Yo...
I'll tell you my 2 cents on this shyt.
20 lbs dumbbell is kinda light...
so if you are lifting until "you can't go anymore"...that tells me one thing..
You're throwing down too many reps. Look, if you're looking to get some mass..you need to get your punk azz to WalMart and buy bigger weights. When you are doing "as many reps" as you can with a light weight, you will actually lose weight. When I try to get women to lose weight..I tell them to do high reps with a low weight...you're doing that shyt right now.. You need to get heavy weights and do LOW reps...because when you are "burning yourself out" with a low weight...you are losing mass (but getting more definition)
Look...you need to STOP WORRYING about the WAY you are working out...and you need to focus on EATING.
If you are small framed, you SHOULD weigh...
155-168
If you are medium framed, you should weigh..
164-178
If you are large framed, you should weight...
172-197
And i disagree with brucevange
His quote:
They are big overall. You cannot have BIG arms without a big everything else. It all grows. And if anyone tells you otherwise, you're wasting time, energy, and precoius, precious protein.
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You CAN get BIG arms...even though you aren't working out everything else. I've seen people who have huge arms but small shoulders and weak legs. I've seen guys with a huge chest...but a weak back...so they slump forward. You CAN get BIG arms and still be unbalanced....but it's not recommended.
I agree with Bruce..in the fact that a body will look STRANGE...when you have big arms and nothing else....
BUT...
it is possible
Anyways....a long story short. Eat more! Don't do high reps with low weights. Do low reps with heavy weights. Go to walmart and buy some heavier weights...or get a deal at a gym.