Mechanical properties of the human Achilles tendon, in vivo M. Kongsgaard a,���, C.H. Nielsen a, S. Hegnsvad a, P. Aagaard b, S.P. Magnusson a,c a Institute of Sports Medicine, Dept. Othopedic Surgery M, Bispebjerg Hospital and Center for Healthy Aging, Faculty of Health Sciences, University of Copenhagen, Denmark b Institute of Sports Exercise and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark c Department of Physiotherapy, Bispebjerg Hospital, Copenhagen, Denmark a b s t r a c t a r t i c l e i n f o Article history: Received 16 November 2010 Accepted 17 February 2011 Keywords: Achilles tendon Ultrasonography Mechanical properties Reproducibility Background: Ultrasonography has been widely applied for in vivo measurements of tendon mechanical properties. Assessments of human Achilles tendon mechanical properties have received great interest. Achilles tendon injuries predominantly occur in the tendon region between the Achilles-soleus myotendinous junction and Achilles-calcaneus osteotendinous junction i.e. in the free Achilles tendon. However, there has been no adequate ultrasound based method for quantifying the mechanical properties of the free human Achilles tendon. This study aimed to: 1) examine the mechanical properties of the free human Achilles tendon in vivo by the use of ultrasonography and 2) assess the between-day reproducibility of these measurements. Methods: Ten male subjects had the Achilles tendon moment arm length, Achilles tendon cross sectional area and free Achilles tendon length determined. All subjects performed isometric plantarflexion ramp contractions to assess between-day reproducibility on two separate days. Simultaneous ultrasonography based measurements of Achilles-soleus myotendinous junction and Achilles-calcaneus osteotendinous junction displacement together with Achilles tendon force estimates yielded free Achilles tendon mechanical properties. Findings: Free Achilles tendon maximal force, deformation and stiffness were 1924 (SD 229) N, 2.2 (SD 0.6) mm and 2622 (SD 534) N/mm on day 1. For between-day reproducibility there were no significant differences between daysfor freeAchilles tendon mechanicalproperties.The between-day correlationcoefficientand typical error percent were 0.81 and 5.3% for maximal Achilles tendon force, 0.85 and 11.8% for maximal Achilles tendon deformation and 0.84 and 8.8% for Achilles tendon stiffness respectively. Last, osteotendinous junction proximal displacement on average contributed with 71 (SD 12) % of proximal myotendinous junction displacement. Interpretation: This study, for the first time, presents an ultrasonography based in vivo method for measurement of free AT mechanical properties. The method is applicable for evaluation of free human Achilles tendon mechanical properties in relation to training, injury and rehabilitation. �� 2011 Elsevier Ltd. All rights reserved. 1. Introduction B-mode ultrasonography has become highly popular for the in vivo measurements of human tendon mechanical properties since the technique is quite inexpensive, non-invasive and easy to use. Especially the assessment of Achilles tendon (AT) mechanical properties has received much attention because of its importance for normal human locomotion (Komi et al., 1987, 1992 Scott and Winter, 1990) and because AT injuries are rather prevalent and difficult to treat (Clement et al., 1984 Cook et al., 2002 Longo et al., 2009 Sandmeier and Renstrom, 1997). However, due to the length of the free human AT, which extends from the calcaneus insertion to the most distal part of the soleus (Calleja and Connell, 2010), B-mode ultrasonography based estimations of AT mechanical properties have been limited. Presently, measurements of the proximal displacement of the distal myotendinous junction of the medial gastrocnemius in relation to an external marker are typically used to give an estimate of AT mechanics (Arampatzis et al., 2007, 2010 Arya and Kulig, 2010 Bryant et al., 2008 Child et al., 2010 Csapo et al., 2010 Hansen et al., 2003 Lichtwark and Wilson, 2005 Magnusson et al., 2001 Muraoka et al., 2005 Rosager et al., 2002). Inherently this method has some methodological problems (Maganaris, 2005). First, the measured deformation will be a product of the individual elongation of both the free AT and the more proximal aponeurosis structures. Since strain behavior and mechanical properties of the free tendon and aponeurosis structures may differ considerably (Lieber et al., 1991 Magnusson et al., 2003), and since some regions of the aponeurosis may even shorten during muscle contractions (Finni et al., 2003 Kinugasa et al., 2008), this may complicate the interpretation and applicability of such data. Secondly, the displacement of the distal myotendinous junction of the medial gastrocnemius is measured in relation to an external marker (Arampatzis et al., 2007, 2010 Arya and Kulig, 2010 Child et al., 2010 Csapo et al., 2010 Hansen et al., 2003 Lichtwark and Wilson, 2005 Magnusson et al., 2001 Muraoka Clinical Biomechanics 26 (2011) 772���777 ��� Corresponding author. E-mail address: mads.kongsgaard@gmail.com (M. Kongsgaard). 0268-0033/$ ��� see front matter �� 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2011.02.011 Contents lists available at ScienceDirect Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech

