Lower-limb neuromechanics of high-intensity running with special reference to fatigue, and their implications for middle-distance performance

Middle-distance runners train at a range of running speeds from steady jogging to sprinting, and race at sustained high intensities that ultimately lead to fatigue. However, there is a lack of research regarding the role of specific lower limb muscle groups for running at different speeds and the neuromechanical changes that occur during fatiguing high-intensity running. Therefore, the purpose of the thesis was to assess the demands and contributions of flexor and extensor leg muscles over a whole stride cycle at different running speeds, to investigate the influence of fatigue on the neuromechanics of high-intensity running and examine the role of plantar flexors in middle-distance running performance. Chapter 3 revealed that the plantar flexors were the major contributor to positive and negative work during stance across a wide range of speeds and for a whole stride cycle at low and medium speeds. However, at fast speeds hip extensors and knee flexors exceeded the contribution of plantar flexors to positiveand negative work over a whole stride, respectively. Chapter 4 showed that high-intensity running to exhaustion led to a reduction in peak plantar flexion moment (-9.0%) and positive plantar flexion work during stance (-13.9%), with a compensatory increase in peak knee extension moment (+10.3%) and positive knee extension work during stance (+33.3%). Chapter 5 investigated the effect of isolated plantar flexor, knee extensor and hip extensor resistance exercise fatigue on high-intensity running neuromechanics. Minor neuromechanical changes were observed during running after the fatiguing resistance exercises, the highest being decreased plantar flexion moment (~2.8%) after knee extensor and hip extensor fatigue, decreased peak vertical ground reaction force (-3.3%) after knee extensor fatigue and reductions in peak anterior-posterior ground reaction forces after all three fatiguing exercises (≤3.8%). Finally, chapter 6 demonstrated that only the Achilles tendon (AT) length from distal insertion to the medial gastrocnemius was related to middle-distance running performance (R=0.50). Other structural (plantar flexor muscle volume, other parts of the AT length, AT moment arm) and functional (isometric strength, peak power, plantar flexor fatigue resistance) characteristics were unrelated to performance. None of the parameters showed a relationship with energy cost either. However, middle-distance runners had a higher normalised plantar flexor volume (to body mass) compared to control participants. Overall, this thesis demonstrated that the plantar flexors have an important contribution to force production in high-intensity running and are a limiting factor in fatiguing high-intensity running due to their reduction in force production whereas knee flexors and extensors play a key role in sprinting. Longer medial gastrocnemius AT suggests being beneficial for middle-distance performance.