Investigations of the Throwing Biomechanics Index in Collegiate

Baseball Pitchers

Hannah Stokes

, Koco Eaton

and Nigel Zheng

Department of Mechanical Engineering and Science, University North Carolina at Charlotte, U.S.A.

Tampa Bay Rays and University of South Florida, U.S.A.

Keywords: Motion Analysis, Shoulder, Elbow, Injury.

Abstract: In the field of sports biomechanics the aim is to improve performance and reduce injury. In this study we

create a novel throwing biomechanics index by using logistic regression to identify the most important and

significant variables that influence injury. Fifteen biomechanics (kinematic and kinetic) variables were

identified using logistic regression and the standards of the throwing biomechanics index were determined

based on the healthy and high performing group (ball speed > 80 mph). Z-scores were used to determine the

index value for each pitcher. Division 1 and 2 collegiate baseball pitchers participated in this study that were

grouped based on their injuries before and after the study compared to the healthy group. The healthy group

had the highest throwing biomechanics index and further analysis will provide more insights on both injury

and performance. The throwing biomechanics index found significant relationships with the pitcher’s height

(p=.0165), mass (p=.0003), age (p=.0099), forearm length (p=.0001), internal flexibility (p=.0015), external

ROM (p=.0002), and external flexibility (p=.0142). There is great value in quantifying a throwing

biomechanics index for both understanding the injury mechanisms and for improved performance.

1 INTRODUCTION

Proper throwing mechanics in baseball pitching are

important to improve performance and reduce

injuries. Baseball pitching is very demanding on the

shoulder; the shoulder internally rotates at about

7,000 degrees per second and the force applied is

greater than 800 Newtons (Zheng et al., 1999).

Throwing arm injuries are common because the

repetitive and high stress motions for pitchers of all

ages, from youth to professional (A Popchak et al.,

2015; Sutter et al., 2018). This prompts the goal to

further understand the mechanism throwing arm

injuries (shoulder and elbow) to enhance preventative

protocols, improve performance, and promote better

rehabilitation practices. This work extends beyond

baseball to other sports and all shoulder and elbow

injuries.

The ability to identify the ideal pitching

mechanics is beneficial to the sport. Thompson et al

found that increasing both shoulder rotation angle and

shoulder angular velocity has shown to increase ball

speed and performance in youth baseball pitching

(Thompson et al., 2018). Further, a previous study

showed that the increase in ball speed and shoulder

external rotation angle was related to increased

shoulder range of motion (Seroyer et al., 2010).

Baseball pitching is a complex movement that puts a

lot of stress on the throwing arm (both the elbow and

shoulder). A previous study found that higher

shoulder joint loading (forces and torques) in

competitive baseball players leads to more injury

incidences (Oyama, 2012). Further, in a study

analysing pitching mechanics, emphasized that poor

pitching mechanics can compound the repetitive

stress placed on the soft tissues of the shoulder and

elbow and has been implicated as a potential risk of

injury (Calabrese, 2013). It is well understood that

high joint loading with repetitive motions can lead to

potential injury. Ultimately there are many factors

that influence the incidence of injury, including the

following: joint loading, flexibility, experience of

pitcher, and pitching mechanics (A. Popchak et al.,

2015). This shows the importance of understanding

of the optimal position of the throwing-arm during

baseball pitching is critical in improving performance

and reducing injuries.

Both throwing arm motions and joint loadings

have been a popular topic in literature; however, it is

unclear which variables are most important and

Stokes, H., Eaton, K. and Zheng, N.

Investigations of the Throwing Biomechanics Index in Collegiate Baseball Pitchers.

DOI: 10.5220/0012255100003587

In Proceedings of the 11th International Conference on Sport Sciences Research and Technology Support (icSPORTS 2023), pages 227-232

ISBN: 978-989-758-673-6; ISSN: 2184-3201

227

influential in impacting both injury and performance.

The purpose of this study is to propose an index that

summarizes all throwing biomechanics variables into

one score that are related to injuries in collegiate

baseball pitchers. Further understanding could be

advantageous for optimizing throwing mechanics,

monitoring shoulder health, and reducing injuries.

2 METHODOLOGY

The study included 177 National Collegiate Athletic

Association baseball pitchers: Division I (n = 117)

and Division II (n = 60) (mean ± SD: age, 20 ± 1

years; height, 186 ± 7 cm; weight, 85 ± 9 kg). The

study protocol was approved by an institutional

review board at the University of North Carolina at

Charlotte, and all pitchers gave informed consent. All

pitchers were healthy at the time of testing, or they

were excluded from the study.

