Keywords
batting - lower extremity - mechanics - trunk - upper extremity
Introduction
Baseball hitting is one of the most difficult skills in sports [1], since it requires precise
coordination of the kinetic chain to achieve optimal batted ball velocity [1]
[2]
[3]. Two commonly used
practice methods to improve offensive performance are tee and front-toss hitting.
When hitting off of a stationary tee, the ball’s preselected and static location
makes it a constructive tool for isolating the development of mechanics without
having to consider pitch variability [4]. For this reason, youth baseball athletes often invest more time hitting
off a tee to master hitting mechanics before eventually progressing to front toss,
which relies more on neuromuscular training and cognitive motor skills associated
with reacting to pitch speeds, movement, and locations [5]. Compared to stationary tee hitting,
front toss hitting may require a batter to adjust their timing and kinematics to
account for pitch location and velocity variances [6]
[7]. The pitch variability in a competitive environment requires the
hitter to have sound coordination of the kinetic chain to achieve optimal contact
and batted ball velocity [8]
[9].
Although tee and front toss can address unique training goals, little is understood
about differences in kinematic parameters between each hitting modality. Ae et al.
[10] examined kinematic differences
in the lower extremities between tee hitting and a machine pitched ball in a sample
of collegiate baseball athletes. The findings showed decreased trunk rotation
velocity and greater overall swing time in the pitched ball condition [10]. Another study by Chen et al. [7] compared collegiate baseball hitting
mechanics across front toss, motor imagery, video projection, and virtual reality
conditions. The results determined greater upper trunk rotation toward the back side
when hitting a front tossed ball compared to hitting off a video projection and
virtual reality conditions [7]. Lastly,
a single softball hitting study examined the kinematic differences between tee and
front toss hitting in collegiate athletes [6]. The findings indicated front-side knee flexion, trunk lateral
flexion, and pelvis and trunk rotation differed between tee and front toss
conditions at various events of the hitting motion [6]. Despite these findings in collegiate
softball athletes, there is a gap in the literature investigating differences in
mechanics between tee and front toss hitting in youth baseball athletes.
Prior baseball and softball hitting research suggests differences in kinematics
across hitting modalities in collegiate athletes; however, it is unknown whether
these differences exist in a youth athletes. A better understanding of the kinematic
differences between each hitting drill would aid coaches in selecting the most
appropriate method for their training goals. Thus, investigative work is necessary
to understand and communicate altered hitting mechanics throughout the kinetic chain
that may depend on hitting drill selection. Therefore, the purpose of the study was
to compare lower extremity, trunk, and upper extremity kinematics between tee and
front toss hitting in youth baseball athletes. Based on the aforementioned baseball
and softball hitting studies [6]
[7], it was hypothesized there would be
differences in upper extremity, lower extremity, and trunk kinematics between tee
and front toss hitting.
Materials and methods
Twenty male youth (9–17 years) baseball players (age: 14.3±2.9 yrs, height:
169.5±16.0 cm, weight: 69.0±17.3 kg) who were active on a team roster and injury or
surgery free for at least six months prior to visiting the lab participated.
Thirteen participants were identified as 'high school' athletes (14–17
years), and seven of the thirteen high school players competed in travel leagues.
Seven participants were identified as youth athletes (9–13 years), and all reported
competing in travel leagues. The high school and youth athletes were combined for
statistical analysis. On average, the participants played competitive baseball for
7.0±2.9 years and indicated they were in-season for baseball 7.3±2.5 months of the
year. Participants were recruited to the laboratory through coaches, players, and/or
family members expressing interest and reaching out to the laboratory to participate
in biomechanical evaluations. Participants arrived for a single visit to the indoor
laboratory in the appropriate athletic attire (loose-fitting t-shirt, athletic
shorts, and preferred pitching tennis shoes). Prior to data collection, all testing
procedures were thoroughly explained by the researcher, and written parental consent
and participant assent were obtained. All testing procedures were approved by the
University’s Institutional Review Board.
Kinematic and kinetic data were collected at 240 Hz with an electromagnetic tracking
system (trakSTAR, Ascension Technologies Inc.; Burlington, VT, USA) synchronized
with analysis software (The MotionMonitor XGen, Innovative Sports Training; Chicago,
IL, USA) [11]
[12]
[13]
[14]
[15]
[16]. Fourteen electromagnetic sensors were attached to the participants
using previously established standards ([Fig.
