1. Biomechanics During a Baseball Swing

2. Biomechanics of the Baseball Swing 2
Introduction
The baseball swing is very complex with regards to the all the different force vectors and
movements involved in the correct and successful swing. It is vital to have proper mechanics
during a baseball swing to achieve maximal mechanic efficiency. To have proper mechanics, a
biomechanical view and diagnosis each of the movements in each phase to fully understand
where the forces will be generated going through the swing.
Trends in methodology that have been studied before usually use video analysis programs
to break the swing down into parts and draw the vectors in these programs to gain proper
technique. The load phase is when the hitter begins to build the most elastic energy before
transferring to the next phase. That next phase is the launch phase, or the actual swing itself to
point of contact, where the elastic energy from the load phase is released by rotating the hips and
bringing the arms and hands down to the ball. The final phase, the follow through, is after
contact is made with the ball when the hands will drive through in a north-south motion,
extending both arms until the top of the follow through. There are factors that affect proper
mechanics such as type/recognition of the incoming pitch, timing, length and weight of the bat,
hand eye coordination, physical ability, mental toughness, and even weather conditions
(Tabuchi, Matsuo, Hashizume, 2007).
As of 2015, there are approximately 17.5 million participants who have played organized
baseball in the past 12 months in the U.S. (Eime, Charity, Harvey, & Payne, 2015). With such a
huge participation in the sport, the purpose of this study is to help players understand where
optimal power comes from during the swing and what their swing should resemble. Another
purpose that can be examined is when the player goes to strength train, they will be able to
mimic the movements that will be described later that will lead to a higher level of play.

3. Biomechanics of the Baseball Swing 3
Load Phase
The main purpose of the load phase is to gain as much potential energy as possible. At
the same time, this phase is used as a timing mechanism to deliver the swing when the ball is
about to cross the plate. We will first see movement in the shoulders and arms on a right-handed
batter as they will extend and move back towards the catcher. This movement lengthens both
arms, but use different muscles for each arm (Escamilla, Andrews, 2009).
The movement of the left arm and shoulder during the load is adduction, which is when
the segment of the body moves toward the midline. The muscles that produce this motion is the
coracobrachialis, pectoralis major, teres minor, and the latissimus dorsi (Escamilla, Andrews,
2009). The coracobrachialis is used in flexion and adduction in the glenohumeral joint which is
inserted on the medial humerus. The coracobrachialis is one of the three muscles, others being
the pectoralis minor and the short head of the biceps brachii, that attach to the coracoid process
of the scapula. The pectoralis major, which is more commonly named as the “pecs” along with
the pectoralis minor, is a large fan shaped muscle that inserts at the bicipital groove of the
humerus but has multiple origins such as the clavicular head that flexes the humerus and
sternocostal head that extends the humerus. This muscle works at both origins to adduct and
medially rotate the humerus, which will be necessary when the load is initiated. The teres minor
is a small thin muscle of the rotator cuff. It is inserted at the greater tubercle of the humerus from
the lateral side of the scapula. This muscle assists in lateral rotation of the arm and helps to
stabilize the humerus which will assist in the extension of the left arm into the load phase. Lastly,
the latissimus dorsi, more commonly known as the “lats” is the widest muscle of the back. It
inserts at the intertubercle groove of the humerus, which separates the greater and lesser
tubercles of the humerus, and attaches from the spinous process of the of vertebrae, iliac crest,

4. Biomechanics of the Baseball Swing 4
inferior 3 or 4 ribs, inferior angle of scapula, and thoracolumbar fascia. The lats internally
rotates, extends, and adducts when the humerus is moved medially and is a major contributor to
the potential energy of the load phase.
At the same time these motions are happening, the left scapula is being protracted and the
shoulders are elevated. Protraction is when the scapula moves forward. The muscle that assists in
elevation of the shoulder is the upper trapezius, the superior part of the trapezius that originates
from the spinous process, the medial third of the occipital bone, and the ligamentum nuchae
inserting at the posterior third of the clavicle. For protraction of the left scapula, the serratus
anterior, a saw-like muscle, is called on which inserts medially on the scapula from the 8 and 9
ribs (Escamilla, Andrews, 2009).
On the opposing side, the right arm abducts to assume the loaded position. Abduction is
the result when the right shoulder and arm move away from the midline of the body. During the
abduction into the load phase on the right side, the supraspinatus, middle deltoid, and upper
trapezius assist in the motion. The supraspinatus is a smaller muscle of the upper back that
inserts at the greater tubercle of the humerus from the supraspinous fossa of the scapula. Its
partner in abduction, the middle deltoid, is a part of the deltoid muscle that inserts at the deltoid
tuberosity of the humerus from the acromion process and the spine of the scapula.
The next motion that happens simultaneously with the shoulders and arms is the rotation
of the spine, making the shoulders and arm more visible from the posterior side of the player’s
stance. Muscles that are activated in rotation of the spine is the multifidus spinae, external
abdominal oblique, and rotatores spinae. These muscles must contract for the rotation away from
the pitcher to occur. The multifidus spinae is a long muscle that fills in the grooves of the
spinous process from the erector spinae, sacrum, PSIS, and the iliac crest (Lin et al., 2013). The