2.1 Pitcher Injury Information

The injury questionnaire was used to record the

pitcher’s team, class, height, weight, history of injury

or surgery, and experience (Table 1). Self-reported

injury questionnaires were filled out by pitchers

during biomechanical testing and at follow-up

(Stokes et al., 2021). Any injury or surgery before

biomechanical testing were referred to as having

injury history. Any injury or surgery after

biomechanical testing noted in a follow-up injury

questionnaire are referred to as having follow-up

injury. Further, eight subjects had both an injury or

surgery before and after. All injuries and surgeries

were on the elbow or shoulder of the pitcher.

Table 1: Pitcher injury group count information.

Injury

Before

Injury

After

Injury

Before

& Afte

Healthy*

n 38 25 8 31

*Healthy pitchers who were also high performing (ball speed

> 80 mph).

2.2 Shoulder Exam

A custom wireless device was developed for testing

purposes and this methodology has been previously

published (Stokes et al., 2021; Zheng & Eaton, 2012).

It utilizes a force sensor and an orientation sensor that

is powered by a rechargeable 9-volt battery. Ten trials

were collected from each pitcher including 5 trials on

external rotation and 5 trials on internal rotation. A 15

second pause was taken between trials. The trials

were averaged and gave the resulting variables of

internal range of motion (ROM), internal flexibility,

external ROM, and external flexibility.

2.3 Motion Capture

Sixteen reflective markers were attached to major

joints for motion capture and biomechanical analysis

based on a previously reported studies (Stokes et al.,

2021; Zheng et al., 2004).

These markers were

attached bilaterally to the distal end of the midtoe,

lateral malleolus, lateral femoral epicondyle, greater

trochanter, lateral tip of the acromion, and lateral

humeral epicondyle on both sides. Additionally, on

the throwing arm, 2 reflective markers were placed

medially and laterally on the wrist and 1 on the back

side of the distal end of the middle metacarpal.

Pitchers were allowed to warm up in any way

they needed primarily by stretching and throwing.

The pitchers threw balls from an artificial portable

mound that was 60 feet 6 inches from home base.

Motion data were collected at 240 Hz using a 10-

camera motion capture system (VICON). Ball speed

was measured using a radar gun, and a rope frame

determined the strike zone for each of the 10 fastball

pitches. Pitchers rested as needed between pitches.

2.4 Baseball Pitching Biomechanics

The 3 fastest strike pitches were digitized, analyzed,

and averaged to represent each pitcher. The whole

pitching motion was divided into 6 phases: windup,

stride, arm cocking, arm acceleration, arm

deceleration, and follow-through (Figure 1) (Dillman

et al., 1993).

For the end of the stride phase, the lead

foot contact was used for normalization and labeled

0%. For the end of arm acceleration, the ball release

was used for normalization and labeled 100%. Data

from –50% to 200% covering, at a minimum, the

stride to arm deceleration phase were analyzed.

Figure 1: The digitized output of the six baseball pitching

phases (wind-up, stride, arm cocking, arm acceleration, arm

deceleration, and follow through).

icSPORTS 2023 - 11th International Conference on Sport Sciences Research and Technology Support

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A custom program (MATLAB; MathWorks) was

created to calculate the throwing arm motion and

loading during the baseball pitching (Fleisig et al.,

1995; Fleisig et al., 1999; Zheng, 2003; Zheng et al.,

1999). The throwing arm motions were identified at

three key times (foot contact, at maximum shoulder

external rotation (i.e., the end of arm cocking phase),

and ball release). A few maximum angular velocities

were also identified. The throwing arm motions at

foot contact were shoulder abduction angle, shoulder

horizontal adduction angle, shoulder external rotation

angle, and elbow angle. The throwing arm motions at

maximum external rotation were initial elbow

extension angle, maximum shoulder horizontal

adduction angle, and maximum shoulder external

rotation angle. The throwing arm motions at ball

release were shoulder abduction angle and elbow

angle. The maximum angular velocities were the

maximum elbow extension angular velocity,

maximum shoulder internal rotation velocity, and

maximum shoulder linear velocity. The peak

throwing arm joint loadings are the forces and torques

of both shoulder and elbow. The peak throwing arm

joint loadings for the shoulder were posterior force,

distal force, inferior force, adduction torque, internal

rotation torque, and horizontal adduction torque. The

peak throwing arm joint loadings for the elbow were

anterior force, medial force, superior force, varus

torque, and extension torque.

2.5 Throwing Biomechanics Index

A multinomial logistic regression was performed

using SPSS to determine which factors were most

influential and related to injury. There were 15

significant variables that include: stride length at foot

contact, foot angle, knee angle at foot contact,

maximum hip angular velocity, trunk forward angle,

maximum spine lateral bend angular velocity,

maximum external rotation angle, elbow angle at foot

contact, maximum elbow angular velocity, peak

anterior/posterior shoulder force, peak

superior/inferior shoulder force, peak medial/lateral

shoulder force, peak internal/external shoulder

torque, resultant elbow force, and valgus/varus elbow

torque. In this initial investigation all variables are

weighted the same of 1 index point so a perfect score

for the throwing biomechanics index is 15.

