In viscoelastic materials, the elastic region changes its shape depending on the strain rate, meaning the speed at which the material is deformed, or more practically, the velocity of the hit. In general, when the material is strained more quickly it is stiffer, so it deforms less. The degree of protection provided by the helmet depends on the velocity of the hit, and performance is poor for the fastest and slowest hits. Air pockets, while absorbing some energy themselves, also help the helmets perform better in very high and low velocity hits. Air chambers have venting systems so the volume and geometry of air chambers is dynamic, helping dampen the viscoelastic effect.