Genetics have a large influence over strength, muscle size and lung capacity, flexibility, and, to some extent, endurance.

One major limitation for endurance athletes is cardiac capacity, or the heart's ability to deliver enough oxygen (via the bloodstream) to the working skeletal muscles. This, too, is largely determined by genetics.

The other limitation for endurance athletes is in the muscles  ability to effectively use the oxygen and create ATP (adenosine triphosphate), the fuel that allows muscular contraction and movement. The efficiency of this process is measured by something called maximum volume of oxygen.

Muscles use something called ATP (adenosine triphosphate) as an energy source. ATP is created in essentially two ways. One is by aerobic metabolism (using oxygen) carried in the bloodstream. This is a sure and steady way to create virtually unlimited sources of energy; however, it has a limit on how fast it can work. The cardiovascular system is limited in its ability to quickly deliver blood and oxygen to the working muscles. Because of this, during high intensity exercise, ATP is also provided by anaerobic metabolism that don't require an ongoing supply of oxygen. Anaerobic pathways can provide ATP immediately, but they also have limited stores that need to be refilled after they are used up. This may cause muscle fatigue.

Accessibility or an 'access focus': In this perspective it is the ability to REACH opportunities that is beneficial, not movement itself. In remote rural contexts gaining access to services, goods and contacts will often require a lot of mobility. However, in many urban contexts accessibility might involve very short trips. And in places like suburban USA policy to enhance accessibility might actually require that we reduce traffic or even reduce the need to travel (or reduce mobility).

The key example is just expanding the roads and building more expressways. It can seem like a good idea but it only ever gives temporary relief (and sometimes not even that). In the long run traffic actually gets worse.

The relationship between transport and development is neither obvious nor simple. Simply building a road does not guarantee that development will take place. Also, studies show that provision of a transport facility does not bring benefit to all social groups equally. The main beneficiaries of a new road, for example, are usually traders and middle-income groups, who have the capacity to take advantage of an expanded network to increase their livelihood. The rural poor (the largest social group in developing countries) usually derive very little benefit from traditional transport systems, and in many cases their livelihood suffers because of them.