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, Repetitive baseball pitching, Pitching biomechanics, Valgus stress, Throwing-related elbow injury
Throwing-related elbow injury is a general term for damage to bones, cartilage, ligaments, and muscle–tendon attachments caused by repetitive throwing motion. Such injuries occur frequently across a wide range of age groups in baseball. Throwing-related elbow injuries can be classified into medial, posterior, and lateral lesions, with medial injuries having the highest incidence.3,15 Most throwing-related elbow injuries are primarily caused by repetitive valgus stress applied to the elbow joint during pitching. The ulnar collateral ligament (UCL) is the main stabilizer that resists valgus stress at the elbow.
The valgus torque on a pitching elbow is reported to be approximately 64 Nm, a force greater than the UCL's failure strength.9 Therefore, the UCL alone, as a static stabilizer, is thought to be insufficient to control valgus stress during pitching fully. Recent studies have focused on the dynamic valgus stabilization mechanism provided by the forearm flexor–pronator muscle group. The flexor digitorum superficialis (FDS), pronator teres (PT), flexor carpi ulnaris (FCU), and flexor carpi radialis (FCR) muscles attach to the medial epicondyle and have been reported to be anatomically connected to the UCL.14 Furthermore, pitchers with UCL deficiency show lower forearm muscle activity during pitching,10,11 suggesting that these muscles play a protective role as secondary dynamic stabilizers against valgus stress. A previous study investigating time-dependent changes in the medial elbow joint space reported that the distance increased after 60 consecutive pitches.12 This suggests that repetitive pitching induces a decline in the valgus stabilization function of the elbow. However, few studies have longitudinally evaluated the function of the forearm flexor–pronator muscles and the medial elbow joint space during a pitching session, and many aspects remain unclear. A detailed analysis of the pitching load is imperative for injury prevention. Therefore, a detailed investigation of the biomechanics of the forearm flexor–pronator muscles during consecutive pitching is essential to improve the prevention and rehabilitation of throwing-related elbow injuries.2,8
This study aimed to investigate temporal changes in the activity of the forearm flexor–pronator muscles and the medial elbow joint space during and after a consecutive pitching session. We hypothesized that, as the number of consecutive pitches increased, medial elbow joint gapping would increase, and the activity of each forearm flexor–pronator muscle would increase reactively.
Materials and methods
Participants
Participants were individuals who had played baseball for at least 5 years while belonging to organized baseball teams from elementary school through adult competitive levels. Organized baseball is defined as participation in officially sanctioned teams with regular practices and competitive games. At the time of the study, all participants actively played baseball and regularly pitched as part of the organized team activities.
Written informed consent was obtained from all participants and the study was approved by the institutional ethics committee. The exclusion criteria were as follows: (1) currently undergoing treatment for an injury or disease affecting the shoulder or elbow joint, and (2) having performed a full-power throw within the preceding 24 hours.
Questionnaire
Participants completed a questionnaire to collect demographic information (age, height, weight, medical history) and baseball-specific data, including years of experience, hand throwing, primary position, ball type used, throwing style (overhand, sidearm, underhand), usual practice participation, and activity within the last 24 hours.
Throwing tasks and measurements
The participants performed 100 maximum-effort pitches toward a protective net 3 m away (Fig. 1). Measurements, including forearm flexor–pronator muscle activity, medial elbow joint space, and subjective fatigue, were obtained before pitching and after every 20 pitches. To minimize variability due to pitching proficiency and ensure data reliability, the following measures were implemented: standardization of pitching intervals and rest periods; intervals between each pitch trial were standardized to 15 seconds. In addition, a 10-minute rest period was implemented after every 20 pitches to eliminate the influence of muscle fatigue on electromyographic signals.
Figure 1.
Measurement environment for the pitching task.
