The brachialis is the smaller muscle, which lies underneath the biceps.
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There's also a little helper muscle tucked under the bicep called Brachialis.
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This is one of the bicep exercises that pull double-duty in terms of targeting that helper muscle Brachialis mentioned earlier.
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To make your biceps look big and proportionate, your bicep flex routine must include exercises for stimulating the two biceps heads, as well as the brachialis.