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Wearable could tell how humans power movement

Steve Rogerson
May 2, 2018

For athletes and weekend warriors alike, returning from a tendon injury too soon often ensures a trip back to physical therapy. However, wearable technology developed by University of Wisconsin-Madison engineers could help tell whether tendons are ready for action.
A team of researchers led by UW–Madison mechanical engineering professor Darryl Thelen and graduate student Jack Martin has devised an approach for noninvasively measuring tendon tension while a person is engaging in activities such as walking or running.
This advance could provide insights into the motor control and mechanics of human movement. It also could apply to fields such as orthopaedics, rehabilitation, ergonomics and sports. The researchers described their approach in a paper published last month in the journal Nature Communications.
Muscles generate movement at joints by pulling on tendons, which are bands of tissue that connect muscles to the skeleton. But assessing the forces transmitted by tendons inside the body of a living person is tricky.
“Currently, wearables can measure our movement, but do not provide information on the muscle forces that generate the movement,” said Thelen, whose work is supported by the National Institutes of Health.
To overcome this, Thelen and his collaborators developed a simple, non-invasive device that can be easily mounted on the skin over a tendon. The device enables the researchers to assess tendon force by looking at how the vibrational characteristics of the tendon change when it undergoes loading, as it does during movement.
This phenomenon is similar to a guitar string, where the tension in the string changes the vibrational response. When a guitar string is plucked, the speed of the wave travelling along the string, and thus the vibration frequency, is related to the tension, or force, applied to the string.
“We’ve found a way to measure the vibrational characteristics – in this case, the speed of a shear wave travelling along a tendon – and then we went further and determined how we can interpret this measurement to find the tensile stress within the tendon,” Thelen said.
The system for measuring wave speed is portable and relatively inexpensive. It includes a mechanical device that lightly taps the tendon 50 times per second. Each tap initiates a wave in the tendon, and two miniature accelerometers determine how quickly it travels.
The researchers have used the device to measure forces on the Achilles tendon, as well as the patellar and hamstring tendons. In each case, they can measure what happens in the tendon when users modify their gait, for example, by changing step length or speed.
By measuring how muscles and tendons behave within the human body, this system could eventually enable clinicians to plan more effective treatments for patients suffering from musculoskeletal diseases and injuries.
“We think the potential of this new technology is high, both from a basic science standpoint and for clinical applications,” Thelen said. “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 technology is being patented through the Wisconsin Alumni Research Foundation. The research was supported by grants from the National Institutes of Health and a graduate research fellowship from the National Science Foundation.