Forearm flexor–pronator muscle activity
A multitelemeter system (WEB7000; Nihon Kohden, Tokyo, Japan) recorded the muscle activities of the FCU, FCR, PT, and FDS (Fig. 2). The skin was shaved, abraded with sandpaper, and cleaned with alcohol to minimize skin impedance. Surface electromyography (sEMG) electrodes were placed on the muscle bellies of the FCU, FCR, PT, and FDS in a bipolar configuration. The electrode placement, orientation, and interelectrode distance were strictly determined in accordance with the recommendations for sEMG sensors and placement procedures by Hermens et al and the guidelines provided by the Surface EMG for the Non-Invasive Assessment of Muscles project (http://www.seniam.org/).13 Each muscle belly position was identified under direct visualization using ultrasound, and correct placement was further verified by manual muscle testing to ensure the isolation of the target muscle and to minimize crosstalk from adjacent musculature. For forearm musculature, where the muscles are densely packed, we focused on the functional isolation of each muscle. Following the experimental framework of Mogk and Keir,16 surface electrodes were positioned over the following landmarks for each target muscle: FCR (one-third of the distance from the medial epicondyle to the distal head of the radius); FCU (on the ulnar border, positioned along the line between the medial epicondyle and the pisiform, at approximately the proximal one-third to one-half of the span); PT (over the oblique path from the medial epicondyle toward the lateral mid-radius, following the diagonal orientation of the muscle); FDS (at the midpoint [approximately one-half] of the forearm, along the line connecting the medial epicondyle to the center of the palmar wrist). This circumferential approach combined with functional validation helped mitigate the risk of common signal content between adjacent synergists.
Figure 2.
Electrode placement for surface electromyography. PT, pronator teres; FCU, flexor carpi ulnaris; FCR, flexor carpi radialis; FDS, flexor digitorum superficialis.
The sampling frequency was 1,000 Hz with a 30-500 Hz bandpass filter. Data were processed using a 100-ms sliding-window root mean square and normalized to maximum voluntary contraction (MVC). The analysis window was defined as the phase from the initiation of the pitching motion to the end of the follow-through phase. The peak and mean %MVC were calculated. For the analysis, data from the first to third, 18th to 20th, and 38th to 40th, 58th to 60th, 78th to 80th, and 98th to 100th pitches were averaged and subsequently referred to as measurements at 1, 20, 40, 60, 80, and 100 pitches, respectively.
Medial elbow joint space
The medial elbow joint space was measured using ultrasonography (Viamo SSA-640A; Toshiba). Participants lay supine with their shoulders abducted to 90° and in maximal external rotation, their elbows flexed to 90°, and their forearms in a neutral position. A physiotherapist with 8 years of experience acquired images of both ulnohumeral joints. Images were captured at rest with the elbow supported to prevent valgus stress and under gravity-induced valgus stress with the support removed (Fig. 3, A and B). The distance between the distal trochlea of the humerus and proximal ulnar coronoid process under rest and valgus stress was measured to the nearest 0.1 mm (Fig. 4). Measurements were taken before pitching and every 20 pitches. The primary investigator assessed the intra- and inter-rater reliabilities of the ulnohumeral joint width measurements. To determine intra-rater reliability, the investigator measured 10 healthy elbows at rest and under valgus stress and then remeasured them 5 days later. The intraclass correlation coefficient was 0.98 for the rest measurement and 0.91 for the valgus stress measurement. For inter-rater reliability, the primary investigator and second investigator examined 10 healthy elbows. The resulting intraclass correlation coefficient was 0.81 at rest and 0.86 under valgus stress.
Figure 3.
(A) Measurement of medial elbow joint space at rest. (B) Measurement of medial elbow joint space under gravity-induced valgus stress.
Figure 4.
Measurement of medial elbow joint space. (A) Medial epicondyle of the humerus. (B) Distal end of the humeral trochlea. (C) Coronoid process of the ulna.
Subjective whole-body fatigue levels related to pitching tasks
A visual analog scale (VAS) was used to assess subjective whole-body fatigue. Participants marked a 100-mm line ranging from “no fatigue” to “maximum fatigue ever experienced.” Measurements were performed before and after every 20 pitches.
