Ultra-endurance athletes suffer no cardiac fatigue, even after six days of non-stop exercise, Swedish study finds

The results, presented at the European Society of Cardiology's Congress 2010 in Stockholm, show no evidence of cardiac fatigue despite the fact that this is a continuous endurance event lasting between five and seven days.

The Adventure Racing World Championship is a true test of endurance. It is held over 800 kilometres of challenging terrain, and competitors have to complete the course non-stop using mountain-biking, trekking, kayaking and in-line skating. The event offered the perfect opportunity to study the unknown effects of the heart's response to prolonged exercise in conditions of sleep deprivation and energy deficiency. 15 athletes were selected for the study (12 male and 3 female), all of whom had hearts within the normal size range. During the event, they exercised almost continuously for approximately 150 hours, at an average work intensity of 40% (in terms of respective VO2 peak).

Commenting on the study, C. Mikael Mattsson from the Karolinska Institutet said that, "Significant interest is emerging in investigating whether extreme workload damages the heart, or if any measured changes are signs of fatigue, similar to that in skeletal muscle. During ultra-endurance competitions, participants have a constantly elevated heart rate for extreme durations, and a possible risk is that heart muscle cells are catabolised, which could lead to severe pathological conditions."

If cardiac muscle were to respond in the same way as skeletal muscle, the expectation would be for lower contraction velocities after exercise. However, the results of the study show that this was not the case. It is worth noting that these results are not consistent with other studies of endurance events such as marathons or triathlons. The researchers believe that this may be because average intensity was relatively low despite competitors exercising for extended durations that averaged 150 hours. Therefore, a possible conclusion may be that it is exercise intensity rather than exercise duration that is the primary source for cardiac fatigue.

This study is one element of a wider project to investigate how the heart responds to ultra-endurance exercise. Other aspects of the project will examine the physiological profile of athletes, circulatory response, heart impact and changes in the work of the heart, as a result of, or adaptation to, ultra-endurance exercise.

Research results have provided new insights into the effects of long periods of exercise, typically over six hours. C. Mikael Mattsson notes, "Some athletes experience increased levels of blood markers, which can indicate cardiac damage, yet these levels decrease rapidly and are back to normal within approximately 24 hours. This temporary elevation may not be a result of cardiac damage, but rather as a means of protecting and regulating growth. It also appears that athletes who performed best and felt the strongest during the latter part of the exercise had less-affected hearts."