Control Systems for commercial bionic legs & feet differ from their upper-limb counterparts. Certainly, they have been more effective. But the desire for increased user control and true sensory feedback persists.
Current Control Systems for Bionic Legs & Feet
With bionic arms & hands, the goal of existing control systems has been to connect the user’s brain to the bionic limb.
However, with bionic legs & feet, control has mostly been delegated to local microprocessors that automatically adjust the prosthesis based on sensor readings. These sensor readings typically include:
- position or angle of the bionic joint;
- walking speed;
- force of impact on the limb;
- slope or nature of the terrain (e.g. stairs).
Automatic adjustments are applied to:
- joint resistance;
- joint angle;
- for joints that offer powered propulsion, the amount of power and the timing of its use.
The goal of these automatic systems is mainly to re-establish a natural walking gait:
Stumble recovery (especially avoiding falls) is also a key goal.
However, these systems are mostly reactionary, i.e. they react only after the residual limb has moved. The prosthesis gets pulled along via a socket or an osseointegrated connector, and only then does the microprocessor make adjustments. These adjustments can occur during both the Swing Phase and the Stance Phase.
Effectiveness of Local Automatic Control
Here is a short video showing a pair of bionic ankles being used to go up and down stairs:
And on rugged terrain:
And navigating a ditch:
And here is a bionic knee being used in a variety of challenging tasks:
It’s difficult to argue with this kind of effectiveness. Local automatic control works.
So what’s missing? Proactive user control and sensory feedback.
Proactive User Control
To move a natural limb, the brain sends signals to motor nerves, which then cause related muscles to contract. It does this even with amputated limbs, although muscle movement may first need to be enhanced through Target Muscle Reinnervation or the implementation of an Agonist-antagonist Myoneural Interface (AMI).
The movement of muscles generates an electrical signal that can be detected by myoelectric sensors either embedded in the muscles or placed against the skin above the muscles. The signals from these myoelectric sensors can then be translated into commands for a bionic device. This type of sensor/control system is the basis for most commercial bionic hands, although it has had its share of problems.
A similar technique is now being explored for bionic legs:
What are the advantages of this type of system? In theory, it can convey user intent the moment that the user’s muscles begin to move.
However, without the use of an AMI interface, control may be less precise than desired. First, the brain has no way to track the position and movement of the limb without visual monitoring, and no way to precisely place a foot without visual guidance. Only AMI addresses these issues.
Also, without power augmentation, the benefits of proactive control are somewhat reduced. Features like automatic flexion resistance and stumble recovery are inherently reactionary anyway. And without advanced sensory feedback and a very fast, trustworthy control system, it is not likely that a human could match the speed or effectiveness of automated responses.
But add propulsive power into the mix and it’s a different story. The strength and timing of myoelectric signals could be used to control power levels more effectively, as well as the transition from the Stance Phase to the Swing Phase (i.e. push off).
So are we looking at some kind of hybrid system, where the user controls limb movements, push-off, and power augmentation while the microprocessor handles reactionary adjustments?
That has yet to be determined. Also, the incorporation of advanced sensory feedback may further blur the line between user vs automatic control.
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Advanced Sensory Feedback
Without some form of sensory feedback to inform the user that the terrain he is walking on has changed, he again must rely heavily on this eyes. This can be mentally taxing.
One solution to this challenge is the same one being explored for upper limbs — a neural interface:
Provide the brain with this kind of feedback and it can take on more sophisticated responsibilities, especially if paired with an accurate and responsive control system.
However, there are significant drawbacks to surgically implanting electrodes to relay sensory feedback to the brain by stimulating nerves. These include:
- cost of surgery;
- the pain of recovery;
- potential nerve damage;
- risk of infection;
- risk of scarring, which may eventually inhibit the ability to stimulate the nerve.
The Role of Osseointegration
While discussing improved user control and sensory feedback, we would be remiss if we did not mention osseointegration.
Osseointegration involves inserting a metal rod into an arm or leg bone with one end of the rod protruding externally. The bone fuses with the internal portion of the rod in a manner similar to fracture healing, while the external portion is used to connect to a bionic limb.
Connecting the bionic limb in this manner improves both user control and sensory feedback. To learn how, see our article on osseointegration.
For a comprehensive description of all current lower-limb technologies, devices, and research, see our complete guide.