Statistical analysis
Statistical analyses were performed using IBM SPSS Statistics (ver. 28.0; IBM Corp., Armonk, NY, USA) with a significance level of P < .05. Data normality was assessed using the Shapiro–Wilk test. Depending on the distribution, changes in forearm flexor–pronator muscle activity and medial elbow joint space across the pitching session were analyzed using either a repeated-measures analysis of variance or the Friedman test. Specifically, for the FCR, normality was confirmed; thus, a repeated-measures analysis of variance was applied. Subjective whole-body fatigue (VAS scores) and intermuscle comparisons at each pitch count were analyzed using the Friedman test. For all significant findings, post hoc pairwise comparisons were conducted using Bonferroni correction. Correlations between changes in the medial joint space and muscle activity or subjective fatigue VAS scores were assessed using Pearson or Spearman correlation coefficients, depending on normality, with changes calculated by subtracting the measurement at the start of an interval from the measurement at the end of that interval.
Results
Participant characteristics
The study included 15 right-handed male baseball players (age, 26.3 ± 1.8 years; mean experience: 11.5 ± 3.8 years). Participants were required to have at least 5 years of experience in organized baseball (officially sanctioned teams) and be actively pitching at the time of enrollment. All were cleared for full athletic activity. All participants utilized an overhand throwing style. Their competitive history spanned elementary to adult levels, with universal experience in both rubber-ball (6.9 ± 3.0 years) and hard-ball (4.5 ± 2.9 years) categories. Furthermore, their positional backgrounds were diverse: 8 pitchers, 5 catchers, 10 infielders, and 6 outfielders. The mean height, weight, and body mass index were 172.0 ± 7.0 cm, 72.0 ± 11.5 kg, and 24.3 ± 3.1, respectively.
Two participants had a history of subjective shoulder pain and medial elbow pain. Subjective shoulder pain and medial elbow pain occurred during high school (10 and 12 years before the study, respectively). For both conditions, neither participant sought medical evaluation nor received a formal diagnosis or treatment. All symptoms resolved with rest, and the players continued baseball participation without symptom recurrence. At the time of enrollment, both participants were asymptomatic and capable of full-effort pitching during regular team activities.
Changes in forearm flexor–pronator muscle activity
Peak %MVC, and mean %MVC of the forearm flexor–pronator muscles during the pitching session are summarized in Tables I and II. The peak %MVC of the FCR was significantly higher at 80 pitches than at 20 pitches (P < .05). The mean %MVC of the FCR also showed a significant overall difference (P < .05), although post hoc comparisons did not reach significance. However, no significant temporal changes in the peak %MVC and mean %MVC were observed for the PT, FCU, and FDS muscles during the pitching sessions. At each measurement point, PT consistently showed significantly higher activity than the other muscles at several time points.
Table I.
Peak muscle activity during pitching.
| Pitch count | PT (%) | FCU (%) | FCR (%) | FDS (%) |
|---|---|---|---|---|
| 1 pitch | 141.7 (98.7-158.3)∗ | 54.2 (46.2-94.1) | 91.8 (66.2-152.3) | 79.8 (57.2-116.5) |
| 20 pitches | 129.7 (112.8-254.5)∗§ | 75.4 (51.1-112.0) | 82.5 (58.6-139.4) | 63.4 (53.1-91.7) |
| 40 pitches | 168.0 (116.5-247.2)∗‡§ | 65.2 (50.8-117.1) | 84.9 (63.8-133.0) | 61.6 (47.3-95.6) |
| 60 pitches | 204.9 (109.2-257.6)†§ | 62.6 (45.9-130.0) | 106.8 (85.6-167.4) | 85.0 (55.6-92.9) |
| 80 pitches | 158.1 (87.6-233.4) | 76.2 (53.1-172.7) | 97.1 (72.3-154.7) | 98.1 (63.1-118.2) |
| 100 pitches | 195.8 (91.6-277.5)† | 83.3 (52.1-129.4) | 147.3 (80.2-199.9) | 80.8 (66.9-128.4) |
PT, pronator teres; FCU, flexor carpi ulnaris; FCR, flexor carpi radialis; FDS, flexor digitorum superficialis; MVC, maximum voluntary contraction.