5. Biomechanics of the Baseball Swing 5
external abdominal oblique is the biggest muscle of the lateral anterior abdomen that attaches at
the iliac crest and pubic tubercle from ribs 5 through 12. The rotatores spinae is a diverse series
of muscles that lie beneath the multifidus spinae and attach at the spinous process from the
transverse process. The contracting side of the lateral rotators of the spine during the load will be
the ones on the left because to rotate to the right, the left side must contract which produces the
desired motion. As the left side of the lateral rotators contract, the right rotators will lengthen to
produce the elastic energy that we want to achieve.
Once abduction/adduction of the arms and rotation of the spine is achieved, we will see
the natural backwards rotation of the pelvic girdle and spine, exposing the front of both hips
from the posterior view due to the momentum from the motions. This will stretch the external
hip rotators to produce elastic energy. These external hip rotators include the gluteus medius,
gluteus maximus, obturator internus, obturator externus, inferior and superior gemellus,
piriformis, and the quadratus femoris (Lin et al., 2013). Mainly, the gluteus maximus and gluteus
medius are the main contributors to the total power of hip rotation. The gluteus maximus is the
largest of the gluteal muscles that attaches at the gluteal tuberosity of the femur and iliotibial
tract from the sacrum, lumbar fascia, gluteal surface of the ilium, and the sacrotuberous ligament.
To achieve the proper balance throughout the load phase, there should be about 60% of the
player’s weight on the back side of the stance, with the remaining 40% on the front side. We will
also see the front foot come off the ground as a timing mechanism that will later trigger the
transition to the next phase. Another way to obtain balance is to flex the knees and hip. Doing so
will produce elastic potential in the knee extensors, hip extensors, and calf muscles in the back
leg which has a heavier load as the player takes their timing step. Knee extensors include the
vastus medialis, vastus lateralis, vastus intermedius, and the rectus femoris. Hip extensors are the

6. Biomechanics of the Baseball Swing 6
gluteus maximus and the hamstrings which consist of the semimembranosus, semitendinosus,
and the long head of the biceps femoris. The gastrocnemius and soleus make up the calf muscles
in which are stretched and fired in the swing.
While the shoulder and arms move into abduction/adduction, the wrist will tilt the bat
back to prepare it to propel forward through the ball to reduce the inertia from the velocity of the
pitch and the ball itself. The movement at this part of the load only occurring in the right wrist
and elbow is pronation, which helps the bat get into the furthest backswing position for optimum
recoil. Pronation is a movement that turns the palm of the hand posteriorly or inwards from the
anatomical position due to medial rotation of the forearm. There are two pronator muscles in the
forearm which is the pronator teres and pronator quadratus. The pronator teres attaches in the
middle of the lateral surface of the radius from the humeral and ulnar heads. The pronator
quadratus is a square shaped muscle that inserts at the lateral anterior surface of the radius from
the medial anterior surface of the ulna. The movement produced at the left wrist and hand during
the tilt of the bat is radial deviation. Radial deviation is when the wrist moves towards the thumb
side of the forearm. The muscles that produce this motion are the flexor carpi radialis, extensor
carpi radialis brevis, and extensor carpi radialis longus. When these forearm muscles contract,
the right elbow supinators and left ulnar deviators will stretch to produce elastic potential.
When muscle is stretched, such as the muscles that are being stretched in this phase, the
muscle spindles associated with the muscles will have an increase in neural activity. This is a
powerful reflex because as the muscle is stretched further, the alpha motor neurons that increase
in activity will contract as a response to the muscle being stretched (Reyes, Crusat, Dolny, 2011).
This is called a myotatic reflex. This reflex can be used the player’s advantage, but only for a

