ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

and estimate in vivo tissue strains during loading (Maganaris, et al. 2002, Maganaris, et
al. 1999, Peixinho, et al. 2008, Maganaris 2003). However, spatial variations in tendon
motion over the cross-section have not been considered. Hence, the purpose of this
study was to investigate variations in tissue motion with eccentric plantarflexor loading.
Our primary hypothesis was that variations in tissue motion would be observed during
lengthening contractions, consistent with the idea that shear deformation is occurring.
We tested subjects in both an extended and flexed knee posture to better understand the
potential for variations in soleus-gastrocnemius load sharing to alter tendon motion
patterns.
3. METHODS
Five healthy young adults were recruited and asked to lie prone on an examination
table, with their foot secured in an inertial loading device. Subjects were asked to cycle
between 15 of dorsiflexion and 15 of plantarflexion at a metronome guided rate of 0.5
Hz. Real-time feedback of ankle angle was provided to subjects to assist them in
producing repeatable, cyclic motion. The device induced lengthening plantarflexor
loading (peak plantarflexor moments of ~10 Nm) with dorsiflexion. Subjects were first
given three minutes of practice to precondition the tendon. Subjects were then tested at
fixed knee flexion angles of 10 and 90, referred to as extended and flexed knee
postures, respectively. Three eight-second trials were collected at each knee angle in a
randomized order.
Cine ultrasound radiofrequency (RF) data were collected with a 10 MHz linear array
transducer (Ultrasonix Corporation, Richmond, BC). A 2 cm thick ultrasound standoff
pad (AquaFlex, CIVCO Medical Solutions, Kalona, IA) was placed over the Achilles
tendon and secured with an elastic ankle wrap. The standoff pad helped to reduce
transducer motion and also provided clear image contrast at shallow depths. The
transducer was positioned to image the distal Achilles tendon throughout the motion
(Fig. 1).
Tissue displacements were computed retrospectively using a 2D cross-correlation based
elastography technique (Konofagou, et al. 1998). RF data were first upsampled by a
factor of 2 and 4 in the directions parallel to, and perpendicular to the ultrasound beam
to increase the spatial density of the
cross-correlation functions (Konofagou,
et al. 1998). A region of interest (ROI)
within the tendon was then manually
defined in an image from the unloaded,
plantarflexed ankle, and a grid of nodes
was defined. Rectangular kernels (2 mm x
1 mm) centered at the nodes were cross-
Fig. 1. The Achilles tendon is shared by
the soleus and gastrocnemius muscles.
Ultrasound data were collected distally
near the calcaneal insertion with a standoff
pad used to diminish transducer motion.
Image adapted from Healthwise,
Incorporated.
correlated with corresponding search
regions in the subsequent frame.
Frame-to-frame nodal motion was
determined by finding the displacements
that maximized the normalized crosscorrelation
function for each kernel