1]) [11]
[12]
[13]
[14]
[15]
[16]
[17]. Sensors were placed
on (1) dorsal aspect of the second metatarsal of the front-side foot (2–3),
bilateral lateral aspect of the shank (4–5), bilateral lateral aspect of the thigh
(6), sacrum between left and right posterior superior iliac spines, (7) posterior
aspect of the trunk at first thoracic vertebrae spinous process (8–9), bilateral
scapula on the flat broad portion of acromion (10–11), bilateral aspect of the
humerus 1–2 cm proximal of the elbow (12–13), bilateral lateral aspect of the distal
forearm, and (14) dorsal aspect of the back-side hand on the third metacarpal. A
15th moveable sensor was attached to a rigid stylus for digitizing
bony landmarks to develop a linked-segment model in accordance with International
Society of Biomechanics standards [18]
[19]. Raw data regarding
sensor position and orientation were independently filtered along each global axis
using a fourth-order Butterworth filter with a cut-off frequency of 13.4 Hz [20]
[21]. The world axis was represented with the positive Y-axis in the
vertical direction. Anterior to the Y-axis and in the direction of movement was the
positive X-axis. Orthogonal and to the right of X and Y was the positive Z-axis.
Position and orientation of body segments were consistent with International Society
of Biomechanics recommendations [18].
Euler angle decomposition sequence of ZX’Y” was used for trunk motion relative to
the world while the YX’Y” sequence was used for shoulder motion relative to the
trunk. Elbow motion was defined relative to the humerus using the Euler angle
decomposition sequence of ZX’Y”. Hip and knee motions were defined as distal segment
relative to proximal segment using the Euler angle decomposition sequence of
ZX’Y”.
Fig. 1 The placement of 14 electromagnetic sensors.
Following sensor attachment, participants were provided unlimited warm-up time to
feel comfortable and ready to perform maximum effort swings [6]. Participants were also asked to use
the same bat they use in a competition to prevent an unfamiliar bat from interfering
with their hitting mechanics [6].
Participants performed three trials of maximal effort swings from a stationary tee
followed by three maximal effort swings from a front toss with a pitcher located
9.14 m in front of home plate. Performing three maximal effort swings per condition
is similar to methods from prior hitting studies [17]
[22]. At least one minute of rest was allotted in between each swing
trial. The tee was placed at a distance from the body to reflect a pitch in the
middle of the strike zone and centered midway between the knee and hip [6]. Hitting trials were deemed successful
if the result was a line drive, front toss location was over the middle of the
strike zone, and the hitter verbally affirmed it was a ‘good’ swing [6]. A line drive was defined as the ball
having a flat trajectory hitting the back net of the batting cage. These procedures
mimic how a strike, tee placement, and a successful swing would be determined in a
practical setting. Although reliability for these measures was not determined,
visual identification of a strike, tee placement, and parameters for a successful
swing are consistent with the methods used in tee and front-toss hitting research
[6]
[17]
[21]
[23]
[24]. Verbal affirmation was required
since swing mechanics can vary between athletes and temporal feel of the swing is
related to successful hitting performance [25]. The participant was instructed to rest in between each maximum
effort swing while the recorded trial was reviewed in The MotionMonitor XGen
software and saved on the computer. Data for each kinematic variable were averaged
across the three trials per modality three trials. Kinematic variables used for
comparison included bilateral knee, hip, and elbow flexion as well as pelvis
rotation, trunk rotation, trunk lateral flexion, trunk flexion, and pelvis to trunk
separation. The front and back-side extremities were those closest to the pitcher
and catcher, respectively.
Prior to analysis, the baseball swing was separated into two phases: the preparatory
phase and the acceleration phase ([Fig.
2]). The preparatory phase was marked by two events: (1) start; the first
1 cm change of posterior displacement of the pelvis in the negative x-direction
toward the catcher; (2) load; maximum posterior displacement of the pelvis in the
negative x-direction toward the catcher. The second event also marked the start of
the acceleration phase and ended with the third event of ball contact defined as one
frame after maximal back hand angular velocity. The events of the swing were marked
in each trial. Using a customized MATLAB (Mathworks) script, data between the start
and end of both the preparatory and acceleration phases were extracted and
normalized to 101 data points which represented 0–100% of each phase of the hitting
motion for each condition (tee and front toss).
