Analysis of windmill pitching underscores risk of injury to biceps in softball pitchers

Contrary to common belief, softball pitching subjects the biceps to high forces and torques when the player’s arm swings around to release the ball, according to an analysis of muscle firing patterns conducted at Rush University Medical Center in Chicago.

Published in the March issue of the American Journal of Sports Medicine, the study of the “windmill” pitching motion appears to explain the high incidence of anterior shoulder pain observed in female softball players.

“The conventional belief has been that the underhand throwing motion of softball places little stress on the arm, but that is not the case,” Nikhil N. Verma, MD, lead author and a specialist in sports medicine at Rush, said in a press release.

In the study, seven women — three collegiate and four professional pitchers — underwent motion analysis and surface electromyography to evaluate the muscle firing pattern of their biceps in the course of a windmill pitch. Electromyography detects electrical potential generated by muscle cells when they contract.

The researchers found that even though the upper arm movement in both baseball and fast-pitch softball gives the ball about the same velocity, the muscle force during the windmill pitch was much higher, according to the press release.

Moreover, the maximum force, or maximum contraction, occurred not when the arm was cocked, as in baseball's overhand pitching, but when the arm circled around from the 9 o’clock position (ie, almost fully extended back) to the 6 o’clock position (ie, perpendicular with the ground), completing the windmill motion with the release the ball.

Consequently, the biceps incurred the majority of the stress, not the elbow, Verma noted in the press release.

“The greatest impact is on the biceps, as the muscle first accelerates the arm and then puts on the brakes, after transferring force to the ball,” he said.

Verma launched his study in Rush's human motion laboratory when he found that female softball players from the local professional team were coming into his practice complaining of pain in the front of their shoulders. He was able to localize the pain to the biceps tendon.

In one case, a pitcher had ruptured her tendon during play, which implicated the long head of the biceps tendon as the source of stress, Verma said in the press release. The study findings correlated with these clinical observations.

According to Verma, female softball pitchers are prone to overuse injury not only because of windmill pitching dynamics, but also because they pitch so many games.

“Competitive female pitchers often pitch in every game during a weekend tournament — the equivalent of 1,200 to 1,500 pitches in as little as 3 days,” Verma said. “This is the opposite of the baseball world, where pitchers receive 3 to 4 days of rest before returning to the mound.”

Reference:
Rojas IL, Provencher MT, Bhatia S, et al. Biceps activity during windmill softball pitching. Am J Sports Med. 37;3:558-565.