Tibialis Posterior (TP) pain and dysfunction, among patients with rheumatoid arthritis (RA), has previously been reported with as high prevalence of 64 % . RA is a chronic polyarticular autoimmune disease that frequently affects the joints and soft tissues of the feet. Foot pain and deformities such as pes planus valgus are common among patients with RA. Because of the mutual prevalence of both aspects, there are multiple theories linking pes planus and TP dysfunction together . However, it is uncertain to which extent pain is the leading cause of the biomechanical alterations observed. Usually patients with RA presents a number of cofounding factors, such as synovitis, deformity and swollen joints that per se can affect gait patterns. Therefore, experimental pain models applied in healthy subjects aims to investigate the effect of pain per se in an, otherwise, healthy system. The aim of this study was to examine how experimental TP muscle pain affect gait pattern in healthy subjects.
Twelve healthy subjects were recruited for this study mean ± SD. Experimental pain was induced by injection of hypertonic (isotonic used as non-painful control) saline into the right TP muscle. Injections were randomised and performed using ultrasound-guidance. Kinematic data during self-paced gait was collected by a Qualisys system (Oqus 300, Qualisys, Sweden). Ground reaction forces and moments were collected at 1000 Hz from force plates installed in the floor (AMTI, USA). Joint moments, and joint forces (Lund et al musculoskeletal model)  were estimated before and during injections. Visual analogue pain scale (VAS) from 0-10 was collected after each trial. Paired t-test and ANOVA with repeated measures was used to analyse data.
The Hypertonic condition’s peak plantar flexion moment was lower compared to all other conditions (P=0.002), and the peak knee medial condyle compression force was lower compared with the other conditions (P= 0.001) (Fig. 1). The self-reported VAS pain after injections was significant lower for the Isotonic condition (1.88±1.4) compared to the hypertonic condition (5.85±1.7), t(11) =2.2, p<0.001.
This study gives insight into the gait antalgic pattern used by subjects with TP muscle pain. The peak plantarflexion moment and peak knee medial compressive was reduced in the hypertonic condition, suggesting a compensation strategy to reduce the joint load in both ankle and knee joint.
The Danish Rheumatism Association financially supported this study.