The Effects of Massage for Pronator Teres on Muscles Stiffness and Kinematics During Pitching in Youth Baseball Players

Background and Purpose: Medial elbow injury is a common problem in youth baseball players. The excessive valgus stress across medial elbow occurs during the transition between the late cocking and acceleration phase of pitching. Pronator teres (PrT) is one of the stabilizers to resist the stress. However, the overused PrT may be stiffness and affect the performance. Therefore, this study aims to examine the effects of PrT massage on muscle stiffness and kinematics during pitching in youth players. Methods: Twelve youth baseball players (mean age: 11.3 ± 0.7 years) were recruited to receive evaluations before and after an eight-minute massage for PrT. The passive and active strain ratio of PrT and extensor carpi radials longus (ECRL) muscle stiffness were evaluated using a sonographic elastography. The muscle activities of PrT and ECRL and the acceleration of forearm during pitching were measured using wireless surface electromyography with IMU. The maximum strength of PrT and ECRL were evaluated by a hand-held dynamometer. Eight-minute manual massage with stroking and kneading techniques was applied for PrT. Two-way repeated ANOVA was used to compare the muscles stiffness (condition*time). Wilcoxon signed-rank test was used to compare muscle strength and activity before and after massage. A significant level was set at p < 0.05. Results: The passive strain ratio (before massage: 21.6 ± 9.3, after massage: 31.2 ± 7.5, p = 0.01) and maximum strength (before massage: 4.4 ± 1.0 kg, after massage: 5.1 ± 1.1 kg, p = 0.01) of the PrT were significantly increased after massage, but no differences were found in muscle activity during pitching and active strain ratio. The muscle activity of ECRL during the windup phase was significantly increased after massage (before massage: 1.6 ± 0.7 mV, after massage 1.9 ± 0.9 mV, p = 0.05). Conclusions: Massage intervention can effectively reduce stiffness and increase strength of PrT, which may be essential for elbow stability during pitching. Massage intervention applying stress and stretch continuously to the subcutaneous soft-tissue may change the relationship of PrT length-tension. Participants may be unable to immediately adapt to the altered PrT stiffness and reduce the pitching performance. Clinical Relevance: Massage before training or competition should be carefully applied to avoid athletes' maladaptation.