Running biomechanical alterations during a 40-km mountain race.

Background: The purpose of this study was to analyze running biomechanical alterations according to different slopes (flat, uphill and downhill) and distance, during a 40-km mountain trail running race.

Methods: Throughout the race, eight runners were equipped with Runscribe inertial units placed on each running shoe. Measurements included spatiotemporal parameters (contact time, step frequency, stride length, running speed), kinetic (impact peak and braking force) and kinematic data (foot pronation velocity and foot pronation excursion). For data analysis, the race was divided in two halves, from which three types of matching segments were extracted: two uphill sections (U1, U2), two downhill sections (D1, D2) and two flat sections (F1, F2).

Results: Intra-section comparisons revealed that during the second part of the race, running speed decreased in all sections. In uphill sections, stride length increased, step frequency and contact time variability decreased. On flat and downhill sections, contact time increased whereas step frequency and stride length decreased. Step frequency and contact time variability increased for both uphill and downhill sections. With regards to downhill sections only, impact peak, horizontal braking force and foot pronation velocity decreased. Foot strike pattern switched from a rearfoot to a midfoot-strike pattern. Contact time, horizontal breaking force pronation velocity and foot strike pattern variability increased.

Conclusions: Over a 40-km mountain trail running race, changes in running biomechanics are important data which should be taken into account by runners and coaches for training preparation, race strategy as well as for injury prevention.