My lower back bends a lot and the weight goes to my arm. What is the problem?
We discussed your position a little in Bicycles Chat. It's possible your frame is one size too small - but we could see a number of possible adjustments that might help.
It looks like your saddle is set too far back and too low. I would start by moving the saddle 5-10mm forward and 10-20mm up.
It also looks like you are pedalling with your toes on the pedal rather than the ball of your foot - this may be (or not) a subconscious compensation for your saddle being so far back.
Making this adjustment may result in needing a longer stem - but try it for a while first and see if you feel cramped.
There are also a couple of factors that are not necessarily related to bike fit.
The rounding of the lower back and 'collapsing' onto the arms can be related to poor core strength. A stronger core and better posture will take weight off the arms.
Finally, your arms are very straight with elbows locked - this doesn't allow your arms to work as shock absorbers and transmits every bump up to your upper arms and shoulders. Keeping a bend in your elbows helps reduce this type of fatigue/soreness (although places more demands on the core muscles).