There is a wealth of material available on how to play tennis. Tennis is classed as an open-skill sport by motor learning experts, which means that players must continually modify their motions to an ever-changing scenario on the court. For example, when serving, players must adjust their swings depending on where the ball is hit. The main muscle groups used during tennis are the legs, back, shoulders, and arms. The muscles involved depend on what type of shot is being played. For example, if a player is hitting a groundstroke he or she will use the legs and feet to swing the racquet and will also need to pull the shoulder back while swinging to generate power. A serve involves more body movement than other shots because the player has to whip the racquet up into the air before throwing it down toward the net. Tennis is a very physical sport that requires constant adjustment of one's movements to avoid injury and maximize performance. As well as having good technique, a player needs to be fit enough to withstand hours on the court without tiring.