Hamstrings for football players are ridiculously difficult injuries to deal with. For one, mild ones are only truly felt when you are making football actions and moves, especially those used by a fast-twitch or deep threat WR. They are notoriously will worsen if you play on them too soon and with the forces on hamstring, they are not an injury that builds up but more one that a sudden movement/jerk can cause a injury.

 

On top of all that depends on how a particular person heals, some people deal with minimal inflammation and scar tissue build up and some people do not. In patient with auto-immune conditions, those are in overdrive all the time. For some people they are in overdrive after injury and can create really pronounced scars called keloids. The only way medically to limit excess inflammation or scarring is with steroids -- which are not generally allowed. 

 

The reason why the inflammation/scarring is important is because more of it can cause pain and tweaks in the recovery process. Some of those can be normal healing. But when players/trainers/etc. are dealing with big injury like a hamstring, they do not want to risk it being a re-injury and just assume it is inflammation. Then on top of that, when that particular muscle or group is healed, the rest of the limb often has less strength and is not particularly balanced which can predispose someone to more injuries.

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