Determination of Achilles Tendon Slack Length Using Supersonic Shear Imaging

Passive mechanical properties of the musculo-articular complex are classically studied using passive torque-angle curves. However, the mechanical properties of each structure of this musculo-articular complex (e.g., muscle, tendon) cannot be dissociated from this relationship. We recently showed that supersonic shear imaging is able to accurately quantify muscle shear elastic modulus of one muscle (Gastrocnemius medialis GM) during passive stretching. This study aimed to determine the slack length of achilles tendon (AT) during passive stretching and to compare it with the slack length of the GM. Relationships between shear elastic modulus and ankle joint angle were characterized for each structure (i.e., GM and AT) at both knee angles (i.e., 0° and 90° of knee flexion). With the knee fully extended, the slack length of AT occurred significantly earlier than that for the GM (42.6 ± 2.6° vs. 23.5 ± 5.4° of plantarflexion; p<0.001). For 90° of knee flexion, the slack length of AT occurred also earlier (43.4 ± 2.4° vs. 14.6 ± 8.5° of plantar flexion; p<0.001). This significant earlier rise in passive tension observed in AT is particularly unexpected. That suggests the potential involvement of monoarticular structures (e.g., soleus, fascia or pulley) prematurely under tension, generating an increase in passive tension within the AT. Further studies are required to better explain this result.