BackgroundThere is evidence that high-volume static stretching training of the lower limbs can increase the range of motion (ROM) while decreasing muscles stiffness. However, to date, there is no evidence on the effects of upper limb stretching training or its effect mechanism. Therefore, this study aimed to investigate the effects of a comprehensive 7-week static stretching training program of the pectoralis major muscle (PMa) on glenohumeral joint ROM, muscle force, and muscle stiffness.MethodsThirty-eight healthy, physically active participants (23 male, 15 female) were randomly assigned to either the PMa-static stretching intervention (PMa-SS) group or the control group. The PMa-SS group performed a 7-week intervention comprising three sessions a week for 15 min per session, including three static stretching exercises of the PMa for 5 min each. Before and after the intervention period, shoulder extension ROM, muscle stiffness of the PMa (pars clavicularis), and maximal voluntary isometric contraction (MVIC) peak torque (evaluated at both long (MVIClong) and short (MVICshort) muscle lengths) were investigated on a custom-made testing device at 45° shoulder abduction.ResultsIn the PMa-SS group, the shoulder extension ROM (+ 6%;p< 0.01;d= 0.92) and the MVIClong(+ 11%;p= 0.01;d= 0.76) increased. However, there were no significant changes in MVICshortor in PMa muscle stiffness in the PMa-SS group. In the control group, no changes occurred in any parameter.ConclusionIn addition to the increase in ROM, we also observed an improved MVIC at longer but not shorter muscle lengths. This potentially indicates an increase in fascicle length, and hence a likely increase in sarcomeres in series.