|dc.description.abstract||Current views of human structure and footwear prescription are encompassed in the poor foot paradigm. An alternative paradigm is proposed which includes evolutionary and anthropological viewpoints that humans are well adapted for life on earth. Early footwear was designed for protection and warmth but has evolved through cultural forces of fashion, finance and mass manufacture. Footwear can change the anterioposterior, mediolateral and vertical orientation of the foot-ground interface, changing somato-sensory information and neuromuscular work needed to keep the human tower upright. A unifying conceptual model of footwear asymmetry patterns influencing the foot-ground interface is proposed for the assessment of footwear and the simulation in the experiments. The aims of this thesis were to determine the frequency and magnitude of mediolateral asymmetry in worn footwear and then simulate quantifiable amounts, based upon clinical observations, of mediolateral asymmetry barefoot and in footwear while measuring the effect on performance and postural stability using dynamic tasks.
It was hypothesised that incremental mediolateral asymmetry (from 1 to 3 mm) would progressively impair neuromuscular performance and postural stability. A pre-existing footwear asymmetry or an extended habituation period with simulated asymmetry was hypothesized to further impair barefoot performance. The two tasks identified as having ecological validity and able to be repeatedly performed without undue fatigue were the single-leg heel-raise (Study 1) and the transition from double- to single-leg stance (Study 2).
Study 1 was a randomized repeated measures cross-over trial of 38 participants (23.0 ± 4.9 years), performing the tasks barefoot and in simulated 1 mm medial or lateral asymmetry. Sustained heel raises, the maximum number of heel-raises and the rate they were performed all had significant (P < .001) decreases in performance of 40.3%, 23.4%, and 10.7% respectively with the medial wedge, simulating lateral heel wear, compared to the control. The rate heel-raises were performed was 6.6% (CI 0.2 to 12.3%, p = .042) better in participants with neutral footwear. Assessment of 294 shoes showed 37.4% were neutral, 4.8% had 1 mm medial, while 57.8% had between 1 mm and 8 mm of lateral heel wear and/or compression.
Study 2 was a blinded randomized repeated measures trial using 106 participants (32.4 ± 13.3 years) performing the task of double- to single-leg stance on two force platforms while barefoot and in their chosen footwear. Medial and lateral footwear asymmetry was simulated using 1, 2 and 3 mm of heel wedging and a 20-minute walk habituation period was included for the last of 8 conditions. In 212 shoes, the frequency of heel asymmetry was biased 57.6% laterally, compared to 0.9% medially. Time to stabilization was 11.9% (CI 4.0 to 19.2%, p = 0.01) quicker barefoot than in footwear. Other postural stability measures were either unchanged or greater barefoot. Barefoot and shod single-leg mediolateral postural stability performance was 7.1% (CI -2.3 to 17.3%, P = .143) and 13.1% (CI 3.3 to 24.0%, P = .008) worse in participants who had asymmetrically worn footwear. The simulation had a significant destabilizing effect post-walk for all conditions except neutral footwear. Incremental simulated lateral asymmetry of 1, 2 and 3 mm systematically reduced participants’ single-leg mediolateral postural stability performances post-walk by 10.3% (CI -0.9 to 22.8%, P = .074), 14.6% (CI 3.0 to 27.6%, P = .013) and 20.9% (CI 8.6 to 34.7%, P = .001). Simulated medial asymmetry had a lessor effect with 1.1% (CI -8.9 to 12.1%, P = .843), 11.6% (CI 0.2 to 24.3%, P = .045) and 4.8% (CI -5.9 to 16.7%, P = .394). A similar effect was measured for the mean velocity of the COP and in the corrected footwear conditions.
The findings indicate that footwear asymmetry is likely to affect human performance and balance. This may have implications for footwear assessment, design and interventions to neutralise mediolateral asymmetry. The conceptual human tower embedded in asymmetric footwear needs neuromuscular compensations in order to remain vertical. Should these compensations fail, abnormal mechanical joint loading and muscular fatigue or inhibition leading to injury and a decrease in skilled performance are possible. Lateral asymmetry was predominantly measured in footwear in this study and individuals wearing these had worse mediolateral stability suggesting a link to a global balance deficit. These may be important factors linked to ankle inversion injuries or falls in the elderly and the development of medial knee osteoarthritis.||