Kinematic adaptation of locomotor pattern in rheumatoid arthritis patients with forefoot impairment

Rheumatoid arthritis (RA) is a leading cause of disability, which affects primarily the forefoot. Moreover, the forefoot is the final ground body interface for transmitting forces produced by the plantar flexors in order to move the body forward. Therefore, a dysfunction in patients with arthritis might induce important changes in gait, such as modifications in the coordination between legs to correct a reduced range of motion (ROM) and to produce smooth stride motions. First, we wanted to investigate the modifications of gait parameters in order to get a deeper understanding of the locomotor adaptation after a distal joint impairment. Second, we wanted to extract the mechanisms used to compensate for these impairments. In order to carry out this study, RA patients with forefoot impairment and healthy subjects were asked to walk along a straight line at two different velocities and were recorded by a motion analysis system. Patients were able to produce an efficient pattern despite a reduction of the ROM of the forefoot. At normal speed, the substantial modification of the locomotor pattern was linked to the adaptation of the lower-limb segment coordination and to the loss of ROM. Compensative mechanisms are the results of an efficient adaptation that offset the effect of the lesions. In contrast, at high speed, all of the kinematic modifications observed at natural speed vanished. It seems that pain and its associated sensory signals help to update the motor command and compel patients to adjust the descending command to the altered representation of distal mobility. Finally, the mechanical consequences of these changes are of particular interest since different levels of force exerted at the hip, knee and ankle might result in a supplementary structural alteration of these joints.