Massage device. a) The massage device had two stepper motors mounted on two bi-directional sliding tracks positioned perpendicular to each other. The compressive forces were related to the z-direction, while the transverse forces were related to the x-direction. b) The massage tip was mounted on a custom designed two-axis force sensor which consisted of two strain gauges. A known gravitational force of a standard weight was used to determine the relationship between the strain and measured voltage.