Lived in Laramie (7,,220 ft) and often worked above 10,000 ft for seven years. The Pokes always played a running game at home. You would see the legs and shots drop off about midway in the second half. Football is worse with all the heavy padding. Opponents would rest their players by having injuries every few plays. That saved time outs. The elevation has a strong psychological effect, even for well conditioned athletes. You breathe much harder, therefore you must be tired. As for altitude sickness, it’s often more about dehydration than depleted oxygen.