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.

Depth-dependent variations in Achilles tendon deformations with age are associated with reduced plantarflexor performance during walking

“…Haraldsson et al (12) suggested that sliding between AT fascicles may allow them to behave as functionally independent structures. Consistent with these observations, we recently discovered in young adults that tendon fascicles presumably arising from the medial gastrocnemius undergo ϳ30% greater peak elongation than those from the soleus during walking (9). These depth-dependent differences became larger with faster walking speed.…”

“…Based on in vivo ultrasound measurements, we recently proposed that sliding between AT fascicle bundles may allow for relatively independent muscle fiber behavior in the triceps surae muscles. This in turn may promote the muscle-tendon coordination necessary for effective force or power generation during pushoff (9). Age-related changes in AT behavior may compromise sliding between adjacent tendon fascicles (36) and thereby alter the coordination of gastrocnemius and soleus muscle actions in walking.…”

Human ankle plantar flexor muscle–tendon mechanics and energetics during maximum acceleration sprinting

“…The influence of this design on muscle fascicle and tendon energetics remains an area of future research. We note here, however, that recent in vivo studies on non-uniform Achilles tendon deformation have observed that tendon fascicles arising from the SO and gastrocnemius may function as independent structures during walking [67,68].…”

Is human Achilles tendon deformation greater in regions where cross-sectional area is smaller?

“…This highlights the complexity of loading on the Achilles tendon with contributions from three independent muscles, which have been suggested to constitute mechanically separate tendon compartments within the Achilles (Bojsen-Moller and Magnusson, 2015). Indeed, ultrasound speckle tracking methods have shown greater elongation within the deeper region of the Achilles tendon compared with the superficial layer during loading (Arndt et al, 2012;Franz et al, 2015;Slane and Thelen, 2015). This may suggest the presence of inter-fascicle sliding and a relative independence of gastrocnemius and soleus tendon components functioning within the Achilles tendon.…”