The harder the forearm flexor-pronator muscles (FPMs) relative to the ulnar collateral ligament (UCL), the less likely it is for UCL laxity to occur with repeated pitching. This study aimed to clarify what selective contraction of the forearm muscles makes FPMs harder relative to UCL. The study evaluated 20 elbows of male college students. Participants selectively contracted the forearm muscles in eight conditions under gravity stress. The medial elbow joint width and the strain ratio indicating tissue hardness of the UCL and FPMs during contraction were evaluated using an ultrasound system. Contraction of all FPMs (in particular flexor digitorum superficialis [FDS] and pronator teres [PT]) decreased the medial elbow joint width compared to rest (p < 0.05). UCL hardens due to the contraction involving the FDS muscle (p < 0.05). FPMs harden due to the contraction of flexor carpi ulnaris (FCU) and FDS (p < 0.05). In the ratio of strain ratio UCL/FPMs, there was no significant difference between rest and each contraction task (p > 0.05). However, contractions composed of FCU and PT tended to harden FPMs relative to the UCL. FCU and PT activation may be effective in UCL injury prevention.
Keywords: baseball; dynamic stabilizers; forearm flexor–pronator muscles; injury prevention; medial elbow joint; selective contraction; ultrasound; youth sports.