%MVC values are presented as the median [first quartile, third quartile] for PT, FCU, FDS, and Between muscle comparisons (at each pitch count).
PT vs. FCU
PT vs. FCR.
PT vs. FDS
PT values were significantly different from FCU values at 1, 20, and 40 pitches (P < .01).
PT values were significantly different from FCU values at 60 and 100 pitches (P < .05).
PT values are significantly different from FCR values at 40 pitches (P < .05).
PT values were significantly different from FDS values at 20, 40, and 60 pitches (P < .01).
Table II.
Mean muscle activity during pitching.
| Pitch count | PT (%) | FCU (%) | FCR (%) | FDS (%) |
|---|---|---|---|---|
| 1 pitch | 44.8 (37.3-55.0)∗ | 23.5 (14.9-36.5)† | 36.7 (24.4-53.6) | 28.9 (23.1-38.7) |
| 20 pitches | 45.9 (38.2-91.5)§ | 31.3 (20.2-45.1) | 39.3 (27.4-53.7) | 26.1 (19.8-38.4) |
| 40 pitches | 59.0 (41.1-80.1)†,‖ | 24.1 (19.6-48.0)† | 32.6 (26.6-50.7)‡ | 26.0 (18.1-39.4) |
| 60 pitches | 67.3 (38.6-78.3)∗,‖ | 2.4 (18.5-49.1)† | 42.5 (33.2-57.5) | 33.2 (16.6-40.7) |
| 80 pitches | 55.2 (37.6-82.6) | 34.2 (21.6-49.5) | 43.9 (30.5-63.0) | 31.1 (24.9-49.6) |
| 100 pitches | 67.9 (41.6-87.5) | 34.1 (25.3-49.5) | 52.8 (27.9-69.7) | 35.5 (27.1-44.1) |
PT, pronator teres; FCU, flexor carpi ulnaris; FCR, flexor carpi radialis; FDS, flexor digitorum superficialis; MVC, maximum voluntary contraction.
%MVC values are presented as median [first quartile, third quartile] for PT, FCU, FCR, and FDS.
Between muscle comparisons (at each pitch count).
PT vs. FCU.
PT values were significantly different from FCU values at 1 and 60 pitches (P < .05).
PT values were significantly different from FCU values at 40 pitches (P < .01) FCR vs. FDS.
FCR values were significantly different from FDS values at 40 pitches (P < .05) PT vs. FDS.
PT values were significantly different from FDS values at 20 pitches (P < .05).
PT values were significantly different from FDS values at 40 and 60 pitches (P < .01).
Changes in the medial elbow joint space
The medial elbow joint space at rest and under valgus stress increased throughout the pitching sessions (Table III). Significant widening at rest was observed after 100 pitches and under valgus stress at 60, 80, and 100 pitches in comparison to prepitching values (P < .05).
Table III.
Medial elbow joint space distance.
| Pitch count | Before pitching | 20 pitches | 40 pitches | 60 pitches | 80 pitches | 100 pitches |
|---|---|---|---|---|---|---|
| Under rest (mm) | 3.5 ± 1.0 | 3.5 ± 0.9 | 3.5 ± 1.0 | 3.7 ± 1.0 | 3.7 ± 1.0 | 3.9 ± 1.0∗ |
| Under gravity valgus stress (mm) | 4.2 ± 1.3 | 4.2 ± 1.3 | 4.4 ± 1.2 | 4.8 ± 1.2† | 4.9 ± 1.2† | 5.0 ± 1.2† |
Values are presented as the mean ± standard deviation (SD).
P < .05.
P < .01: significant difference relative to before pitching.
