The aim of this study was to examine how running experience affects leg stiffness (Kleg) 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). Kleg 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). Kleg was strongly related to Fpeak for trained runners only (r = 0.79; untrained runners, r = 0.34). Kleg 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.
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