Non-uniform displacements within the Achilles tendon observed during passive and eccentric loading

Abstract

The goal of this study was to investigate Achilles tendon tissue displacement patterns under passive and eccentric loading conditions. Nine healthy young adults were positioned prone on an examination table with their foot secured to a rotating footplate aligned with the ankle. Subjects cyclically rotated their ankle over a 25° range of motion at 0.5 Hz. An inertial load geared to the footplate induced eccentric plantarflexor contractions with dorsiflexion. Passive cyclic ankle motion was also performed over the same angular range of motion. An ultrasound transducer positioned over the distal Achilles tendon was used to collect radiofrequency (RF) images at 70 frames/s. Two-dimensional ultrasound elastographic analysis of the RF data was used to track tendon tissue displacements throughout the cyclic motion. Non-uniform tissue displacement patterns were observed in all trials, with the deeper portions of the Achilles tendon consistently exhibiting larger displacements than the superficial tendon (average of 0.9-2.6mm larger). Relative to the passive condition, eccentric loading consistently induced smaller tissue displacements in all tendon regions, except for the superficial tendon in a flexed knee posture. Significantly greater overall tissue displacement was observed in a more extended knee posture (30°) relative to a flexed knee posture (90°). These spatial- and posture-dependent displacement patterns suggest that the tendon undergoes non-uniform deformation under in vivo loading conditions. Such behavior could reflect relative sliding between the distinct tendon fascicles that arise from the gastrocnemius and soleus muscles.

Keywords: Achilles tendon; Non-uniform motion; Ultrasound elastography.

Figure 1

An overview of the experimental setup and image analysis. Inertial disks were used to induce eccentric plantarflexor contractions when moving from a plantarflexed to dorsiflexed position. An ultrasound transducer placed over the distal Achilles tendon was used to collect RF data throughout the cyclic trials. In post-hoc analysis, initial nodal positions were defined within the tendon from an image collected in the most plantarflexed position. Two-dimensional elastography was then used to track the subsequent motion of nodes located in superficial, mid and deep portions of the tendon.

Figure 2

Average (±1 s.d.) temporal patterns of the plantarflexion angle and internal ankle moment over an eccentric task motion cycle, with 0% corresponding to the most plantarflexed position. Ankle range of motion and peak ankle moment were not significantly different when performed with 30 and 90 deg of knee flexion.

Figure 3

The average (across 9 subjects) peak nodal displacements (negative corresponds with distal) at different tissue depths for passive (Pass) and eccentric (Ecc) loading tasks. Larger tissue displacements were observed when the knee was extended (Ex; 30 deg) compared with when the knee was flexed (Fl; 90 deg).

Figure 4

Average (+1 s.d.) peak tissue displacements (negative corresponds with distal) when moving from plantarflexed to dorsiflexed positions. Knee posture (Fl: flexed, Ex: extended) and loading conditions significantly affected tissue displacements in all layers, except for the superficial tendon which exhibited similar displacements under both loads when the knee was flexed. *p < 0.05.

Figure 5

Superficial, mid and deep tissue displacements, relative to the average ROI displacements for each condition. Depth-dependent differences in relative tissue displacements are evident for each of the tasks, though differences between the mid and deep layers are only significant in the passive flexed knee trial. *p < 0.05.