Visual analog scale assessment of subjective whole-body fatigue
Subjective whole-body fatigue, as assessed by VAS, increased significantly after 40, 60, 80, and 100 pitches compared to the prepitching level (P < .05, Table IV).
Table IV.
VAS score of fatigue levels during pitching.
| Pitch count | Before pitching | 20 pitches | 40 pitches | 60 pitches | 80 pitches | 100 pitches |
|---|---|---|---|---|---|---|
| VAS (mm) | 0.0 (0.0-0.0) | 1.5 (1.1-2.3) | 3.0∗ (2.3-3.6) | 4.5∗ (3.8-5.8) | 6.2∗ (5.2-7.8) | 7.5∗ (7.2-8.2) |
VAS, visual analog scale.
Values are presented as medians (first quartile, third quartile).
P < .01: Significant difference from the prepitching value.
Correlations between medial joint space, muscle activity, and visual analog scale of fatigue levels
No significant correlation was observed between changes in the medial joint space and subjective fatigue VAS scores at the 20-pitch intervals (Table V), although a positive trend was noted from 40 to 100 pitches. Significant positive correlations were observed between the change in the medial joint space under valgus stress and the change in peak %MVC of the PT and FCU from the prepitching state to 20 pitches. A significant positive correlation was also found between the change in the medial joint space from 80 to 100 pitches and the change in the peak %MVC of the FDS (Table VI). The change in the medial joint space from the prepitching state to 20 pitches was also positively correlated with the change in the mean %MVC of the PT (Table VII).
Table V.
Correlation between change in MEJS (ΔMEJS) and change in subjective fatigue VAS (ΔVAS) per 20-pitch segment.
| Pitch interval | ΔVAS-ΔMEJS (r) |
|---|---|
| 1-20 | 0.32 |
| 21-40 | 0.20 |
| 41-60 | 0.47 |
| 61-80 | 0.49 |
| 81-100 | 0.49 |
VAS, visual analog scale; MEJS, medial elbow joint space.
Values are presented as correlation coefficients (r).
No significant correlation was observed.
Table VI.
Correlation between change in MEJS (ΔMEJS) and change in peak %MVC (Δpeak %MVC) for each muscle per 20-pitch segment.
| Pitch interval | Δpeak %MVC of PT-ΔMEJS (r) | Δpeak %MVC of FCU-ΔMEJS (r) | Δpeak %MVC of FCR-ΔMEJS (r) | Δpeak %MVC of FDS-ΔMEJS (r) |
|---|---|---|---|---|
| 1-20 | 0.71† | 0.67† | 0.45 | 0.34 |
| 21-40 | −0.09 | 0.33 | 0.14 | 0.35 |
| 41-60 | −0.03 | 0.14 | −0.42 | 0.40 |
| 61-80 | 0.27 | 0.01 | −0.10 | −0.16 |
| 81-100 | −0.11 | 0.10 | 0.11 | 0.56∗ |
PT, pronator teres; FCU, flexor carpi ulnaris; FCR, flexor carpi radialis; FDS, flexor digitorum superficialis; MEJS, medial elbow joint space; MVC, maximum voluntary contraction.
The values are correlation coefficients (r). Significant correlations are indicated by ∗P < .05, †P < .01.
Table VII.
Correlation between change in MEJS (ΔMEJS) and change in mean %MVC (Δmean %MVC) for each muscle per 20-pitch segment.
| Pitch interval | Δmean %MVC of PT-ΔMEJS (r) | Δmean %MVC of FCU-ΔMEJS (r) | Δmean %MVC of FCR-ΔMEJS (r) | Δmean %MVC of FDS-ΔMEJS (r) |
|---|---|---|---|---|
| 1-20 | 0.61∗ | 0.24 | 0.05 | 0.34 |
| 21-40 | −0.02 | 0.49 | −0.07 | 0.16 |
| 41-60 | 0.06 | 0.08 | 0.17 | 0.20 |
| 61-80 | 0.33 | 0.06 | −0.22 | −0.29 |
| 81-100 | 0.46 | 0.07 | −0.01 | −0.11 |
PT, pronator teres; FCU, flexor carpi ulnaris; FCR, flexor carpi radialis; FDS, flexor digitorum superficialis; MVC, maximum voluntary contraction; MEJS, medial elbow joint space.
