Chapter 17: Osteoporosis—physical activity for bone health

INTRODUCTION

A recurrent theme in this volume of Clinical Sports Medicine is the irrefutable data that regular physical activity prevents a host of chronic diseases.^{1, 2} With respect to bone, various international position statements spell out that weight-bearing physical activity is essential to build a strong and healthy skeleton during childhood, maintain bone mass and strength during adult years, and mitigate skeletal decline as we age.^{1, 3, 4}

In this chapter we:

• summarise the key evidence demonstrating that physical activity promotes bone health

• discuss how to maximise bone health during maturation

• discuss the prevention of bone mineral loss and osteoporosis in adults

• highlight contemporary non-pharmacological and pharmacological approaches for treating bone loss across the age spectrum.

KEY EVIDENCE—HOW BONE ADAPTS TO PHYSICAL ACTIVITY AND EXERCISE

Bone has a remarkable ability to adapt its mass, structure and strength to match functional demands including those associated with weight-bearing activity. Functional adaptation of bone is determined by strain (deformation of bone tissue), with muscle forces imposing the largest strain on the skeletal system during voluntary activities.^{5, 6}

A ‘mechanostat’ feedback system keeps bone strains within a ‘physiological window’, by adjusting bone mass and structure. The key load-sensing cells in bone are osteocytes, and they react strongly to changes in their environment to initiate key cell signalling pathways involved in mechanotransduction (the process by which bone converts a biophysical force into a cellular response). Osteocytes accommodate (switch off) when receiving steady-state signals.⁷ Thus, in the absence of varied stimuli, bone cells desensitise to loading.

These principles of bone adaptation to loading are summarised in the late Professor Charles Turner’s three rules for bone adaptation to mechanical stimuli:

• bone adaptation is driven by dynamic rather than static loads (i.e. cyclic not continuous)

• only a short duration of loading is needed to initiate bone adaptation

• bone cells accommodate and become less responsive to routine loading.

These three rules should guide exercise prescription that targets bone health across the lifespan.⁸ If we use these three rules within the FITT principles (Chapters 3 and 4), an effective exercise program for bone health will have the following features:

• Frequency—Short, frequent bouts of weight-bearing activity are effective. This refers to daily, or multiple times daily, load (physical activity or exercise).

• Intensity—High-impact exercises (or high strain from muscle on bone) promote bone adaptation better than low-intensity activities.

• Time—Short bouts (5–10 min or less) separated by rest periods are more effective than a continuous bout with the same amount of total load.

• Type—Activities that include multidirectional and irregular loading patterns (‘surprises’) are more effective ...