TENS works through two main mechanisms. At higher frequencies, it activates the gate control theory of pain — the electrical signals compete with pain signals travelling to the brain, effectively closing the "gate" and reducing the perception of pain. At lower frequencies, it stimulates the production of endorphins, the body's natural pain-relieving chemicals. Neither mechanism produces a muscle contraction — the current targets sensory nerves, not motor nerves.

Motor nerves fire in response to electrical signals from the brain. NMES mimics this by delivering a calibrated electrical pulse to the motor nerve, causing the muscle to contract in the same way it would during voluntary movement. The electrical parameters — pulse width, frequency, intensity, and on/off timing — can be adjusted to target specific muscles and replicate specific movement patterns. Some devices allow the stimulation to be triggered at a precise moment during a movement, which is how protocols like foot drop rehabilitation work.

Muscle fibres generate electrical activity when they contract (and when they relax). Surface electrodes placed over a muscle pick up this activity and send it to the device, where it's processed and displayed — typically as a bar graph, waveform, or audio tone that changes with muscle effort. This gives both the clinician and the patient objective, real-time information about what the muscle is actually doing. The clinical value is significant: patients cannot always feel whether a muscle is activating correctly, and clinicians cannot always see it. EMG makes it visible.