The values are correlation coefficients (r). Significant correlations are indicated by ∗P < .05.
Discussion
A key finding of our study was the potential for significant positive correlations between changes in medial elbow joint gapping and flexor–pronator muscle activity during 100 maximum-effort pitches. In the early phase of pitching (1-20 pitches), significant positive correlations were observed between changes in medial elbow joint gapping and PT and FCU activities. While previous studies have highlighted the valgus-stabilizing functions of PT and FCU, few have investigated their contribution during actual pitching.14,17,19 These findings appear to align with those of previous reports. The correlation observed during the first 20 pitches might be attributed to the demanding protocol of 100 maximal-effort pitches; in the early stage, pitching mechanics may have been unstable, potentially necessitating increased PT and FCU activity for valgus stabilization. Previous studies have indicated that motor variability is driven significantly by neural fluctuations during the planning stage, rather than being solely a product of peripheral muscular noise.4,5 In the early phase of pitching, the brain's "preparatory state" may not yet be fully calibrated. This lack of optimization, characterized by high trial-to-trial neural variability, leads to kinematic instability. Central to this process is quenching, the rapid reduction of cortical variability at the onset of a task. It is suggested that during the first 20 pitches, this neural convergence is incomplete, meaning that the brain is still refining the optimal motor commands, which manifests as the observed instability in movement.
Significant medial elbow joint gapping occurred only after 60 pitches, suggesting that structural changes in the elbow may not be directly associated with injury risk in this early phase. Notably, between 80 and 100 pitches, FDS peak %MVC appeared to be related to medial elbow joint gapping, whereas FDS mean %MVC did not, potentially indicating that momentarily high FDS activation might contribute to controlling valgus stress. Anatomical studies have reported tendon septa connecting the PT, FDS, and FCU to the medial epicondyle, and their close association with the UCL.6,14 The FDS lies directly over the UCL at 30° elbow flexion, with the PT and FCU located anteriorly and posteriorly, respectively.6,14 These arrangements suggest that FDS is potentially well suited for dynamic valgus stabilization.
However, the observed correlation between FDS peak %MVC and medial joint gapping between 80 and 100 pitches may reflect compensatory overactivity resulting from fatigue-induced reductions in forearm flexor–pronator function and increased mechanical load from pitching variability. The correlation observed in the late stage (80-100 pitches) suggests that maintenance of the medial elbow condition, which remained stable during the mid-phase, was compromised by the physical constraint of muscle fatigue. To compensate for the fatigue-induced decline in muscle contractile force, the brain likely issued higher motor commands, resulting in synchronization of muscle activity with the biological response to the mechanical load of increased gapping. If the participants had been limited to experienced pitchers, they might have utilized an efficient kinetic chain capable of minimizing compensatory movements, even amidst fatigue. As a result, the pitching motion could be maintained at 80-100 pitches, potentially leading to a lower correlation compared with non-pitchers.
