I Don't know 100% what volleyball knee pads are made for or Motocross
but I would stick to paintball protection if your playing paintball they are designed for the type of impact you might get when diving around kneeling on stones etc
Like I said I don't know exactly what volleyball and motocross are made for without researching it, I would imagine Motocross pads would be very hard padding to try take the impacts of a fall