Limitations to performance will vary depending on the distance concerned. Limitations to performance over 50km will be different to 100 miles, 48 hours or 1000 miles.
No1 is the brain’s control on performance. Our brain doesn’t allow us to push ourselves to death (as a rule). There is always a cushion by which the brain insures that homeostasis is maintained. That cushion cannot be encroached. We can train and compete in such a way that the brain will allow us to use more of our resources, our work capacity. This can lead to improved performances.
and then in no particular order:
– Rate of muscle glycogen depletion.
– Local muscular endurance, strength and power.
– Ability to take on sufficient fluid and energy throughout the duration of the event to prevent levels of dehydration and energy depletion from adversely affecting performance.
– Psychological ability to maintain power output against a tide of rising fatigue as race progresses.
– Physiological/running efficiency – the ability to run at race pace with the minimum of energy expenditure.
The following extract from a presentation (see Ref No1) by scientists from Copenhagen’s University Hospital Research Centre is so important that I have reproduced it in full (highlights are mine):
“An important and overlooked aspect of running efficiency is that, in contrast to cycling, where efficiency is reasonably maintained, a marked deterioration is seen with time in running. Furthermore, it appears that for the better runners a more moderate drift in oxygen uptake is occurring during running. There are two obvious conditions causing a drift in VO2 at constant exercise intensity: the gradual switch to more fat being used as a substrate for oxidative metabolism and the slight increase in ventilation. In cycling these changes can to a large extent explain the drift in VO2. In running, the elevation in VO2 is much greater and even in well trained athletes amounts to 0.3-0.5l/min.
A major difference between cycling and running is that there is no eccentric component in the former, but even when running on the flat the eccentric impact on the muscle is noticeable. Thus, one possibility for the mentioned difference could be that elastic recoil vanishes in prolonged running due to the eccentric component that causes mechanical wear and tear. In the process of fatigue, structural alterations may also occur at the molecular level thereby changing the elastic recoil capacity which may lead to a less efficient stride. The magnitude of increase over a two – three hour run is in the order of0.3-0.5l/min or about 10%.
In more extreme situations such as in a 100km race, a given speed costs 1 l/min more oxygen at the end of the race than at the start – this represents an elevation in energy cost of 30-40%. It is worth noting that the reduction in running pace is not due to a lower fractional use of VO2max which is kept quite constant, instead there is an inverse relationship between the increased metabolic costs and running speed.”
– The ability to pace the race effectively. This means to run as near even pace as possible although effort will be very uneven – starting very low and rising steadily throughout the event until extremely high effort levels are required to maintain the same pace that could be achieved with easy effort at the start of the race.
1) Muscle metabolic factors decisive for marathon running by Helge, Hall & Saltin from “Marathon Medicine” edited by Dan Tunstall Pedoe; published by The Royal Society of Medicine Press 2001.