Biplanar ultrasound investigation of in vivo Achilles tendon displacement non-uniformity

Abstract

The Achilles tendon is a common tendon for the medial and lateral gastrocnemius and soleus muscles. Non-uniform Achilles tendon regional displacements have been observed in vivo which may result from non-uniform muscle loading and intra-tendinous shearing. However, prior observations are limited to the sagittal plane. This study investigated Achilles tendon tissue displacement patterns during isometric plantarflexor contractions in the coronal and sagittal planes. Fourteen subjects (5 female, 9 male, 26±3 yr) performed maximal isometric plantarflexor contractions with the knee in full extension and flexed to 110°. An ultrasound transducer positioned over the free Achilles tendon collected beam formed radio frequency (RF) data at 70 frames/s. Localized tissue displacements were analyzed using a speckle tracking algorithm. We observed non-uniform Achilles tendon tissue displacements in both imaging planes. Knee joint posture had no significant effect on tissue displacement patterns in either imaging plane. The non-uniform Achilles tendon tissue displacements during loading may arise from the anatomical organization of the sub-tendons associated with the three heads of the triceps surae. The biplanar investigation suggests that greatest displacements are localized to tissue likely to belong to soleus sub-tendon. This study adds novel information with possible implications for muscle coordination, function and muscle-tendon injury mechanisms.

Keywords: speckle tracking; sub-tendon; triceps surae; ultrasound.

Figure 1.

Measurement set-up. Ultrasound probe was attached over the posterior (sagittal plane, black probe) or lateral (coronal plane, gray probe) aspect of the free Achilles tendon. Two load cells attached in the cables of the foot fixation device measured the force exerted by ankle plantarflexors.

Figure 2.

An example of the placement of tracking nodes from a single subject. The ultrasound image shown is from the beginning of the contractions. White dots indicate the locations of the nodes placed over this image. Black dots indicate the locations of the nodes at the instant of peak displacement. White dotted lines depict borders of the tendon. A gel pad and ultrasound gel were used in the sagittal plane between the probe and the skin (dark area in top of the image). In the coronal plane, only ultrasound gel was used.

Figure 3.

The Achilles tendon peak tissue displacements in different regions for sagittal and coronal plane imaging. Data obtained in extended knee posture are shown in blue and in flexed knee in red. Greatest displacement were observed in anterior (sagittal plane imaging) and in medial (coronal plane imaging) regions of the Achilles tendon irrespective for knee joint posture. Statistical differences between regions are not shown for clarity. Refer to the text for the statistical differences.

Figure 4.

Average force generation profiles of the isometric plantarflexor contractions performed in extended and flexed knee postures. Peak force generation (foot reaction force) in the ramped isometric contractions did not differ between trials performed in extended or flexed knee postures either in sagittal (left) or in coronal (right) plane imaging.