BDSM participants are expected to understand practical safety aspects. For instance, they are expected to recognize that parts of the body can be damaged, such as nerves and blood vessels by contusion, or that skin that can be scarred. Using crops, whips, or floggers, the top's fine motor skills and anatomical knowledge can make the difference between a satisfying session for the bottom and a highly unpleasant experience that may even entail severe physical harm. The very broad range of BDSM "toys" and physical and psychological control techniques often requires a far-reaching knowledge of details related to the requirements of the individual session, such as anatomy, physics, and psychology. Despite these risks, BDSM activities usually result in far less severe injuries than sports like boxing and football, and BDSM practitioners do not visit emergency rooms any more often than the general population.