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[Game of Thrones] How on earth did Jaime Lannister keep up with Brienne in a sword fight?

He's been a prisoner for over a year, his hands  are shackled, he probably hasn't had a good meal or sleep in months, and he spent the majority of that time sitting in a cage with no exercise. And somehow he can keep up and give a good fight with someone almost twice his size.
gpt
1. Jaime was one of the most skillful and knowledgeable sword fighters in Westeros.  He was knowledgeable enough to point out in their fight that Brienne has a tell when she attacks (she tends to grimace), which is why he can defend against her attacks.  

2. Brienne was taking it easy on Jaime because she's meant to be bringing him to Sansa alive.  As Jaime noted, this meant she would hesitate to use lethal force against him.  

3. He kinda didn't keep up with Brienne.  Their early exchanges were fairly even, yes, but soon enough he was starting to take hits from Brienne, and their fight was concluded when Jaime exhausted himself and he sorta just collapsed at her feet.  Fortunately by then she was distracted by the arrival of the Bolton men, which is why she didn't follow up with a hit on Jaime's exposed back.