Wearable robotics assisting stroke, cerebral palsy patients

The worlds of electronics and medicine collided last week, with not one but two US research groups announcing successful studies into wearable robotic devices for patients struggling with abnormal gaits.

On 26 July, researchers from Harvard and Boston Universities unveiled a lightweight, soft, wearable ankle-assisting exosuit to help reinforce normal gait. The team had already demonstrated that their exosuit technology could deliver assistive forces during walking and jogging in healthy people — now they wanted to know if these benefits could be observed in patients who had suffered a stroke. Their work has been published in the journal Science Translational Medicine.

Up to 80% of stroke patients experiencing a phenomenon called hemiparesis — where one limb loses the ability to function normally — and those who recover their mobility typically retain abnormalities in their gait. Recovering patients develop compensatory walking strategies to deal with their inability to clear the ground with their affected limb and to ‘push off’ at the ankle during forward movement. They typically have to lift their hips (hip hiking) or move their foot in an outward circle forward (circumduction), rather than in a straight line during walking.

Rigid plastic braces worn around the ankle can be prescribed to help with walking, but they do not help overcome these abnormal gait patterns. Exosuits, on the other hand, are anchored to the affected limb of a hemiparetic stroke patient via functional apparel and provide gait-restoring forces to the ankle joint by transferring mechanical power via a cable-based transmission from battery-powered actuators that are integrated into a hip belt or an off-board cart located next to a treadmill.

“In treadmill experiments we found that a powered exosuit improved the walking performance of seven post-stroke patients, helping them to clear the ground and push off at the ankle, thus generating more forward propulsion,” said Jaehyun Bae, a co-first author on the study from Harvard’s School of Engineering and Applied Sciences (SEAS). The team also observed a reduced functional asymmetry between the paretic and non-paretic limbs of participants and found that the exosuit’s assistance enabled them to walk more efficiently.

The team went on to assess exosuit-provided benefits in an overground walking experiment, which typically has different dynamics to a treadmill. Co-first author Lou Awad, who at the time of the study was a postdoctoral fellow under team leader Conor Walsh, said, “It was extremely encouraging to see that an untethered exosuit also had the ability to facilitate more normal walking behaviour during overground walking.

“This is a key step toward developing exosuits as rehabilitation devices for patients to use outside of the clinic and in their normal lives.”

The ankle-assisting soft exosuit can facilitate normal overground walking. Image credit: Rolex Awards/Fred Merz.

The team is currently looking to further personalise exosuit assistance to specific gait abnormalities, investigate assistance at other joints such as the hip and knee, and assess longer-term therapeutic effects of their technology. Kathleen O’Donnell, a staff member at Harvard’s Wyss Institute for Biologically Inspired Engineering, is currently leading efforts to translate the technology to the clinic with industrial partner ReWalk Robotics.

That same day, scientists at New York’s Columbia University announced a robotic training method that improved posture and walking in children with cerebral palsy (CP) — another condition that results in abnormal gait patterns. Their work has been published in the journal Science Robotics.

Slow walking speed, reduced range of motion of the joints, small step length, large body sway and absence of a heel strike are among the difficulties that children with CP experience. A subset of these children also exhibit crouch gait, which is caused by a combination of weak extensor muscles that do not produce adequate muscle forces to keep posture upright, coupled with tight flexor muscles that limit the joint range of motion.

The soleus, an extensor muscle that runs from just below the knee to the heel, plays an important role in preventing knee collapse during the middle of the stance phase when the foot is on the ground. The researchers knew that this major weight-bearing muscle is activated more strongly among the lower leg muscles when more weight is added to the human body during gait. They reasoned that strengthening the soleus might help children with crouch gait to stand and walk more easily.

“We hypothesised that walking with a downward pelvic pull would strengthen extensor muscles, especially the soleus, against the applied downward pull and would improve muscle coordination during walking,” said study leader Professor Sunil Agrawal. “We took an approach opposite to conventional therapy with these children: instead of partial body weight suspension during treadmill walking, we trained participants to walk with a force augmentation.”

The team tested their hypothesis using a robotic system called a tethered pelvic assist device (TPAD), invented in Professor Agrawal’s Robotics and Rehabilitation (ROAR) Laboratory. The TPAD is a wearable, lightweight, cable-driven robot that can be programmed to provide forces on the pelvis in a desired direction as a subject walks on a treadmill.

“TPAD is a unique device because it applies external forces on the human body during walking,” said Jiyeon Kang, lead author of the paper. “The training with this device is distinctive because it does not add mass/inertia to the human body during walking.”

The researchers worked with six children diagnosed with CP and exhibiting crouch gait for 15 short training sessions over six weeks. While the children walked on treadmills, they wore the TPAD as a lightweight pelvic belt to which several wires were attached. The tension in each TPAD wire was controlled in real time by a motor placed on a stationary frame around the treadmill, based on real-time motion capture data from cameras.

The researchers programmed the TPAD to apply an additional downward force through the centre of the pelvis to intensively retrain the activity of the soleus muscles. They used a downward force equivalent to 10% of body weight, based on the results of healthy children carrying backpacks — the minimum weight needed to show notable changes in posture or gait during walking.

The team examined the children’s muscle strength and coordination using electromyography data from the first and last sessions of training and also monitored kinematics and ground reaction forces continuously throughout the training. They found that their training both enhanced the children’s upright posture and improved their muscle coordination. In addition, their walking features, including step length, range of motion of the lower limb angles, toe clearance and heel-to-toe pattern, improved.

“Feedback from the parents and children involved in this study was consistent,” said Professor Heakyung Kim, who treated the patients. “They reported improved posture, stronger legs and faster walking speed, and our measurements bear that out. We think that our robotic TPAD training with downward pelvic pull could be a very promising intervention for these children.”

The researchers are planning more clinical trials to test a larger group and changing more variables. They are also considering studying children with hemiplegic/quadriplegic CP.

Top image caption: Rear view of the Harvard–Boston ankle-assisting exosuit with its soft robotic elements and the cable that transfers mechanical power from actuators operated from the hip to the ankle joint. Image credit: Rolex Awards/Fred Merz.