Do Mound Height and Pitching Distance Affect Youth Baseball Pitching Biomechanics?

Abstract

Background:

Pitching injuries continue to be a serious problem, with adolescents now representing the group with the most injuries. Some have proposed that lowering or eliminating the pitching mound in youth baseball may reduce joint stress and subsequent injuries. Another potential risk factor is advancing from youth to adult pitching distance without an intermediate distance.

Hypotheses:

It was hypothesized that for a group of young pitchers, pitching kinetics and kinematics would change with mound height. It was also hypothesized that pitching kinetics and kinematics would change with pitching distance.

Study Design:

Controlled laboratory study.

Methods:

Twenty-one young (12.6 ± 0.5 years) baseball pitchers pitched 5 full-effort fastballs each from 5 different conditions, in random order: 14.02-, 16.46-, and 18.44-m distances from a 25 cm–high mound, 16.46-m distance from a 15 cm–high mound, and 16.46-m distance from flat ground. Pitching biomechanical values were collected with a 12-camera automated motion capture system. Ball velocity and 31 other parameters were computed for each pitch. Data were compared between the 3 mound heights at 16.46 m by use of repeated-measures analysis of variance and paired post hoc t tests (P < .05). Similarly, data were compared between the 3 distances from the 25-cm mound via repeated-measures analysis of variance and paired post hoc t tests (P < .05).

Results:

No differences were found in ball velocity, shoulder kinetics, or elbow kinetics associated with mound height. Ten kinematic parameters differed with mound height, including 8 parameters at lead foot contact. Maximum shoulder horizontal adduction torque and maximum shoulder anterior force increased with pitching distance. Only 3 kinematic parameters showed significant differences with pitching distance.

Conclusion:

The hypothesis that shoulder and elbow kinetics would change with mound height was not supported by the data. Several kinematic differences were found, but the majority were at lead foot contact before the rapid, dynamic phases of pitching. Change in pitching distance was associated with slight increase in shoulder kinetics as well as a few kinematic differences.

Clinical Relevance:

Lowering or eliminating pitching mounds in youth baseball would not significantly decrease joint stress and injury risk to young pitchers. However, when available, transition from 14.02-m to 16.46-m to 18.44-m pitching distance may reduce stress on the young throwing shoulder.

Keywords

shoulder elbow kinematics kinetics force torque

Pitching injuries are an epidemic in baseball, from the adolescent level^5,20 to professional leagues.^3,4 While pitching injuries often appear to have acute onset, imaging and visual observation during surgery usually reveal tissue damage consistent with chronic, accumulated trauma.^{3,12,18,20,22} The amount of pitching has been shown to be a primary risk factor in adolescent baseball,⁸ leading to the establishment of pitching limits.¹⁵ Pitching mechanics,^2,7 pitch type,^6,8 ball velocity,^18,21 and mound height^1,16,17 have also been suggested to influence elbow and shoulder stress on each pitch and, subsequently, the risk of overuse injury. In Little League Baseball, players younger than 12 years play on smaller fields with shorter base distances, shorter pitching distance, and lower mounds (15 cm) than standard adult fields. Players older than 12 years play on fields with adult height (25 cm) mounds.¹³ Some have suggested that lowering the mound in youth and adolescent baseball would be more “natural” and may lessen the stress on developing arms.^1,16,17

For players 12 years and younger, baseball is played on small fields with a 14.02 m (46 ft) pitching distance.¹³ As youth pitchers become older, taller, and heavier, they graduate to adult fields with 18.44 m (60.5 ft) pitching distances. A recent study by Fleisig et al¹¹ found that this is also the age when pitching biomechanics change to be more like adult biomechanics. Some organizations, such as Little League Baseball, give options for playing on intermediate size fields with 15.24 m (50 ft) or 16.46 m (54 ft) pitching distances.¹³ The premise of using intermediate size fields is to serve as a transition step for players to adjust to longer pitching and base path distances. Some parents, coaches, and organizations believe that intermediate size fields provide a stepwise transition in player development, while others believe that such a transition is not necessary and young players can advance directly from youth to adult size fields. Unfortunately, no previous study has investigated the effect of field size on baseball biomechanics for young players.

The purpose of this study was to determine whether variations in mound height and pitching distance affect pitching kinetics and kinematics. It was also hypothesized that shoulder and elbow kinetics would increase with mound height, and kinematics would vary as well. Finally, it was hypothesized that shoulder and elbow kinetics would increase with pitching distance, and kinematics would vary as well.

