Just wondering why handlebars sweep back/up.
I was told it is more ergonomic and I can see that on really wide bars but not for narrower ones.
Also, wouldn't an agreesive sweep put too much pressure on the outside of your palms?
Is it a fashion thing?

i think it brings the bar closer to your body.

they're alot more comfortable than straight bars, but that is just my opinion

Aesthetics.

I'm a slave to it too.

Sweep is a rider comfort/control thing. Most riser bars have both a rearward and a slightly upward sweep when mounted in their neutral position. The angle chosen (6-9 rearward) is the average person's most neutral grip on the bars. Most bicycle bars are set sweep figures; but sweep is a very personal preference thing. A lot of motocross bar makers are starting to sell bends that are preferred by their star racers and will differ from normal production specs (Like, say, a Ricky Carmichael bend).

The rearward sweep is fine, but I can't work out the upward sweep at the ends? Just put your hands out in front of you like you're holding something and you'll see that it should be a downward sweep at the ends if anything. Coming from an old flat barred bike, the current sweep of the bars is killing me.

Grip a pencil in each hand and hold your arms out like you're reaching for your bars. Look at the angles. You'll see the ends of the pencils pointing slightly ahead and down. That is your neutral grip that 'sweep' is trying to meet. Everyone's neutral is different, though. But even an XC riser bar has 3 degrees of upsweep. If you are uncomfortable, you may not have your bars set right.

My rule of thumb is to set the rise bend section (viewing from the side) parallel to your fork rake and head tube angle. Then slightly tweak up or down (a degree or two) from there for your comfort.

The upward sweep ones I have are bmx and are to be very upright -leverage,
The more down sweep ones are to be back, closer to the body -these put pressure on the outside of my palms run.
I now run a riser bar with backsweep, but flat horizontal.
I imagine a bar without sweep would stress the wrist and elbow as your arm position would change.?

There's another factor to bar comfort that a lot of people don't think about and that's the angle of your levers. A lot of people set them too horizontal - - or angled properly for when they are seated. But adjustement should be for your agressive standing position for descending (especially if you're a downhiller and you 'cover' your brake levers a lot). Standing situations are where you're going to need your brake reach the most; and if your levers aren't adjusted for that, your wrists will fatigue much quicker. Just a thought . . . .

Yep, I agree. The reaction time is quicker if they are down closer to the fingertips. You shouldn't need to change your wrist position to grab some brake. Your wrist\arm should be at the optimum angle during the stop.

i once got a Doug Henry bend and hated it

i think you might be over analyzing it. with your elbows up and flexed and levers adjusted accordingly...everything falls inline pretty well on swept bars