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ABSTRACT
Objectives: This study aimed to determine adjustments in spring-mass model characteristics, plantar loading and foot mobility induced by an exhaustive run.
Design: Within-participants repeated measures.
Methods: Eleven highly-trained adolescent middle-distance runners ran to exhaustion on a treadmill at a constant velocity corresponding to 95% of velocity associated with VO^sub 2^^sub max^ (17.8 ± 1.4 km h^sup -1^, time to exhaustion = 8.8 ± 3.4 min). Contact time obtained from plantar pressure sensors was used to estimate spring-mass model characteristics, which were recorded (during 30 s) 1 min after the start and prior to exhaustion using pressure insoles. Foot mobility magnitude (a composite measure of vertical and medial-lateral mobility of the midfoot) was measured before and after the run.
Results: Mean contact area (foot to ground), contact time, peak vertical ground reaction force, centre of mass vertical displacement and leg compression increased significantly with fatigue, while flight time, leg stiffness and mean pressure decreased. Leg stiffness decreased because leg compression increased to a larger extent than peak vertical ground reaction forces. Step length, step frequency and foot mobility magnitude did not change at exhaustion.
Conclusions: The stride pattern of adolescents when running on a treadmill at high constant velocity deteriorates near exhaustion, as evidenced by impaired leg-spring behaviour (leg stiffness) and altered plantar loading.
© 2014 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.
Keywords:
Fatigue
Spring mass model
Stride mechanics
Medial arch
1. Introduction
From a mechanical point of view, running is a typical human movement where the musculo-tendinous structures of the lower limbs alternately store and return elastic energy during the so-called stretch-shortening cycle.1 Accordingly, the lower limbs can be considered as springs loaded by the weight and inertia of the body mass. This paradigm refers to the "spring-mass model" (SMM) and has been applied increasingly in recent years to describe the lower limb neuromuscular behaviour (stiffness regulation) during fatiguing runs.2-4 Studies examining changes in SMM characteristics during ultra-long distance events have reported increased leg and/or vertical stiffness and step frequency with fatigue development.5-7 Conversely unchanged peak vertical forces and constant or decreased step frequency and vertical stiffness over multiple running sprints have been observed.2,8'9
Part way between ultra-long distance and sprint events, studies of SMM characteristics during middle-distance running...