The Physiology of a Distance Runner

The Marathon, a 42.2km distance race is enough to make the fittest amongst us tremble. Standing at the finish line you can’t help to observe the variety of runners who choose to tackle this tremendous challenge, both large and small, muscular and lean, young and old. If you’re at all inquisitive like myself you’ve probably wondered what sets apart the great runners from the not so great, and how can the most unfit looking individual finish alongside a clearly more athletic counterpart.

What factors makes a great distance runner

If you were to ask an expert what makes a fast racing car you’d quickly realise the more you delve into the question the more complex it becomes. On the surface a car with a big engine and a low body weight will be an advantage, as will aerodynamics and gearing ratios. But then the educated car enthusiast would have a more exhaustive list, excuse the pun. The same can be said about the question what makes a good distance runner.

Some of the key factors are Aerobic fitness compared to body weight, cellular or enzyme dynamics which are multifactorial but ill place this one under Anaerobic Threshold and Running Economy. .

Before I delve into each these three factors lets have an exercise physiology catch up. Muscles contract by breaking down a molecule known a Adenosine Triphosphate or ATP, ATP replenishment occurs through two pathways. Anaerobic replenishment which is an incomplete pathways results in the build up of metabolic bi-products, think of  a cheetah, a high power output but a short staying power. Aerobic energy production which is a complete pathway is the opposite, being able to provide prolonged energy without fatigue. The main requirement of aerobic energy production is the delivery and utilization of oxygen by a cell. That brings us back to our question of what makes a good distance runner.

Aerobic Capacity

You’ve probably heard to term VO2max mentioned in running circles before. VO2max is a measure of aerobic capacity and for runners we measure this per kg of body weight (ml’s/kg/min). Unlike cycling during running the body weight is self-supported as result the energy cost of running rises with increasing body weight.

A high aerobic fitness per body weight enables large quantities of oxygen to be delivered to the cells. A positive for you as a runner is that aerobic fitness despite having genetic influences is largely modifiable. Men have higher aerobic fitness levels due to larger hearts, more developed vasculature and higher haemoglobin or red blood cell counts. These are predominately influenced by testosterone both in development and current life. Heart size, haemoglobin levels and blood vessels development all increase with training, contributing to improvements in performance. Interestingly unlike with muscle size and strength, you cannot see VO2max, the only possible sign is a smaller frame which possibly will mean a higher VO2max per body weight. This is why amongst the top 20 percent of runners you’ll find a series of physical outliers who you never would have picked as making it to the finish line let alone finishing at the pointy end of the field

Running Economy

Running economy refers to how much oxygen is required to cover a given distance, and again we measure this per kilogram of body weight. Because peak oxygen consumption (VO2max) is a contributing factor of running performance reducing the oxygen cost of running a given distance enables you to cover that distance faster. Here’s an example, if you use 180 units of oxygen to cover a kilometre and your VO2max is 60 units of oxygen per minute, the it is impossible to cover that kilometre any faster than in 3 minutes (180/60=3). Assuming you have some super heavy shoes and a limp you may now use 240 units of oxygen to cover the same kilometre. With the same VO2 max the fastest time you can run a kilometre in is 4 minutes.

So what effects running economy? Running is a skill and like most skills it can be improved. Studies show the more someone runs generally the more efficient they become. On top of this specific strength training, technique modification and footwear choices all influence economy. 

Anaerobic Threshold

We talked of the aerobic and anaerobic pathways for energy production, these pathways are never independent, there is a small amount of anaerobic production even a low levels of exercise. The anaerobic threshold is the point at which metabolic bi-products or acidosis begins to increase, this is due to a rising anaerobic contribution. The anaerobic threshold is measured as a percentage of VO2max and has a strong correlation with fatigue. The anaerobic threshold has been shown to relate closely to  a muscles oxidative enzyme levels. Interestingly training volume seems to also produce steady increases in the oxidative capacity of the muscles providing support to the analogy of getting some solid miles in the legs. Muscle oxidative capacity also relates to slow twitch fibre type percentages. As slow twitch fibres increase there is also a rise in the oxidative capacity of the muscle. A higher oxidative capacity means less acidosis and subsequent increase in the time before fatigue sets in

The Key Performance Predictor

A person’s Aerobic Fitness, Running Economy and Anaerobic Threshold are all important predictors of middle distance running performance. There’s a key performance indicator that combines these variables and has a high correlation with middle distance running ability. This is the velocity at anaerobic threshold. This combines the effects of VO2max, running economy and anaerobic threshold and ties them neatly into one performance indicator.

So when you’re next out watching a race or heading for a run and you see someone glide past, you’ll now that they either have a big engine, are highly efficient or that the fatiguing effects of acidosis begin at much higher running speeds. If they were to combine these factors with a lower body weight then you’ve got a performance runner on your hands