Simultaneous Quantification of Ankle, Muscle, and Tendon Impedance in Humans

Abstract

Objective: Regulating the impedance of our joints is essential for the effective control of posture and movement. The impedance of a joint is governed mainly by the mechanical properties of the muscle-tendon units spanning it. Many studies have quantified the net impedance of joints but not the specific contributions from the muscles and tendons. The inability to quantify both muscle and tendon impedance limits the ability to determine the causes underlying altered movement control associated with aging, neuromuscular injury, and other conditions that have different effects on muscle and tendon properties. Therefore, we developed a technique to quantify joint, muscle, and tendon impedance simultaneously and evaluated this technique at the human ankle. Methods: We used a single degree of freedom actuator to deliver pseudorandom rotations to the ankle while measuring the corresponding torques. We simultaneously measured the displacement of the medial gastrocnemius muscletendon junction with B-mode ultrasound. From these experimental measurements, we were able to estimate ankle, muscle, and tendon impedance using non-parametric system identification. Results: We validated our estimates by comparing them to previously reported muscle and tendon stiffness, the position-dependent component of impedance, to demonstrate that our technique generates reliable estimates of these properties. Conclusion: Our approach can be used to clarify the respective contributions from the muscle and tendon to the net mechanics of a joint. Significance: This is a critical step forward in the ultimate goal of understanding how muscles and tendons govern ankle impedance during posture and movement.

The nervous system leverages the non-linear properties of the Achilles tendon to regulate ankle impedance during postural control

“…Muscle short-range stiffness describes the initial response to small, fast displacements prior to reflexive or volitional muscle activation and is critical in the control of posture and limb stability [35,36]. This finding is consistent with our previous results that demonstrated that our muscle stiffness estimates are similar to measurements of muscle short-range stiffness scaled to the triceps surae [20]. We do note, however, that if the stretch within the muscle or tendon was larger or slower, we would expect to observe more complex viscoelastic behavior.…”

“…We refer to the relationship between MTJ displacement and the angular rotations of the ankle as the translation ratio. Specifically, to characterize ankle, muscle, and tendon impedance, we estimated ankle impedance and the translation ratio, and used these quantities to compute muscle and tendon impedance [20]. We previously demonstrated that the magnitude of the frequency response functions was nearly constant from 1 to 3 Hz, indicating that stiffness was the dominant contributor to impedance over this frequency range [20].…”

“…Specifically, to characterize ankle, muscle, and tendon impedance, we estimated ankle impedance and the translation ratio, and used these quantities to compute muscle and tendon impedance [20]. We previously demonstrated that the magnitude of the frequency response functions was nearly constant from 1 to 3 Hz, indicating that stiffness was the dominant contributor to impedance over this frequency range [20]. As such, we computed the stiffness component of ankle, muscle, and tendon impedance by averaging the magnitude of the respective frequency response functions from 1 to 3 Hz.…”

“…We evaluated the sensitivity of ankle stiffness to changes in muscle and tendon stiffness at different levels of force. We first consider that ankle stiffness (KA) is determined by the serial connection of the muscle and tendon and can be described as a function of these stiffnesses [20],…”

“…The objective of this study was to determine how the triceps surae and Achilles tendon contribute to the impedance of the ankle during conditions relevant to postural control. We used an innovative technique that combines joint-level perturbations with B-mode ultrasound to quantify ankle, muscle, and tendon impedance [20]. Given the limited and conflicting experimental data reported in the literature, we tested the null hypothesis that the muscle and tendon contribute equally to ankle impedance to determine which structure was most dominant over contraction levels ranging from 0 to 30% MVC.…”