When you break a bone, in most cases you can count on a six-week healing period after which the bone is stronger. Soft tissue injuries are more problematic than bones. When you sprain, strain, or tear a tendon, recovery time varies. Doctors must make a highly subjective call when deciding if the healing is sufficient for the person to start using the limb again. Whether athlete or weekend warrior, getting back into action too soon can extend recovery and even worsen the injury.

University of Wisconsin-Madison engineers developed wearable technology that noninvasively measures tendon tension while a person is walking or running.  Most exercise wearables measure movement, but not force. When tendons are injured, moving the muscles at joints pulls on the tendons which connect the muscles to the skeleton. Too much force applied to a tendon too early during recovery means the person is headed back to the couch. The engineering team designed a small device that can be mounted on the skin directly over a tendon. The device uses two accelerometers to measure the vibrations of a moving tendon. A mechanical device taps the tendon 50 times a second and the accelerometers determine the speed of the shear waves that result from the taps. As described in a paper published in Nature Communications, the Wisconsin-Madison team has used the device to measure forces on Achilles, patellar, and hamstring tendons. The researchers can measure changes that occur in the tendons when subjects modify their step length or speed.

As research continues, the team hopes it will add to the understandings of human movement. The knowledge could then help with rehabilitation from musculoskeletal diseases and injuries. The better understanding of tendons and force also would have implications for physical therapy, orthopedics, sports training, ergonomics, and human factors study.