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Posted: October 11, 2004

Ironman Triathlon: How To Determine The Optimum Intensity

A new paradigm, 'the ultraendurance threshold' is needed to replace the anaerobic threshold and other variables used to govern performance intensity and to predict finish times in events beyond the marathon. That's the intriguing premise suggested by a pair of Canadian sports scientists in a review published in Sports Medicine.

Paul Laursen and Edward Rhodes from the University of British Columbia point out that conventional measures like maximal oxygen uptake (VO2max), anaerobic threshold (AT), economy of motion and the fractional utilisation of oxygen uptake (VO2) correlate highly with normal endurance performance. However, in ultraendurance events performance becomes harder to predict, partly because of the impact of other factors - such as gastric emptying rates and fluid and electrolyte imbalances - and partly because of the evidence that peak AT cannot be maintained beyond four hours.

Their review focuses on the Ironman or 'ultraendurance' triathlon, comprising a 3.8k swim, then a 180k cycle ride, followed by a conventional marathon. 'The UET [ultraendurance triathlon] is a gruelling event that takes between 8 and 17 hours to complete, depending on the calibre of the athlete,' point out Laursen and Rhodes. 'The successful triathlete is one who has the ability to perform each sequential event at an optimal pace without creating fatigue that will hinder performance in the next event.

'However, the duration, intensity and extreme environmental conditions encountered during the UET combine to produce physiological stress not seen in shorter races' While the sport of triathlon has received much research attention, an optimum performance intensity has not been established that takes into consideration the specific UET problems.'

So what are the peculiar physiological stresses affecting ultraendurance athletes? The researchers describe them as follows:

l. Extreme caloric expenditure. Expenditure for the UET can range from 8,500 to 11,500 kcal, which calls for significant fuelling from all sources, including carbohydrate (CHO), fat and protein. Although CHO needs to be the main fuel in order to maintain blood glucose levels, it is best taken in liquid form during the event because this speeds up the rate of gastric emptying and hence absorption into the bloodstream - especially with cold drinks.

2. Fluid loss. Sweat rates can reach up to 2L/h in the heat, creating fluid and electrolyte disturbances that can lead to elevated core temperatures and reduced performance. According to the Canadian researchers, dehydration is the most common reason for a triathlete in the Hawaiian Ironman Triathlon needing medical assistance, while hyponatraemia, a form of sodium imbalance, is the predominant electrolyte disturbance. Fluid and sodium consumption during a UET are therefore important. However, hyponatraemia is also associated with 'overdrinking', leading to fluid retention - a particular risk for slower runners who have a greater opportunity to ingest fluids.

3. Cardiovascular drift. This slow but steady increase in heart rate (HR) is seen during prolonged endurance exercise at a constant work rate and seems to be caused by a progressive decrease in stroke volume - the amount of blood pumped out with each heart beat - which is itself associated with dehydration and reduced blood glucose levels. Because CV drift occurs during triathlon events, HR during a UET may not be an accurate reflection of the actual work rate.

4. Cumulative effects of events. 'The effects of a preceding event on a subsequent event probably contribute to the decrement in performance during the later stages of a triathlon,' explain the researchers.

The evidence suggests, they conclude, that there is a need to identify an exercise intensity below the anaerobic threshold during the UET that allows for a conservation of energy and a balanced utilisation of metabolic fuels, so hypothetically eliciting improved times. They suggest that future research could examine this theory, using highly trained ultraendurance triathletes, measured month to month over repeated laboratory ultraendurance cycle-run performance, or during field work at the UET race itself.

'This paradigm would be called the ultraendurance threshold, and would be defined as the optimum intensity at which an ultraendurance athlete can perform during a specific ultra-distance event.

'Although the ultraendurance threshold will exist at an intensity below the AT, it may prove to be proportionate. For example, individuals with higher ATs may have higher ultraendurance thresholds' Further research is required to determine adequately whether a critical intensity exists relative to the AT in ultraendurance athletes.'

Sports Med 2001 31(3), pp 195-209

Isabel Walker

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