Ulnar collateral ligament (UCL) rupture is one of the most common throwing arm injuries for throwing athletes. Reconstructive surgery known as Tommy John surgery is often performed to restore joint stability [1]. According to the 2002 Major League Baseball Disability Analysis, almost 70% of players on the disabled list are pitchers and throwing arm related injuries account for 53% of all disabled list placements. To reach a high ball speed, pitchers cock, or excessively externally rotate their pitching arm to or near an extreme ROM of 180° [2]. The shoulder is then immediately internally rotated at over 7000°/s after the leading foot contact. The excessive external rotation ROM and astonishing internal rotation velocity are thought to contribute to throwing arm injury [3]. Repeated exposure to the large valgus torque may cause excessive laxity and catastrophic rupture of UCL [2]. A recent study showed that uninjured pitchers with higher elbow valgus torque exhibited UCL thickening whereas uninjured pitchers with lower elbow valgus torque did not have such adaptation in UCL appearance [4]. It is believed that microtear and catastrophic rupture of UCL are related to higher elbow valgus torque [2]. However, it is not clear how the conditions of the UCL are related to the elbow valgus torque during throwing. Therefore, it is our interest to investigate risk factors to throwing arm injuries. In this study, we investigated the elbow joint loading during throwing among subjects without UCL injury at the time of testing and after testing, with UCL reconstruction (UCL-R) at the time of testing, and UCL reconstruction after testing (PUCL-R). It was hypothesized that there was no significant differences in elbow joint loading between subjects with UCL-R, PUCL-R and uninjured groups. Findings from this may improve our understanding of UCL injury and assist us to identify risk factors for UCL injury.

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