The standards of the throwing biomechanics were

determined based on the healthy and high performing

group (ball speed > 80 mph). For each variable the

mean and standard deviation was taken of the healthy

and high performing group. From there each pitcher

was compared to the mean of the healthy and high

performing group for each variable. The z-score for

each variable was used to calculate the probability to

give a continuous value compared to the healthy and

high performing group which we defined as the index

value for each variable. For the kinematic variables a

two tailed approach was used because the mean of the

healthy and high performing group was the ideal

value. For the kinetic variables a one tailed approach

was used because the lower the force or torque the

better and the risk is found in the higher values.

A bar graph was used to visually compare the

throwing biomechanics index across the three injury

and the healthy and high performing groups. A table

shows the mean and standard deviations for the three

injury and the healthy and high performing groups.

One-way ANOVA with post-hoc tests and Pearson

correlation tests were performed to compare the

throwing biomechanics index using SPSS. The alpha

value was set at 0.05.

3 RESULTS

The results show the healthy and high performing

group had the highest throwing biomechanics index

compared to the other groups (Figure 2). The injury

before group’s throwing biomechanics index is 7.19

± 1.75, the injury after group’s throwing

biomechanics index is 7.59 ± 1.31, the injury before

& after group is 7.57 ± 1.24, and the healthy group

was 7.69 ± 1.67 (Table 2). When comparing across

the three injury groups and the healthy and high

performing group’s index score there was no

statistically significant difference.

Figure 2: The throwing biomechanics index for the before,

after, before + after injury groups and the healthy and high

performing group.

Investigations of the Throwing Biomechanics Index in Collegiate Baseball Pitchers

229

Table 2: The mean and standard deviation for the index

score for the before, after, before + after injury groups and

the healthy and high performing group.

Injury Groups

Before After Before + After Healthy

Mean 7.19 7.59 7.57 7.69

Standard

Deviation

1.75 1.31 1.24 1.67

Further analysis investigated the individual index

scores for all fifteen variables that were used to create

the index (Table 3). When comparing across the three

injury groups and the healthy and high performing

group’s index score for each of the fifteen variables

there was no statistically significant difference. The

healthy and high performing group has the largest

mean for six of the fifteen variables.

Table 3: The mean and standard deviation for the individual

index scores (maximum of 1) for all fifteen variables that

were used to create the index for the before, after, before +

after injury groups and the healthy and high performing

group.

Injury Groups

Before After

Before +

Afte

Healthy

Stride length at

foot contac

0.49±0.3 0.40±0.3 0.28±0.2 0.52±0.3

Foot angle

0.45±0.4 0.42±0.3 0.42±0.3 0.47±0.3

Knee angle at

foot contac

0.58±0.3 0.58±0.2 0.68±0.2 0.47±0.3

Max hip angular

velocit

0.47±0.3 0.57±0.3 0.59±0.3 0.62±0.3

Trunk forward

0.39±0.3 0.43±0.3 0.45±0.3 0.50±0.3

ax spine latera

bend angular

velocit

0.47±0.3 0.52±0.3 0.58±0.3 0.50±0.3

Max external

rotation an

0.44±0.3 0.44±0.3 0.44±0.3 0.52±0.3

Elbow angle at

foot contac

0.40±0.3 0.50±0.3 0.25±0.2 0.46±0.3

Max elbow

ular velocit

0.41±0.3 0.51±0.3 0.40±0.3 0.55±0.3

Anterior/

posterior

shoulder force

0.43±0.3 0.44±0.3 0.68±0.1 0.50±0.3

Superior/

inferior shoulde

force

0.54±0.3 0.61±0.2 0.47±0.3 0.51±0.3

Medial/lateral

shoulder force

0.57±0.3 0.49±0.3 0.72±0.3 0.51±0.3

Internal/external

shoulder torque

0.55±0.3 0.61±0.2 0.43±0.3 0.52±0.3

Resultant elbow

force

0.49±0.3 0.46±0.3 0.65±0.2 0.50±0.3

Valgus/varus

elbow torque

0.52±0.3 0.59±0.2 0.52±0.2 0.52±0.3

The results show that many of the pitcher

demographic and shoulder exam variables are related

to the throwing biomechanics index (Table 4). The

height, mass, age, forearm length, and external

flexibility all have a moderately strong negative

statistically significant correlation with the throwing

biomechanics index. Showing that the larger

throwing biomechanics index was related to smaller

variables and the smaller throwing biomechanics

index was related to the larger variables. The internal

flexibility and external ROM all have a moderately

strong positive statistically significant correlation

with the throwing biomechanics index. Showing that

the smaller throwing biomechanics index was related

to smaller variables and the larger throwing

biomechanics index was related to the larger

variables.

