Short communicationAchilles tendon moment arms: The importance of measuring at constant tendon load when using the tendon excursion method
Introduction
Knowledge of Achilles tendon (AT) force is important for the understanding of its role for elastic energy storage and return, injury prevention and the mechanical contribution of the triceps surae muscles to locomotion. Due to the invasive nature of measurement techniques, it is difficult to quantify muscle and tendon forces in humans directly; however, by utilizing external joint moments and tendon moment arms, a useful estimate of AT force can be obtained (An et al., 1984). AT moment arms have been measured in vitro using cadaver specimens (Hintermann et al., 1994, Spoor et al., 1990) as well as in vivo using 2D ultrasonography (Fath et al., 2010, Fath et al., 2013, Karamanidis et al., 2011, Lee and Piazza, 2009, Maganaris, 2003), MRI (Fath et al., 2010, Fath et al., 2013, Maganaris et al., 1998, Maganaris et al., 2000, Rugg et al., 1990), and digital photography (Kongsgaard et al., 2011, Scholz et al., 2008).
Two common calculation methods to measure AT moment arms are the center of rotation and the tendon-excursion methods. The center of rotation method requires knowledge of joint geometry, and allows the moment arm to be measured directly using graphical analysis (Reuleaux, 1875) and images of the joint at different positions (e.g. X-ray, MRI). This method is particularly sensitive to the location of the joint center and the line of action of the tendon, both of which can change during contraction (Maganaris et al., 1998, Maganaris et al., 2000). This sensitivity can be avoided by using the tendon-excursion method where the moment arm is measured as the ratio of the tendon displacement to the angular joint excursion (An et al., 1984, Storace and Wolf, 1979) for states where the tendon length is the same. Although the tendon-excursion method can be applied to both MRI (Maganaris et al., 2000) and 2D ultrasound techniques (Fath et al., 2010, Karamanidis et al., 2011), the difficulties of controlling and measuring joint torques during MRI make ultrasound more suitable for measuring moment arms during active contractions. The tendon stretch should be kept constant, and this can be achieved by controlling the muscle force and tendon loading rate (Pearson et al., 2007, Theis et al., 2012). However, previous studies have used the contraction effort as a proxy for muscle force (Fath et al., 2013, Maganaris et al., 1998, Maganaris et al., 2000), yet due to the force–length relationship within a muscle (Gordon et al., 1966) the activation level is not directly related to muscle force.
The objective of this study was (A) to introduce calculation methods that would allow AT moment arms to be calculated using the different assumptions of constant force, joint torque, or contraction effort, and (B) to assess whether the calculation method affects the estimated moment arms.
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Participants
Eight healthy male subjects (23.0±0.8 years, 173.4±9.5 cm, 80.3±12.2 kg, mean±SEM) provided written informed consent approved by the Office for Research Ethics at the University.
Experimental protocol
Participants were seated in a dynamometer (System 3, Biodex, New York, USA). The lateral malleolous was aligned with the axis of rotation of the dynamometer and the knee was fully extended, with a relative hip angle of 100°. B-mode ultrasound (Echoblaster 128, Telemed, Vilnius, Lithuania) was used to image the
Results
During isometric contraction, the AT lengthened causing the MTJ to move in a proximal direction along the leg (Fig. 1). At high levels of contraction the muscles appeared to bulge, moving the AT line of action. Root-mean-square errors between repeat trials of the MTJ were 1.20±0.08 and 1.35±0.17 mm for the x-position for the MG and LG, respectively, and 0.31±0.05 and 0.24±0.03 mm for the y-position for the MG and LG, respectively (mean±SEM, N=8 subjects).
There was a significant effect of subject,
Discussion
Achilles tendon moment arms, calculated using the tendon-excursion method, vary with ankle angle and level of muscle contraction. This study shows that estimates of the AT moment arm also vary depending on the calculation method: whether force, torque, or effort is constrained across the range of ankle angles. The tendon-excursion method is sensitive to tendon stretch. Tendon stiffness may decrease when loaded slowly (Pearson et al., 2007, Theis et al., 2012), and this creep poses a challenge
Conflict of interest statement
None.
Acknowledgments
We thank NSERC, Canada for financial [Discovery Grant 261262-2008].
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