Topography of motor deficit and spasticity in UMNS

As a medical student, I have been told that upper motor neurone syndrome is a cause of motor deficit and spasticity. The motor deficit is said to be affecting predominantly the extensor apparatus at the upper limbs and the flexor apparatus at the lower limbs. To the contrary, spasticity is flexural at the upper limbs and extensive at the lower limbs.
Can someone explain me why the motor deficit and spasticity affect differently the upper and lower limbs ? I have found no useful reference dealing with the subject. Many thanks in advance.