Sensitive traps and arch back

I have been working out for three years..

When I started I did not notice these problems until recently my good buddy pointed out the problem I faced when I am having my shoulder workout and back workout.

"Hyper active" traps
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My traps are extremely sensitive due to my constant arching of my back and usage of PC. I tend to slouch forward sometimes.

Whenever I have a deltoids or upper back workout, my traps would be very sensitive and i tend to shrug more often than usual. Thus result in a inefficient workout on my upper and deltoids as I am not able to isolate the muscles.

Any suggestions to curb this problem especially when I am working out and tends to go heavy..

Arched back
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I found out recently that I am not able to perform a good barbell row or deadlift as my back tends to arch and my traps seemed to get more workout than usual.

I have problems maintaining the right posture and I do not want to be seen arching my back while performing "t" bar row as well.

How do I solve this problem that has been bothering me for months?

My forearms are extremely weak. Is that one of the reasons that caused these problems?

Yeah fill up a backpack with bricks and walk around with that on everywhere. Seriously though you shouldn't be asking how to workout better with this problem. You need to fix your posture! Make a conscience effort to not slouch, stand up tall when walking and when at the PC get a new chair, they make posture correcting chairs. Bad posture can be extremely dangerous in the gym! Not to many exercises require you to round your back during thier execution.