Non-uniform in vivo deformations of the human Achilles tendon during walking

Abstract

The free Achilles tendon (AT) consists of distinct fascicles arising from each of the triceps surae muscles that may give rise to non-uniform behavior during functional tasks such as walking. Here, we estimated in vivo deformations of the human AT during walking using simultaneous ultrasound and motion capture measurements. Ten subjects walked at three speeds (0.75, 1.00, and 1.25 m/s) on a force-measuring treadmill. A custom orthotic secured a linear array transducer in two locations: (1) the distal lateral gastrocnemius muscle-tendon junction and (2) the free AT, on average centered 6 cm superior to calcaneal insertion. We used motion capture to record lower extremity kinematics and the position and orientation of the ultrasound transducer. A 2D ultrasound elastography algorithm tracked superficial and deep tissue displacements within the free AT. We estimated AT elongation (i.e., change in length) relative to the calcaneal insertion by transforming the orthotic, transducer, and calcaneus kinematics into a common reference frame. Superficial and deep regions of the free AT underwent significantly different longitudinal displacements and elongations during walking. For example, we found that the superficial AT exhibited 16-29% greater peak elongation than the deep AT during the stance phase of walking (p < 0.01). Moreover, superficial-deep AT tissue deformations became less uniform with faster walking speed (p < 0.01). Non-uniform deformations of the free AT, which could reflect inter-fascicle sliding, may enable the gastrocnemius and soleus muscles to transmit their forces independently while allowing unique kinematic behavior at the muscle fiber level.

Keywords: Elastography; Gait; Plantarflexor; Triceps surae; Ultrasound.

Copyright © 2014 Elsevier B.V. All rights reserved.

Figures

Figure 1

We used a custom orthotic and 2D ultrasound elastography speckle tracking method to track superficial and deep tissue displacements within the AT on average centered 6 cm superior to the posterior calcaneus marker on the right leg. Subjects walked at three speeds (0.75, 1.00, and 1.25 m/s) on a force-measuring treadmill.

Figure 2

Group mean (standard error) longitudinal displacements of superficial vs. deep regions of the free AT during walking plotted over an averaged gait cycle, from toe-off to toe-off. Negative values for longitudinal displacements indicate motion distal to the reference position at toe-off. Single asterisks (*) indicate superficial vs. deep significantly different in post-hoc pairwise comparisons (p

Figure 3

Group mean (standard error) calf muscle activities and elongation (i.e., change in length) of the free (superficial and deep) and aggregate AT plotted over an averaged gate cycle, from toe-off to toe-off. We estimated AT elongation as the change in length between the calcaneus marker and the average nodal positions or muscle-tendon junction.. Single asterisks (*) indicate superficial vs. deep free AT significantly different (p