Premature ageing in myositis: Impairments in physical function, muscle power and body composition.

OBJECTIVE: Myositis negatively impacts physical function, muscle power and body composition. However, the extent of these deficits relative to the general population remains unclear. Therefore, this study aimed to quantify physical impairments in individuals with myositis compared to healthy age-matched controls. METHODS: Two myositis cohorts (inclusion body myositis (IBM), n=22; non-IBM idiopathic inflammatory myopathies (IIM), n=32) were compared with an age- and sex-matched healthy cohort from The Copenhagen Sarcopenia Study (total n=1305, IBM-matched n=414, non-IBM IIM-matched n=1144). Physical function (30-s sit-to-stand (STS) performance), maximal leg extensor power, and body composition (via DEXA) were compared. RESULTS: Both myositis cohorts exhibited significantly reduced STS performance and leg muscle power compared to the healthy cohort (IBM ∼ -50 to -70%, p< 0.05; non-IBM IIM: ∼ -17 to -36%, p<0.05, respectively), corresponding to values in healthy individuals ∼15-30 years older. Further, males with IBM had significantly lower arm- and leg-specific muscle indices than the healthy cohort (∼ -17%, p<0.05; ∼ -12%, p<0.05, respectively), corresponding to levels in ∼15-20 years older healthy individuals. Females with non-IBM IIM demonstrated lower arm-specific muscle and fat indices (∼ -11%, p<0.05), corresponding to values in healthy individuals ∼45 years older. CONCLUSION: Both myositis cohorts exhibited significant impairments in physical function, maximal muscle power and muscle mass compared to an age-matched healthy cohort, reflecting substantial deterioration and muscle deficit. These findings underline the need for targeted, individualised and evidence-based interventions to support and improve the musculoskeletal health and functional status in individuals with myositis.