University of Witwatersrand and Xsens Model The Biomechanics of a Paralympian

Biomechanics is complex. The body moves in a three-dimensional space, pulling and rotating. We see athletes perform stunning movement combinations on TV in a variety of sports. A basketball player pivots, fades back, and fires a three-pointer. A badminton player jumps, twists, and snaps their arm and racket like a whip onto a shuttlecock. A shot putter compresses, extends, and explodes out a cannonball from their arm.

Such complexity is hard to break down into its important parts simply by watching the action. At the highest levels of sport, a few extra degrees of leg flexion during a jump may give an athlete just enough of an edge to secure an Olympic medal. It’s a major challenge for coaches to identify these optimal movement parameters and translate them for their athletes to apply in competition.

Paraplegic athletes provide a unique set of biomechanics challenges that make coaching even more complex. Each athlete has a unique disability that requires their body to move in a very specific way for optimal performance. A person with a below-knee amputation will have different biomechanics than a person with an above-knee amputation, for example.

Tyrone Pillay is such an athlete. He was born with a birth defect of his left leg that’s required him to wear a prosthetic limb since birth. His knee is nonfunctional so he’s classified as an above-knee amputee in the Paralympics.

“It’s a gift that allows me to be who I am today,” said Tyrone.

Tyrone won a bronze medal for South Africa at the Rio 2016 Paralympic Games in shot put and is aiming for gold at Tokyo 2020. He threw a 6 kilogram shot put 13.91 meters then and is aiming for higher.

“I’m as good as I am, but there must be something that can make me better,” said Tyrone.

Tyrone met Jacobus Liebenberg, a biomechanics professor at University of Witwatersrand in Johannesburg, South Africa, in a stadium through a Paralympic athlete journalist friend of Jacobus’s.

“I said to Tyrone: let’s work with Xsens and see if we can collect the data,” said Jacobus.

At that time, Jacobus had been researching biomechanics of cricket players using a motion-tracking suit from Xsens. The suit was loaded with sensors that allowed for motion tracking in the field, away from the restrictions of cameras and closed labs.

“The suit has 17 IMU trackers that each have an accelerometer, magnetometer, and gyroscope,” said Felix Wolbert, a product specialist and business development manager from Xsens. “We can reproduce movements in real time to analyze joint angles and segment data like the acceleration of hand. The main benefit for IMUs is you’re not limited to space. Because there are no cameras, we don’t have to be in a laboratory. The accelerometer measures gravity to measure up and down movements. The magnetometer trackers magnetic north and gives us a heading. The gyroscope tracks angular velocity. We use foot contact detection to determine position. By combining this data together, we have orientation as an output for every tracker.”

Jacobus and his team reconstructed the accelerations and angles of Tyrone’s limbs and combined the data into a viewable model. From this data, they saw that Tyrone could improve by bending his right knee less in the load up and by optimizing his balance over his center of gravity.

Credit: Jacobus Liebenberg, University of Witwatersrand. The figures on the left show Tyrone’s throwing form. On the right, the graphs measure acceleration of Tyrone’s right upper leg, and his right knee joint angle. The blue line shows the amount of flexion, which the team used to determine that Tyrone was bending his leg too much.

Here’s an example recording of Tyler’s shot put taken using the Xsens system:

To Tyrone, this advice was understandable and valuable. Jacobus translated the large amount of technical data into advice that Tyrone and his coaching team could understand.

“What Jaco gave us with the data so far was actually very good and it was readable to us. He gave us three simple points. It’s not like I have to read a 100 page document to figure out what I need to do.”

The team overcame a few challenges in adapting the suits for the Paralympian to get this data.

“Putting the suit over the prosthetic leg was challenging. And I’m not the world’s smallest person. I think Jaco and I underestimated how big my leg was. The fact that I sweat like a grease monkey also didn’t help much, so the [EMG] sensors kept shifting here and there,” Tyrone joked.

Tyrone’s prosthetic was essentially a rigid leg without much flexion in neither the knee nor foot. Fitting the suit over the limb was the biggest challenge. They removed the foot from the leg and put the suit on from the bottom. After getting suited, Tyrone calibrated it in less than a minute by standing and walking.

They ran their trial to set up connections and settings in the lab then headed out to the track. Tyrone was fitted into his suit again, warmed up as if he were in a competition, and then he threw six shots at maximum effort. The team recorded the data, and were done. Overall, it took about an hour on the track from start to finish.

Tyrone enjoyed the Xsens system better than previous motion trackers.

“Previous systems stuck these pods on my hands, but it altered my throw. As an athlete, if you’re gonna be doing testing, you want to be in your real state. The problem with previous systems was that I had to throw a 6kg shot put with a pod stuck on my hand. Now, that’s very difficult and complicated. One of the nicest things here was that Jaco put a little band on my wrist, and it never altered my throwing style. The Xsens is a lot more user friendly and a little bit more comfortable.

Tyrone is positive about the future of sport for Paralympic athletes. He first witnessed the Paralympics in Beijing 2008 and has been pushing for more ever since.

“The whole process was fun and exciting. I hope I can get to do more testing to try and help. Paralympic sport is becoming bigger across the world. I can openly say Tokyo 2020 is going to be the most outrageous ever. Where Paralympic games are going, there’s a place for this testing. We can make athletes so much better.”

The Xsens is available for many applications with athletes in rugby, cricket, cycling, and medical applications such as for the detection of tremors and rehab for stroke.

Edit on February 6, 2019: A previous version of the article stated that Tyrone Pillay was an above-leg amputee. In correction, he had a birth defect that doesn’t allow him use of his knee, which classifies him as an above-knee amputee in the Paralympics.

Photo credits: Lianri Malan

Link: Xsens homepage…

University of Witwatersrand homepage…

Flashbacks: Toward Preventing Shoulder Injuries Using 3-D Motion Analysis…3-D Body Suit Put to Use in Healthcare Research…