Heather Anne McKay
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2019)
All healthy children pass through the same stages of growth, yet they do so at distinct times and tempos. Consequently, there can be large maturational differences between children of the same chronological age. Thus, it is essential to measure maturity as study results may be confounded by biological age. Yet assessing maturity is difficult. As a result, methods to predict maturity exist, however, the validity of these models have been questioned. Further, the influence of maturational timing on bone mass, density, structure, and strength remains unclear. In this dissertation, I utilized data from healthy children who were participants in the Healthy Bones Study III (HBSIII). Three studies comprise my dissertation. First, given the need for accurate prediction of maturity, in Chapter 5 I assessed the utility of models that estimate maturity offset (MO) and age at peak height velocity (APHV), common indicators of somatic maturity. I discovered errors in the development of the original equations and subsequently, I developed and validated new models. Second, given known growth and maturation differences between ethnicities, in Chapter 6 I examined whether there were differences between Asian and white children who lived in the same community (Metro Vancouver). I observed that growth and maturation differences existed between Asian and white children, despite living in the same neighborhoods. I also assessed the validity of new maturity prediction equations for Asian children, as previous equations were developed in white children. I recalibrated the equations to better predict MO and APHV in this ethnic group. Third, given the unknown influence of maturity on post-pubertal bone health, in Chapter 7 I examined the relationship between maturational timing and bone mass, density, structure, and strength in late adolescence. Though some evidence suggests naturally delayed maturation is deleterious to bone in post-puberty, I found that post-pubertal adolescents who were considered late-maturing youth (mostly) ‘caught-up’ with those that were considered early-maturing, although there were sex- and site-related differences. Collectively these studies enhance our understanding of maturation, provide maturity prediction models for both white and Asian children, and clarify the complex relationship between maturational timing and bone mass, density, structure, and strength.
Introduction: Physical activity (PA), the movement of one’s body, and mobility, moving one’s body through space using a variety of modes, allow older adults to participate in their communities, cultivate social connections, maintain their health, and access services. Segments of the population, however, have been overlooked. While a growing body of research has focused on older adults, we know surprisingly little about the PA and mobility of foreign-born older adults (FBOAs).Objective: This dissertation uses a focused ethnographic approach to characterize the PA and mobility of 49 visible minority FBOAs in South Vancouver, Canada. Methods: The research was conducted in Hindi, Punjabi, Cantonese, Mandarin, and English. Forty-nine participants completed surveys about their PA habits; of these 49, 46 wore accelerometers and 18 completed in-depth interviews. I also developed a novel interview tool “interactive interpreted interviews”, neighbourhood walking interviews that included professional interpreters, which 13 participants completed. Results: Participants’ mean daily step count was 7,876 (women: 8,172; men: 7,164; Chinese: 8,291; South Asian: 7,196). The bulk of their time is spent in light and sedentary activities. Physical activity is principally acquired through walking for errands and work performed in/around the home. Participants walk for physical and mental wellbeing, and have access to a supportive social environment, which includes culturally familiar and linguistically accessible shops and services.Conclusions: This study challenges the assumption that FBOAs are less active than their non-immigrant peers and confirms the role of “nonexercise” and low activity, rather than moderate to vigorous activity, in older adults’ PA acquisition. Building on the Webber (2010) model of mobility in older adults, this study also highlights how gender and personal biography, including work history and family context, impact participants’ PA and mobility behaviours.
