The success of an endurance exercise training regimen is dependent on the acute (short-term) and chronic (long-term) physiological adaptations. When you exercise, consider all the changes your body goes through to make your training session feasible.
As illustrated below in Figure 1, during exercise, your body control systems increase the activity of the nervous, cardiovascular, respiratory, muscular and integumentary (skin) systems while partially shutting down the gastrointestinal and urinary systems to make more blood available to the working muscles. The precision of such changes allows you to exercise in all the intensity domains and, over time, change your physiology for better performance in your next A-race.
For years, sports scientists around the globe have been studying the physiological adaptations after training sessions or weeks of training. Exercise is a potent disturbance for homeostasis, defined as a self-regulating process by which biological systems maintain stability while adjusting to changing external conditions[1]. To cope with these exercise-dependent stressors, your body is constantly adapting (allostasis[2]) and getting better prepared for the next training session, be it a long endurance ride, a high-intensity workout with VO2max intervals, or a race.
Pedagogically, these adaptations can be split into central (e.g., nervous, cardiovascular) and peripheric (e.g., muscular, skin) or, more broadly, acute or chronic, depending on how long you can count on them to remain faster and metabolically fitter. In this article, we will dig deeper into some of the adaptations occurring in the human skeletal muscle.
Boosting Your Powerhouses
A recent systematic review and meta-regression study discussed the effects of endurance exercise training (up to 12 weeks) on mitochondrial and capillary growth in human skeletal muscle[3]. These research studies follow laborious methodological steps to summarize through statistical tests and models all the previous peer-reviewed studies on a specific topic.
Mitochondria are tiny powerhouses inside the muscle fibers that make more ATP (your energy currency) available for their contractile functions. The powerhouses need a constant supply of energy substrates (e.g., fatty acids and glucose) and oxygen to make ATP at high rates during exercise. The capillary bed (tiny vessels around the muscle fibers) is another piece of the performance equation as they deliver the energy substrate to and remove the metabolic byproducts of the skeletal muscle fibers (Figure 2). According to current research techniques, measurements of mitochondria and capillaries are a good proxy when checking for muscular adaptations or endurance performance after completing a training regimen.
In this study, the authors compared different exercise training methods, including low- or moderate-intensity continuous endurance training (ET), high-intensity interval training (HIT), and sprint interval training (SIT), on their effects on mitochondrial content and capillarization.
Results indicated that mitochondrial content increases similarly with ET, HIT, and SIT, and these increases are not significantly influenced by age, sex, menopause or disease. SIT was found to be more efficient per total hour of exercise in increasing mitochondrial content compared to HIT and ET. This efficiency makes SIT an attractive option for individuals with limited time, as it can lead to significant physiological adaptations in a shorter duration. Capillaries per fiber increase similarly across all training types, but capillaries per mm² increase more with ET[3].
VO2max improvements were similar across all training types, with HIT showing a tendency for greater improvement. Higher training frequencies and lower initial fitness levels were associated with higher mitochondrial content, capillarization, and VO2max improvements.
No significant differences in exercise-induced adaptations in mitochondrial content, capillarization, or VO2max were found between individuals with metabolic diseases, cardiovascular diseases or COPD and healthy individuals. Despite these findings, exercise training is not consistently included in primary, secondary and tertiary medical interventions.
This could be due to a lack of awareness or understanding of the adaptability to exercise across various populations, including those with chronic diseases[4]. The study’s results highlight the potential for exercise training to be a beneficial component of medical prescriptions, as it can lead to significant health improvements regardless of age or disease. The lack of inclusion may also stem from logistical challenges, such as designing individualized exercise programs and ensuring patient adherence, which require more resources and infrastructure than traditional medical treatments[5].
The Key Exercise Regimen Takeaway
How about those New Year’s resolutions for those starting over or beginning an exercise regimen? Individuals with lower initial fitness levels experienced superior percentage improvements in mitochondrial content, capillarization, and VO2max. The initial training phase can lead to rapid gains, which can be highly motivating for individuals looking to improve their health and fitness.
