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With input from Philip Tan & Fabian Lim
One does not become a marathoner overnight. Endurance running requires an extended period of training. The training provides a stimulus to induce adaptations in our bodies to make us more efficient runners.
Let us first understand how our body works when we run. They are all related, but I’d like to simplify things by looking at the four key systems that enable us to run: the musculoskeletal, energy, cardiorespiratory, and cooling systems.
Training Adaptations
 The whole purpose of training is to induce adaptations in the four systems we just discussed. Each system and each structure adapts at a different rate. For example, muscle adapts faster than bone. Adaptation takes time and cannot be rushed beyond an optimal rate. For adaptations to occur, there must be a stimulus or stress beyond what the system or structure is accustomed to (i.e. the principle of training overload), and the stimulus must be specific to the system or structure (i.e. the principle of training specificity). For example, cycling, like running, stimulates the cardiovascular system; but it is not specific enough for runners as cycling is non-impact and does not provide the necessary stress to the bones to help it adapt to the impact forces encountered during running. The following are the progressive changes that you benefit from when you run regularly.
Cardiovascular System
The earliest change to occur is the increase in plasma (your blood minus the cellular components) volume. The increased plasma volume is extremely helpful to performance because it leads to an increased cardiac output (see formula on "The Human Running Machine Part 1"). This expansion in plasma volume is detectable within a week of commencing your training. But easy come, easy go, as they say — when you stop training, your plasma volume starts to contract within two days, demonstrating the training principle of reversibility.
The haemoglobin content in the blood also increases, boosting the blood’s ability to transport oxygen.
Progressively, your heart’s muscle mass increases, in particular the left ventricle, which is the most important part of the pump. Hence, endurance athletes are known to have ‘big hearts,’ literally. With a more powerful pump, the stroke volume and hence the cardiac output increases, delivering more blood to the working muscles during running.
Musculoskeletal System
One of the most important adaptations is the increase in the number of capillaries surrounding each muscle fibre (i.e. capillarity). This would allow greater exchange of gases, nutrients and waste products between the blood and the working muscle fibres. Myoglobin content within the muscle fibres has also been shown to increase by 75 to 80 per cent, allowing more oxygen to be shuttled from the cell membrane to the mitochondria. Finally, the mitochondria also increase in number, size and efficiency, improving the muscle’s capacity to produce ATP.
The muscles also become stronger and more resilient to damage during training. Likewise, the tendons also become stronger, firmly anchoring the muscles to the bone.
Running is a high impact activity. To tolerate such repeated impact, the bones have to become more resilient to the impact, otherwise stress fractures occur. Within our bones, there are microscopic scaffolds that give it its strength. With repeated impact, these scaffolds are able to remodel, whereby they are resorbed (meaning they are broken down and then re-assimilated) and new ones form so that the bones can better tolerate the stress lines. The diameter of long bones increases, enabling them to tolerate heavier loads. The mineral content, namely calcium, also increases — that is why impact activities are recommended for the prevention of osteoporosis or loss of bone mass. Bones adapt slower than muscles do, so we have to build up our mileage gradually — if you have been running 10 km per week, it will take you a few years before you can tolerate a mileage of 100 km per week.
Energy System
It was mentioned earlier that carbohydrates are our main energy source, and we only have a limited carbohydrate store. Fortunately, with consistent training, your liver and muscle will be able to stores considerably more glycogen. With a bigger fuel tank, you will be able to run longer.
Another training adaptation is the ability to mobilize fat faster, thus supplementing the energy from carbohydrates. If we are able to better mobilize fat, we then have a lower reliance on carbohydrate during exercise. This carbohydrate-sparing ability can save you from hitting the wall.
These biochemical adaptations are often forgotten despite being crucial for distance runners. These adaptations, brought about by long slow runs, differentiate distance runners from middle distance runners and sprinters.
Cooling System
An improvement in thermoregulation will result in a lower body temperature when running, enabling the runner to exercise longer. Thermoregulation can be improved through a process known as heat acclimatization. By running in the heat daily for 10 – 14 days, the body can be conditioned to run faster, under hotter environmental conditions. Physiological adaptations can be observed after just three to four days, and minimal adaptations are observed after 14 days.
During heat acclimatization, sweat rates increase. Hence, training adaptation keeps our temperatures down by producing more, not less, sweat. As the human body has no means of storing water, heat acclimatized runners need to drink more water to compensate for the increased sweat rates. We are most hydrated at the start of the race, and from there on, we get more and more dehydrated if we do not replace our losses. Hence, hydration strategies during the race are critical for optimal performance.
Electrical impulses travel through our nerves and muscles, and for this electrical system to be stable, the electrolyte (e.g. sodium, potassium, calcium, magnesium) concentration around the nerves and muscles must also be kept within a tight range. For example, hypocalcemia (calcium levels in the blood falling below the normal range) causes our muscles to go into spasms; hyponatraemia (low sodium concentration in the blood) leads to swelling in the brain and induces coma. As the electrolyte concentration is critical to the body’s function, our body adapts to regular training by reducing the amount of electrolytes lost through sweating, thus conserving electrolytes. |
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