Hockey players leave the ice after well under a minute of play. The shift length is set by how quickly the body's immediate energy supply runs out.
The energy system involved
Repeated maximal skating draws on stored energy in the muscle that is available immediately but present only in small quantities.
That supply supports something on the order of half a minute of near-maximal work before output begins to fall regardless of how hard the player tries.
Once it is depleted, the body switches to a slower system, and the difference is visible in stride length and in how quickly a player closes on an opponent.
Why the game does not slow down instead
Other sports handle this by pacing, with players jogging between efforts and recovering while play continues elsewhere on the field.
Hockey has no equivalent low-intensity state, because the surface is small and possession changes so quickly that every player is involved almost continuously.
The only available recovery is leaving the ice entirely, which is why the sport permits substitution during live play rather than only at stoppages.
Changing on the fly
Substitutions occur while the puck is in play, with the departing player heading for the bench as his replacement steps over the boards.
The exchange is legal only within a defined distance of the bench and while the departing player is off the ice, so timing is coached carefully.
A change made at the wrong moment leaves the team briefly outnumbered, and goals conceded during a bad change are among the most preventable in the sport.
Lines rather than individuals
Players are grouped into lines that change together, because three forwards who have skated together understand each other's positioning without needing to communicate.
Defensive pairs rotate on a separate cycle, since their work involves less repeated sprinting and they can sustain slightly longer shifts.
Coaches match lines against opposing ones where the rules allow, which turns substitution into a tactical contest running underneath the game itself.
What happens when shifts run long
A player trapped on the ice by sustained pressure loses speed first and decision quality shortly afterwards, and the second loss is more damaging than the first.
Teams that can clear the puck and force a stoppage buy their tired players a change, which is why simply getting the puck out has real value.
Conversely, a team that keeps the puck in the attacking zone is not only creating chances but grinding down opponents who cannot get off the ice.
