Process of Adaptation to Training – how can coach and athlete understand?
By Sarah Willis, PhD
For adaptation (or change) to happen, there first needs to be stimulus or stress. This is just like life, where we often need a challenging situation, circumstance, or conversation to stimulate us to grow and understand ourselves better. Back to our training mindset…we need a stressful environment in our bodies (i.e., via training such as aerobic/anaerobic/strength/etc.) which is specific to a goal in that it helps to improve performance.
Similarly, rest and recovery need to be prioritized (i.e., off day, cross-training, relaxation, etc. – methods to decrease stress and also life stresses such as work, finances, relationships, family, etc.). If we don’t allow the body to rest (i.e., insufficient recovery), then our efforts are impaired, and we are overloading our body which in the end will decrease performance. More rest is sometimes the key needed to improve fitness; however, this is quite individual and changes frequently even in the same person. Thus, it is vital to understand the perceptions of our fatigue/stress/discomfort. This comes with experience. Note: it could be helpful to use a variety of metrics to individually learn this…some examples might include measuring resting heart rate and heart rate variability, completing questionnaires – feeling before/during/after training, sleep quality, nutrition, training load, etc. There are several technologies available to begin identifying some of this information. Most important is that we learn from a period of data collection (insight to our perceptions) and implement strategies/adjustments in training-life stress to improve our overall status (fatigue, stress…) to help us as athletes and coaches understand how our bodies work and best ways for adaptation to occur.
Over time with the consistent stressful training stimulus and recovery cycles, the body will adapt physiologically to perform better (faster and stronger). This can be seen in a trend to increase the baseline into a supercompensation phase. The ability to improve performance is related to increasing the availability of oxygen, improving oxygen transport throughout the body from lungs to cells, related to oxygen efficiency in mitochondria and cellular function to improve energy metabolism. We adjust our baselines and over the years, this baseline should be gradually progressing towards greater fitness and reliable individual perceptions of performance related to training load and recovery. This is another reason that periodization is so important (i.e., training zones, macro cycles of the year). We need the training stimulus but also the recovery – otherwise we just beat our bodies up with no chance to recover. Periodization and varied training along with intentional recovery allows us to reap the benefits of the training we did to improve our fitness and performance.
What happens to our bodies when we exercise? (physiological response)
Over time, our body gets used to the stimulus (training) and we get better at it (think of a skill – playing instrument, shooting a basketball, etc.). Just like that our body adapts to be able to better handle the stress of performing this task and improves the response to the metabolic demand. This means what energy is being asked by the body to complete the specific task, i.e., using aerobic or anaerobic energy metabolism – depending heavily on exercise intensity and duration, improves in efficiency related to oxygen transport and utilization.
Long-term adaptations to exercise training (improvements in performance) are made to our cardiovascular (heart and vessels) and skeletal muscle system, along with several other physiological systems for general health. To name a few, these adaptations include decreased heart rate, increased stroke volume (blood 
volume ejected from heart each beat), increased cardiac output (systemic blood flow), increased volume of oxygen consumed (VO2 – via the Fick equation), increased hemoglobin in blood (molecule to carry oxygen through bloodstream), increased muscle hypertrophy (size and strength), increased myoglobin (carry oxygen in muscle), increased mitochondria (powerhouse of the cell – energy for cell to survive and produce ATP for energy metabolism), increased aerobic/anaerobic enzymes depending on the specific training, increased blood lactate thresholds, etc. These are all related to efficiency of movement and becoming more economical to consume less oxygen to move at that pace/vertical velocity/power output.
Specifically, the cardiovascular system allows us to increase our VO2, which happens via increasing contractility of the heart (greater cardiac muscle firing rates) and thus increasing the size and strength of our heart muscles (cardiac hypertrophy). Further, we can therefore move more blood around our heart (increased blood volume) due to efficiency and strength of each stroke, and this increases the stroke volume (amount of volume each stroke).
While the muscle can also adapt in size and strength (hypertrophy) via increased blood volume and additionally the muscle can convert to using more aerobic metabolism for energy supply. Thus, increasing muscle glycogen stores and improving stroke volume.
Why are these changes so important – how do they translate to improving performance?
