There are not many volleyball robots (even prototypes) out there. Most motors cannot react fast enough to cause a ball to bounce, nor do they have any internal mechanism to dampen the impact (the the motors are stressed during every blow). Suggest a custom humanoid (or modified commercial humanoid) where you replace the shoulder servos with much faster ones. LEGO Mindstorms would not be powerful enough nor fat enough (nor have enough degrees of freedom) to make a walking robot with arms…