Changes in Running Mechanics During a 6-Hour Running Race.

PURPOSE: To investigate changes in running mechanics during a 6-h running race.

METHODS: Twelve ultraendurance runners (age 41.9 ± 5.8 y, body mass 68.3 ± 12.6 kg, height 1.72 ± 0.09 m) were asked to run as many 874-m flat loops as possible in 6 h. Running speed, contact time (tc ), and aerial time (ta ) were measured in the first lap and every 30 ± 2 min during the race. Peak vertical ground-reaction force (Fmax ), stride length (SL), vertical downward displacement of the center of mass (Δz), leg-length change (ΔL), vertical stiffness (kvert ), and leg stiffness (kleg ) were then estimated.

RESULTS: Mean distance covered by the athletes during the race was 62.9 ± 7.9 km. Compared with the 1st lap, running speed decreased significantly from 4 h 30 min onward (mean -5.6% ± 0.3%, P < .05), while tc increased after 4 h 30 min of running, reaching the maximum difference after 5 h 30 min (+6.1%, P = .015). Conversely, kvert decreased after 4 h, reaching the lowest value after 5 h 30 min (-6.5%, P = .008); ta and Fmax decreased after 4 h 30 min through to the end of the race (mean -29.2% and -5.1%, respectively, P < .05). Finally, SL decreased significantly (-5.1%, P = .010) during the last hour of the race.

CONCLUSIONS: Most changes occurred after 4 h continuous self-paced running, suggesting a possible time threshold that could affect performance regardless of absolute running speed.