Viewpoint: On the hysteresis in the human Achilles tendon.

ELASTIC HYSTERESIS IS A PROPERTY of tendon and describes the energy dissipated due to material viscosity. The amount of tendon hysteresis is important for efficiency of locomotion. Higher hysteresis is associated with greater energy dissipation as heat, and thus less energy can be recoiled to propel our movements. Classical papers report hysteresis of 7% in the plantaris tendon of sheep (9) and 10% in tendons of different mammals (3, 24). Although greater hysteresis values have been presented especially in human studies in vivo (e.g. 10–20), several authors have suggested that the low hysteresis values are likely to be realistic because they ensure greater elastic recoil and minimize heat damage (1, 3, 9). Since the 1990s ultrasound imaging has become a popular tool when assessing in vivo tendon properties in humans. It is possible to measure tendon properties from isometric loadingunloading cycles (e.g. 20, 25) and even during natural locomotion such as hopping (18). The most often reported tendon property is stiffness, a very relevant parameter regarding the potential to store elastic energy. However, the amount of energy dissipation that occurs after storage (i.e., hysteresis) also affects efficiency of our locomotion. This raises questions about why there are far more studies reporting stiffness than hysteresis. For example, the PubMed search term “tendon stiffness” returns 1,689 hits compared to just 69 for “tendon hysteresis.” This Viewpoint was stimulated by two observations: 1) the statistical skewness whereby numerous articles have reported tendon stiffness and Young’s modulus, but far fewer have reported tendon hysteresis; 2) in vivo human studies seem very often to report hysteresis values greater than 10%, suggesting either that there are methodological differences between in vivo and in vitro studies or that human tendons in vivo have a much poorer capacity to store and reutilize elastic energy. In this article we focus on the healthy human Achilles/gastrocnemius tendon (AT) because it has an important locomotor function, and clearly a low AT hysteresis would allow elastic recoil for efficient locomotion (1, 27). Figure 1 shows the mean hysteresis values from selected animal studies and from the majority of human studies in the last 30 years. Two observations are evident from the figure: 1) animal studies report smaller values than human studies; 2) in the human data there is a very large range of hysteresis values. The variability in human studies may be explained by several methodological factors. First, the definition of tendon length and assessment of length change both vary. For example, the tendon can also include parts of the aponeurosis and not only the “free” external tendon, which appears to have lower hysteresis (30) than the gastrocnemius tendon (Fig. 1). In the literature there are about five different ways that have been used to assess tendon length change during voluntary contractions by ultrasonography: 1) Movement of a medial gastrocnemius (MG) muscle fascicletendon crosspoint is traced using ultrasonography. The displacement of this point is taken as the change in tendon length (e.g., 12–14). This early method has disadvantages that the following methods account for partly or completely: a) “tendon length” includes aponeurosis with differing properties, b) it does not account for displacement at the insertion of the tendon, and c) absolute tendon length is not assessed. 2) Movement of both MG muscle-tendon junction and calcaneus are tracked. The difference between the displacements of these points denotes the change in tendon length (e.g., 20). Free AT length change has been obtained similarly by tracking the soleus muscle-tendon junction, but video analysis was used instead of ultrasonography to track calcaneal movement (30). 3) MG muscle-tendon junction is tracked with corrections including calcaneal rotation that has been determined during passive movement (e.g., 7, 8, 21). 4) MG muscle-tendon junction is tracked using ultrasonography with motion analysis recording of both the heel and the ultrasound probe positions, and the linear distance between the tendon origin and insertion is calculated (5, 18). 5) The same as in no. 4 but including the curvature of the tendon (e.g., 2, 28). Second, measurements without tendon preconditioning may be one source of the greater hysteresis (4, 19). Third, tendon force measurements contain uncertainties that arise from the estimations and assumptions required in calculating the forces. A common assumption is that all of the plantar flexor moment is transmitted via the Achilles tendon, although contributions from other muscles (synergistic and agonistic) are likely to occur (6). Furthermore, the moment arm values used can affect the force values considerably, and mean values from the literature may distort the individual variability. Uncertainties in tendon length and force measurements affect not only hysteresis but also other measures of tendon