The ice hockey body check is the most emphatic defensive tactic available in the game. Like the slam dunk in basketball, a "big hit" often changes the momentum of a contest. A body check is an attempt by a defensive player to stop or limit the progress of an offensive player through the use of his/her body. The rules of ice hockey provide that this check may only be directed toward a player who has possession of the puck: the rules also provide some latitude as to how long and in what manner a player may be said to "finish the check," by maintaining physical contact after the offensive target has lost possession of the puck. The defensive player administering a body check must use the shoulders or hips to deliver the desired contact; the use of the stick, gloves, forearms, or knees are prohibited by rule, but with the speed of movement on the ice it is difficult in some circumstances to accurately determine the exact mechanisms of contact.
A number of factors contribute to the amount of force that will be created and absorbed as a result of the collision created between two players in any given body check, including:
The forces created in an ice hockey check are those governed by the basic physical principle, a product of the mass of the players and the speed with which they are traveling at impact. Ice hockey players informally grade the quality of a check by the use of expressions such as "getting a piece" of an opponent where the check delivered was incomplete, to "a great hit" to describe the type of contact that results in either knocking the opponent to the ice or the otherwise successful elimination of the opponent from that particular sequence of play.
The boards surrounding a modern ice hockey surface, which include the plastic glass attached to them, are not rigid structures. The boards are designed to absorb a considerable degree of force from the bodies that are directed into them. A completely fixed and rigid structure that did not permit absorption of the forces of a body check would create a significantly greater risk of injury for the player taken to the boards. The boards will play a significant part in the measure of the forces created in a check, as often the check target is relatively stationary when the blow is delivered. Depending on the angle of the players' bodies at the point of contact, the player receiving the check may sometimes have his/her head knocked into the glass partition, creating a greater potential for concussion.
The more forceful checks are those delivered in the open ice of the playing surface. Often, these collisions occur when an offensive player looks down at the ice to see the puck in preparation for an offensive maneuver, as he/she approaches the opponent's zone. These checks are often executed when the offensive player is momentarily not aware of the position of the oncoming defender. The defensive player will move forcefully, accelerating toward the offensive player and, as he/she prepares to make contact, the defender will generally lower his/her shoulders and hips to create better stability in preparation for the forces delivered and those that will be absorbed from the target. The defensive player does not have time in these high speed encounters to precisely determine what part of the offensive player he/she will strike. Any contact that results in a trip of the offensive player with the defender's stick, a punch delivered in conjunction with the check, or an apparent blow to the head of the offensive layer will usually result in a penalty being assessed against the defender.
Modern shoulder pads are constructed with a plastic composite surface that creates an extremely rigid and inflexible point of contact with an opponent. Given the weight of a typical professional