Researching tendon strain with In Vivo testing

Posted on 1 August 2019

In Vivo testing to measure strain on tendons during treadmill running

In a first in the world trial, Dr Edwin Dillon of Mediclinic Winelands Orthopaedic Hospital together with a team from Stellenbosch University, Dr JH Müller and Llewellyn Groenewald, Prof Hellen Bayne (University of Pretoria) and Dr Izabel Moore (Cardiff Metropolitan University) are investigating the strain placed on the patella and Achilles tendons during a treadmill running session. While this may not seem remarkable in itself, the method of measuring the strain may well be.

By making use of optic fibre inserted into the respective tendons, researchers are using the optic fibre (and resulting changes to the flow of light through these channels) to measure the change in tendons during a running session. In vivo (or testing on a live subject) requires ethical approval, and this has been granted by the Health Research Ethics Committee at Stellenbosch University.

“The aim of this research is to firstly assess whether the computational models we have regarding these types of movements are accurate. If this is proved true, the computational modelling can be developed further and actively used in planning surgeries,” says Dr Dillon. “While the working principles of the tendons are understood, it is still uncertain what constitutes optimal tendon loading and unloading in terms of running economy. It is also unclear whether the loading and unloading behaviour can be modified through change of running metrics, such as stride frequency and stride length. This knowledge can then be used by physiotherapists or biokineticists when planning patient rehabilitation and in injury prevention.”

This trial follows on two previous studies conducted by Dr Dillon in collaboration with Stellenbosch University and MIT. These studies have been published in the American Journal of Sports Medicine and the Journal of Biomechanics respectively.

In this case, the study of each of the five athletes undergoing In Vivo testing begins with the insertion of very thin optic fibre into the patella and Achilles tendons, which in turn is connected to detectors. “Once the fibre is inserted inside the tendon, the tendon fibres compress the optic fibre as the tendon loads. This will reduce the amount of light that is detected by the detector. Once the tendon unloads, the geometry of the optic fibre will return to its original form and the light signal intensity will increase,” explains Llewellyn Groenewald. “As the light intensity changes, the electrical signal output will change. Through a process known as calibration, the change in electrical output can be related to the change in the load that the tendon carries. The team can therefore monitor the change in tendon load by recording the change in electrical output as the volunteer runs on the treadmill.”

In addition to the optic fibre, supplementary measurement equipment is also required. This includes motion capture, electromyography, cardiopulmonary exertion and finally an instrumented treadmill.

Motion capture utilises infrared cameras to track small reflective spheres in the capturing volume. As the volunteer moves, the position of the spheres are recorded and sent to a processing unit, which then calculates the joint angles and body position of the volunteer.

The second tool used for In Vivo testing is electromyography, which allows for measurement of the skeletal muscle activity through probes that detect muscle activation signals, indicating the level of muscular activation. Although the level of activation and amount of force generation are not linearly proportional, models exist that can be used to estimate the force magnitudes. For the study, the team used surface electrodes that attach to the skin.

The third measurement tool is cardiopulmonary exertion, which was utilised during certain running exercises on treadmill. The amount of oxygen that is inhaled and the amount of carbon dioxide that is exhaled is measured along with ECG probes monitoring heart rate. This data is then used to assess the running economy using the principle of gas exchange.

“The final key to measurement is the instrumented treadmill, which has special sensors located below the running bands. The sensors record the ground reaction forces during the stance phase of the running cycle. By recording the ground reaction forces, the research team will be able to monitor the muscle response before and after heel strike. This has been shown to be an important measure in injury rehabilitation,” explains Dr Dillon.

“The state of the art equipment we have available at the Neuromechanics Unit Central Analytical Facilities of Stellenbosch University has made this all possible,” says Groenwald. “We believe that we can provide active value in validating models, while also identifying solid building blocks when considering athlete rehabilitation.”

According to Dr Dillon, “We will invest a lot of time over the next few months to analyse the data sets and a number of research publications will be submitted to prominent journals. We are hoping to gain funding for a follow on study to be done outside the laboratory in specific sporting activities, which will add more specific knowledge to the injury patterns of overuse injuries.”