Some women AREN'T as naturally slim as e_mcmaster - I can bulk up! One year I was going to be racing lots so I did a major strenthening program with lots of heavier weights and my shoulders went ballistic! Never again! I looked like a shot-putter.
Biceps and triceps are harder to bulk up as a woman...but having said that my cousin hates her muscley arms!
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Warning here!
Never again! I looked like a shot-putter. 