While exercise initially leads to significant gains, adaptations can eventually plateau, particularly with high-intensity training like SIT[3]. This plateau occurs because the body becomes more metabolic efficient, increasing the relative stress and stimulus for further adaptation. However, even when visible gains slow down, continued exercise is beneficial as it maintains the adaptations achieved and contributes to overall health and fitness. At this point, it’s important to vary the training stimulus by adjusting intensity, volume, or exercise frequency to continue progressing.
Remember the elite-level endurance athletes? There’s a high chance you have heard they train 20 or more hours per week. Higher training volumes, characterized by increased weekly training hours and training weeks, are associated with more training adaptations at the skeletal muscle.
A higher training load provides a more consistent and prolonged stimulus for physiological changes, such as increases in mitochondrial content and VO2max. ET and HIT significantly increased mitochondrial content and VO2max with higher training volumes, while SIT adaptations plateaued sooner[3]. This suggests high-intensity training can lead to rapid initial gains, but sustained improvements may require a larger training volume.
Applying the Research to Your Training
The Performance Management Chart (PMC) from TrainingPeaks is a tool for tracking and managing training load, fitness, and fatigue over time. It helps athletes and coaches visualize how training volume and intensity affect performance, such as in your power profile. The chart uses metrics like the Training Stress Score (TSS) to quantify the training load and its impact on your fitness and fatigue levels. By tracking these metrics, you can distinguish when you are reaching a plateau in performance and adjust your training plan to continue progressing.
The PMC also helps balance the training load to avoid overtraining, providing that you receive adequate recovery to maximize adaptations (they happen when you’re resting). Remember that higher training volumes lead to more substantial adaptations but require careful management to prevent excessive fatigue and injuries and deliver continued improvement.
Athletes looking to optimize their performance can benefit from personalized training guidance. This is where services like TrainingPeaks’ Coach Match and structured training plans come into play. A coach can help you interpret data from tools like the PMC and your Power Profile to adjust training loads, ensuring that you work at the right training load to promote continuous improvements in fitness and performance. A coach can also help you navigate performance plateaus by introducing new training stimuli or adjusting recovery periods.
Alternatively, you can choose from training plans available on TrainingPeaks. Experienced coaches designed these plans, catering to different goals, fitness levels, and sports. A structured training plan can provide a roadmap, ensuring you progressively increase the training volume and intensity to promote optimal adaptations while minimizing the risk of overtraining. Both options offer structured training approaches to help you achieve your goals more efficiently. They leverage the insights from scientific research and coaching experience on short- and long-term training adaptations.
References:
[1] Billman GE. Homeostasis: The Underappreciated and Far Too Often Ignored Central Organizing Principle of Physiology. Front Physiol 2020;11:200. https://doi.org/10.3389/fphys.2020.00200.
[2] McEwen BS, Karatsoreos IN. Sleep Deprivation and Circadian Disruption Stress, Allostasis, and Allostatic Load. Sleep Med Clin 2022;17:253–62. https://doi.org/10.1016/j.jsmc.2022.03.005.
[3] Mølmen KS, Almquist NW, Skattebo Ø. Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression. Sports Med 2024:1–30. https://doi.org/10.1007/s40279-024-02120-2.
[4] Russell AP, Foletta VC, Snow RJ, Wadley GD. Skeletal muscle mitochondria: A major player in exercise, health and disease. Biochim Biophys Acta (BBA) – Gen Subj 2014;1840:1276–84. https://doi.org/10.1016/j.bbagen.2013.11.016.
[5] Hossri CAC, Araujo FC, Baldi BG, Otterstetter R, Uemoto VR, Carvalho CRR, et al. Association among cardiopulmonary and metabolic rehabilitation, arrhythmias, and myocardial ischemia responses of patients with HFpEF or HFmrEF. Braz J Méd Biol Res 2024;57:e13174. https://doi.org/10.1590/1414-431x2024e13174.
