In one embodiment, an apparatus for monitoring locomotion of a person that can be secured to the person's shoe includes a mounting unit, a housing unit, and a motion-sensing device. The mounting unit is adapted to be secured to an outer surface of the shoe. The housing unit is adapted to releasably attach to the mounting unit. The motion-sensing device is attached to the housing unit such that, when the housing unit is detached from the mounting unit, the motion-sensing device remains attached to the housing unit. In another embodiment, a motion-sensing apparatus includes a circuit board and an accelerometer. The accelerometer has an acceleration-sensing axis, and is mounted on the circuit board such that the acceleration-sensing axis is oriented at an acute angle with respect to a surface of the circuit board. In yet another embodiment, a motion-sensing apparatus includes a housing unit and an accelerometer. The housing unit is adapted to be secured to an instep portion of a shoe. The accelerometer is supported by the housing unit such that, when the housing unit is secured to the instep portion of the shoe, an acceleration-sensing axis of the accelerometer is oriented to be substantially parallel to a bottom surface of the shoe or substantially perpendicular to the bottom surface of the shoe.