In vivo characterization of Achilles subtendon function and morphology within the tendon cross section and along the free tendon

Abstract

The Achilles tendon is composed of three distinct fascicle bundles, or subtendons, each originating from the head of one of the three triceps surae muscles. In a healthy tendon, these subtendons slide relative to each other during muscle contractions. This subtendon sliding is reduced in older adults and individuals who suffer from an Achilles tendon injury. However, subtendon sliding is challenging to quantify in low-load scenarios that are critical for monitoring subtendon biomechanics in patients with mechanically compromised tendons, such as following an Achilles tendon rupture and repair. The purpose of this study was to develop a reliable method to characterize subtendon behavior in vivo using combined transverse plane ultrasound imaging and neuromuscular electrical stimulation of individual gastrocnemii. We used a Kanade-Lucas-Tomasi point tracking algorithm to quantify tendon displacement during isolated muscle stimulations. Next, we applied k-means clustering to characterize heterogeneous subtendon behavior within the tendon cross section. The tendon cross section displayed differential displacement patterns depending on the stimulated muscle (P < 0.0001), and these displacements differed along the tendon length during lateral gastrocnemius stimulations (P = 0.004). These results reflect possible differences in load-sharing between adjacent subtendons and differing muscle-tendon dynamics among the triceps surae muscles. Finally, this method confirmed no bilateral differences in subtendon behavior and demonstrated high intersession reliability (intraclass correlation > 0.74). Overall, this study furthers our understanding of the differential muscle-tendon dynamics of individual Achilles subtendons within the tendon cross section and along the tendon length.NEW & NOTEWORTHY Achilles subtendon function and morphology are challenging to characterize in vivo. This study used transverse plane ultrasound imaging and neuromuscular electrical stimulation to characterize the behavior of individual subtendons both within the tendon cross section and along the free tendon. It is the first study to demonstrate functional behavior of the Achilles subtendons using these combined tools. In addition, the lack of bilateral differences in healthy individuals presents this tool's potential to quantify altered subtendon function post injury.

Keywords: Achilles subtendons; muscular stimulation; ultrasound.

Figure 1.

Experimental setup. A) Placement of NMES stimulation electrodes and EMG recording electrodes. B) Ultrasound imaging setup with ankle in a neutral position and transverse ultrasound imaging at five locations along the free tendon. Vector fields overlaid on ultrasound images of the Achilles tendon cross section demonstrate differing responses of the tendon to NMES along the free tendon in response to GL and GM stimulations. C) Point tracking and k-means clustering workflow. D) Example mean trajectories of points within the identified clusters. Displacement values relative to the previous frame rapidly increase upon stimulation onset and rapidly decrease after the stimulation ends and the tissue relaxes. Artwork from Biorender was used in this figure illustration.

Figure 2.

Mean direction of peak displacement of clusters during GM and GL stimulations. *p<0.05, **p<0.01, ***p<0.0001. Each data point represents a single leg from one subject. Artwork from Biorender was used in this figure illustration.

Figure 3.

Mean cumulative displacement of points during A) GL stimulations and B) GM stimulations. **p<0.01, ***p<0.001. Each data point represents a single leg from one subject.

Figure 4.

A) Medial-lateral position of the centroid of the most displaced cluster during GM and GL stimulations. B) Anterior-posterior position of the centroid of the most displaced cluster during GM and GL stimulations. *p<0.05, **p<0.01, ***p<0.001, ***p<0.0001. Each data point represents a single leg from one subject.

Figure 5.

Area of the cluster representing the most displaced region within the tendon cross section, represented as a fraction of the tendon cross sectional area, measured at five locations along the free tendon during both GM and GL stimulations. *p<0.05, **p<0.01. Each data point represents a single leg from one subject.

Figure 6.

Achilles tendon cross-sectional area, width, and thickness did not differ between right and left legs. Data shown in each plot represent images taken at all positions along the free tendon for all subjects. Lines connect data points from the imaging position from same subject.

Figure 7.

Measurements of cluster centroid positions and area fractions did not differ between right and left legs. Panels A, B, and C display bilateral comparisons of cluster medial-lateral position, anterior-posterior position, and cluster area fraction, respectively, during GL stimulations. Panels D, E, and F display bilateral comparisons of cluster medial-lateral position, anterior-posterior position, and cluster area fraction, respectively, during GM stimulations. Lines connect data points from the same subject.