Three-dimensional deformation and transverse rotation of the human free Achilles tendon in vivo during isometric plantarflexion contraction

Abstract

Freehand three-dimensional ultrasound (3DUS) was used to investigate longitudinal and biaxial transverse deformation and rotation of the free Achilles tendon in vivo during a voluntary submaximal isometric muscle contraction. Participants (n = 8) were scanned at rest and during a 70% maximal voluntary isometric contraction (MVIC) of the plantarflexors. Ultrasound images were manually digitized to render a 3D reconstruction of the free Achilles tendon for the computation of tendon length, volume, cross-sectional area (CSA), mediolateral diameter (MLD), anteroposterior diameter (APD), and transverse rotation. Tendon longitudinal and transverse (CSA, APD, and MLD) deformation and strain at 70% MVIC were calculated relative to the resting condition. There was a significant main effect of contraction on tendon length and mean CSA, MLD, and APD (P < 0.05), but no effect on tendon volume (P = 0.70). Group mean transverse strains for CSA, MLD, and APD averaged over the length of the tendon were -5.5%, -8.7% and 8.7%, respectively. Peak CSA, MLD, and APD transverse strains all occurred between 40% and 60% of tendon length. Transverse rotation of the free tendon was negligible at rest but increased under load, becoming externally rotated relative to the calcaneal insertion. The relationship between longitudinal and transverse strains of the free Achilles tendon during muscle-induced elongation may be indicative of interfascicle reorganization. The finding that transverse rotation and strain peaked in midportion of the free Achilles tendon may have important implications for tendon injury mechanisms and estimation of tendon stress in vivo.

Three-dimensional morphology and strain of the Achilles free tendon immediately following eccentric heel drop exercise

“…MVIC, we cannot discount small changes in volume (minimal detectable change for 3DUS volume ∼0.2 ml; Obst et al, 2014b) or redistribution of fluid along the tendon length (Sun et al, 2015). Regardless, our findings demonstrate that acute changes in tendon transverse morphology after exercise may be more pronounced when measured under tensile load, and that such changes may largely be dependent on changes in tendon length due to mechanical creep.…”

“…Increased strain of the Achilles free tendon measured at the same external load after exercise has been demonstrated following running (Lichtwark et al, 2013) and repeated eccentric heel drops (Obst et al, 2015), and is suggestive of early stage tendon fatigue due to mechanical creep (Fung et al, 2010). Because increased tendon longitudinal strain during tensile loading is closely associated with changes in tendon transverse morphology and strain (Obst et al, 2014b;Pokhai et al, 2009;Reeves and Cooper, 2014;Vergari et al, 2011), a similar relationship might be expected for the changes in tendon morphology and strain that occur in response to an exercise bout. In view of the high rate of exerciserelated injuries of the Achilles free tendon and the potential role short-term changes in tendon transverse dimensions could have on the local mechanical and biological environment (Heinemeier and Kjaer, 2011;Shim et al, 2014;Smith et al, 2013), there is a need to better understand the acute effects of exercise on the tendon 3D morphology and strain.…”

“…Total scan time for each trial was 8-12 s (average scan speed of 10 mm s −1 ) with an approximate between-frame interval of less than 0.2 mm. Achilles free tendon length determined using a combination of sagittal, transverse and frontal image re-slices and was defined as the shortest distance between two 3D points corresponding to the most distal edge of the calcaneal notch and the SOL muscle-tendon junction (MTJ) (Obst et al, 2015(Obst et al, , 2014b. Tendon cross-sections were manually segmented between these two landmarks at ∼5-10 mm intervals and interpolated using a shape-based interpolation method to create a 3D surface representation of the Achilles free tendon (Treece et al, 2000) (Fig.…”

“…1A-C). The corresponding 3D point cloud was then exported into Matlab (version R2012a, The MathWorks, Natick, MA, USA) for determination of tendon transverse morphology using methods described by Obst et al (2014b). Preand post-exercise longitudinal and transverse strains at 70% MVIC were normalised to the corresponding pre-exercise resting lengths.…”

“…The purpose of this study was to examine the immediate effect of a single bout of eccentric heel drop exercise on the 3D morphology of the Achilles free tendon at rest, and during a submaximal isometric plantarflexion contraction, using a 3D ultrasound (3DUS)-based method (Obst et al, 2014b). We selected the eccentric heel drop as it remains an integral part of Achilles tendon rehabilitation (Beyer et al, 2015;Habets and van Cingel, 2015;Kjaer and Heinemeier, 2014), induces high tendon strains (∼8.5%; Jeong et al, 2014) and has been shown to affect immediate changes in mechanical (Obst et al, 2015) and 2D morphological (Grigg et al, 2009) properties of the Achilles free tendon in healthy adults.…”