Background: We reported that humeral head abnormality at asymptomatic phase was the lesions which were near-related to the consecutive onset of throwing shoulder injury. We also reported that the risk-factor of humeral head abnormality on dynamics was shoulder joint reaction force ratio at the acceleration phase and we developed the simulation system using this risk-factor which could predict the probability of the lesion with high accuracy. The purpose of this study is to evaluate how the hijisagari tends to form the humeral head abnormality using this simulation system.
Methods: The subjects were 13 asymptomatic collegiate baseball players. We made the pitching motion hijisagari on the computer system. The abduction angle of the throwing shoulder was 20 degrees lower than that of the original motion. The shoulder joint reaction force ratio was calculated using multi-body dynamics analysis with a musculoskeletal model. The probability of the existence of humeral head abnormality was calculated using the regression formula we developed before. Then we evaluated the difference between the original motion and hijisagari.
Results: On this simulation, the shoulder joint reaction force ratio of the hijisagari showed an increase of 30 percent over the original (p<0.01) in all the cases. The probability of the existence of humeral head abnormality of the hijisagari showed an increase of 25 percent over the original (p<0.01) in all the cases.
Conclusion: If a player has the pitching motion hijisagari, he will have more probability of the humeral head abnormality caused by the internal impingement and he may suffer from throwing shoulder injury.