Fig. 2 Events of the swing and phases analyzed. (1) Start; initial
displacement of pelvis towards the catcher; (2) Load; maximal displacement
of pelvis towards the catcher; (3) Ball contact; maximal back hand angular
velocity.
Statistical Analysis
To conduct multiple comparisons across each phase of the swing, 1-dimensional
statistical parametric mapping (SPM) multivariate analysis of variance (MANOVA)
within-subjects testing were performed. Statistical parametric mapping is a
novel approach to hitting research given its unique ability to examine mechanics
across an entire phase rather than limiting analysis to a single time point.
Initial use of within-model SPM{F2} MANOVAs enabled hypothesis testing at
the multivariate level to be performed over time (0–100% of each phase) [26]. For multivariate testing,
kinematic variables were sorted by body segment/joint which resulted in the
following kinematic variable groups: a) trunk; b) knee; c) hip; and d) elbow.
Kinematic variables that comprised each group were as follows: a) trunk
rotation, lateral flexion, flexion, and pelvis to trunk separation; b) knee
(flexion for both front and back legs; c) hip (flexion for both front and back
legs; and d) elbow (flexion for front and back arms). For every phase,
1-dimensional SPM MANOVAs compared the combined dependent kinematic variables of
each variable group (a-d) and condition (tee and toss). A total of eight
SPM{F2} MANOVA tests were performed in MATLAB 2020 A (Mathworks)
using the open-source software package spm1d
[27]. An alpha level of 0.05 denoted
statistical significance. In the case of a significant MANOVA test, follow-up
paired-samples t-tests were performed in MATLAB 2020 A (Mathworks) using
SPM{t} tests for each kinematic variable associated with the
significant MANOVA omnibus test. Post hoc testing using SPM{t}
tests then permitted comparisons between tee and front toss conditions using
continuous data (101 data points) for each kinematic variable [27]
[29]
[29]. To account for multiple
comparisons when performing follow-up paired samples SPM{t} tests, a
Sidàk stepdown correction was applied to each observed p-value [30].
Results
The average batted ball velocity for tee and front toss were 115.1 ± 21.2 km/h and
112.7 ± 20.8 km/h, respectively.
Preparatory Phase
The preparatory phase within-subjects SPM{F} MANOVA indicated a
statistically significant difference in the combined dependent kinematic
variables of the trunk between conditions expressed over 100% of the preparatory
phase ([Fig. 3]). According to
SPM{t} follow-up testing, differences in trunk rotation between hitting
conditions were found for 100% of the preparatory phase (p=0.001) ([Fig. 4]). SPM{t} revealed
that trunk was more rotated toward the back side (catcher) throughout 100% of
the preparatory phase when hitting off the tee compared to the front toss
condition (p<0.001) ([Fig.
4]
[5]). No significant
differences between tee and front toss conditions were determined for the
remaining trunk variables (trunk flexion, trunk lateral flexion, and pelvis to
trunk separation) throughout the preparatory phase. SPM{F} MANOVAs did
not find any other statistically significant differences in the preparatory
phase for the combined dependent variables of knee, hip, or elbow kinematics
between tee and front toss conditions; therefore, follow-up SPM{t} tests
were not performed.
Fig. 3 Trunk SPM MANOVA for the preparatory phase. The black line
represents the SPM{F2} test statistic at each point in time
throughout the preparatory phase; horizontal dashed lines represent the
test statistic critical threshold; SPM=statistical parametric mapping.
Fig. 4 Trunk rotation (a) SPM t-test and (b)
kinematic plots for the preparatory phase. The black line
represents the SPM{t} test statistic at each point in time throughout
the preparatory phase; horizontal dashed lines represent the test
statistic critical threshold; SPM=statistical parametric mapping.
(b) Comparison of trunk rotation between tee (blue line) and
front toss (red line) conditions; − 180°=facing catcher, 0°=facing
pitcher.
Fig. 5 Trunk rotation of the preparatory phase for tee and toss
conditions (mean trunk rotation at start and end of phase illustrated).
*p<001; significant difference between tee and front toss
conditions across 100% of the preparatory phase. Hitting off the tee
condition displayed significantly greater trunk rotation towards the
backside.
