Applied anatomy and kinesiology; the mechanism of muscular movement . Fig. 57 ELEVATION OF THE ARM 103 when it is taken when lying on the face, the posterior deltoid actsin some objects. When the arm is raised forward there is this difference in themechanism of the movement: the middle deltoid is replaced duringthe first 90 degrees of elevation by the anterior deltoid and the. Fig. 58Figs. 56, 57 and 58.—Positions of scapulae in arm elevation above the the scapulre turn forward along the chest wall the full amount of rotationcannot be showTi in such a series of pictures. upper
Applied anatomy and kinesiology; the mechanism of muscular movement . Fig. 57 ELEVATION OF THE ARM 103 when it is taken when lying on the face, the posterior deltoid actsin some objects. When the arm is raised forward there is this difference in themechanism of the movement: the middle deltoid is replaced duringthe first 90 degrees of elevation by the anterior deltoid and the. Fig. 58Figs. 56, 57 and 58.—Positions of scapulae in arm elevation above the the scapulre turn forward along the chest wall the full amount of rotationcannot be showTi in such a series of pictures. upper half of the pectoralis major; above the horizontal there isno difference. Observation of elevation diagonally betw^een for-ward and sideward shows that action of the deltoid is not neces-sarily divided into the three divisions usually named, for portionsof the anterior and middle sections act in this case. 104 MOVEMENTS OF THE SHOULDER-JOINT Many persons are unable to raise the arms above 135 degreeswithout moving the head forward and elevating the chin, showingstrong action of the upper part of the trapezius and weakness orlack of control of the muscles that hold the head erect. Somewriters say that the rhomboid acts in the later stages of arm eleva-tion, but according to Sherringtons law of coordination it oughtto be fully relaxed, so as to permit complete upward
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