Mechanical determinants of the energy cost of running at the half-marathon pace.

Background: The aim of this study was to determine the influence of spring mass model characteristics (e.g. stiffness) and Achilles tendon properties in determining the energy cost of running in half marathon runners.

Methods: Achilles tendon characteristics (i.e. cross-sectional area -ATCSA- and resting length -ATL-) were measured on 32 males by means of an ultrasound apparatus the day before a half marathon race. After these measurements the energy cost of running (C) was determined while the subjects run on a treadmill at the speed (vT) they were expected to maintain during the race (vR); the vertical (kvert) and leg (kleg) stiffness were calculated based on kinematic data.

Results: No differences were observed between vT and vR. Higher values of vT were associated with larger values of kleg and kvert. The faster runners (with larger vT) were the ones with the lower C (r=-0.43, P<0.05) and the larger ATCSA (r=0.46, P<0.01). No relationship was found between C and ATCSA but C was lower in runners with longer ATL (r=-0.52, P<0.001). Finally, no relationship was found between kleg or kvert and C, but runners with larger kvert were also those with the larger ATCSA (r=0.45, P<0.01).

Conclusions: These findings underline the correlation between spring-mass model parameters and Achilles tendon characteristics in half-marathon runners; they further show how these parameters influence the half marathon pace and the energy cost of running at this pace.