For me, it comes down to the oxygen transport system (cascade – like a waterfall). As oxygen is transported from the air to our lungs, pulmonary capillaries, bloodstream, heart, aorta-arteriole-arteries-capillaries, diffusion into cells, and used by cells-mitochondria to generate energy (ATP), we loose partial pressure. This means that as our oxygen moves throughout our body getting closer to the tissues and cells for which it will be used for function, we will have less and less availability of oxygen. The better we train our body and cardiovascular system to improve the effectiveness of the transport of our oxygen, the greater pace/power output/performance we can obtain. Here we are referring to the cells in our muscles (skeletal and heart muscles, other organs for function), ability of our respiratory system to regulate the ventilation and perfusion of our alveoli and get oxygen into our bloodstream, cardiac function (heart) to pump oxygenated blood to the rest of the body and return deoxygenated blood to the heart (so lungs can expire). The way in which our body makes oxygen available to our working tissues is vital to our function and ultimately athletic performance.
How long does it take for these adaptations to happen?
Generally, we are looking at a range of 4 to 16 weeks of consistent training for our body to adjust and adapt to the training stimulus/metabolic stress. This is quite varied depending on previous frequency of completing the exercise. If it is something you have done before, adaptation time will be shorter than if it is a new activity. It also depends on the type of exercise (strength – related to neuromuscular response versus endurance training – related to interval type and rest interval duration, etc.), altitude, environmental conditions (cold, heat), time for recovery, etc.
Initially (1-4 weeks), the body is working to deliver more oxygen to the tissues by developing the heart and circulation within the vascular system and creating more capillaries for improved oxygen availability. An intermediary phase (4-8 weeks) allows mitochondrial biogenesis (production of more mitochondria), where enzymes are busy working to produce more energy aerobically (with the presence of oxygen, which is more efficient). Our bodies then rely on more fat metabolism and spare glycogen (carbohydrate) while boosting the glycogen stores and enhancing our endurance capacity. Finally (8-12+ weeks), the more advanced adaptations could include improvements in the lactate threshold which enable us to sustain higher exercise intensities for longer (exercise intensity where the body can no longer keep up with the production and removal of blood lactate, and where the body produces excess in response to higher metabolic demands --- anaerobic metabolism, energy produced without presence of oxygen). Further, we can think of VO2max (maximal oxygen consumption), fatigue resistance, muscle fibers shifting towards higher proportion of Type 1 (slow-twitch) fatigue-resistant fibers, resilience of connective tissue-tendons-ligaments related to force of movements. All of these factors contribute to endurance capacity and injury prevention. Along these lines, we also know that a good training plan follows progression and includes different periods with various goals dialing in these specific focuses.
Are there other benefits? – Health (obvious to many, but not forgotten)
It is obvious to many, but not to be forgotten, that exercise training will improve our health. This is related to the function of systems we discussed (heavily on cardiovascular, respiratory, musculoskeletal) for properly functioning organs and oxygen transport as we age. Some parameters of great interest for our health that are improved with consistent exercise are: resting heart rate and blood pressure, blood sugar levels, lipid/cholesterol levels, mental health/production of endorphins and other chemicals that prevent depression/etc. Striving to achieve a higher level of activity will improve our muscle strength and endurance, along with flexibility, agility, etc. Daily tasks and activities of life will become easier as we will have more energy for them. It must also be noted that we can generally achieve greater physiological benefits/health with higher intensity or metabolic demand of exercise.
How do we know that we have adapted / are adapting well?
Incorporating a regular check or monitoring of how your body feels will give you indications if it is getting easier (think breathing, muscles, mentally, technique more relaxed for same pace, etc.). Even just checking-in with your perception of the effort required to complete the same task will give you insight. You can also consider monitoring physiological parameters (pace/velocity, HR, blood lactate, rating of perceived exertion, oxygen consumption (VO2), resting values HR, etc.), more psychological parameters with questionnaires of your fatigue, recovery, nutrition, etc. As you improve, you can alter the duration of intervals, reduce recovery needed between sets, increase the number and/or frequency of high-intensity efforts, etc. Ideally, the task will be getting easier in the future (may take several weeks, months, or even years) and you will have an increased ability to cope with the new load/exercise/movement, indicating that you have or are undergoing adaptation.
Remember the importance of prioritizing rest, sleep, hydration, nutrition, mental health, life stress, etc. To gain the most from your training sessions, you can increase your training volume over time, and also the amount of intensity. The best adaptations are made when intensity is done in a quality way i.e., in the right intensity zone, and for the appropriate amount of on-time. You might also consider changing the style of interval (sometimes experimentation is best to see how an individual adapts and what works for their body/related to goals).
Looking forward to following and imagining how your body has been adapting this year!