With recent advances in imaging technologies, we are acquiring a better understanding of the complex hierarchy of bone and how bone adapts its geometry, microarchitecture and ultimately, its strength to withstand the loads imposed upon it during adolescent growth. Thus, in this thesis, I examine the influence of physical activity (PA), sedentary time, maturity and sex on estimated bone strength and its determinants (i.e., microarchitecture, geometry and density) across adolescence.This thesis is based on the UBC Healthy Bones III Study (HBSIII), a mixed longitudinal cohort of healthy girls and boys age 8-12 years at study entry. We assessed bone strength, geometry and density at the tibial shaft using peripheral quantitative computed tomography (pQCT) and bone strength, microarchitecture, geometry and density at the distal tibia and radius using high-resolution pQCT (HR-pQCT). We assessed PA and sedentary time using accelerometry. Four studies comprise this thesis. First, I investigated cross-sectional associations between sedentary time and bone strength and its determinants at the distal tibia by HR-pQCT. I found no associations between sedentary time and bone parameters.Second, I examined maturity- and sex-related adaptations of bone geometry and strength at the tibial shaft using pQCT. I found that larger bone area in boys provided them a greater bone strength advantage compared with girls across adolescence. Third, I examined maturity- and sex-related adaptations of bone strength and its determinants by HR-pQCT at the distal tibia and radius. I found greater bone strength in boys across adolescence was underpinned by greater trabecular bone volume and total bone area. Fourth, I examined prospective associations between PA, sedentary time and bone strength and its determinants at the distal tibia and radius using HR-pQCT. I observed greater bone strength and trabecular bone volume in participants engaging in more PA and lower total bone area in participants engaging in more sedentary time.Collectively, these studies enhance our understanding of how bone is gained during adolescence and add a unique perspective to the benefits of PA for bone strength and its determinants.
Mobility is vital to healthy aging. The built environment is central to mobility as it is the setting where mobility occurs. Older adults of low socioeconomic status (SES) may be especially reliant on built environments that support non-motorized mobility. Despite this, older adults of low SES are underrepresented in research. Therefore, this dissertation applies quantitative methods to describe the mobility (capacity and enacted function) and investigate the association between the built environment and non-motorized mobility (walking for transportation and physical activity) of older adults of low income. The studies within this dissertation are set in Metro Vancouver; the last three utilize data from a cross-sectional study of 161 older adults of low income. The first study investigates the reliability of a novel approach (virtual audits) for measuring the built environment. I found that virtual audits reliably measure macroscale (neighbourhood-level) built environment features that promote older adult walking, but may be inappropriate for measuring fine-grain details of the microscale (street-level) built environment. The second study provides an in-depth description of the mobility of older adults living on low income. I noted that participants generally had the capacity to be mobile and made a high proportion of walking trips, although these did not together serve to meet physical activity guidelines for most. The third study analyzes travel diary data to identify destinations that older adults most commonly visit in a week (i.e., grocery stores, malls, restaurants/cafes); I also found that each 10-point increase in Street Smart Walk Score (measure of the built environment) was associated with a 20% increase in walking for transportation (IRR = 1.20, 95% CI = 1.12, 1.29). The fourth study analyzes accelerometry and self-report data to investigate associations between the built environment and physical activity and walking for transportation. Odds of any walking for transportation were 1.45 (95% CI = 1.18, 1.78) times greater for each 10-point increase in Street Smart Walk Score. There were no other built environment-mobility associations. Taken collectively, these studies fill methodological gaps in the literature and provide data on an understudied population that may be especially reliant on built environments that support walking.
