Shemmy

My first bike was used and I rode it for a long time not having a proper bike fitting professionally done. I recently bought a brand new bike where they fitted me to the bike. I am sore all of the sudden in my back, more muscle soreness pain than injury. Is it common for this to happen?

guadzilla

No, not normal. Looks like you were fitted to the bike, as oppose to the bike being fitted to you.

noimagination

It's impossible for anyone on the internet to provide you with an accurate answer. We don't know what type of bike you had previously and how you were fitted on it. We don't know what type of bike you purchased. We don't know how fit you are, or if you have physical limitations that might affect fit. We are not professionally trained fitters (so, even if we can fit ourselves on a bike because we have lots of experience, we are useless at providing advice on fit to others). Etc.

Soreness could be due to a poor fit. It could also be because the fit is for a different type of bike (for example, going from an upright posture on a hybrid to a lower position on a road bike) and you're muscles just aren't used to it.

Why not go back to the fitter and have him/her address your concerns? If they're any good, they are aware that they are not perfect and that fitting a person to a bike is an empirical process, not an exact science.

Iride01 

Change up some position you were in on one bike and it can take a while for your body to adjust. Also, did you change anything about how you ride or how long you ride. That might be some issue too.

Also, bike fits are not just for comfort. They are also to put you in better positions to efficiently provide power to the bicycle and reduce your wind resistance. These all can't be in their best setting. So compromises have to be made. If you want to be fit for comfort, then you need to make sure the fitter knows that.

You do need to go back to the fitter and discuss your issue. Fitting isn't a mathematical formula that applies to everyone equally.

Chubby715

I agree with Noimagination's post. The lower back soreness would be from riding in a much lower position than on your old bike. And if your gearing on the new bike is different, then your legs need to adjust.

eduskator

It's almost impossible to provide any advices/guidance virtually. If you had a LBS do your fitting, and unless they are incompetent, I would let some time for your body to adapt.

-If you experience pain, I'd consider trying a different fit.
-If you experience discomfort during or soreness after rides, I would wait a few more rides prior to see if your body is only adapting.

For instance, I experienced lower back pain (not discomfort) when I got my new bike and I immediately knew it wasn't normal. I flipped my stem which resulted in higher handlebars, and the pain went away. I was simply too leaned forward.

Perhaps your LBS or the person who did the fit would be able to help you? As other said, it's your bike that should be fitted to you, not the opposite.

Classtime 

How much had you been riding your old bike? You went to the trouble of getting a fitting on your brand new bike and perhaps rode it for a longer than usual ride. That will cause some discomfort.