Factors affecting the energy cost of running during an ultra-endurance race.

Purpose: to investigate: 1) the role of V'O2max, fraction of it (F) and metabolic cost of transport (CoT) in determining performance during an ultra-endurance competition and 2) the effects of the race on several biomechanical and morphological parameters of the lower limbs that are likely to affect CoT. Methods: Eleven runners (age: 29-54 years) participated in an ultra-endurance competition consisting of three running stages of 25, 55 and 13 km on three consecutive days. Anthropometric characteristics, body composition, morphological properties of the gastrocnemius medialis, maximal explosive power of the lower limb and V'O2max were determined before the competition. In addition, biomechanics of running and CoT was determined, before and immediately after each running stage. Results: Performance was directly proportional to V'O2max (r=0.77), and F (r=0.36) and inversely proportional to CoT (r=-0.30). Low CoT values were significantly related to high maximal power of the lower limbs (r=-0.74), vertical stiffness (r=-0.65); and low foot-print index (FPI, r=0.70), step frequency (r=0.62) and external work (r=0.60). About 50% of the increase in CoT during the stages of the competition was accounted for by changes in FPI, which represents a global evaluation of medio-lateral displacement of the foot during the whole stance phase, which in turn are associated with the myotendinous characteristics of the lower limb. Conclusions: lower CoT values were related to greater muscular power and lower FPI, suggesting that a better ankle stability is likely to achieve better performance in ultra endurance running competition.