Planar covariation: a new key for the early detection of Duchenne muscular dystrophy walking behaviour

Three-dimensional gait analysis objectively quantifies lower-limb function and has been used to describe the gait pathomechanics in Duchenne muscular dystrophy (DMD) and to assess treatments. However, elevation angles, non-redundant sagittal-plane measures relative to the vertical axis, have not been examined in children with DMD, nor have gait differences between overground and treadmill walking been investigated in this population. This study investigated elevation angle profiles, planar covariation, spatiotemporal parameters, and joint kinematics in young children with DMD and typically developing (TD) children walking overground and on a treadmill. Kinematics were recorded using a Vicon motion capture system during self-selected overground and treadmill walking in twenty-five age-matched boys (fourteen with DMD; mean age 6.43 ± 1.45 years). Results showed that normalized step length and walking velocity were higher overground than on the treadmill in both groups. DMD children showed consistently wider normalized step width than TD peers. Joint kinematics did not distinguish groups across conditions, whereas elevation angles and planar covariation revealed significant overground differences in DMD children who exhibited reduced foot elevation angles bilaterally during terminal swing. They also showed smaller percent of variance on the first principal component (PV1) and larger percent of variance of the second principal component (PV2) values, with shorter, wider loops, suggesting weaker primary coordination and greater compensatory adjustments. These findings indicate that planar covariation can detect subtle gait alterations in young DMD children during overground walking, highlighting early compensatory coordination strategies not observable under treadmill pacing. Foot elevation angle modulation might serve as a sensitive marker for disease progression and treatment evaluation.