To investigate changes in running mechanics during a 6-h running race.
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 (t c), and aerial time (t a) were measured in the first lap and every 30 ± 2 min during the race. Peak vertical ground-reaction force (F max), stride length (SL), vertical downward displacement of the center of mass (Δz), leg-length change (ΔL), vertical stiffness (k vert), and leg stiffness (k leg) were then estimated.
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 t c increased after 4 h 30 min of running, reaching the maximum difference after 5 h 30 min (+6.1%, P = .015). Conversely, k vert decreased after 4 h, reaching the lowest value after 5 h 30 min (–6.5%, P = .008); t a and F max 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.
Most changes occurred after 4 h continuous self-paced running, suggesting a possible time threshold that could affect performance regardless of absolute running speed.