Darryl Thelen speaks about the mechanical properties of tendons

By Aaron Bishop

Darryl Thelen, Spangler professor of Mechanical Engineering at the University of Wisconsin-Madison, presented a seminar titled “Gauging Tendon Behavior by Observing Tissue Dynamics” on November 10th, 2017 in the UW-Madison Mechanics Seminar Series. Thelen runs the UW Neuromuscular Biomechanics Laboratory (UWNMBL) and is the Associate Dean for Research in the College of Engineering.

The UWNMBL focuses on two major topics: knee mechanics and in-vivo muscle-tendon mechanics. Much of the lab’s funding comes from the National Institutes of Health (NIH) and other clinically focused sources so most projects focus on improving clinical outcomes. The knee mechanics group works closely with the Gillette Children’s hospital to improve surgical outcomes for children with cerebral palsy induced crouch gait, study alignment effects in total knee arthroplasty, and suggest tendon repair methods to help reduce the risk of osteoarthritis following knee injury.

After briefly introducing the knee mechanics work, the UWNMBL’s in vivo muscle tendon mechanics projects were the focus of Thelen’s talk. The lab group has worked through three different methods of tracking tendon mechanics: speckling tendons with digital image correlation, shear wave electrography to determine material properties, and wave speed analysis during gait to determine kinetic effects. In general, tendon exhibit crimp with very little hysteresis and acts as a spring in the body. However, tendons are not as simple as a single spring, they can attach multiple muscles to a single attachment point and exhibit shear forces between bundled fascicles.

Recently, the lab group has been able to show that, at high strains, wave speed propagation is directly proportional to tendon strain. With this understanding UWNMBL has developed non-invasive methods of observing tendon strain by watching wave propagation speed with skin based sensors. These sensing methods could be used to help inform best surgical practices on a patient specific level. They have also been used to better understand tendon stress in running to show how stride rate effects strain in the patellar tendon and how running speed effects hamstring tendon strain. Interestingly, Thelen’s data suggests that the highest strain in the hamstring tendon occurs early in the gait cycle, contrary to common thought that hamstring rupture usually occurs in late swing phase.
Even with the advances of these testing methods, the properties and mechanics of tendons are still far from fully understood. Much more work needs to be done at the microscopic level since many macro-level studies are contradictory to each other. The work done in the UWNMBL shows that tendon strain can be measured non-invasively with skin based accelerometers. Future work using this technology promises to improve understanding of tendon injury and aging.