The ulnar collateral ligament loading paradox between in-vitro and in-vivo studies on baseball pitching (narrative review)

Abstract

Ulnar collateral ligament (UCL) weakening or tears occur in 16% of professional baseball pitchers. To prevent players from sustaining a UCL injury, it is important to understand the relationship between the UCL properties and elbow stabilizers with the load on the UCL during pitching. Invitro studies showed that the ultimate external valgus torque of 34 Nm would rupture the UCL, which is in apparent conflict with the reported peak valgus torques in pitching (40-120 Nm). Assuming both observations are correct, the question rises why 'only' 16 out of 100 professional pitchers sustain a UCL rupture. Underestimation of the effect of other structures in in-vivo studies is most likely the explanation of this mismatch because the calculated in-vivo torque also includes possible contributions of functional and structural stabilizers. In-vitro studies show that the flexorpronator mass has the potential to counteract valgus torque directly, whereas the elbow flexorextensor muscles combined with the humeroradial joint might have an indirect effect on valgus torque by increasing the joint compression force. Accurate experimental electromyography data and a more detailed (musculoskeletal)mechanical model of the elbow are needed to investigate if and to what extent the structural and functional stabilizers can shield the UCL during pitching.

Establishing the Role of Elbow Muscles by Evaluating Muscle Activation and Co-contraction Levels at Maximal External Rotation in Fastball Pitching

“…The in-vitro studies in combination with the results of our study may indicate that the UCL might not resist the entire valgus torque by itself, but that the forearm flexor pronator muscles are able to counteract the valgus torque at least partly during baseball pitching as well. Elbow flexor and extensor muscles, together with the joint articulation, could also indirectly affect the mechanical resistance of the external valgus torque at the elbow during pitching (Van Trigt et al, 2021). In-vitro cadaver studies and forward dynamic model studies showed that the biceps and triceps are important in stabilizing the elbow joint (Morrey et al, 1991;Seiber et al, 2009;Rahman et al, 2018).…”

“…Elbow flexor and extensor muscles, together with the joint articulation, could also indirectly affect the mechanical resistance of the external valgus torque at the elbow during pitching (Van Trigt et al, 2021 ). In-vitro cadaver studies and forward dynamic model studies showed that the biceps and triceps are important in stabilizing the elbow joint (Morrey et al, 1991 ; Seiber et al, 2009 ; Rahman et al, 2018 ).…”

“…The literature shows that not only the UCL but also functional stabilizers, such as muscles, are able to counteract the external valgus torques, either direct or indirect (Van Trigt et al, 2021).…”

“…The literature shows that not only the UCL but also functional stabilizers, such as muscles, are able to counteract the external valgus torques, either direct or indirect (Van Trigt et al, 2021 ). In vitro studies show that the flexor pronator muscle group (FPM), which consists of the m. pronator teres, m. digitorum superficialis, m. flexor carpi ulnaris and the m. flexor carpi radialis, is a significant contributor to counteract an external valgus torque (Park and Ahmad, 2004 ; Lin et al, 2007 ; Seiber et al, 2009 ; Udall et al, 2009 ).…”

“…The forearm flexor muscles could have a direct effect in counteracting the external valgus torque during pitching. In addition, the interaction between the functional stabilizers and the elbow joint geometry could have an indirect effect on the valgus torque by increasing the joint compression force (Van Trigt et al, 2021 ). Several in-vitro studies showed that simulated loading of the biceps and triceps brachii significantly decreased the ulnohumeral joint space and thus resist the external valgus torque (Morrey et al, 1991 ; Seiber et al, 2009 ).…”