Neuromechanics of middle-distance running fatigue: a key role of the plantar flexors?

Identifying the neuromechanical changes during high-intensity running to fatigue may highlight the biomechanical limitations to performance and indicate mitigation/training strategies. PURPOSE: To investigate the changes in lower limb kinematics, kinetics and muscle activation during a high-intensity run to fatigue (HIRF). METHODS: 18 male and female competitive middle-distance runners performed a HIRF on an instrumented treadmill at a constant, but unsustainable middle-distance speed (~3 min) based on a preceding maximum oxygen uptake (V[Combining Dot Above]O_2max) test. Three-dimensional kinematics and kinetics were collected and compared between the start, 33%, 67%, and the end of the HIRF. Additionally, activation of eight lower limb muscles of each leg were measured with surface electromyography (sEMG). RESULTS: Time to exhaustion was 181 ± 42 s. By the end of the HIRF (i.e. vs the start): ground contact time increased (+4.0%); whilst flight time (-3.2%), peak vertical ground reaction force (-6.1%) and vertical impulse (-4.1%) decreased (All, P<0.05); and joint angles at initial contact became more (dorsi-)flexed (ankle +1.9°; knee +2.1°; hip +3.6°; All P<0.05). During stance, by the end of the HIRF: peak ankle plantar flexion moment decreased 0.4 N·m·kg^-1 (-9.0%), whereas peak knee extension moment increased 0.24 N·m·kg^-1 (+10.3%); similarly, positive ankle plantar flexion work decreased 0.19 J·kg^-1 (-13.9%), whereas positive knee extension work increased 0.09 J·kg^-1 (+33.3%; both P<0.05) with no change in positive hip extension work. Hip extensor sEMG amplitude increased during the late swing phase (+20.9-37.3%; P<0.05). CONCLUSION: Running at a constant middle-distance pace led primarily to fatigue of the plantar flexors with a compensatory increase in positive work done at the knee. Improving fatigue resistance of the plantar flexors might be beneficial for middle-distance running performance.