To investigate possible alterations on scapular muscle strength in subjects with traumatic anterior glenohumeral instability.Cross-sectional study.Laboratory setting.Fifty-two subjects of both sexes: 26 healthy and 26 with traumatic anterior glenohumeral instability.Subjects performed maximal isometric and concentric isokinetic contractions of shoulder protraction and retraction in scapular and sagittal planes, at slow (12.2 cm/s) and fast (36.6 cm/s) speeds.Subjects with glenohumeral instability presented lower peak force of protraction and retraction during isometric and fast speed tests in the scapular plane; and of isometric protraction in the sagittal plane.People with traumatic anterior glenohumeral instability present muscle weakness of scapular protractors and retractors. Considering the importance of the scapulothoracic muscles for the dynamic stability of the glenohumeral joint, strengthening of these muscles is recommended for rehabilitation of traumatic anterior glenohumeral instability.