Musculo-Tendinous Changes in Spastic Plantar-Flexor Muscles After Spinal Cord Injury

To identify the underlying mechanisms of time-course stiffness reduction in plantar-flexor muscles after chronic spinal cord injury (SCI) with spasticity.

Cross-sectional study.

Research laboratory.

Volunteer sample of 24 participants (N=24), including 14 participants with SCI and 10 participants without SCI.

The participants' ankles were rotated from 10° plantar flexion (PF) to 20° dorsiflexion by a dynamometer at 5°/s with the hip and the knee fixed at right angles. PF torque, ankle angle, and ultrasound images of the gastrocnemius (gas) muscle belly and muscle tendon junction (MTJ) were recorded. Changes in muscle fascicle length (MFL), pennation angle, and excursions of the MTJ were obtained.

Stiffness indices (SI) were calculated for the whole plantar-flexor muscles (SI whole PF), outer tendon (SI tendon), proximal portion of MTJ (SI MTJ), and muscle fascicle (SI MFL). In addition, calf circumference and medial gastrocnemius (MG) muscle thickness were evaluated.

SI whole PF, SI MTJ, and SI MFL were negatively correlated with injury duration, and time constants of the changes were 92.5, 36.4, and 85.4 months (∼8, 3, and 7y, respectively). In terms of the partial correlation coefficients controlled, the morphologic variables (calf circumference and MG), age, and injury localization, only 2 parameters (SI whole PF and SI MTJ) retained significance.

These results indicated that a reduction in the elasticity of plantar-flexor muscles in spastic SCI is partly explained by alterations of tissues in the proximal portion of the gas MTJ (aponeurosis and/or connective tissues), especially in the first several years of the chronic stage.