Acceleration Phase
The acceleration phase within-subjects SPM {F2} MANOVA further indicated a
statistically significant difference in the combined dependent kinematic
variables of the trunk between conditions expressed over 67% of the acceleration
phase ([Fig. 6]). According to
SPM{t} follow-up testing, differences in trunk rotation between
hitting conditions were revealed from 0–67% of the acceleration phase
(p<0.001). SPM{t} revealed the trunk was more rotated towards the back
side (catcher) from 0 to 67% of the acceleration phase when hitting off the tee
compared to the front toss condition (p<0.001) ([Fig. 7]
[8]). No significant differences
between tee and front toss conditions were determined for the remaining trunk
variables (trunk flexion, trunk lateral flexion, and pelvis to trunk separation)
throughout the acceleration phase. SPM{F} MANOVAs did not show any other
statistically significant differences in the acceleration phases for the
combined dependent variables of knee, hip, or elbow kinematics between tee and
front toss conditions; therefore, follow-up SPM{t} tests were not
performed.
Fig. 6 Trunk SPM MANOVA for the acceleration phase. The black line
represents the SPM{F2} test statistic at each point in time
throughout the preparatory phase; horizontal dashed lines represent the
test statistic critical threshold; SPM=statistical parametric mapping.
Fig. 7 Trunk rotation (a) SPM t-test and (b)
kinematics plots for the acceleration phase. (a) SPM plot; black
line represents the SPM{t} test statistic at each point in time
throughout the preparatory phase; horizontal dashed lines represent the
test statistic critical threshold; SPM=statistical parametric mapping.
(b) Comparison of trunk rotation between tee (blue line) and
front toss (red line) conditions; − 180°=facing catcher, 0°=facing
pitcher.
Fig. 8 Trunk rotation of the acceleration phase for tee and toss
conditions (mean trunk rotation at start and end of phase illustrated).
*p<001; significant difference between tee and front toss
conditions from the start to 67% of the preparatory phase. Hitting off
the stationary tee condition displayed significantly greater trunk
rotation towards the backside.
Discussion
The purpose of this study was to compare lower extremity, trunk, and upper extremity
kinematics between tee and front toss hitting in youth baseball athletes. The
results indicated there were significant differences in trunk rotation when hitting
off a tee versus front toss. This partially confirmed the hypotheses, since other
upper extremity, lower extremity, and trunk kinematics were not found to be
significantly different. Specifically for the trunk, athletes demonstrated more
rotation toward the back side (catcher) during the tee compared to the front toss
condition during the preparatory and acceleration phases of the swing.
The findings suggest that youth baseball athletes modify trunk rotation depending
on
whether the ball is pitched or in a stationary position. Youth athletes may be more
inclined to achieve greater trunk rotation towards the catcher during the tee
position, since they can primarily focus on hitting the ball with a high exit
velocity. Conversely, youth athletes may be less inclined to achieve the same degree
of counter rotation towards the catcher during the front toss condition, since they
must multi-task to attain a visual of the incoming ball, tracking its speed and
location, and control their movement to achieve optimal contact.
The current study’s findings contrast the collegiate softball hitting study by
Washington et al. that reported greater trunk rotation toward the back side for the
front toss compared to the tee condition [6]. However, the discrepancy is likely attributed to analyzing different
competition levels, since prior research has shown that mechanics vary between youth
and elite athletes [31]. In addition,
the current study performed an SPM analysis that examined kinematics over the entire
preparation and acceleration phases, while Washington et al. examined trunk rotation
at single events during the swing [6].
A prior study by Chen et al. [7] compared
collegiate baseball hitting mechanics across various modalities (front toss, motor
imagery, video projection, and virtual reality). Athletes achieved greater upper
trunk rotation toward the back side when hitting a front tossed ball compared to
when athletes attempted to swing and hit a ball during video projection and virtual
reality conditions [7]. The front tossed
ball was the condition in which the ball was delivered at a slower speed, while the
video projection and virtual reality conditions were delivered at faster speeds.
Therefore, the current study and the study by Chen et al. [7] illustrate a pattern of decreasing
trunk rotation toward the back side when the ball is pitched at increased speeds.
Modifying trunk rotation may account for the timing needs of the batter to quickly
transition from linear displacement during the preparation and early acceleration
phases to a high rotation velocity of the trunk and shoulders during the remaining
acceleration phase [32].
