OnplayerUpdate(playerid)
{
    if(GetPlayerHealth(playerid) < 25.0 && GetPVarInt(playerid, "CrackAnimDone") == 0)
    {
         //ApplyCrackAnimHere
         //Set here a variable so that the crack animation doesn't keep applying. Something like 
         SetPVarInt(playerid, "CrackAnimDone", 1);   
         TogglePlayerControllable(playerid, false);
    }
    else if(GetPVarInt(playerid, "CrackAnimDone") == 1 && GetPlayerHealth(playerid) > 25.0)
    {
         //remove the animation
         SetPVarInt(playerid, "CrackAnimDone", 0);   
         TogglePlayerControllable(playerid, true);
    }
    return 1;
}

// put this at the top of onplayerupdate
new Float:Health;
GetPlayerHealth(playerid,Health);

// replace the other GetPlayerHealth(playerid) with 
Health