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First published online December 2, 2005
Journal of Experimental Biology 208, 4715-4725 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01950

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In vivo mechanical properties of the human Achilles tendon during one-legged hopping

G. A. Lichtwark^1,* and A. M. Wilson^1,2

¹ Structure and Motion Laboratory, Institute of Orthopaedics and Musculoskeletal Sciences, University College London, Royal National Orthopaedic Hospital, Brockley Hill, Stanmore, Middlesex, HA7 4LP, UK
² Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Herts, AL9 7TA, UK

View larger version (75K): [in a new window]   Fig. 1. (A) Anatomy of the triceps surae muscle group and the Achilles tendon (adapted from Gray's Anatomy). (B) Photograph of the attachment of the ultrasound probe to the leg. Three markers are rigidly attached to the probe by way of a fibreglass mould. This setup allowed for images of the gastrocnemius muscle–tendon junction to be visualised in the sagittal plane of the leg (see Fig. 2).

View larger version (72K): [in a new window]   Fig. 2. Images of the Achilles tendon junction with the lateral gastrocnemius. The tendon is the thick white structure to the left of the arrow, while the muscle fascia branches off this white structure to the right. The arrows represent the point that was tracked in the image for each frame during the hopping movement. (See Video 1 in supplementary material).

View larger version (102K): [in a new window]   Fig. 3. Photograph of the Perspex phantom emerged in water and imaged by the ultrasound probe (with rigidly attached markers). The positions of the grooves relative to the laboratory were recorded by tracking the tip of a wand as it move along the grooves. The positions of the grooves were also determined with the ultrasound probe using the technique described and a comparison made between the two.

View larger version (20K): [in a new window]   Fig. 4. (A) Three-dimensional (3D) reconstruction of the position of the Perspex® grooves of the phantom by tracking the tip of a wand with motion analysis (blue) and embedding the position of the grooves when visualised by an ultrasound probe into the laboratory frame of reference (red). The line represents a 3D regression line placed through the motion analysis data for each groove. The inter-quartile range (IQR) of the perpendicular distance of the motion analysis measured coordinates along the Perspex® grooves from the regression line in both 2D (x–z plane) and the 3D were 0.44 mm and 2.12 mm, respectively. (B) Two-dimensional (2D) comparison of the position of the grooves as determined by regression through the motion analysis data and the position determined by the ultrasound technique for each Perspex® plate. This was measured in the same plane (x–z) that was used to image the muscle–tendon junction during the hopping experiments. (C) Histogram of the error of the ultrasound technique compared to the calculated regression line that represents the position of the phantom grooves in both 2D and 3D. The IQR of the perpendicular distance from the measured coordinates of the grooves measured with the ultrasound technique to the linear regression line in both 2D (x–z plane) and 3D were 0.98 mm and 1.15 mm, respectively.

View larger version (24K): [in a new window]   Fig. 5. (A) Achilles tendon (AT) length and force measurements against time for five consecutive hops. AT length is shown in the raw form (blue) and after applying a fourth order, 5 Hz low-pass Butterworth filter (green). Note that only periods of contact have been displayed and subsequent data during time off the ground were removed. (B) Force against length for the same five trials as in A. Arrows represent the general trend for rise and fall of the force against length. (For an animation of stick figure, see Video 2 in supplementary material)

View larger version (11K): [in a new window]   Fig. 6. Average change in Achilles tendon length (dotted line), gastrocnemius muscle–tendon length (solid line) and approximate muscle length (broken line) during a single hop. The muscle length represents both the length of the fibres and the also other serial elastic tissues including the proximal tendon and aponeurosis.

View larger version (17K): [in a new window]   Fig. 7. Average Achilles–tendon force vs the change in Achilles tendon length (relative to the length at 200 N) for the same subject over three separate measurement occasions (blue, trial 1; green, trial 2; red, trial 3).

View larger version (26K): [in a new window]   Fig. 8. (A) Average Achilles tendon force–length relationship for 10 individual participants. (B) Average Achilles tendon force vs change in length (relative to the length at 200 N) for each of the 10 participants.

View larger version (17K): [in a new window]   Fig. 9. (A) The average stress–strain relationship for three participants across the range of elastic moduli determined. The elastic modulus for each individual was determined by fitting a linear regression through the stress–strain data, as displayed. (B) Group average stress–strain relationship at five points during both loading (filled diamonds) and unloading (unfilled diamonds) during the hopping movement and the standard deviation of stress and strain measurements (error bars indicate ±1 S.D.).