In the context of youth baseball athletes, the current study indicated that hitting
a
tossed ball produced less trunk rotation towards the backside. Therefore, variations
in trunk mechanics across the preparation and acceleration phases of the swing may
limit the transferability of hitting off a tee to more game-like contexts. Awareness
of these differences in trunk kinematics between hitting modalities can help coaches
structure a hitting program unique to an athlete’s needs. While hitting off a
stationary tee can still be advantageous for youth hitters needing to isolate their
focus on improving specific swing mechanics, coaches should ensure athletes are
maintaining consistent mechanics across all hitting modalities that transfer to live
pitching. Coaches seeking additional assistance to help youth control their trunk
rotation across various hitting modalities may also seek out resistance training
since strength and power exercises have been shown to aid performance in hitting
sport-specific tasks [33]
[34]. For example, medicine ball rotation
throws performed specific to the ranges of motion of the swing may be used to
enhance power development while aiding drill transferability of hitting a ball at
increased speeds [33].
Lastly, it is important to note the degree of trunk rotation is a single factor in
a
series of precise movements contributing to the baseball swing. It is also important
to consider factors such as the timing of rotation and rotational velocities when
seeking to improve performance. Rather than directly comparing the relationship
between the degree of trunk rotation and hitting performance, studies have primarily
compared trunk mechanics across competition levels. Studies have demonstrated
differences in the degree of trunk rotation across competition levels as well as
higher trunk rotation velocities in more skilled athletes [4]
[35]. The results suggest coaches should consider both the degree of trunk
rotation and rotational velocity when training hitters. Additional research should
compare the degree of trunk rotation as well as trunk rotational velocities between
front toss and tee hitting modalities in youth baseball athletes.
Important study limitations should be noted. The first is pitch location was deemed
in the center of the strike zone through visual observation only. Although some
variability can be expected between strike zone and tee placement based on visual
observation, the current study’s other criteria for a recorded trial served as
de-limitations. Additionally, the tee was placed in the middle of the strike zone
and centered midway between the knee and hip for all participants. Other study
criteria including line drive hit trials only and verbal affirmation from the
participants helped to ensure tee placement was considered close to the middle of
the strike zone. Second, a standard bat was not used across participants, yet this
was similar to prior softball hitting methodologies that allowed participants to use
the bat they were comfortable swinging in a competition setting to make the practice
experience more game-like and individualized [6]
[17]. Results may have been
confounded by a bat weight or length that was unaccustomed to the athlete for which
altered mechanics could have been a result of a change in mass-moment of inertia in
the swing, or potentially insufficient or greater strength relative to bat weight
and length. Lastly, the age range can be considered a limitation of the study;
however, all athletes reported playing on travel league teams, which is considered
a
higher level of competition than recreational leagues.
In conclusion, there was a significant difference in trunk kinematics between tee
and
front toss hitting conditions in youth baseball athletes. Specifically, there was
greater trunk rotation toward the back side in the tee condition throughout the
preparatory phase and 67% of the acceleration phase compared to the front toss
condition. There were no other significant differences between tee and front toss
conditions for all other lower extremity, upper extremity, and trunk kinematics
across the preparatory and acceleration phases of the swing. Though it is understood
that coaches utilize various training modalities when seeking to enhance hitting
performance, the potential differences in trunk mechanics should be considered when
seeking to develop fundamental hitting techniques in youth baseball athletes.
Hitting off a stationary tee can still be advantageous for youth hitters needing to
isolate their focus on improving specific swing mechanics; however, coaches should
ensure athletes maintain consistent mechanics across all hitting modalities to
ensure skill sets translate to live pitching.
Lastly, future studies should consider utilizing statistical parametric mapping (SPM)
as a novel method for investigating hitting mechanics. This method has gained
popularity in baseball [36] and softball
[37]
[38] pitching research given its unique
ability to examine mechanics across an entire phase rather than limiting analysis
to
a single time point. As a growing number of studies seek to further understand
hitting mechanics, SPM can provide a more comprehensive interpretation of mechanics
throughout the phases of the swing. Considering that the current study only compared
mechanics between tee and front toss hitting conditions, additional research is
needed to compare mechanics during live hitting to improve the transferability of
findings to a competitive environment.
