Pondering Exhaustion

I love the simplicity of most OSR games, and spend quite a lot of time thinking about them and their rules in quiet moments. You know, waiting for someone to pick up the phone, on the toilette, in the queue at the post office. Anyway these random thought often lead me into coming up with ways of complicating the simple and making the quick slower. 

So with that in mind I was pondering how people get puffed out when fighting, worn down by the effort expended. I was wondering how you might replicate that in an OSR fight and here's what I came up with.

• You track the number of rounds your character is in the fight. 
• Once the number of rounds equals the characters CON, you make a save throw (roll D20 vs CON). 
• If you fail the save you take a -1 on all future combat attacks. 
• Fail or succeed, you reset the count.  
• A success after a failure does not undo the penalty. 
• Multiple failed saves accumulate penalties. 
• Once combat ends, and the character gets a chance for a breather, you stop counting and discard any penalties.

Thus a character with CON:9 would attempt the save every 9 rounds.