Commentaries on viewpoint: On the hysteresis in the human Achilles tendon.

TO THE EDITOR: Ultrasound imaging has provided an invaluable tool for investigating muscle and tendon mechanics, particularly for measuring tissue strain. However, as Finni and colleagues (1) argue, some physiological measurements are not as consistent or of the magnitude we would expect from isolated tissue preparations. Although tendon hysteresis is often not reported in studies of tendon mechanical properties using ultrasound, the analysis presented in this Viewpoint demonstrates some important technical limitations of combining stress and strain measurements using this technique. Unusually large hysteresis values ( 10%) determined through loading and unloading of tendons are likely to be a problem related to the technique rather than a physiological property of the tendon. Although Achilles tendon hysteresis has not been directly calculated from isolated mechanical testing, research by Wren et al. (2) clearly demonstrates low levels of hysteresis in this tendon ( 7%) that are consistent with tests on other tendons. Temporal differences between stress and strain measurements can be easily determined, for example, by generation of an event that can be visualized with B-mode imaging and also detected simultaneously by other measurement devices, e.g., A/D converter. However, other factors are more difficult to control, particularly those pertaining to the estimation of tendon force from joint torque measurements that are influenced by neural control of both agonist and antagonist muscle activity, intramuscular force transmission, and changes in moment arm. We must therefore be cautious when making conclusions about changes in tendon stiffness or hysteresis across or within groups when using ultrasound methods.