Spring-mass characteristics during human locomotion: Running experience and physiological considerations of blood lactate accumulation

Abstract

The aim of this study was to examine how running experience affects leg stiffness (K_leg) and spring-mass characteristics during running stages associated with the onset of blood lactate accumulation (OBLA). Seven trained (66.9?±?4.8?kg; 182?±?4.0?cm; 23.1?±?3.1 years) and 13 untrained (78.5?±?7.6?kg; 182?±?3.0?cm; 20.3?±?1.5 years) runners completed an incremental treadmill run. Running velocity was increased by 1 km^.h^?1 every four minutes and blood lactate samples were taken at every stage, in addition to a 10?s video recording using ‘Runmatic’. Once 4?mmol^?L^?1 (OBLA; the second lactate turn point) had been reached one more stage was completed. Spring-mass characteristics across groups and at pre-OBLA, OBLA and post-OBLA were compared. The velocity at OBLA was higher for the trained runners compared to the untrained runners (18?±?0.7 vs 11?±?1.3 km^.h^?1, p?<?0.001). K_leg was similar between untrained and trained runners across each stage (15.8?±?0.3 vs 14.3?±?0.3 kN^.m) and did not change between stages, yet spring-mass characteristics differed between groups. Vertical stiffness increased in the trained runners from pre-OBLA to post-OBLA (45.5?±?3.35–51.9?±?3.61 kN^?1), but not in untrained runners (35.0?±?5.2–39.6?±?5.7 kN^?1). K_leg was strongly related to F_peak for trained runners only (r?=?0.79; untrained runners, r?=?0.34). K_leg was unaffected by physiological training status and was maintained across all OBLA stages. Trained runners appear to have optimised their spring-mass system in a homogenous manner, whilst less consistent spring-mass characteristics were observed in untrained runners.