Two simple, reliable and valid tests of proximal muscle function, and their application to the management of idiopathic inflammatory myositis

Abstract

To develop objective, isotonic, reliable and valid tests of upper (UL) and lower (LL) limb proximal muscle function for use in the management of idiopathic inflammatory myositis (IIM). The '1 kg arm lift' test was devised to assess UL function and the '30 s chair stand' test was used for LL function. The tests were performed in 30 control subjects to determine short (24 h) and long (2 week) interval test-retest reliability. Thirty-two patients with IIM were assessed over a period of 2 yr. In the control group both tests showed excellent test-retest reliability; Spearman correlation >0.8 for both tests over both time intervals. Twenty-four of the 32 IIM patients remained in remission over a mean period of 20.7 months throughout which scores varied by <5 for the '1 kg arm lift' test and <3 for the '30 s chair stand' test from each patient's mean score. Eight patients relapsed with scores falling by >5 for the '1 kg arm lift' test and by >3 for the '30 s chair stand' test from the mean remission score in each case. Both UL and LL scores correlated inversely with serum creatinine kinase. Two isotonic tests of proximal muscle function are described. They exhibit excellent test-retest reliability and demonstrate construct validity in IIM. Both tests are responsive to changes in disease activity, offer physiological and practical advantages over existing tests of muscle function and are suitable for use in clinical practice.