上臂内收 meaning in Chinese

adduction of upper limb
medial adduction of the upper arm

Frame # 13 : left hand / forearm force application is backward and powered by upper arm adduction
画面13 ：左手/小臂向后用力，并由上臂内收提供动力。
Frame # 6 : the vigor of the right arm pull is evidenced by the amount of upper arm adduction that has occurred since the previous frame
画面6 ：从上帧画面开始的上臂内收继续加大，这也使得右臂的划水力增大。