A knee exoskeleton is a wearable robotic device designed to augment or restore human movement. These external frameworks provide mechanical assistance, support, and stability to the leg, helping individuals perform tasks they might otherwise find challenging or impossible. They are often custom-fitted and constructed from lightweight materials like aluminum or carbon fiber.
How Knee Exoskeletons Work
Knee exoskeletons operate through a combination of mechanical components, sensors, and control systems. The basic structure includes a frame that attaches to the thigh and calf, motors, and various sensors. These sensors monitor the user’s movements, such as knee angle and leg orientation, and the forces applied during activities. This data then informs the motors on how to provide appropriate assistance or resistance.
Exoskeletons can be categorized into active (powered) and passive (unpowered) systems. Active exoskeletons use external power sources, like batteries, to drive motors and actively assist movement, offering greater strength and mobility. For example, a powered knee exoskeleton might use a brushless motor to provide up to 17.4 Nm of output torque to the knee joint, powered by a portable lithium polymer battery. Passive exoskeletons, in contrast, rely on mechanical elements such as springs, rails, or weights to provide support, often by storing and releasing the user’s kinetic energy. These passive systems are lighter and offer more flexibility, balancing the load rather than actively powering movement.
Rigid exoskeletons feature a stiff frame that provides direct mechanical support and house the motors and sensors. Soft exoskeletons are made from flexible materials, offering a less restrictive and more comfortable fit. These soft components work alongside rigid frame elements to provide a more comfortable and adaptable user experience.
Key Applications
Knee exoskeletons find diverse applications across rehabilitation, mobility assistance, and specialized industrial or military uses. In rehabilitation, these devices are employed to aid individuals recovering from conditions like stroke or spinal cord injuries. They facilitate early mobilization and intensive training, aiding muscle strength, range of motion, and functional recovery. For instance, a powered knee exoskeleton can reduce peak knee torques by 6.24% on the unaffected side and 11.9% on the affected side in stroke survivors during sit-to-stand transitions, also decreasing muscle activation by up to 46.7% in the affected quadriceps.
For mobility assistance, knee exoskeletons help individuals with impaired movement due to aging or chronic conditions. These devices can restore or enhance the ability to walk and perform daily activities, offering mechanical support to reduce the load on compromised knee joints. They are also being developed for children.
In industrial settings, knee exoskeletons help workers perform demanding tasks, such as prolonged squatting or repetitive lifting, by reducing muscle strain and fatigue. For example, a knee-assistive robotic exoskeleton (KARE-1) was designed to support body weight during production line tasks, utilizing a four-bar link mechanism with a rotary actuator and gas spring to efficiently support static sitting and movement between standing and sitting. Military applications also leverage these devices to enhance soldier performance, reduce musculoskeletal injuries, and improve endurance during heavy load carrying and extended missions.
Benefits for Users
Knee exoskeletons offer advantages that improve functional independence. These devices can restore or enhance mobility by providing mechanical support, which helps reduce the load on the knee joint during movement. For individuals with conditions like hemiparesis after a stroke, exoskeletons can significantly improve their ability to stand up, making the movement more symmetrical and requiring less effort. This support translates to increased independence in daily activities.
The reduction in physical effort and fatigue is a key benefit. Exoskeletons can decrease muscle strain, which is particularly helpful for tasks involving repetitive squatting or heavy lifting. They can reduce quadriceps muscle fatigue, allowing users to maintain proper lifting posture even when tired. This improves overall endurance.
Improved balance and stability are also observed with exoskeleton use. For stroke survivors, a powered knee exoskeleton can improve medial-lateral balance by 61.2% to 81.6% during sit-to-stand transitions, based on metrics like degree of asymmetry and center of pressure. This enhanced stability helps prevent falls and promotes safer movement. In rehabilitation settings, the devices support faster recovery by enabling early and intensive training, promoting neuromuscular activation, proprioception, and coordination, leading to improved gait patterns.