Methods

Participants

This study was approved by the institutional review board at St. Vincent’s Health System (Birmingham, Alabama, USA). To be eligible for the study, each player had to be currently participating as a pitcher on a baseball team, feel “100% healthy” for pitching in his opinion, and be 12 or 13 years old. This age range was selected because Little League teams transition from 14.02-m pitching distance to either 16.46 m or 18.44 m at this age.¹³ Furthermore, any pitcher with a history of shoulder or elbow injury during the previous 12 months was excluded. Twenty-one youth baseball pitchers voluntarily participated in this study. The mean ± SD age for the participants was 12.6 ± 0.5 years. Their mass and height were 49.6 ± 14.6 kg and 1.62 ± 0.09 m, respectively.

Data Collection

Testing was conducted in an indoor biomechanics laboratory. After assent and consent were obtained from the participant and his parent or guardian, respectively, the athlete dressed in tight-fighting Spandex shorts, socks, and baseball spikes. Thirty-eight reflective markers were then attached to the participant.⁷ After warming up, the participant pitched sets of 5 full-effort fastballs from 5 different conditions, in random order. The pitcher was allowed warm-up pitches for each condition, if desired, before data collection. The 5 conditions were as follows:

25 cm–high mound to a strike zone target located over a home plate 14.02 m away

25-cm mound with a 16.46-m distance

25-cm mound with a 18.44-m distance

15-cm mound with a 16.46-m distance

Pitching rubber on flat ground with a 16.46-m distance

The 25 cm–high mound was a commercially available portable mound with dirt areas for both feet at the rubber and for the lead foot at landing (Athalonz). The downhill slope of the mound, starting 15 cm in front of the pitching rubber, was 4.8° per the standard adult baseball mound specifications of “1 inch drop per 1 foot for six feet, starting 6 inches in front of the rubber.”¹⁴ The 15 cm–high mound was custom-built by the same company (Athalonz) with a 2.9° drop, starting 15 cm (6 in) in front of the rubber. Thus, both mounds were built with a slope to reach ground level 3.20 m in front of the rubber.

The motion capture volume was large enough for 2 pitching mounds next to each other, each with a separate strike zone and home plate (Figure 1). Based on each participant’s random test order, the first of the 2 mound height conditions were set up before his arrival. For each pitching distance condition, the home plate and the strike zone target above it were moved back the appropriate distance, while the mound and motion capture system remained in place.

Biomechanical Computations

Ball velocity for each pitch was recorded with a radar gun (Stalker Sports Radar), while 3-dimensional motion data were collected with a 12-camera automated motion capture system sampling at 240 Hz (Motion Analysis Corporation). Marker position time data were filtered with a fourth-order, Butterworth low-pass filter with a cutoff frequency of 13.4 Hz.

Twenty-five kinematic parameters were calculated for each pitch using BioPitch software (ASMI). The global X direction was defined as a vector from the pitching rubber to home plate, global Z was defined as a vector pointing vertically, and global Y was the cross-product of the Z and X. Eleven of the parameters were measured at the instant of lead foot contact (stride length, lead foot position, lead foot angle, lead knee flexion, pelvis rotation, trunk axial rotation, upper trunk lateral tilt, shoulder external rotation, shoulder abduction, shoulder horizontal abduction, and elbow flexion). Magnitudes and timing for maximum pelvis angular velocity and maximum upper trunk angular velocity were computed as the trunk rotated to face the target. The timings were reported on a normalized time scale, where 0% was the time at lead-foot contact and 100% was the time at ball release. During the arm cocking phase, maximum shoulder external rotation, shoulder horizontal adduction, and elbow flexion were measured. Maximum elbow extension velocity and maximum shoulder internal rotation velocity were determined during arm acceleration. Lead knee flexion, trunk forward tilt, trunk contralateral tilt, shoulder abduction, and elbow flexion were measured at the instant of ball release.

Six kinetic parameters were calculated from the motion data, estimated mass properties, and standard inverse dynamics calculations.²³ Near the instant of maximum shoulder external rotation, maximum values were computed for elbow varus torque, shoulder internal rotation torque, shoulder horizontal adduction torque, and shoulder anterior force. Near the instant of ball release, maximum elbow flexion torque and maximum shoulder proximal force were determined.

