The upper and lower designation of motor neurons does not describe what parts of the body they contol, but the level of refinement or order of movement. Upper motor neurons govern fine motor control (although exactly how is a bit of a mystery).
Think in terms of an infant, and the level of control they have over their bodies. An infant can move their arms and legs, but lack the motor control to thread a needle or write their name. When UMN damage occurs the body loses its coordination and fine motor control. This is why "primitive" reflexes (similiar to those you see in an infant like the startle response and Babinski's sign) remerge with significant UMN damage.
I hope this helps, and someone certainly can explain it better than I,