Date of Award

2015

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Doctor of Philosophy

School

School of Exercise and Health Sciences

Faculty

Faculty of Health, Engineering and Science

First Supervisor

Professor Robert U. Newton

Second Supervisor

Dr Sophia Nimphius

Third Supervisor

Jason Weber

Abstract

A paucity of research exists to characterise and investigate lower-body musculoskeletal characteristics and morphological adaptations in elite Australian Footballers with the aim to improve screening, monitoring and load management practices. Given the high prevalence of lower-body skeletal injuries in Australian Football; and the ability to measure, modify and train muscle and bone strength and their derivatives; this project served to extend scientific understanding of musculoskeletal morphology and bone strength characteristics in elite level field-based team sport athletes through a series of research studies using Dual-energy X-ray Absorptiometry (DXA) and peripheral Quantitative Computed Tomography (pQCT). In particular, studies one and two provided normative and comparative lower-body musculoskeletal profiles of elite Australian Footballers, stratified by training age (exposure), limb function (asymmetry) and injury incidence (stress fracture), while study three quantified the morphological changes and magnitude of adaptation and maladaptation experienced by Australian Footballers following an in-season and off-season annual phase. The general conclusion provided by the collective studies of this thesis promotes the importance of bone structure and geometry as potent contributors to skeletal robustness, and bone strength. Athletes with higher levels of training exposure and greater physical resilience exhibited higher tibial mass and cortical density with thicker cortical walls and larger muscle and bone cross-sectional areas. Asymmetrical adaptations from differential loading patterns between limbs through-out an in-season and off-season generate vastly different unilateral load tolerance capabilities when extrapolated overtime. The high-impact gravitational loads experienced by the support limb appear to optimise the development of robust skeletal properties specific to bone structure and geometry which may serve as a loading model to prophylactically enhance bilateral musculoskeletal strength and resilience.

Study one provided a set of normative and comparative lower-body musculoskeletal values to describe and compare muscle and bone morphology between less experienced and more experienced athletes (training age); and differential loading patterns between the kicking and support limbs (limb function). Fifty-five athletes were stratified into less experienced (≤ 3 years; n = 27) and more experienced (> 3 years; n = 28) groups in accordance with their training age. All athletes underwent whole-body DXA scans and lower-body pQCT tibial scans on the kicking and support limbs respectively. More experienced players exhibited greater tibial mass, trabecular vBMD, cortical vBMD and total vBMD (p < 0.009; d ≥ 0.79); greater cortical thickness and cortical area (p < 0.001; d ≥ 0.92), and larger stress-strain indices and absolute fracture loads (p ≤ 0.018; d ≥ 0.57) than less experienced players. More experienced players also exhibited greater muscle mass and muscle cross-sectional area (p ≤ 0.016; d ≥ 0.68). Differences were also observed between limbs, with greater material (tibial mass and cortical vBMD), structural (trabecular area, cortical area, total area, periosteal area and cortical thickness) and strength (stress-strain index and absolute fracture load) characteristics evident in the support leg comparative to the kicking leg of more experienced players (d ≥ 0.20); with significantly higher asymmetries in tibial mass and cross-sectional area evident in more experienced players than less experienced players as a product of limb function over time. The findings of this study illustrate that training exposure and continued participation in Australian Football produced greater lower-body material, structural and strength adaptations; with chronic exposure to asymmetrical loading patterns developing differential morphological changes between the kicking and support

Study two provided a retrospective and comparative set of lower-body musculoskeletal data to describe and compare muscle and bone morphology between injured and non-injured Australian Football athletes, in addition to injured and non-injured limbs within injured players, in order to identify musculoskeletal characteristics which may predispose athletes to stress fractures or highlight skeletal fragility. Fifty-five athletes were stratified into injured (n = 13) and non-injured (n = 42) groups. All athletes underwent whole-body DXA scans and lower-body pQCT tibial scans across both limbs. Injured players exhibited lower tibial mass (p ≤ 0.019; d ≥ 0.68), cortical vBMD (d ≥ 0.38) and marrow vBMD (d ≥ 0.21); smaller cortical area and periosteal area (p ≤ 0.039; d ≥ 0.63); smaller trabecular area, marrow area, total area, endocortical area and cortical thickness (d ≥ 0.22); lower stress-strain indices, absolute fracture loads and relative fracture loads (support leg: p ≤ 0.043; d ≥ 0.70, kicking leg: d ≥ 0.48) than non-injured players. Injured players also exhibited lower muscle cross-sectional area and muscle mass (p ≤ 0.034; d ≥ 0.79), yet higher muscle density (d ≥ 0.28) than non-injured players. Differences between injured and non-injured limbs internal to injured players were also observed, with lower material (tibial mass and total vBMD), structural (cortical area and cortical thickness) and strength (stress-strain index and relative fracture load) in the injured limb comparative to the non-injured limb (d = 0.20 – 0.70). Muscle density was lower in the injured limb (d = 0.54). The findings of this study illustrate a general inferiority and global musculoskeletal weakness in injured players, with non-injured players ~10-12% stronger across both limbs. Injured players were skeletally slender with smaller muscle and bone cross-sectional areas and thinner cortices. Similarly, injured limbs of injured players also exhibited smaller structural proportions, highlighting the importance of cortical area and cortical thickness as key structural and geometric skeletal properties with potent contributions to bone strength and resilience. limbs. Indeed, routine high-impact, gravitational load afforded to the support limb preferentially improves bone structure and geometry (cross sectional area and thickness) as potent contributors to bone strength and skeletal fatigue resistance.

Study three provided a seasonal investigation into lower-body musculoskeletal adaptations over the course of a ~26 week in-season and ~10 week off-season period in Australian Football. Forty athletes (n = 40) and twenty-two athletes (n = 22) were recruited to quantify morphological changes in muscle and bone following the in-season and off-season periods respectively. All athletes underwent whole-body DXA scans and lower-body pQCT tibial scans for the kicking and support limbs at the commencement and conclusion of each season. Australian Football athletes exhibited increases in trabecular vBMD, total vBMD and cortical thickness in the kicking leg; with increased cortical vBMD, total vBMD, trabecular area, total area, periosteal area, cortical thickness and reduced endocortical area in the support leg following the in-season period. Percent changes between limbs were significantly different for trabecular vBMD, cortical vBMD, total vBMD and trabecular area (p ≤ 0.049; d ≥ 0.46), despite similar increments in bone strength (~44 – 50 N), demonstrating asymmetrical morphological responses to differential loading patterns in-season. Conversely, Australian Football athletes exhibited material decreases in tibial mass, trabecular vBMD, cortical vBMD and total vBMD in both limbs over the off-season by similar yet opposite magnitudes to the benefits accrued during the in-season, in addition to reduced muscle area, highlighting a general musculoskeletal de-training effect. Structural adaptations were mostly maintained or increased for both limbs over the off-season, with bone strength completely reversed in the kicking leg, yet wholly preserved in the support leg; a lasting adaptation from regular high-impact, gravitational loading specific to the support leg. The findings of this study illustrate the osteogenic potential of a ~26 week in-season, and the de-training potential of a ~10 week off-season. Specifically, the kicking and support limbs continued to show asymmetrical morphological adaptations to differential in-season and off-season loading and de-loading patterns.

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