The ability of the extrinsic foot muscles, intrinsic foot muscles, and foot ligaments to modulate foot arch rigidity may influence the transmission of energy generated through ankle plantar flexor work to the ground. However, the extent to which the function of these structures may affect the ankle plantar flexor work performed during vertical jumping is not well understood. The purpose of this thesis was threefold: First, to examine the effects of modifying centre of pressure (COP) location during jump propulsion on vertical jump height, and distribution of ankle, knee, and hip work during the subsequent landing. Second, to estimate the contributions from the extrinsic foot muscles, intrinsic foot muscles, and foot ligaments to foot arch support. Third, to examine the effects of 11-weeks of heel-raise exercise performed on an incline plane compared to a flat box on vertical jump performance, ankle plantar flexor strength, and toe flexor strength. Three studies were undertaken in pursuit of these purposes. In study 1, female volleyball (n = 17, age = 18 ± 2 yrs., height = 1.76 ± 0.07 m, mass = 67.1 ± 7.3 kg) and ice hockey players (n = 19, age = 17 ± 2 yrs., height = 1.64 ± 0.06 m, mass = 62.9 ± 7.8 kg) performed vertical countermovement jumps while recorded using motion analysis. When jumping with the COP close to the forefoot, participants performed more ankle plantar flexor and knee extensor work, compared to jumps with the COP close to the heel (p < 0.05). Jumping with the COP close to the forefoot also resulted in greater jump height and more ankle plantar flexor work (p < 0.05), but less hip extensor work being performed during landings (p < 0.05). In study 2, a musculoskeletal model was developed and used to estimate the contribution of the extrinsic foot muscles, intrinsic foot muscles, and foot ligaments to the midfoot moment in four different loading conditions, with and without maximal voluntary toe flexor activation. Input data was obtained from 6 female (age: 23 ± 3 yrs., stature: 1.62 ± 0.05 m, mass: 57.3 ± 5.6 kg) and 6 male
(age: 27 ± 6 yrs., stature: 1.78 ± 0.08 m, mass: 78.7 ± 8.8 kg) participants who volunteered for iii the study. Midfoot net joint moment increased with increasing loads, while midfoot angles decreased with increasing load (p < 0.05). The static optimization algorithm used to estimate muscle and ligament contribution to midfoot moment could only find viable solutions to the force sharing problem when a specific tension of 60 N/cm2 was used. The extrinsic foot muscles were the largest contributors to the midfoot net joint moment, followed by the intrinsic foot muscles and foot ligaments, for all external loads and with and without maximal voluntary toe flexor contraction. In study 3, female volleyball players completed 11 weeks of heel-raise exercise performed on an incline plane (n = 14, age = 16 ± 1 yrs., stature = 1.77 ± 0.08 m, mass = 67.1 ± 11.1 kg) or a flat box (n = 11, age = 17 ± 2 yrs., stature = 1.80 ± 0.07 m, mass = 70.3 ± 7.2 kg). Vertical countermovement and 3-step approach jump performance before, after 7 weeks, and after 11 weeks of training was assessed using motion analysis. Ankle plantar flexor and toe flexor strength was assessed using dynamometry. Both exercise groups improved vertical countermovement and 3-step approach jump height and hallux flexion strength over 11 weeks of training (p < 0.05). However, no changes were observed in ankle plantar flexor strength or work performed during vertical or approach jumps (p > 0.05). This work provides evidence that the extrinsic and intrinsic foot muscles may contribute to foot arch support, and that the hallux flexors can be strengthened using heel-raise exercise. Further, this thesis suggests that ankle plantar flexor work is an important determinant of vertical jump height. However, the mechanism through which an improvement in ankle plantar flexor strength affects jump height remains unclear.
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