et al., 2005 Rosager et al., 2002), and thus any vertical marker dis- placement will lead to a faulty estimation of the tendon���aponeurosis deformation (Maganaris, 2005). Thirdly, only the displacement of the proximal end of the tendon is measured, which probably yields a significant overestimation of strain (Shin et al., 2008). Ideal assessment of free AT elongation, defined as the actual displacement between the calcaneus and the soleus-AT myotendinous junction, does not include deformation of any aponeurosis structures, and possible displacement of the calcaneus is accounted for, thus rendering the use of an external marker unnecessary. Evaluation of free human AT mechanical properties has previously been performed by tantalum bead implantation in conjunction with Roentgen stereophotogrammetric analysis (Schepull et al., 2007, 2010) and by the used of advanced magnetic resonance imaging techniques (velocity-encoded, phase-contrast MRI) (Kinugasa et al., 2010 Shin et al., 2008). However, the aforementioned techniques have limited applicability since they are invasive and/or quite technically advanced, respectively. To the best of our knowledge, ultrasonogra- phy based in vivo assessments of the human AT mechanical properties based on free AT elongation have not previously been performed. However, recent technological advances have made the production of longer ultrasound heads, and therefore also a complete visualization of the entire free Achilles tendon, possible. Therefore, the purposes of the present study were: 1) to examine the mechanical properties of the free human AT, in vivo, by the use of B-mode ultrasonography and 2) to assess the between-day reproducibility of these measurements. 2. Methods 2.1. Subjects Ten male subjects with a mean age, body weight and height of 30.6 (SD 6.1) yr, 78.9 (SD 6.4) kg and 183 (SD 5) cm volunteered for the study. All subjects were recreational athletes who engaged in various physical activities. No subjects had a history of previous or present AT or lower limb/foot injuries. 2.2. Design On day 1 all subjects attended measurements of AT moment arm length, free AT length, AT cross sectional area (CSA) and free AT mechanical properties as described in details later. In all subjects, the foot/leg of the preferred leg was used for assessments. To assess the between-day reproducibility of the measurement of the free AT mechanical properties, all subjects attended re-measurements one week following the first test-day. On day 2 the Achilles tendon structural properties were not assessed since reproducibility of these measures were not a part of the purpose of this study. Instead, the Achilles tendon structural properties obtained at day one were used for the calculation of tendon stress, strain and modulus for day 2 assessments. All subjects were investigated at the same time of day on day 1 and day 2. 2.3. Experimental set-up 2.3.1. AT moment arm As previously described in detail (Scholz et al., 2008 Zhao et al., 2008) the AT moment arm was measured as the perpendicular distance from the center of rotation (axis trough the inferior tip of medial and lateral malleoli) (Scholz et al., 2008) to the AT line of action (LoA). In brief, the inferior tip of the medial and lateral malleolus was marked, and two lines (D1 and D2, Fig. 1) extending from each malleoli to the posterior aspect of the AT were drawn on the skin. Subsequently, the foot was photographed in the lateral and medial sagital planes (Panasonic Lumix DMC-TZ5, Digital Camera) (Fig. 1). The length of lines D1 and D2 were then measured using ImageJ version 1.42 software (http://rsbweb.nih.gov/ij/download.html), and the mean of these two measurements (M1) was calculated. Thereafter, a Hitachi EUB-6500 ultrasound scanner (Hitachi Medical Corporation, Tokyo, Japan) equipped with a 10 MHz, 100 mm long, linear array B-mode transducer (Hitachi, Model: EUP- L53L) fitted with a water pad, was used to produce a sagittal plane ultrasonography (US) image of the AT. In this image the intersection of lines D1 and D2 was indicated with a steel needle (Fig. 1) and the perpendicular distance (M2) from the D1���D2 intersection to the AT LoA was measured using ImageJ software (Fig. 1). The AT moment arm was calculated as: MA =M1 ���M2 (Zhao et al., 2008). Pilot trials (see later discussion) indicated that the degree of joint rotation, with the current set-up, was negligible and therefore deemed to not significantly affect moment arm length during contraction. Thus, no attempts were made to correct for changes in moment arm during contraction. 2.3.2. AT structural properties On day 1 subjects were seated with a 90�� hip and knee angle and a neutral ankle joint angle (0�� dorsiflexion). A sagittal US image of the AT was obtained and the length of the free AT was measured using ImageJ software as the shortest vertical distance between the calcaneal notch and the soleus-Achilles myotendinous junction (MTJ) (Fig. 1). Following this, the location of Achilles-calcaneal OTJ and the soleus-Achilles MTJ were marked on the skin. Three points indicating 25% (proximal), 50% (mid) and 75% (distal) of free AT length was Fig. 1. Achilles tendon (AT) length and moment arm measurements. (A) Two horizontal lines (D1 and D2) were drawn from the inferior tip of the medial and lateral malleoli respectively, to the posterior aspect of the AT. The mean distance of D1 and D2 was calculated as M1. (B) A Hitachi EUB-6500 ultrasound scanner equipped with a 10 MHz, 100 mm long transducer was used to visualize the intersection of lines D1 and D2 (steel needle) and the AT line of action (LoA). From this image the distance from the D1���D2 intersection to the AT LoA was measured as M2. The AT moment arm was calculated as MA =M1 ���M2. The AT length was measured as the shortest vertical distance between the soleus MTJ and the calcaneal notch. 773 M. Kongsgaard et al. / Clinical Biomechanics 26 (2011) 772���777