Tendons, which connect muscles to bones, exhibit a distinctive behavior under human physical activity: they become wider and narrower appearing fatter and thinner as they are stretched and contracted. This phenomenon reflects their unusual auxetic behavior, characterized by lateral expansion when stretched longitudinally. Tendons are composed of tough, high-tensile-strength bands of dense fibrous connective tissue. They play a critical role in transmitting mechanical forces generated by muscle contractions to the skeletal system. Especially during abrupt movements such as jumping, tendon dynamics are essential not only for joint positioning and control but also for energy absorption and redistribution. However, tendons are susceptible to damage, particularly under excessive strain or repetitive stress, which often results in injuries. To mitigate such risks and reduce the mechanical burden transmitted through tendons, the development of an auxetic musculoskeletal assistive structure is proposed. This structure utilizes viscoelastic, form-fitting properties, enabling it to simultaneously contract and expand in two perpendicular directions during bodily movements. Such a capability closely mimics the natural auxetic response of tendons, offering both protection and support.

In designing a macro-scale auxetic structure with elastic flexibility suitable for musculoskeletal assistance, it is crucial to develop an auxetic unit cell that allows for adjustable stiffness. This adjustability ensures the structure can adapt to various mechanical demands during motion. Auxetic structures are particularly advantageous in aiding human movement due to their reversible and adaptive configurations, which accommodate dynamic changes in shape and force. Furthermore, the periodic cellular architectures employed in auxetic designs were initially explored in the field of lightweight structural engineering because of their remarkable properties, including high specific stiffness, enhanced damping, and superior energy absorption. These properties now offer promising benefits for biomechanical applications, especially in assistive devices designed to support or augment human musculoskeletal function.