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Relationship between forearm flexor-pronator muscles activity and medial elbow joint gapping distance during

Wataru Machida¹, Rie Nakazawa², Yota Abe^2,3

¹Department of Rehabilitation, Takashimadaira Chuo General Hospital, Tokyo, Japan.

JSES International

|May 13, 2026

Abstract

Background:

Valgus stress on the elbow during pitching can exceed the tensile strength of the ulnar collateral ligament and is the primary cause of throwing-related elbow injuries. The forearm flexor-pronator muscles act as dynamic stabilizers against elbow valgus stress. However, the temporal changes in activity during repetitive pitching and their relationship with valgus stabilization remain unclear. This study aimed to investigate changes in the activity of the forearm flexor-pronator muscles and medial elbow joint gapping during repetitive pitching, and to examine their association.

Methods:

Fifteen healthy adult males with baseball experience performed 100 maximal-effort pitches. Surface electromyography (EMG) of the pronator teres (PT), flexor carpi ulnaris (FCU), flexor carpi radialis (FCR), and flexor digitorum superficialis (FDS) was recorded. The mean and peak EMG activities were calculated as percentages of maximal voluntary contraction. Medial elbow joint gapping was assessed at rest and under valgus stress using ultrasonography. Subjective fatigue related to the pitching task was measured using a visual analog scale. All measurements were taken before pitching and after every 20 pitches. The EMG analysis focused on the phase from the lead leg contact to the end of the pitching motion.

Results:

Peak FCR activity was significantly higher in the 80th pitch than in the 20th pitch. PT showed higher mean and peak activity than FCU, FCR, and FDS at several time points. Medial elbow joint gapping increased significantly at rest after 100 pitches and under valgus stress from the 60th pitch onward. Subjective whole-body fatigue increased significantly after 40 pitches. Positive correlations were observed between the change in medial joint gapping from pitches 1 to 20 and the changes in peak activity of the PT and FCU, as well as the mean activity of the PT. A positive correlation was also found between the change in gapping from pitches 80 to 100 and the change in FDS peak activity.

Conclusion:

Forearm flexor-pronator muscle activation patterns change with increasing pitch count, highlighting the dynamic role of these muscles in elbow valgus stabilization.

Keywords:

Forearm flexor–pronator muscles Medial elbow joint gapping Pitching biomechanics Repetitive baseball pitching Throwing-related elbow injury Valgus stress

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