Statistical Analyses

For each kinematic and kinetic parameter, the values for the 5 trials of each participant for each condition were averaged. The mean values were then compared for all participants pitching 16.46 m from the 0-, 15-, and 25-cm mounds, by use of repeated-measures analysis of variance. When a significant difference (P < .05) was found among the 3 mound heights, pairwise differences were analyzed with a post hoc Tukey test (P < .05). Similarly, the mean values were compared for all players between pitches thrown 14.02 m, 16.46 m, and 18.44 m from the 25-cm mound, by use of repeated measures analysis of variance. When a significant difference (P < .05) was found among the 3 distances, pairwise differences were analyzed with a post hoc Tukey test (P < .05).

Results

No statistical difference (P = .95) was found in ball velocity, with a mean value of 25.5 m/s from each mound height. Ten kinematic parameters showed significant differences with mound height, including 8 at the instant of lead foot contact: stride length, lead foot position, lead foot angle, lead knee flexion, pelvis rotation, upper trunk lateral tilt, shoulder external rotation, and elbow flexion (Table 1). The other 2 significant differences were timing of maximum upper trunk angular velocity and trunk forward tilt at ball release (Table 1). Post hoc analyses revealed that the differences occurred between flat ground pitching and the other 2 conditions. No significant differences were found for any kinetic parameter.

Table 1

Significant Differences With Pitching Mound Height

	Mean Values
	0-cm Mound	15-cm Mound	25-cm Mound	Standard Error	P Value
At the instant of lead foot contact
Stride length, % height	68.9	69.5	70.8	1.9	.032^a
Lead foot position, cm	8.5	7.6	6.1	2.2	.002^a
Lead foot angle, deg	18.0	22.0	20.3	2.5	.036^b
Lead knee flexion, deg	52.8	46.8	44.9	1.9	<.001^a,b
Pelvis rotation, deg	27.4	20.9	25.0	2.4	.033^b
Upper trunk lateral tilt, deg	8.4	5.1	5.7	1.6	.002^a,b
Shoulder external rotation, deg	62.5	52.4	58.4	6.8	.0199^b
Elbow flexion, deg	95.7	92.1	92.2	3.8	.031
After lead foot contact
Time of maximum upper trunk angular velocity, % pitch complete	44.8	50.9	50.0	2.6	.012^b
Trunk forward tilt at ball release, deg	29.6	30.98	31.4	2.1	.004^{a , b}

Post hoc significant difference between pitching from 0-cm and 25-cm mounds.

Post hoc significant difference between pitching from 0-cm and 15-cm mounds.

Mean ball velocity was 25.5 m/s at 14.02 m, 25.5 m/s at 16.46 m, and 25.6 m/s at 18.44 m (P = .10). Of the 25 kinematic parameters tested, 3 showed significant differences with pitching distance: maximum pelvis angular velocity, maximum elbow flexion, and trunk forward tilt at ball release. Neither elbow torque parameter showed significant differences; however, 2 of the 4 shoulder kinetic parameters showed significant differences with distance: maximum shoulder anterior force and maximum shoulder horizontal adduction torque. Means and standard errors for parameters with significant differences are shown in Table 2.

Table 2

Significant Differences With Pitching Distance

	Mean Values
	14.02-m Distance	16.46-m Distance	18.44-m Distance	Standard Error	P Value
Kinematic parameters
Maximum pelvis angular velocity, deg/s	583	589	599	15	.004^a
Maximum elbow flexion, deg	104.5	105.1	106.2	2.3	.035^a
Trunk forward tilt at the instant of ball release, deg	32.2	31.4	30.5	2.1	<.001^a
Kinetic parameters
Maximum shoulder horizontal adduction torque, N·m	37.5	37.9	38.3	2.8	.022^a
Maximum shoulder anterior force, N	153	154	158	11	.003^{a , b}

Post hoc significant difference between 14.02 m and 18.44 m.

Post hoc significant difference between 16.46 m and 18.44 m.

