The caster chooses to wither either the arms or the legs of the subject.
Book of Vile Darkness 3.5
By Monte Cook
Level: Cleric 2, Sorcerer/Wizard 2
Components: V, S
Casting Time: 1 action
Range: Close (25 ft. + 5 ft./2 levels)
Target: One humanoid creature with limbs
Duration: 1 round/level
Saving Throw: Fortitude negates
Spell Resistance: Yes
Withered legs force a character to fall prone and make it impossible for her to move more than 5 feet per round. Withered arms make it impossible for a character to use objects or cast spells with somatic components, and the subject must drop anything she was holding. At the end of the spells duration, the limbs return to normal.