Who Controls the Prosthetic Leg? Voluntary Control Beyond the Microprocessor Knee

Microprocessor-controlled knees (MPKs) have meaningfully advanced the safety and confidence of many transfemoral prosthesis users, yet a persistent clinical paradox remains: improved device intelligence does not automatically translate into greater everyday activity or full functional recovery. This editorial argues that the value of an advanced prosthetic leg is realized not when the device assumes control on the user's behalf, but when it is coupled with the user's own voluntary control and human-device coordination. Drawing on evidence that objective activity does not consistently increase after MPK provision, that hip strength alone poorly predicts walking and balance, and that attention to the prosthesis increases with use, I propose that rehabilitation should invest in the human control loop as deliberately as in device technology. The microprocessor knee serves here as the representative case for a broader principle that may plausibly extend to microprocessor feet, hip disarticulation components, and powered limbs, though this extension is offered as a hypothesis for future investigation rather than an evidence-based conclusion. My organizing proposition is that, in the age of intelligent prostheses, the human remains the origin of control.