Past research has shown that a horizontal force applied to the trunk greatly assists the user's gait by reducing metabolic cost and reducing the horizontal ground reaction force. Different from the literature which describes a constant applied, horizontal force, the authors hypothesized that a horizontal tether force will not be constant but will oscillate due to the periodic nature of human walking gait. This hypothesis was tested by analyzing the tether force, ground reaction forces, and gait kinematics and kinetics of six able-bodied human participants. An assistive device was designed by attaching a spring-tether to the user's trunk and the other end was affixed to the treadmill. The user naturally found a position on the treadmill to stretch the spring tether. Multiple tethers with different stiffnesses were used. The subjects were asked to walk on a treadmill at 1.2 m·s-1 while wearing the assistive device. Motion data, volumetric rate of oxygen consumption (Vo2)data, and the tether force data were collected. Three iterations of the tests were performed per participant, and the data was averaged. The horizontal assistive force was found to be periodic with twice the frequency of the gait cycle. Also, (Vo2) was found to be lower when wearing the device. The tether with the lowest stiffness was found to be the most effective in terms of reducing the metabolic rate. The authors concluded that the assistive device supplied power just at push-off for each foot while reducing the metabolic rate by 17% on average.