muscle origin is where the bone and muscle are attached, but do not move during contraction and the muscle insertion is where bone and muscle are attached and move during contraction.
The muscle fiber direction points from the origin to the insertion point of a muscle. When a muscle contracts it shortens along that line and pulls the insertion point towards the origin.
but…
In houdini the muscles get scaled in that direction so its the opposite. Draw from the insertion to the origin
origin - around the scapular
insertion - on the forearm after the elbow (radius)
action - Flexion of elbow
origin - around the scapular
insertion - on the forearm after the elbow (ulna)
action - Extension of elbow, extension of shoulder
Pectoralis major (clavicular head)