In this paper is given a design of device for safe and effective shoulder muscle exercise based on determinated biomechanical requirements which such a device must fulfill. Prior to the biomechanical analysis, complex anatomy of shoulder girdle is shown. Muscles, bones and joints which are part of the shoulder are listed and their role in the shoulder girdle functionality is explained. Influence of the activity of certain deltoid muscle head on shoulder movement is shown by biomechanical analysis using the simplifed 3D model of humerus, deltoid muscle and shoulder joint. Furthermore, in paper is given an insight into glenohumeral joint load, importance of rotator cuff muscles for shoulder stability and position changes of joint rotation axes. The most common injuries of shoulder, as well as the causes and consequences of these injuries, are explained. Shoulder muscle exercises with dumbells, elastic bands and barbell are described. Existing shoulder muscle exercise devices are shown and a review of their functionality, ergonomics and fulfillment of preestablished biomechanical requirements is given. Based on all the guidelines established by biomechanical analysis, an optimal arm movement for deltoid muscle exercise is defined with the emphasis on strengthening anterior and posterior delts. Exercises which fulfill all of biomechanical requirements are presented. All of three presented designs of shoulder muscle exercise devices are based on the same arm motion. Exercise is achieved by lifting the supported barbell. The difference between these three devices are the way device adapts to the anthropometric characteristics of the user and the different way of load selection. The advantage of these devices is enabling of effective and safe deltoid muscle exercising, with the emphasis on anterior and posterior deltoid.