Our study also suggests that the peak %MVC and Mean %MVC of the flexor–pronator muscles may differ among individual muscles. Although the peak %MVC of the FCR significantly increased at the 80th pitch, the other muscles (PT, FCU, and FDS) did not show significant temporal changes. This muscle-specific response could potentially reflect a compensatory role of the FCR in maintaining kinetic chain efficiency, as the pitching motion varies with fatigue. Whether its sustained high activity is inherently required or reflects its intrinsic endurance properties remains a subject for further investigation. Overall, these differences in muscle activity ratios suggest complex compensatory actions owing to changing pitching mechanics.8
In this study, the medial elbow gapping distance at rest increased significantly after 100 pitches in comparison to the prepitch values. Repeated pitching likely causes cumulative valgus stress, affecting both the UCL and forearm flexor–pronator muscles, which serve as static and dynamic valgus stabilizers, respectively. Although Erickson et al7 reported no association between preseason valgus angles and in-season injuries in professional pitchers, our results reflect immediate postpitch changes, and longitudinal studies are needed to assess the long-term injury risk. Under gravity-induced valgus stress, medial gapping increased significantly after 60 pitches, which is consistent with Hattori's findings in high school players.12 Cadaveric studies indicate that the UCL alone cannot resist the valgus torque generated during pitching,1,9 suggesting that repeated pitches may reduce the functional capacity of both static and dynamic stabilizers, thereby increasing medial gapping. Shanley et al18 found that preseason medial gapping was greater in professional pitchers who later developed UCL injuries. Our findings highlight that repeated pitching increases medial gapping, indicating a load-induced effect on the elbow's valgus stabilizers. Future longitudinal studies are required to clarify the association between increased gapping and injury occurrence.
Subjective whole-body fatigue related to the pitching task increased significantly from the 40th pitch onward, likely reflecting the cumulative effect of the repeated whole-body pitching motion. No significant correlations were observed between changes in medial elbow joint gapping and subjective fatigue at 20-pitch intervals. However, from the 40th pitch onward, a trend toward a positive correlation was noted, although it was not statistically significant. The absence of a significant correlation may be attributable to the assessment method, which subjectively evaluates the overall body fatigue. Future studies should use specific questionnaires to assess subjective fatigue in the forearm flexor–pronator region, as well as mental fatigue, and examine their relationship with medial elbow joint gapping.
This study had several limitations. First, the sample included both pitchers and non-pitchers, which may have affected pitching mechanics and muscle activity. Non-pitchers may have been less familiar with throwing from the set position, potentially influencing valgus stress on the elbow. Future studies should focus on a larger cohort of baseball pitchers and examine the interactions among muscles, timing of peak muscle activity, and the relationship between muscle activity and pitching mechanics to provide novel insights for preventing pitching-related elbow injuries under more specific pitching-simulated conditions.
Second, this study employed sEMG, but it cannot be denied that its specificity is lower than that of fine-needle EMG (fEMG) in densely packed muscle areas such as the forearm. While prior research reported high selectivity with fEMG, it also noted the risk of electrode displacement during vigorous dynamic movement.11 In this study, the determination of electrode placement, orientation, and interelectrode distance adhered to the international guidelines recommended by Hermens et al and the Surface EMG for the Non-Invasive Assessment of Muscles project (http://www.seniam.org/). In addition, manual muscle testing was used to confirm the contraction of each muscle, verify electrode placement validity, and minimize crosstalk from adjacent muscles.16 Furthermore, the fact that significant changes were observed only in the FCR in this study supports the relative selectivity of our electrode placement. While definitive conclusions regarding individual deep muscle fibers would require fine-wire EMG, future research should include the simultaneous measurement of muscle thickness changes using ultrasound and verification using fEMG.
Third, although 2 participants reported a history of shoulder and elbow pain over 1 decade prior, they were asymptomatic and pitched at full effort at the time of the study. Although this remote history could have influenced muscle activity or joint gapping, all symptoms resolved without medical intervention. Future studies should include participants with no history of throwing-related pain to clarify these effects further.
Conclusion
This study investigated the effects of increasing pitch count on forearm flexor–pronator muscle activity and medial elbow joint space in 15 healthy adult male baseball players. Changes in the peak %MVC of the PT and FCU and the mean %MVC of the PT from pitches 1 to 20 were positively correlated with changes in the medial joint space. In addition, changes in the peak %MVC of the FDS from pitches 80 to 100 were positively correlated with changes in the joint space. These findings indicate that forearm flexor–pronator muscle activity may help dynamically stabilize the elbow against valgus stress during pitching.
Disclaimers:
Funding: No funding was disclosed by the author(s).
Conflicts of interest: The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
Footnotes
Gunma University Ethical Review Board for Medical Research Involving Human Subjects Number: HS2025-075.
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