Objective: To study the etiology of pes plano valgus in rheumatoid arthritis (RA) patients using ultrasound (US) and electrophysiological methods. Patients and Methods: In a cross-sectional study, RA patients having pes plano valgus were investigated for etiology. Twenty patients with RA, mean±SD (range) disease duration of 10.93±7.66 years (2–25 years), and 20 control subjects were recruited. The mean±SD Disease Activity Score in 28 joints was 5.81±1.06. The role of tibialis posterior (TP) muscle tenosynovitis was investigated using power Doppler US scanning, as well as electrophysiological procedures to uncover the role of peripheral nerves and muscles. The findings were compared with that recorded in healthy volunteers with comparable age, sex and general somatic feature. Results: Statistically significant decrease of TP muscle activity was recorded in RA group compared to control group during maximum voluntary contraction (MVC) (P=0.024). Tibialis posterior tendon thickening, edema and power Doppler signals of inflammation were observed in the majority of RA patients. Conclusion: The study demonstrated, low TP muscle EMG activity of RA patients having PPV and US-confirmed TP tenosynovitis. Altered muscle function occurred in conjunction with moderate levels of tendon pathology, and active disease. Targeted therapy may be warranted to reduce pain, inflammation and mechanically off-load diseased tendon states.