Exploring Variable Achilles Tendon Loading Via Active Acoustics

Non-invasive, direct Achilles tendon (AT) load measurements have been a long-standing objective in biomechanics toward elucidating underlying muscle-tendon dynamics during normal and pathological human locomotion. However, traditional methods fail to capture subcutaneous changes due to the disconnect between externally measured forces and internal tendon dynamics. In this study, we introduce Active Acoustics (AA) as a non-invasive approach for measuring AT loads, building on recent developments by leveraging continuous mechanical stimulation in place of intermittent taps or bursts. We assessed AA's performance against Inverse Dynamics (ID) in 10 healthy subjects. We collected data from 13 tasks designed to capture a wide range of AT force, displacement, and velocity conditions. AA successfully tracked dynamic changes in AT loading while maintaining low computational complexity, achieving the shortest filtering latency in synthetic benchmark tests compared to prior methods. Our benchmark evaluation demonstrated a strong Pearson correlation (r=0.95 ±0.02, n=6) during active, isometric contractions, supporting the feasibility of continuous stimulation for AT load measurements. Across the 13 tasks, AA captured task-specific variations in AT loading, revealing nuanced effectiveness dependent on the specific ankle joint dynamic conditions, and the need for improved under-the-skin reference signals. The continuous stimulation approach enabled a higher output frequency (500 Hz vs 50/5 Hz previous work) and demonstrated consistent performance at an experimentally optimized stimulation frequency of 750 Hz. This study underscores AA's potential for real-time, low-latency applications in assistive and rehabilitative devices. Future research should explore sensor optimization, expanded task sets, and application-specific features to further enhance AA's utility.