Changes in Passive Mechanical Properties of the Gastrocnemius Muscle at the Muscle Fascicle and Joint Levels in Stroke Survivors

(A) Experimental setup. The knee-ankle evaluation device consists of 2 motors and a linkage between. The JR3 force/torque sensors were mounted on the motor shaft at both joints to measure the joint torques/forces. With the knee flexion axis aligned with the knee motor, the ankle motor can be adjusted along the leg linkage to align it with the ankle flexion axis. (B) Longitudinal ultrasonic images of the medial gastrocnemius muscle at rest. The skin is on the top of the image, and the left side corresponds to proximal. The muscle tendon junction represented the musculo-tendon (muscle aponeurosis) junction. α and β are the posterior and anterior pennation angles, respectively. The medial gastrocnemius muscle tendon junction was taken as the distal reference point.

Passive stiffness at the ankle joint for both healthy subjects (dark line) and patients poststroke (gray line) at 4 knee positions: (A) full knee extension, (B) 30° knee flexion, (C) 60° knee flexion, and (D) 90° knee flexion. ^*Significant differences between the 2 populations with P<.05 (t test). Abbreviations: DF, dorsiflexion; PF, plantarflexion.

Muscle fiber lengths of both healthy subjects (dark line) and stroke survivors (gray line) at 4 knee positions: (A) full knee extension, (B) 30° knee flexion, (C) 60° knee flexion, and (D) 90° knee flexion. ^*Significant differences between the 2 populations with P<.05 (t test). Abbreviations: DF, dorsiflexion; PF, plantar-flexion.

The relationship between the medial gastrocnemius fascicle length and the passive gastrocnemius force (thick line) and the medial gastrocnemius fascicle force (gastrocnemius force × 0.610 / cos(θ_pennation); thin line) for the stroke and control groups (the group average with 1-sided SE was shown for both fascicle length and muscle/fascicle force). The medial gastrocnemius fascicle length was determined at full knee extension with ankle between 20° plantar flexion and 15° dorsiflexion.

Anterior pennation angles of both healthy subjects (dark line) and stroke survivors (gray line) at 4 knee positions: (A) full knee extension, (B) 30° knee flexion, (C) 60° knee flexion, and (D) 90° knee flexion. ^*Significant differences between the 2 populations. P<.05 (t test). Abbreviations: DF, dorsiflexion; PF, plantarflexion.