Mechanical and neural changes in plantar-flexor muscles after spinal cord injury in humans

Study design:Cross-sectional study.Objectives:To determine the effect of injury duration on plantar-flexor elastic properties in individuals with chronic spinal cord injury (SCI) and spasticity.Setting:National Rehabilitation Center for Persons with Disabilities, Japan.Methods:A total of 16 chronic SCI patients (age, 33±9.3 years; injury localization, C6-T12; injury duration, 11-371 months) participated. Spasticity of the ankle plantar-flexors was assessed using the Modified Ashworth Scale (MAS). The calf circumference and muscle thickness of the medial gastrocnemius (MG), lateral gastrocnemius and soleus were assessed using tape measure and ultrasonography. In addition, the ankle was rotated from 10° plantar-flexion to 20° dorsiflexion at 5 deg s-1 with a dynamometer, and the ankle angle and torque were recorded. After normalizing the data (the initial points of angle and torque were set to zero), we calculated the peak torque and energy. Furthermore, angle-torque data (before and after normalization) were fitted with a second-and fourth-order polynomial, and exponential (Sten-Knudsen) models, and stiffness indices (SI SOP, SI FOP, SI SK) and Angle SLACK (the angle at which plantar-flexor passive torque equals zero) were calculated. The stretch reflex gain and offset were determined from 0-10° dorsiflexion at 50, 90, 120 and 150 deg s-1. After logarithmic transformation, Pearson's correlation coefficients were calculated.Results:MAS, calf circumference, MG thickness, peak torque and SI FOP significantly decreased with injury duration (r log-log=-0.63,-0.69,-0.63,-0.53 and-0.55, respectively, P<0.05). The peak torque and SI FOP maintained significant relationships even after excluding impacts from muscle morphology.Conclusion:Plantar-flexor elasticity in chronic SCI patients decreased with increased injury duration.