Originally Posted by grolby

Regarding handlebar height: it's commonly understood that lower, further away handlebars give an aggressive position that may be worse for the back and that higher, closer bars give a more comfortable, back-friendly position. But this is a dangerous generalization. You can have the bars closer and higher, but because of saddle height and setback have your spine under a lot of tension. Or you can have the bars lower and/or further away but with a neutral spine under a lot less stress. A lot of riders are surprised to find that they are more comfortable after a re-fitting that actually moves them into a more aerodynamic, lower position. It's important to remember that handlebar position is just one of three contact points in a bike fit (hands, in this case), and its effects can't be considered without the relationship between that and saddle height (butt) and setback (feet). How hard the rider typically goes is also important, as torque from the hips can also take some of the weight off of the hands. Whether you're a racer or just cruising, though, the key is that your position be balanced and keep your back as neutral as possible.

As an example, I recently changed my position by moving my saddle up and back a bit, while putting on a 1 cm longer stem. I'm actually more comfortable with this position as it has caused my hips to tilt forward on the saddle, releasing my lower back from a lot of tension. The major adaptation is not my back or shoulders, but my diaphram and core, which were used to being scrunched up rather than elongated. It has also closed up my hip angle a bit, which helps with power in a sprint, but can hurt aerobic output a bit. As a result, I find myself hopping out of the saddle on climbs more often to open it up a bit.

I would suggest consulting a fitter who focuses on racers or performance-oriented cyclists if you want to get lower. If your saddle setback is appropriate for your current bar position, this change will probably require that your saddle come up and forward a bit - effectively rotating you forward without altering your position so much. A fitter can help you move it the right amount. On the other hand, if your position is already pathological, as mine was (hence my moving the saddle back, not forward), the fitter can help you get it right, enabling you to get not only lower but more comfortable and less prone to injury as well.