabolic stress, something each individual athlete can handle in order to 1) execute the workout, and 2) recover from the effort so we can continue training/performing again and again. There are many moving components of training load and how our bodies adjust and adapt, it is a never-ending process of learning ourselves and our athletes. Here’s to the adventures of adaptation ahead!

Børsheim, E., Bahr, R. Effect of Exercise Intensity, Duration and Mode on Post-Exercise Oxygen Consumption. Sports Med 33, 1037–1060 (2003). https://doi.org/10.2165/00007256-200333140-00002.

Bonnar D, Bartel K, Kakoschke N, Lang C. Sleep interventions designed to improve athletic performance and recovery: a systematic review of current approaches. Sports Med 2018, 48:683-703 http://dx.doi.org/10.1007/s40279-017-0832-x.

Burke LM, van Loon LJC, Hawley JA. Postexercise muscle glycogen resynthesis in humans. J Appl Physiol (1985) 2017, 122:1055-1067 http://dx.doi.org/10.1152/ japplphysiol.00860.2016. 

Greer BK, O'Brien J, Hornbuckle LM, Panton LB. EPOC Comparison Between Resistance Training and High-Intensity Interval Training in Aerobically Fit Women. Int J Exerc Sci. 2021 Aug 1;14(2):1027-1035. PMID: 34567357; PMCID: PMC8439678.

Osterberg KL, Pallardy SE, Johnson RJ, Horswill CA. Carbohydrate exerts a mild influence on fluid retention following exercise-induced dehydration. J Appl Physiol (1985) 2010, 108:245-250 http://dx.doi.org/10.1152/ japplphysiol.91275.2008.

Peak JM. Recovery after exercise: what is the current state of play? Current Opinion in Physiol 2019, 10:17-26 https://doi.org/10.1016/j.cophys.2019.03.007.