Characterizing the high-rate viscoelastic behavior of tendons using time-temperature and time-ethanol superposition

Tendons can act like biological springs in animal movements. When loaded by a muscle, tendons store elastic energy, which, when unloaded, assists animals in running, jumping, etc. The energy efficiency of this movement depends on the loading and unloading rate of the tendon, where rapid unloading results in higher energy loss. To fully predict the tradeoff between energy efficiency and speed in ultrafast elastically-driven movements, we characterize tendon viscoelasticity across a broad range of frequencies. We perform dynamic mechanical analysis (DMA) measurements of American Bullfrog plantaris tendons. To access the high-frequency response of the tendon, we take two approaches. First, we use DMA and time-temperature superposition as a well-established baseline to measure high-frequencies; however, the measurement is limited to above-freezing temperatures due to the water content in the tendon. As a secondary approach, we use DMA and time-ethanol superposition, where a higher ethanol concentration probes higher frequencies by reducing the effective free volume of molecules in the tendon. Our results suggest that although there is general qualitative agreement between the two approaches, there are remaining challenges in applying time-ethanol superposition to measure tendon viscoelasticity.