Sprained Leg is a status effect giving a negative buff that is gained through fall damage. It will decrease the player's speed by 20%, and leave them more susceptible to further fall damage which could possibly result in a Broken Leg. It has a duration of 2400 seconds or 40 minutes in real time.
It is possible to heal a Sprained Leg using a splint, however, this has no real effect on a player's speed as the speed modifier used by the splint will also leave the player with a speed reduction of 20%. Using a splint will help protect a player and should the player receive any fall damage while the splint is applied then they will again end up of with a Sprained Leg or, if the fall is of sufficient height, a Broken Leg.