Ankle stiffness varies with medial gastrocnemius fascicle velocity

Ankle impedance plays a critical role in how we interact with our environment. Understanding the mechanisms underlying these interactions is especially important when joint function is impaired. Ankle impedance is proportional to ankle torque and muscle activation during postural control tasks; however, during dynamic tasks this relationship no longer holds. While muscles crossing the joint likely have a large contribution to joint impedance during dynamic tasks, their role is unknown. Therefore, we sought to determine how musculotendon kinematics vary with ankle impedance during dynamic tasks. Subjects completed different tasks where ankle angle and torque were systematically altered, resulting in concentric and eccentric contractions with varying torque demands. Our main findings indicated that during eccentric muscle contraction, ankle stiffness was inversely related to fascicle velocity. This study will ultimately provide further insight into the relationship between ankle impedance and musculotendon kinematics.