Forearm Muscle Oxidative Capacity And Biomechanical Property Testing As Clinical Tools For Rheumatoid Arthritis

Rheumatoid arthritis (RA) commonly impairs hand and wrist function. RA skeletal muscle is characterized by weakness, sarcopenia, and altered oxidative metabolism; however, clinical tools for assessment of RA upper extremity muscle dysfunction are understudied. PURPOSE: To identify if novel non-invasive testing of RA forearm muscle oxidative capacity and biomechanical properties associate with traditional clinical and physiologic parameters. METHODS: RA participants (n = 12; mean age = 59.5 ± 8.0) underwent assessments in a cross-sectional design. Forearm flexor muscle oxidative capacity was assessed via near-infrared spectroscopy (NIRS) post-exercise oxygen consumption recovery (k). Passive muscle tone (oscillation frequency, Hz), stiffness (N/m) and elasticity (inverse of logarithmic decrement) of unilateral forearm flexor musculature were assessed using mechanical deformation with myotonometry via the MyotonPro® device. Additional assessments included RA disease activity (DAS-28), BodPod® body composition, hand grip dynamometry (kg), and cardiorespiratory fitness via cardiopulmonary exercise testing (VO2 peak; mL/kg/min). Correlations were compared using Spearman’s rho. RESULTS: RA forearm flexor muscle oxidative capacity (mean k = 1.11 ± 0.32) was strongly correlated with RA disease activity (mean DAS-28 = 3.76 ± 1.1; rho = 0.61, p = 0.034). Tone (mean = 16.8 ± 2.1 Hz) was strongly correlated with lean body mass (mean = 55.9 ± 10.9%;rho = 0.83, p = 0.001). Stiffness (mean = 316.4 ± 65.8 N/m) was strongly correlated with grip strength (mean = 24.1 ± 9.6 kg; rho = 0.59, p = 0.042). Elasticity (mean decrement = 1.25 ± 0.22) was strongly correlated with cardiorespiratory fitness (mean VO2 peak = 21.1 ± 4.5 mL/kg/min; rho = 0.61, p = 0.034). CONCLUSIONS: Unexpectedly, greater forearm muscle oxidative function associated with greater RA disease activity, possibly reflecting local muscle metabolic alterations to compensate for more severe hand and wrist arthritis. Conversely, greater RA forearm muscle tone, stiffness, and elasticity associated with better systemic muscle quality. These findings highlight the clinical and integrative physiology research potential for longitudinal monitoring of RA skeletal muscle metabolic and biomechanical health with non-invasive assessments.