The number of seniors is rising worldwide. Exoskeleton devices can help seniors regain their lost power, balance, and agility, thus improving their quality of life. Exoskeleton devices and control strategies assist human gait. A common strategy is to use oscillator-based controllers, which “lock in” with the gait and help the subject walk faster using a phase lead characteristic. Such strategies are limited to gait assist only and are less effective in more general movements. These controllers can be detrimental in critical cases such as when the leg needs to execute a fast reactive stepping to stop a fall. We present a control strategy for a hip exoskeleton, which assists human leg motion by providing motion amplification at the hip joint. The controller is “neutral” because it assists any leg motion, not only a gait, and can help avoid falls by assisting reactive stepping. Our control strategy modifies the joint dynamics of the coupled human-exoskeleton system such that the desired dynamic response is achieved while guaranteeing stability. We define assistance as reducing the impedance and increasing the admittance of the coupled system. The dynamic response of the leg is defined by the frequency response profile of the magnitude of integral admittance (torque-to-angle relationship) of the coupled human-exoskeleton system, and assistance occurs when this profile is higher than that of the unassisted leg for all frequencies of interest. Our controller produces hip joint motion amplification and results in larger and faster leg swing motions, and can help recover the seniors' power and agility.