Arching your back pushes your seat bones back and out from under you, making it hard to ride any of the gaits with a following seat. It also has the tendency to lock your hip joint. Most riders with an arched back have the appearance of stiffness because of the two points above.
Without you posting a photo of yourself riding, it's impossible to say how big or small this flaw is. Since your posting about it, I'm guessing it's a concern.
It's a very hard habit to unlearn. The best resource I've seen is Centered Riding by Sally Swift. You don't have to do every exercise in the book to get value from it, just skip the ones that don't work or make sense to you. Pay particular attention to the lower back, abdomen and general body awareness exercises.
Some people have a more naturally arched back....some people are taught to ride with a small arch in their back.
If you have a naturally arched back, its going to be a bit more difficult to correct, because that is how your body is. However, if you have been taught to do this, it can be "unlearned" its just going to take a lot of effort to undo it.
Do you have a picture of how arched your back gets while riding? It would be much easier to say...oh yeah, that's fine....or no, that's too much.
I have a naturally arched back so I too ride with one, only not because I choose to. I have back problems period. However, it is not to the extreme. Over the fence, I have to make my back feel like it's being "hallowed" to make it flat over fences. It's a disability, not major, but I have taught myself to ride with it and have learned how to achieve a flat back. I think a small arch is fine, but never ride with a major arch. I think it looks tacky.(: Posted via Mobile Device
I have another question; why would you have your leg on as hard as you can get it? Is this something which is taught in jumping? I would think that woudl be exhausting and hard on your horse, too.
One thing I notice about some hunter riders who have too much bend (hollow) in their lower back (this is what you are talking about,, no?), it's that they also have really rigid arms and elbows and are trying to reach as far forward as possible with their hands , hence they also lean forward. They do this because they need to have the rein short enought to be able to effect the horse, but since the horse is running along with no real connection to the bit, no give in his poll nor bend in his neck, it makes it hard to have short enought reins without the rider reaching way up the neck.
Oh, sorry about the video. I really have no idea how to get videos that were tagged on FB to save on your computer:/
Tinyliny- From what everyone is describing I would say I only have a slight arch. It's just enough to make me feel "tall". Also, I've been trained to have a tight leg. For example it shouldnt be swinging around at the canter, it should be 'stuck to the horse'. This tightness also helps your leg not swing back at jumps; which would cause you to fall forward.
From a dressage perspective, it is important to have a steady and quiet leg... but the idea is not to hold it there tightly. As my instructor puts it, it is far better to simply let your legs "drape" around the sides of you horse with the weight sinking down through your heels. You ought to find that it happens pretty naturally when you don't think about it. Too much rigidity is never a good thing. When your legs are properly relaxed and your seat is following the motions of the horse fluidly, you should find that your leg will not move at the canter and so on.