The term “myoelectric” combines “myo,” from the Greek word for muscle, and “electric,” for electricity. This technology harnesses the faint electrical impulses naturally produced when muscles contract. While the principle of detecting these signals, known as electromyography, has various applications, its primary use is in advanced prosthetic limbs. These devices translate muscle signals from a residual limb into movement, offering a way for users to regain function.
The Science of Myoelectric Control
Myoelectric control is based on the body’s neural communication system. When a person intends to move a limb, the brain sends electrical signals through motor neurons to the muscles, commanding them to contract. Even after an amputation, these signals continue to be sent to the remaining muscles in the residual limb. Myoelectric technology leverages this persistent biological process.
To capture these commands, surface electrodes are placed on the skin over specific muscles in the residual limb. These electrodes detect the weak electrical signals, known as the electromyogram (EMG) signals, generated by muscle contractions. Because the raw EMG signals are faint, they are sent to an amplifier within the prosthesis to boost their strength for processing.
Once amplified, the signals are fed into a microprocessor, a tiny computer housed within the prosthetic device. This processor analyzes the incoming electrical patterns to decipher the user’s intent. The processor translates a specific muscle flex into a command, such as “open hand,” while another muscle’s contraction might signal “close hand.” This command then activates motors that drive the joints of the prosthesis, moving the hand, wrist, or elbow as directed.
Applications in Prosthetics
Myoelectric technology is most commonly applied to upper-limb prosthetics, providing users with active control over their artificial limb. The complexity of these devices depends on the level of amputation. A transradial prosthesis, which attaches below the elbow, is less complex than a transhumeral prosthesis, which fits above the elbow and must also replicate elbow function.
These prostheses can perform a range of functions, from basic, strong grips to dexterous movements. Simpler devices may offer a single grip pattern, like a pincer or hook, which is robust for many daily tasks. More advanced models are multi-articulating hands with individually powered fingers capable of executing multiple grip patterns. This allows for nuanced tasks, such as picking up delicate objects or holding items of various shapes.
Hybrid prosthetics also exist, combining myoelectric controls with body-powered components. For instance, a user might have a myoelectric hand for fine motor control, paired with a body-powered elbow that is operated by a harness system. While the primary application remains in upper limbs, research continues to explore myoelectric control for lower-limb prostheses to help manage functions like ankle rotation or foot flexion.
Learning to Use a Myoelectric Prosthesis
Operating a myoelectric prosthesis is a learned skill that requires practice and professional guidance. A prosthetist designs and fits the custom socket of the prosthesis to ensure proper contact between the electrodes and the user’s skin. Following this, the user works with an occupational therapist to learn how to control the device effectively.
The initial phase of training focuses on mastering basic muscle control. The user must learn to consciously isolate and contract the specific muscles that will operate the prosthesis. For example, they practice flexing one muscle to generate a clear “open” signal and another for a “close” signal. This stage often involves using a training unit with visual feedback, like a computer game, to help the user see the strength of their muscle contractions.
As the user becomes proficient, training progresses to functional use. The focus shifts to integrating the prosthesis into daily activities, such as grasping objects of different weights and textures or performing bimanual tasks. Over time and with dedicated practice, these actions can become more intuitive, allowing the user to operate the limb without intense conscious thought.
Technological Evolution and Variations
The field of myoelectric prosthetics is advancing beyond foundational two-signal control systems. A development is the use of pattern recognition technology. Instead of relying on just two electrode sites, pattern recognition systems use an array of electrodes to capture complex patterns of muscle activity. Software then learns to associate a user’s unique signal patterns with specific intended movements, allowing for more fluid and intuitive control.
For individuals with higher-level amputations, a surgical procedure known as Targeted Muscle Reinnervation (TMR) offers another path to enhanced control. In TMR, surgeons reroute the residual nerves that once controlled the amputated limb to reinnervate other muscles, such as those in the chest or upper arm. These reinnervated muscles then act as biological amplifiers for the nerve signals.
This procedure creates new locations for placing electrodes, increasing the number of available control signals. A patient who has undergone TMR can, for example, think about closing their missing hand, and the reinnervated chest muscle will contract, sending a signal to the prosthesis to perform that action. This allows for the simultaneous control of multiple joints, such as moving the elbow and rotating the wrist at the same time.