Table 4: The Pearson correlation values for the pitcher

demographic and shoulder exam variables and the throwing

biomechanics index, where p< .05 is bolded.

Demographics

Hei

ht -0.2368 .0165

Mass -0.3490 .0003

BMI -0.1604 .1073

e -0.2542 .0099

Forearm Len

th -0.3747 .0001

Upper Arm Len

th -0.0148 .8826

Years Playe

-0.1024 .3058

Shoulder

Exam

Internal ROM -0.1627 .1023

Internal

Flexibility

0.3105 .0015

External ROM 0.3642 .0002

External

Flexibility

-0.2422 .0142

4 DISCUSSIONS

The kinematic and kinetics variables have significant

relationships with the injury groups in collegiate

baseball pitchers. When combined the significant

variables to create a throwing biomechanics index

there were trends but there were no statistically

significant differences between the for the before,

after, before + after injury groups and the healthy and

high performing group. This methodology helps

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create an index that is useful in being able to evaluate

the athletes. Findings of this study indicated that

throwing arm injuries are complicated and often due

to overuse and faulty mechanics of throwing.

Tracking changes of a throwing athlete with throwing

biomechanics index may provide insight into

implications of throwing mechanics and injuries.

Further diving into the relationship of the

throwing biomechanics index with other variables

such as the demographic variables, performance

variable, and shoulder exam variables showed

statistically significant relationships. For the

relationship between the throwing biomechanics

index and the pitcher demographic variables there

was a statistically significant relationship between

height, mass, age, and forearm length. For the

relationship between the throwing biomechanics

index and the pitcher shoulder exam variables there

was a statistically significant relationship between

internal flexibility, external ROM, and external

flexibility. For all the other variables there was not

statistically significant relationship with the throwing

biomechanics index. Kinetic variables are dependent

on height and weight so seeing those connections is

obvious with the throwing biomechanics index;

however, the dominant arm internal SRF, dominant

arm external ROA, non-dominant arm external ROA,

and dominant arm external SRF must be related to the

throwing biomechanics kinematics components,

which means this single index is a well reflection of

collegiate throwing biomechanics.

This novel methodology leaves room for further

research. The value in the index is that it gives a

quantitative way to summarize all the throwing

biomechanics variables. Many papers have

investigated specific or certain types of variables but

very few have investigated a way to quantify a

summary variable. The index is useful in monitoring

rehabilitation protocols as well as monitoring the

athlete’s injury risk. The higher the index the closer

the athlete is to both healthy and high performing as

we quantify by the ball speed. The results we see

show that those athlete’s that had the injury after and

the injury before + after have very similar index

scores, while the injury before group had the lowest

index score. It is interesting to consider that the those

who had injuries before may be using different

mechanics after their rehabilitation period and this

could be impacting the overall index score. This could

follow the idea of those athlete’s that are more injured

have a lower index and those who are healthy have a

higher index and more investigations will help give a

clear picture of what is going on. It is important to

note that there were eight subjects in the before and

after injury group so some variation may come from

the sample size. Variables seen throughout the kinetic

chain spurs on more investigations of how even the

wind up may influence pitcher’s potential injury and

performance potentials.

There are several limitations of this work as initial

exploration. Majority of them will be revised with

further and future work. One challenge is the sample

size, to fully test and evaluate our methodology it

would be advantageous to have more cases of

pitcher’s who have had injuries before, injuries after,

and both injuries before and after. The inherit

challenge when working with athlete’s is the fact that

every athlete is different and there are exceptions.

Further case studies may help fully understand the

injury mechanisms. Another consideration is with

testing a larger sample size it would help identify and

confirm the validity of the index. Understanding

optimal mechanics, we could adjust the index more

specifically if we can determine thresholds of healthy

and high performing players for all these variables.

This index is intended to evaluate both injury

mechanisms and performance so real game data

would be useful to compare the throwing

biomechanics index to quantifiable performance

parameters.

Future work would include using a larger sample

size and further looking into all the variables. This

study could then be extended beyond collegiate

baseball pitchers to those of youth, high school, and

professional baseball pitchers. This index could then

further be investigated or expanded to all throwing

athletes for example American football quarterbacks,

track and field javelin throwers, and many others.

Connecting the throwing biomechanics index to

demographics, shoulder rotational properties, and

other clinical tests to increase the knowledge in this

field. There is great value in quantifying a throwing

biomechanics index for both understanding the injury

mechanisms and for improved performance.

ACKNOWLEDGEMENTS

This study is funded by a Clinical Research Grant

from Major League Baseball.

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231

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