<p>Tensed biceps brachii and brachialis muscles, with the forearm supinated and the elbow in a flexed position. Photo of right arm of a male 27 year old wrestler. (23K)
The majority of the bulk here is the brachialis. 
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See also the related image <a href="//commons.wikimedia.org/wiki/File:Arm_flex_pronate.jpg" title="File:Arm flex pronate.jpg">Image:Arm flex pronate.jpg</a>.
No machine-readable author provided. <a href="//commons.wikimedia.org/wiki/User:EncycloPetey" title="User:EncycloPetey">EncycloPetey</a> assumed (based on copyright claims).
CC BY 2.5
No machine-readable source provided. Own work assumed (based on copyright claims).