Tracking Day-To-Day Achilles Tendon Loading Progression During Rupture Recovery: A Case Study
Authors: Baxter Josh R. , Song Ke , Cone Stephanie G. , Zellers Jennifer A. , Thelen Darryl G. 
Published: November 2022

Category: Sports; Basic Sciences/Biologics Introduction/Purpose: Achilles tendon rupture patients often suffer long-term functional deficits. Although progressive tendon loading improves healing, it is difficult to reliably measure in the real world and thus be precisely prescribed in rehabilitation. Modern wearable sensors, including instrumented shoe insoles, provide powerful platforms to measure real-world biomechanics. Using simple algorithms, we showed that these instrumented insoles can accurately quantify Achilles tendon loads. Yet, the feasibility of using wearables to enhance targeted Achilles tendon therapeutics remain unknown, partly due to the challenges of obtaining data on many patients over the long recovery process. Therefore, we leveraged a case study to verify whether instrumented insoles are able to track the longitudinal progression of Achilles tendon loading in daily life, and relate to clinically relevant events of tendon health. Methods: A patient (F, 30y/o) who underwent a right Achilles tendon repair was enrolled after consent in this IRB-approved study. During a standard rehabilitation protocol, an immobilizing boot fit with a 3-sensor instrumented insole (Loadsol) was worn starting from week 4 post-surgery. The insole remained in footwear after switching from boot to shoe, and data collection continued until week 22. The patient recorded >10 seconds of insole data during gait each day, logged daily step count, and documented any event that may be related to tendon health, including pain and notable daily living activities. For each step identified by insole, we estimated Achilles tendon loads using our established algorithm, including load peak, impulse over a step, and load rates. We compared the daily average of these variables to self-reported events to explore their inter-relationships. We also correlated the load variables to determine whether they each provide unique insights into tendon loading. Results: The insole recorded up to 70 steps each day on a total of 116 days. Peak Achilles tendon load increased gradually yet non-linearly along the recovery process, with large variations between week 12-19 (Figure). 'Sharp' loading changes often corresponded to tendon health-related events, such as days of pain and swelling following rapid load increases in late week 12. A day of high step count (20k+) in week 18 was followed by a large decrease of the peak tendon load at the end of the same day. The insole also identified tendon load changes according to patient instructions, including a 65% increase in peak load by intentionally 'trying to push off' during gait. Finally, tendon load rates and impulse strongly correlated with peak load (R2 >0.85). Conclusion: Longitudinally monitoring Achilles tendon loading is challenging because 1) it is limited by the scarce frequency of clinical visits, and 2) lab-based gait measurements do not faithfully reflect real-world biomechanics. Our first-of-kind case study shows the feasibility of using instrumented insoles to track day-to-day Achilles tendon loading in the real world. The results suggest Achilles tendon loading can be sufficiently tracked using peak load, and modified instantly and interactively via instructions or biofeedback. Our innovative paradigm empowers future studies to leverage accessible tools and deliver personalized rehabilitation according to quantitative guidance, thereby optimizing long-term Achilles tendon healing and functional recovery.