The extensive review of the literature revealed a research gap in evaluating the isometric and isokinetic torque and electromyographic (EMG) activity of shoulder’s flexor and extensor muscles in movements adapted to the gymnastics. The aim of the present study was to examine the reliability and the validity of new and original specific laboratory strength measurements of the shoulders' flexor and extensor muscles, when activating both arms simultaneously with the elbows fully extended. Specifically, the study examined isometric and isokinetic torque, conventional (flexors/extensors) and functional (eccentric/concentric) muscles’ strength ratios, EMG activity, neuromuscular efficiency (torque/HMG ratio, NME) and antagonist/agonist co-activation ratio (MCA) of the flexors and extensors of the shoulder joint. In addition, it evaluated the performance on selected gymnastics skills and its relationship to the strength parameters tested in the laboratory. Fifteen high competitive male gymnasts (GYM) and eighteen physical education students (PE) participated voluntarily in the study (age: 19.37 ± 2.27 and 21.27 ± 2.41 years, respectively). To verify the reliability of peak torque (PT) measurements, two identical measurements with one week interval were assessed using the isokinetic Humac Norm 770 dynamometer at three angles (45º, 90º, and 135º) for isometric and at three angular velocities (60º/s, 180º/s, and 300º/s) for isokinetic concentric and eccentric action modes. All isokinetic measurements were conducted in a range of motion of 10º to 180º (anatomical position in 0º, with the arms by the sides of the body and palms facing forward), in supine position, bilaterally, with the elbows fully extended. During isometric and isokinetic torque measurements, the EMG activity of the muscles: anterior deltoid, clavicular head of the pectoralis major and biceps brachil (flexors) and posterior deltoid, latissimus dorsi and long head of the triceps brachil (extensors) was measured at the same time in the Biopac ΜP100 electromyography system. The relative and absolute reliability of isometric and isokinetic dynamometer measurements was supported by the ICC, SEM, SEMe, CV and Bland-Altman analyses. From the examination of the relations between the strength parameters and the performance in the gymnastics skills, the internal and external validity of the measurements was supported. From multivariate analyses of variance (MANOVAs) and multivariate analyses of covariance (MANCOVAs), with physical characteristics and somatometric parameters as covariates, statistically significant higher PT values were obtained for GYM athletes compared to PE students for all variables tested (with some exceptions in which the covariate was the cross-section area of the right arm). On the other hand, statistically significant differences in shoulders F/E ratios (conventional and functional) were not observed. From the results of EMG analyses it appeared that GYM athletes compared to PE students tended to have higher NME values and lower MCA…