Background/Purpose: Physical activity monitors (PAM) are increasing being used to objectively quantify free-living movement in clinical research, especially musculoskeletal diseases. Given that patients with idiopathic inflammatory myopathies (IIM) have proximal muscle weakness which affects mobility, PAM may be a helpful tool for clinical assessment. We examined the validity, reliability and responsiveness of the activity measures obtained from a research grade PAM, Actigraph, as an outcome measures in IIM.

Methods: The 6 validated myositis core set measures (CSM; manual muscle testing [MMT], physician global disease activity [MD global], patient global disease activity [Pt. global], extra-muscular global disease activity [Ex-muscular global], HAQ and muscle enzymes) and 3 functional measures (six-minute walk [6MWD], timed up-and-go [TUG] and sit-to-stand tests [STS]) and Patient-Reported Outcome (PRO) Measurement Information System physical function short form (PROMIS-PF) were completed on outpatients with IIM [DM, PM, necrotizing myopathy (NM) or anti-synthetase syndrome] at baseline, 3 and 6 months. Patient were instructed to wear, Actigraph, a PAM device, on the waist for 7 consecutive days monthly for 6 months, and PAM measures including average of daily step count (controlled for length of time the device was word) and average of maximum step counts in a minute (surrogate measure for speed of walking), were assessed. Pearson correlations were used for cross-sectional analysis with rho >0.5 considered strong, 0.35-0.5 moderate, and 0.2-0.35 weak correlation. We examined the test-retest reliability as well as responsiveness. Mixed liner model was used for longitudinal analysis and validation against all CSMs.

Results: Fifty patients [mean age, 53.6 (±14.6); 30 females/20 males] were studied. Eleven of 50 had PM, 27 had DM, 7 had NM and 4 had anti-synthetase syndrome. Cross-sectional results at baseline showed Actigraph maximum steps per minute had moderate to strong correlation with MD global, MMT, Pt-global, HAQ and Ex-muscular global, whereas average daily steps showed moderate to strong correlation with MD global, Pt-global and HAQ. Both measures of actigraph showed significant longitudinal association with MD global, MMT, pt. global and HAQ demonstrating construct validity for the measure, whereas muscle enzyme and Ex-muscular global didn’t show significant results (Table 1). There was significant association of both Actigraph measures with functional tests and patient reported outcomes (Table 1). Both measures increased significantly in patients with improvement and remained stable for patients with stable disease over 0, 3, and 6 months, demonstrating responsiveness and test-retest reliability of the measures.

Conclusion: Actigraph’s average daily steps and maximum steps per minute both showed good construct validity, reliable and responsive longitudinally.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/physical-activity-monitors-new-tool-to-assess-improvement-in-myositis/