Abstract
This study introduces a noninvasive wearable system for investigating tendon loading patterns during outdoor locomotion on variable terrain. The system leverages shear wave tensiometry, which is a new approach for assessing tendon load by tracking wave speed within the tissue. Our wearable tensiometry system uses a battery-operated piezoelectric actuator to induce micron-scale shear waves in a tendon. A data logger monitors wave propagation by recording from two miniature accelerometers mounted on the skin above the tendon. Wave speed is determined from the wave travel time between accelerometers. The wearable system was used to record Achilles tendon wave speed at 100 Hz during 1-km outdoor walking trials in nine young adults. Inertial measurement units (IMUs) simultaneously monitored participant position, walking speed, and ground incline. An analysis of 5108 walking strides revealed the coupled biomechanical effects of terrain slope and walking speed on tendon loading. Uphill slopes increased the tendon wave speed during push-off, whereas downhill slopes increased tendon wave speeds during early stance braking. Walking speed significantly modulated peak tendon wave speed on uphill slopes but had less influence on downhill slopes. Walking speed consistently induced greater early stance wave speeds for all slopes. These observations demonstrate that wearable shear wave tensiometry holds promise for evaluating tendon tissue kinetics in natural environments and uncontrolled movements. There are numerous practical applications of wearable tensiometry spanning orthopedics, athletics, rehabilitation, and ergonomics.
Highlights
Wearables are transforming the field of biomechanics, with the capacity to measure real-world movement enabling new applications in sports [1,2], ergonomics [3,4,5], and rehabilitation [6,7,8,9].The overwhelming majority of studies to date have used wearable inertial sensors to measure movement kinematics
We successfully used shear wave tensiometry to track Achilles wave speed patterns during outdoor walking on variable terrain
The average maximum and minimum wave speeds over all gait cycles were 66 and 16 m/s, respectively, with the peak wave speed aligning with the push-off, and minimal values tending to occur just after toe-off, as shown in gait characteristics are summarized in Appendix A, Table A2
Summary
The overwhelming majority of studies to date have used wearable inertial sensors to measure movement kinematics. These common raw measurements have been analyzed in many ways, such as quantifying sports performance [10,11], monitoring symptoms of movement disorders [12,13], analyzing gait [14,15,16] or losses of balance [17], or estimating markers of injury risk, such as tibial shock [18,19], all in non-laboratory settings. Muscle-tendon kinetics are of particular interest for quantifying performance, assessing injury risk, and evaluating rehabilitation progress. There remain no viable approaches to measure the forces transmitted by muscle-tendon units during human movement outside a laboratory
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