Bone and muscle are interconnected tissues forming the ‘muscle-bone unit’. Sarcopenia and osteoporosis are common conditions in older adults;​ the characterisation of the relationship between muscle and bone is fundamental to the development of potential novel preventive and therapeutic strategies that benefit both tissues. This thesis presents data from two cohorts, the Hertfordshire cohort study (HCS) and Southampton Longitudinal Study of Ageing (SaLSA) that consider relationships between muscle and bone in later life.

Using existing data available from the Hertfordshire Cohort Study (HCS), the association between sarcopenia or/and osteoporosis with frailty was assessed. Co-existence of sarcopenia and osteoporosis were associated with a much higher risk of frailty than either condition alone, while sarcopenia and osteoporosis were both closely linked with multimorbidity. The overall prevalence of frailty was 8.1% with the risk increasing with age, while corresponding figures for pre-frailty were 57.5%, with the risk increasing with age only in females.

Furthermore, possible determinants, such as demographic and anthropometric, of muscle density were considered and the relationships of muscle density measures to the clinical outcomes of falls and fractures were reported. Demographic and anthropometric (female sex, older age, and lower adiposity), rather than lifestyle factors such as physical activity and diet, were associated with lower muscle density, approximately 11 years later. Forearm muscle density was associated with previous fracture, rather than falls history.

In addition, the relationships between other sarcopenia components including muscle size, strength and function were considered with the clinically important outcomes of falls and fractures in HCS. Observed relationships between muscle mass and strength but not function with falls and fractures were reported;​ sexual dimorphism was also described in the above-mentioned relationships.

Finally, we present results from a new community-based cohort of older adults in Southampton, SaLSA. Initially we have investigated the impact of the COVID19 pandemic on lifestyle factors associated with musculoskeletal health in older adults living in their own homes. Greater nutritional risk and sarcopenia risk were associated with being in a worse category for change in diet quality in SaLSA during the 1st year of the pandemic.

Investigating the factors leading to these changes, understanding whether they are reversible, and recognising the consequences to musculoskeletal health is required. Future work is described using this new cohort of older adults that will allow the investigation of muscle bone interrelationships in greater detail than has been previously possible. Given the interrelation between bone and muscle, future studies, such as SaLSA, might allow us to better understand muscle-bone crosstalk, with the aim of developing preventative strategies to retard or prevent deterioration of both tissues with age.