The research team strapped a portable ultrasound system to the calves of participants and imaged their muscles as they performed a series of walking tasks. (Credit: The Harvard Biodesign Lab/Harvard SEAS)

People rarely walk at a constant speed and a single incline. We change speed when rushing to the next appointment, catching a crosswalk signal, or going for a casual stroll in the park. Slopes change all the time too, whether we’re going for a hike or up a ramp into a building. In addition to environmental variably, how we walk is influenced by sex, height, age, and muscle strength, and sometimes by neural or muscular disorders such as stroke or Parkinson’s Disease.

This human and task variability is a major challenge in designing wearable robotics to assist or augment walking in real-world conditions. To date, customizing wearable robotic assistance to an individual’s walking requires hours of manual or automatic tuning — a tedious task for healthy individuals and often impossible for older adults or clinical patients.

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a new approach in which robotic exosuit assistance can be calibrated to an individual and adapt to a variety of real-world walking tasks in a matter of seconds. The bioinspired system uses ultrasound measurements of muscle dynamics to develop a personalized and activity-specific assistance profile for users of the exosuit.

Previous bioinspired attempts at developing individualized assistance profiles for robotic exosuits focused on the dynamic movements of the limbs of the wearer. The SEAS researchers took a different approach.

There is not necessarily a direct mapping between the movement of the limbs and that of the underlying muscles driving their motion. So, in order to study the muscle dynamics, the team strapped a portable ultrasound system to the calves of participants and imaged their muscles as they performed a series of walking tasks.

From these pre-recorded images, they estimated the assistive force to be applied in parallel with the calf muscles to offset the additional work they need to perform during the push-off phase of the walking cycle. The new system only needs a few seconds of walking — even one stride may be sufficient to capture the muscle’s profile.

For each of the ultrasound-generated profiles, the researchers then measured how much metabolic energy the person used during walking with and without the exosuit. They found that the muscle-based assistance provided by the exosuit significantly reduced the metabolic energy of walking across a range of walking speeds and inclines.

The exosuit also applied lower assistance force to achieve the same or improved metabolic energy benefit than has been recorded in previously published studies. “By measuring the muscle directly, we can work more intuitively with the person using the exosuit,” said graduate student Sangjun Lee. “With this approach, the exosuit isn’t overpowering the wearer, it’s working cooperatively with them.” When tested in real-world situations, the exosuit was able to quickly adapt to changes in walking speed and incline.

Next, the research team aims to test the system making constant, real-time adjustments.