A biomechanical energy harvester is presented that generates electricity during human walking. The key feature of this device is that the power generation adds only a minimal extra effort to the user. The knee-mounted devices accomplish this by selectively engaging power generation at the end of the swing phase when knee flexor muscles act to brake knee motion. Analogous to regenerative braking in hybrid cars, the device assists deceleration of each leg within each stride while generating electrical power. We developed a control system to engage/disengage power generation based on the measured knee kinematics during a gait cycle. Experimental results show that generative braking generated 4.8 plusmn 0.8 W of electrical power with a minimal increase in metabolic cost.