Physical activity (PA) benefits bone strength in children but little is known of the effects of PA on bone strength in adolescents. In this thesis, my primary aim was to determine the effect of a secondary school based PA intervention on bone strength, structure and density in adolescents. This 8-month cluster, randomized-controlled, whole school-based intervention study had four intervention and five control schools. Participants were 210 Grade 10 students who were 15.3 years old, on average, at baseline. The Health Promoting Secondary Schools (HPSS) intervention was a choice-based model based on self-determination theory that aimed to increase PA, promote healthy eating and reduce screen time in adolescents. I used peripheral quantitative computed tomography (pQCT) to assess bone strength, structure and density at the distal and shaft sites of the tibia and radius. I assessed PA using a validated PA self-report questionnaire and I measured a sub-set of participants’ PA objectively using accelerometry. Part I is a systematic review and narrative synthesis of PA and pediatric bone literature. High-quality randomized-controlled trials (RCTs) with weight-bearing PA increased bone strength in children. Bone structure adaptations in response to PA were more common than adaptations in bone density (RCTs and observational studies). Only one RCT involved adolescents (average age 13.8 years) and studies often overlooked the influence of muscle on bone responses to PA.In Part II, moderate-to-vigorous PA (MVPA), vigorous PA (VPA) and grip strength positively influenced bone strength in boys and girls after controlling for ethnicity, maturity, limb length and muscle mass. Sedentary time (SED) negated the positive influence of MVPA, but not VPA, on bone strength in girls. In Part III, the HPSS intervention did not lead to significant gains in bone strength, structure or density in adolescents. The external factor of a province-wide teacher job action possibly hindered the execution of the HPSS intervention. In summary, MVPA and VPA benefit bone strength in adolescents but further investigations are warranted to determine the effects of SED on bone strength. It remains to be determined the effects of a choice-based intervention on bone strength adaptations in adolescent boys and girls.
Introduction: Physical activity (PA) and sedentary behaviours are important modifiable risk factorsfor cardiovascular disease in adults; however, we know much less about their role for enhancing andsustaining children’s cardiovascular health. The aim of this thesis is to evaluate the relation between PA, sedentary time, and cardiovascular health in children. Methods: Participants were volunteers in a two-year randomized controlled dissemination trial of a whole school PA model (30 schools; 1,529 children). Activity intensity was measured via accelerometry (n=629), arterial compliance by applanation tonometry (n=250), and cardiovascular fitness (CVF) using a 20-meter shuttle run test (n=1,319). Results: In study 1, epoch length influenced the volume of activity recorded; however, the direction and magnitude of the bias depended on activity intensity and volume. In study 2, girls accumulated less moderate-to-vigorous PA (MVPA) and more sedentary time compared with boys, except during physical education. Fewer girls than boys met PA guidelines during school, recess, and lunch. Similarly few boys and girls (
Master's Student Supervision (2010 - 2018)
Introduction: Overweight children have greater bone mass than their healthy weight peers; however, they sustain more fractures. Thus, there is a need to better understand the relation between body fat and bone strength and aspects of bone quality such as bone microstructure that contribute to bone strength.Methods: I aimed to determine the cross-sectional relationship between fat mass (FM) and aspects of bone quality (strength, geometry, density and microstructure) at the distal radius and distal tibia in boys (n = 137, 15.6 ± 3.3 yrs on average) and girls (n = 157, 14.5 ± 3.9 yrs) in the context of the functional model of bone development (after adjusting for lean mass (LM)). FM and LM were measured using dual energy X-ray absorptiometry and bone quality was measured using high resolution-peripheral quantitative computed tomography.Results: In boys, FM negatively predicted bone strength at the radius but not at the tibia. Conversely, FM did not significantly predict bone strength in girls. In both boys and girls, FM negatively predicted total area at the radius but not the tibia. In girls but not boys, FM positively predicted cortical bone mineral density at the tibia but not the radius. For bone microstructure, FM did not significantly predict many variables; however, FM negatively predicted cortical thickness at the tibia in boys, trabecular thickness at the tibia in girls, and cortical porosity at both the radius and tibia in girls. In nearly all cases, LM mediated the relationship between FM and bone quality, whereby prior to adding LM to regression models FM positively predicted bone quality; however, after adjusting for LM the positive associations became non-significant or negative.Conclusions: The relation between fat and bone is complex and appears to be sex- and site-specific. My results also highlight the important influence of lean mass on bone strength. Longitudinal studies with larger cohorts and more overweight and obese participants would clarify the sex-specific muscle-fat-bone relationships during growth and into adulthood. The potentially hazardous influence of high levels of fat mass on child and youth bone health must be considered among the adverse consequences of overweight and obesity.