Passive versus powered
Most artificial legs, even sophisticated ones, are passive. They can store and release energy in springs, and some adjust how stiff a knee is, but they do not add energy. A person walking on one has to supply the extra work, usually by using the hip muscles harder. A powered or “bionic” leg has a motor and battery that put in energy at the knee, the ankle or both. The idea is that the device does some of the work a natural muscle would do, such as pushing off at the end of a step or lifting the body up a stair.
The word “bionic” is more a description of marketing than a technical term. Here it is used for legs with active motors and electronic control. Passive computer-controlled knees, which came earlier, only adjust resistance and do not add energy.
Why walking needs power
Biomechanics studies showed why power matters. During normal walking, the calf muscles and Achilles tendon deliver a burst of energy at push-off, and the knee extensors add power during activities such as climbing stairs or standing from a chair. Researchers measuring gait in the late twentieth century found that passive ankles could return only part of the energy of a natural ankle, and that people with leg amputations tended to use more energy than other walkers. These findings, along with improvements in small motors, encouraged work on active devices.
The university research phase
The first serious powered legs came out of university laboratories in the 1990s and 2000s. A group at Vanderbilt University in Tennessee worked on a powered knee and ankle unit, and a group at the Massachusetts Institute of Technology, led by Hugh Herr, developed a powered ankle. Herr lost both legs below the knee after a mountaineering accident in 1982 and has spoken openly about wanting better devices; his lab’s work gave powered ankles a public profile. Their early prototypes used electric motors, springs and sensors, and they were heavy and had short battery life, but they demonstrated that a prosthesis could push off actively.
A powered knee reached the market in the mid-2000s, and a powered ankle followed. Several companies and research groups have since produced devices of varying design. Specifications and claims differ, and prosthetic users and clinicians generally treat them as one option among many.
How they work
A bionic leg typically contains several parts working together.
- Sensors measure joint angle, load, and the orientation and motion of the leg.
- A small computer interprets the sensor data to decide what the user is doing, such as walking, standing or climbing, and what the joint should do next.
- Motors and gearing apply torque at the joint, with springs often used in series to smooth the output.
- A rechargeable battery, usually lithium-ion batteries, powers the system for a day of use, with limits depending on activity.
The control problem is difficult. The leg has to detect the user’s intent without a direct link to the nervous system, using only the motion of the body. Many designs use patterns in the gait cycle to guess the next step, which works for steady walking but can be less reliable when the user changes speed, turns, or does something unexpected.
What studies have found
Research on powered legs has produced mixed and cautious results. Some studies have found that powered ankles reduce the metabolic cost of walking in certain conditions or help users walk at a more natural speed. Others show smaller benefits, or benefits only for specific tasks, such as slopes or stairs. People differ in age, activity level, and level of amputation, so a result for one group may not apply to another. These devices also cost much more than passive ones, weigh more, need charging, and require maintenance, and clinicians weigh these factors against likely benefits.
Beyond motors
Researchers have tried to link powered legs more directly to the body. One approach uses electrodes on the remaining leg muscles so that the user’s own signals help set the joint’s movement, borrowing from myoelectric arms. Another surgical approach, developed at MIT, aims to keep muscles connected in a way that gives the nervous system better feedback about the position of a prosthetic joint. These ideas are still being tested, and they are part of the wider story of neuroprosthetics.
Alongside powered legs, robotic exoskeletons are being developed to help people who still have their limbs but have difficulty walking, which uses much of the same motor and sensor technology. And fine socket fit still sets the limit of what any advanced leg can achieve: a motor that cannot be comfortably attached cannot help the user.
A cautious view
The bionic leg is best seen as an engineering step, not a finished solution. It shows what is possible when sensors, computing and compact motors are combined, and it points to more capable devices ahead. But the history so far is one of gradual improvement, and for many users a well-fitted passive limb remains the practical choice.