Muscle contraction drives our movement and, to do so, muscle cells need ready access to energy held in glucose, glycogen or fats. Just as a £50 note is of little use to feed a parking meter, the energy within each of these food molecules is far too large to use directly for muscle contraction. Instead the tiny fibrils of muscle use a smaller energy packet in the form of a molecule called ATP (adenosine triphosphate) - in our currency analogy ATP might be a 50p coin. ATP molecules supply just the right kick for little fibrils in muscle to dig in and slide past one another, thereby shortening the muscle and making it pull on its attachments. Teeny tiny structures called mitochondria are responsible for the process of aerobic respiration that generates ATP from glucose and/or fatty acids. Mitochondria can only do this in the presence of oxygen. So for muscle contraction we need:
- semi-processed food molecules ie small enough to travel in the blood stream and be delivered into cells. Glucose is the main candidate.
- oxygen (again this is mainly delivered via the bloodstream, but muscles have a small emergency supply)
- muscle cells organised to form a cohesive muscle mass and large enough to do useful work
- mitochondria (located within each muscle cell)
Good training addresses each of these essential factors.
Any activity that accelerates the heart rate for a sustained period will improve the body's ability to rapidly deliver well-oxygenated, food-rich blood to the muscles. What constitutes a "sustained period" remains open to debate, but is reasonably interpreted as 20-30mins exercise at least 3 times per week. Such activity helps build the heart muscle itself so the heart contracts more strongly with each beat.
Likewise any activity that increases breathing depth and accelerates breathing rate will trigger physiological adaption to make these more sustainable and tolerable.
Incorporate activities which use the muscles you need for running. Performing such activities will damage the muscles being used, but upon recovery the muscle will be better than before - it could contain more muscle cells (thereby getting bigger) and each of these cells may have a higher density of mitochondria, thereby better at supplying ATP for contraction. N.B. Recovery is vital. Just using the muscle will cause more and more damage.
Include sessions where you make the muscles work hard despite them feeling tired. Tempo runs, the hard efforts within a fartlek or interval sessions will cause discomfort and even pain as your limits to deliver oxygen to muscle are reached. Without oxygen the muscle can temporarily switch to anaerobic respiration, a process which permits the recycling of ATP so some is available for muscle contraction. However, anaerobic respiration releases damaging lactic acid as a by-product. Unlike carbon dioxide, that other toxic respiratory product, lactic acid cannot be exhaled. Nor is it excreted elsewhere. It will just sit around doing damage unless you get sufficient oxygen available to recycle it into non-damaging alternatives. Training for lactic acid production would thus seem madness, but in forcing your body to work at its limits with regard to aerobic v anaerobic respiration, you force it to respond by generating more mitochondria. Mitochondria - those glorious ATP generating machines.