Effects of spinal cord injury on lower-limb passive joint moments revealed through a nonlinear viscoelastic model

We developed a mathematical model to describe the lower-limb passive joint moments to investigate and compare these moments in a small sample of able-bodied volunteers and individuals with long-standing motor complete paraplegia. Isokinetic tests, which were performed on a sample of four subjects with spinal cord injuries (SCIs) and five uninjured individuals, measured the passive moments at the ankle, knee, and hip joints throughout their ranges of motion in the sagittal and coronal planes. We fitted an 11-parameter nonlinear viscoelastic model to the acquired passive moment data (mean square error ranging from 0.020 to 5.1 Nm2) to compare subject populations and to determine the influences of joint velocity and passive coupling between adjacent joints. Although the passive moment curves of the SCI and able-bodied groups exhibited many similarities in shape, a repeated measures analysis of variance (ANOVA) that compared the passive moment curves of the two groups indicated a statistically significant (p < 0.01) difference for every joint except the knee. This new model for passive joint moments should prove to be useful in examining how changes in passive properties affect bipedal function and movement.