7. Biomechanics of the Baseball Swing 7
very short period as this it only lasts about 0.5 seconds. Therefore, timing is a crucial element to
receive the benefits of the reflex.
Launch Phase
This phase of the swing is from the top of the backswing to point of contact. It is
important to understand the biomechanics of the launch phase because this is the phase that the
player gets to see how much potential energy they produced if the timing and contact is pure.
The first movement is the planting of the front foot which signals for the contractile phase of the
external hip rotators in the left leg of the right-handed player which will cause the internal hip
rotators to stretch. This will indicate that it is time for the hips to open and release the elastic
energy from the muscles what were stretched in the hips, thighs, and calves on the right side
during the load phase, resulting in the anterior part of the player facing the pitcher. Once the hips
begin to open, the external hip rotators of the right leg will contract to assist in the rotation of
that right side that extends the right hip and knee. This motion puts the right foot into
plantarflexion, extension of the gastrocnemius and soleus resulting in the pointing of the toes
towards the ground, due to the immense amount of energy being transferred to the front side
after being stored for contact with the ball. Forward rotation of the spine is also another product
of the firing of the right side of those spinous muscles.
As the lateral rotators of the right side of the spine contract due to the firing of that right
side, the energy stored in the arms combined with the energy in the legs will assist the arms
through the hitting zone through contact (Tago, Ae, Tsuchioka, Ishii, Wada, 2010). Due to the
positioning of the hands on the bat, there will be two different forces acting against each other
(Escamilla et al., 2011). Therefore, the left hand, the bottom hand, will pull the bat and the top
hand, the right hand, will push the bat through the hitting zone. “The wrist and hand joints are

8. Biomechanics of the Baseball Swing 8
the strongest in the neutral position, as this will give the hitter the best chances of solid contact”
(pg. 208). As the player is about to start the forward swing, there should be flexion at the elbows
to adjust to any off-speed or inside/outside pitches. The muscles involved in abduction and
extension of the left arm as it extends to contact are the anconeus, triceps, rhomboids, middle
trapezius fibers, posterior deltoid, teres minor, and infraspinatus. The anconeus is a small muscle
on the posterior side of the elbow joint that attaches at the lateral surface of the olecranon
process and the superior proximal part of the posterior ulna from the lateral epicondyle of the
humerus. The other elbow extensor, the triceps brachii, is attached at the infraglenoid tubercle of
scapula and above and below the radial sulcus. The rhomboids are rhombus-shaped muscles that
retract the scapula and hold it to the thoracic wall. It is attached to the medial border of the
scapula from C7 to T5 on the spinous process. The posterior deltoid helps in shoulder abduction
as well as flexion and extension and is attached at the deltoid tuberosity of the humerus from the
posterior spine of the scapula. The infraspinatus is another triangular muscle like the deltoid that
laterally rotates the arm and stabilizes the humerus. It attaches at the greater tubercle of the
humerus from the infraspinous fossa of the scapula.
Follow Through Phase
The follow through phase is the least examined phase of the baseball swing mainly
because at this point in the swing, the player has already made contact with the ball. It includes
the time the ball leaves the bat all the way until the swing is naturally ended or the player begins
to run to first base. Internal rotation is when the body segment is rotated towards the longitudinal
axis of the body. This is allowed during the follow through because the right and left thigh both
internally rotate as the medial hip rotators are stretched and wanting to return to the shorter, more
relaxed position. Another reason internal rotation of both hips occur is because the left leg will

9. Biomechanics of the Baseball Swing 9
end up fully extended with credit to the quadriceps femoris muscle (Tago et al., 2010). The
quadriceps femoris is a muscle group that as in the name consists 4 different muscles. These are
the rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis which all attach at the
tibial tuberosity from the quadriceps tendon. Consequently, inversion of the feet to occur. As the
hips open more, the spine will continue to rotate as the lateral rotators of the spine contract. The
left arm and shoulder will continue to abduct with contraction of the rhomboid, posterior deltoid,
and middle fibers of the trapezius, infraspinatus, and teres minor. The right arm will continue to
extend and adduct due to contraction of pectoralis major, serratus anterior, coracobrachialis, and
the anterior deltoid. The right wrist will end up in a pronated position, because of the contraction
of the pronator teres and pronator quadratus, compared to its supinated beginning position. The
left forearm will end up in supination given that the contraction of the biceps brachii and
supinator.
Conclusion
To obtain optimal swing speed and power, the muscles that contract and lengthen
described must be worked on and incorporated into a player’s workout plan. It is understood that
not every player has the same swing and form, but understanding what muscles will help make
them a better player can produce positive results. Timing is an extremely important aspect for a
player as they must decide if they want to swing at the pitch, then they must start the positive
segment of the swing and make solid contact with the ball in that 0.5 second gap their muscles
start contracting after being stretched. Needless to say, this is the reason why hitting a 90 mile
per hour fastball is the hardest thing to do in all of sports.

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11. Biomechanics of the Baseball Swing 11
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