Effects of keiser-assisted Nordic hamstring curl training on hamstring flexibility, mechanical properties, and pitching performance in high school baseball pitchers

[Purpose] This study aimed to investigate the effects of 10-week Keiser-assisted Nordic hamstring curl (NHC) training on the mechanical properties, muscle strength, and flexibility of the hamstrings, as well as shoulder range of motion (ROM) and pitching performance in high school baseball pitchers.[Methods] Sixteen high school baseball pitchers performed Keiser assisted NHC training for 10 weeks. The pre- and post-training measurements included the mechanical properties of the hamstrings (muscle tone, stiffness, elasticity, creep) measured using MyotonPRO, isometric muscle strength, trunk and shoulder ROM, and pitching performance (velocity and spin rate).[Results] After training, hamstring muscle tone, stiffness, and elasticity significantly decreased (p < 0.01), whereas creep and isometric strength significantly improved (p < 0 0.001) (effect size dz = 0.84-2.90). Additionally, the trunk flexion range significantly increased (p < 0.001, dz = 2.12), and the glenohumeral internal rotation deficit in the throwing shoulder significantly improved (p < 0.05). The pitching velocity (p < 0.001, dz = 2.61) and spin rate (p < 0.01, dz = 0.89) significantly increased.[Conclusion] Keiser-assisted NHC training improved hamstring mechanical efficiency, isometric strength, and trunk flexibility, with concurrent gains in pitching velocity and spin rate. As this was a single-group pre-post study without a control group, these findings require confirmation through controlled trials.