Question: which action shortens the fibers of the serratus anterior

title: Serratus anterior muscle
text: In primates , the thoracic cage is wide and the scapula is rotated onto its posterior side to have the glenoid cavity face laterally . Additionally , the clavicle takes care of medial forces . In cursorial mammals ( for example the horse and other quadrupeds ) , the scapula is hanging vertically on the side of the thorax and the clavicle is absent . Therefore , in climbing animals , the serratus anterior supports the scapula against the reaction forces of the free limb and exerts high bending forces on the ribs . To sustain these forces , the ribs have a pronounced curvature and are supported by the clavicle . In cursorial animals , the thorax is hanging between the scapulae at the serratus anterior and pectoralis muscles .