Discussion

As hypothesized, pitching kinematics changed with pitching mound height. Most of the differences were at the instant of lead foot contact. Although the 3 pitching heights were fairly evenly spaced (0, 15, 25 cm), post hoc testing revealed no differences between the 2 raised mound conditions but several differences between flat ground pitching and the other 2 conditions. On flat ground, the pitcher’s lead leg landed with a shorter stride and more knee flexion. These findings are consistent with the results from a similar study by Nissen et al,¹⁷ who tested 15 youth pitchers (ages 12.7 ± 1.3 years) pitching from flat ground and from a 25 cm–high mound. Results from the current study also showed that from flat ground, the pitcher landed with his lead foot position more closed (toward the third base side, for a right-handed pitcher) and lead foot angle more open (rotated more toward pointing at home plate). The upper trunk was tilted about 3° more uphill relative to the horizontal plane when pitching from flat ground, but since the mounds were sloped downhill 3° and 5°, upper trunk tilt relative to the pitching surface was actually relatively similar among the 3 conditions.

The hypothesis that kinetics would change with mound height was not supported by the data. This is in contrast to results from Nissen et al,¹⁷ who found that pitching was less stressful from flat ground than from a 25-cm mound. Similar to the current study, Nissen et al¹⁷ found no difference in ball velocity between flat ground (23.3 m/s) and mound (23.5 m/s) pitching. However, they reported significantly less elbow varus torque (31.4 vs 33.3 N·m) and shoulder internal rotation torque (31.7 vs 33.6 N·m) when pitching on flat ground. Future research may add insight into whether flat ground pitching requires less torque. Ideally, future studies would allow for several weeks of acclimation to pitching from each condition.

Perhaps more important than the finding that 3 kinematic parameters differed with pitching distance was the fact that 22 other kinematic parameters did not differ. Thus, in general, pitchers used similar mechanics regardless of the pitching distance. The increased elbow flexion and pelvis angular velocity with increased pitching distance may indicate slight increased effort and involvement in pelvis rotation. The recent study by Fleisig et al¹¹ showed a gradual increase in pelvis angular velocity as pitchers matured from 10 to 15 years of age, although the finding was not statistically significant. The decreased trunk forward tilt may be a compensation technique for the increase in pitching distance, as decreasing trunk forward tilt can increase the trajectory and distance of the pitch (Figure 2).

The increased shoulder kinetics with increased pitching distance may have clinical implications. Maximum shoulder horizontal adduction torque and maximum shoulder anterior force both occur during the arm cocking phase, as the upper trunk rotates forward and the throwing arm rotates backward (Figure 3).⁹ Shoulder torque and force during pitching can lead to proximal humeral epiphysitis or other injury to the immature shoulder,¹⁹ and the increased kinetic magnitudes with longer pitching distance may increase the risk.

While results from this biomechanics study suggest that 16.46 m is a reasonable intermediate pitching distance for adolescent pitchers, other factors must be considered. Fields with 14.02-m pitching distance are readily available (often shared with softball). Fields with 18.44-m pitching distance are likewise readily available, as these fields are used by baseball players from older adolescents to adults. However, many towns and sports complexes do not have 16.46-m pitching distance fields available. One alternative solution used in many locations is modification of larger or smaller fields by use of portable pitching mounds placed 16.46 m from home plate. Other factors beyond the current study are the effects of field size on batting, fielding, and baserunning.

A possible limitation of this study was that participants were asked to throw from various mound heights and distances to which some of the players were not accustomed. Although participants were allowed time to practice and warm up from each condition, it is likely that their mechanics might change over time if they played with that condition for months or years. Another possible limitation could be fatigue during testing. To minimize that possibility, the order of the test conditions was randomized. No participants mentioned fatigue during testing, and the number of pitches for the study (25) was well below the pitch count limits for their baseball leagues. Another issue of the study design was that many (32) kinematic and kinetic parameters were analyzed, raising the possibility of type I error. Some differences that were statistically significant were of small magnitude, less than the standard error. It is not surprising that some parameters had relatively large standard errors, as young pitchers have the greatest biomechanical variability from pitch to pitch.¹⁰ We chose to report all significant differences, but because of the limitations of the statistical model, particular attention was given to assess clinical significance. Furthermore, the participants in this study were limited to 12 to 13 years of age, as this is the typical age when leagues might change pitching distances. Studying pitchers of younger and older age groups might be of value.

In conclusion, this study does not support the theory that lowering or eliminating the pitching mound in youth baseball would reduce the stress and injury risk to young pitchers. However, using an intermediate distance (16.46 m) when available may reduce the stress on the young throwing shoulder.

Footnotes

One or more of the authors has declared the following potential conflict of interest or source of funding: This research was funded by a grant from Major League Baseball. G.S.F. is a research injury advisor for Major League Baseball. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

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