Researchers from the University of Wisconsin-Madison have detailed a wearable device that can better measure muscle-tendon tension during certain activities, such as walking or running.

The prototype device, which has so far been tested on the Achilles, patellar, and hamstring tendons, represents a feasible, non-invasive alternative to tendon tension measurement, and with additional work could find a place in rehabilitation or orthopedics, according to the team.

"We think the potential of this new technology is high, both from a basic science standpoint and for clinical applications," Darryl Thelen, a mechanical engineering professor at UW-Madison, said in a statement. "For example, tendon force measures could be used to guide treatments of individuals with gait disorders. It may also be useful to objectively assess when a repaired tendon is sufficiently healed to function normally and allow a person to return to activity.”

The system works by observing the speed of a vibration, in this case shear wave propagation, that occurs across a tendon when it is exerted, Thelen explained. By quantifying this vibration and translating it to a corresponding measure of tendon tension, the team is able to indirectly measure the force being placed upon certain parts of the body.

Of note, this strategy would allow researchers or specialists to measure the internal muscle-tendon forces of movement without surgically implanting force sensors — a strategy previously used in animal studies but withheld for use of humans.

“Currently, wearables can measure our movement, but do not provide information on the muscle forces that generate the movement,” Thelen said. ”We've found a way to measure the vibrational characteristics … and then we went further and determined how we can interpret this measurement to find the tensile stress within the tendon.”

The device created by the researchers consists of a mechanism that taps the tendon it is placed over 50 times per second. Each tap creates a rippling wave through the tendon, which is measured with two accelerometers. With this data, the researchers can see noticeable changes in wave speed when the wearer alters their gait.

“Such information could conceivably be used as biofeedback to guide gait retraining, and to track tissue load changes with injury and treatment,” the researchers wrote. “

“We envision numerous applications for investigating the motor control underlying normal movement and for identifying the biomechanical factors contributing to movement disorders,” the researchers wrote in Nature Communications. “Further, muscle–tendon forces induce much of the loading and deformation that occurs in ligament, cartilage, and skeletal tissues. Thus, muscle-tendon load information could be used to guide rehabilitative interventions, plan orthopedic procedures, assess tissue healing following treatment, monitor activity, and engineer viable tissues that can restore lost or impaired motor function.”