Chapter 9Skeletal health

Chapter overview

• Introduction

• Biology of bone

• Osteoporosis: definition, prevalence and consequences

• Physical activity and bone strength

• Physical activity and fracture risk

• Physical activity in prevention and management of osteoporosis

• Physical activity and osteoarthritis

• Summary

Structure of a long bone

• an average adult has 10–12 kg of bone;

• bone offers ‘strength with lightness’;

• cortical bone – dense, ivory-like;

• trabecular bone – lattice of thin, calcified struts

• membrane, the periosteum, covers surface of cortical bone.

Remodelling in trabecular bone

• Osteoclasts resorb (digest) old bone;

• osteoblasts fill this cavity with new bone;

• new bone undergoes remineralization.

Measuring the structural properties of bone

• Dual energy X-ray absorptiometry (DXA) – measures bone mineral content and bone mineral density (BMD).

• Quantitative ultrasound – measurements reflect bone architecture as well as bone mineral.

• Quantitative computed tomography – measures bone mineral content, BMD and axial cross-sectional area.

Changes in bone mineral density over the life-span

Adaptation to load bearing

• Bone is deposited according to the load it must bear;

• strains produced during loading stimulate an adaptive, osteogenic response;

• response is determined by the magnitude, rate and distribution of strains, as well as the number of repetitions (strain cycles);

• immobilization and space flight both lead to net bone loss.

Osteoporosis

A skeletal disorder characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in fragility and susceptibility to fracture.

Osteoporotic changes in lumbar vertebrae

Normal, good weight-bearing Osteoporotic, loss of weight-bearing competence

What determines likelihood of osteoporosis or osteopenia?

• Peak bone mass as a young adult

– childhood and adolescence therefore a ‘window of opportunity’;

• rate of bone loss experienced with ageing

– dietary factors

– physical activity level;

• genetic variability.

Side-to-side differences in humerus of female tennis and squash players

Effect of 18 months of high-impact training in women aged 35–45

RCT of high-impact training in postmenopausal women

Clinical endpoint, hip fracture – Nurses’ Health Study (2002)

• 55% lower risk in postmenopausal women reporting > 24 MET-h per week, compared with < 3 MET-h per week;

• risk was 6% less for each increase in activity of 3 MET-h per week;

• in women who did no other exercise, walking for at least four hours per week was associated with a 41% lower risk than among those who walked for less than one hour per week.

Summary I

• Functional loading is the most important influence on bone remodelling.

• Strain rate and an unusual strain distribution largely determine its osteogenic effects.

• Bone mass increases during growth and reaches a peak towards the end of the second decade.

• Age related loss of bone can lead to osteopenia and osteoporosis, compromising strength and increasing the risk of fracture.

Summary II

• In premenopausal women, the effect of exercise is mainly conservation of bone. In older women it is to reduce the rate of loss.

• Physically active women have a lower risk of osteoporotic fracture of the hip and maybe of the spine.

• Regular exercise may decrease the risk of fall-related fractures.

• Moderate amounts and intensities of exercise have a favourable effect on pain and function in osteoarthritis of the knee, but sports involving high-intensity impacts or torsional types of stress increase risk.