Exercise to improve the endurance for back muscles

two questions.

Does anyone know any back muscle exercise that doesn't require machine or any equipment?

I started to get back to shadowboxing last Friday after about 2 months of no exercise. Whenever I practiced shadowboxing or if I'm in my boxing stance after 3 rounds my back begins to sore/fatigue. My boxing stance is from the crazy ****** defense boxing like the guy in the brown shirt in this link http://farm3.static.flickr.com.....76.jpg?v=0 .

Is it natural for someone to feel fatigued in their back if they haven't practiced boxing for a while?

 

Maybe you're too tight?
I had this problem

 

pull ups 4 strength but make sure you do lots of flexibility work

 

That doesn't require machine!

 

lay on ur stomach and bend up, lifting ur feet at the same time(kind of forming a U shape with ur body)... it works ur lower back really good

 

Yeah that's what I was going to say. We had to do those at boxing and it's a killer. No equipment needed either.