Hand grip endurance moderating the effect of grip force on functional ability and disease activity in rheumatoid arthritis patients: a cross-sectional study.

To examine the effect of endurance on the relationship between grip force and measures of functional capacity and disease activity, we performed a cross-sectional study at the University Department of Rheumatology, Physical medicine and Rehabilitation from January 2017 to August 2018. Functional capacity of the hand was measured by ABILHAND-RA questionnaire and disease activity was assessed by the Disease Activity Score (DAS-28-CRP). All participants underwent dynamometric measurements of maximal grip force and hand grip endurance during repeated gripping. We analyzed the data from 34 RA patients at the median (IQR) age of 57 (51-61), 31 (91%) of them women, and 44 healthy participants at the age of 55 (50-59), 39 (89%) of them women. The moderating effect of endurance on the correlation between maximum grip force and the ABILHAND-RA score was not significant in healthy participants (b = 0.000, 95% CI - 0.005-0.004, p = 0.862), but it was in RA patients (b = 0.003, 95% CI 0.000-0.005, p = 0.027). In RA patients, the effect of maximum grip force on the ABILHAND-RA score increased with the increase in hand grip endurance. In RA patients, the interaction between endurance and grip force significantly explained the 15% more variance of the disease activity than main effects of these two measures, age, gender and body mass index alone. Hand grip endurance during repeated gripping affects the correlation between maximum grip force and the ABILHAND-RA score in a pattern that differs in RA patients and in the healthy population. In RA patients, hand grip endurance significantly moderates the correlation between maximum grip force and the DAS-28-CRP.