By Courtney Semkewyc (RPP Bio-mechanist Intern, PhD Candidate Biomedical Engineering at Rutgers University)
In assessing a motion capture session one of the most important things to examine is the kinematic sequence. This consists of the angular velocities and corresponding timing of the pelvis, torso, shoulder, and hand. These angular velocities can reach speeds upwards of 5000 º/s, and as such cannot be measured through standard 2-D video analysis thus requiring a 3-D motion capture system. This simple graph below off our Qualisys Motion Capture system is key to determining how efficiently an athlete is transferring energy from the ground up, through their body, and into the ball.
The above chart is a kinematic sequence from set up to release. Where the blue vertical line is foot plant, the red vertical line is release, and the color coding of red, green, purple, and yellow corresponds to angular velocities of the pelvis, torso, shoulder, and hand, respectively. Here is a closer look at the final 200 ms of a typical delivery.
The release of this energy into the ball directly correlates to the pitching velocity, and thus increasing energy results in an ultimate increase in velo. This relationship between angular velocity and pitch velocity is supported by a study conducted by Stodden, Fleisig et al. on elite pitchers that found that pitch velocity increased as pelvis angular velocity increased during the arm cocking phase and torso angular velocity increased during the acceleration phase (Relationship of Pelvis and Upper Torso Kinematics to Pitched Baseball Velocity. Journal of Applied Biomechanics, Stodden, Fleisig et al.)
Not only can the kinematic sequence provide insight into energy transfer and reasons for leakages in velocity, it can also help provide insight into potential injury. For example, if a pitcher isn’t effectively using their lower body and transferring energy up the kinetic chain, they might compensate through overuse of their arm, which can in turn increase the torque on their arm and the risk of injury.
As a result, it is important to understand the kinematic sequence, and how it translates to the pitching motion. The three key factors to look for in the graph are the:
- The Sequence (timing)
- Shape of the Curves (accel/decel)
1. The Sequence (timing)
When a pitcher’s foot hits the ground during foot plant, the ground exerts force back onto the foot through the point of contact. This is commonly known as ground reaction force (GRF) and is the start of the energy transfer up and through the kinetic chain. To take this energy from the ground and transfer it to the ball, the energy must first be transferred through the body.
The most efficient way to transfer energy from the ground to the ball is to start with the lower body and then move the energy through to the upper body. Here is a brief summary of each color code curve in the sequence.
Pelvis (red curve) – At foot plant, the ground reaction force will arise from the ground up through the lead leg driving the femur into the hip and driving it backwards into rotation. This backwards rotation is used to accelerate the pelvis around the lead hip resulting in the pelvis angular velocity curve.
Torso (green curve) – At this point the pelvis rotation in combination with scap load will help produce the hip-shoulder separation that causes the torso to uncoil and begin to rotate thus generating its angular velocity.
Shoulder (purple) and Hand (yellow) – This trend then continues through the shoulder via the lat and para spinals (purple curve) into the hand (yellow curve), and ultimately into the ball.
As a result, when examining a sequence, the proper progression is pelvis, torso, shoulder, and hand, where the arrows represent the start of each segment’s acceleration.
This means that we want to see the pelvis angular velocity accelerate or increase first, followed by the torso, shoulder, and hand, respectively. When these curves are out of sequence this means that a pitcher is losing energy throughout their delivery. This can lead to a reduced velocity or can cause a pitcher to try and make up this energy with another segment, which can lead to injury. An example of this is when a pitcher is forced to overuse their arm due to early trunk rotation.
Next, we want the energy to build as we move from the larger proximal muscles (lower body) to the smaller distal muscles (upper body). Since the formula for rotational kinetic energy is rotational inertia multiplied by angular velocity squared, this means that the energy at each segment is proportional to its angular velocity squared.
This means that even small changes in angular velocity can lead to large changes in rotational energy. So, to determine if the energy is building between segments, we can look at the peak velocity of each segment as a representation of the energy in that segment.
An efficient transfer of energy is seen in increasing peaks from the pelvis to the hand. In the image below, there are two examples of kinematic sequences with “Inefficient Transfer” (left) demonstrating a lack of energy growth, and “Efficient Transfer” (right) sequence demonstrating increased energy growth.
In Figure A (left), it can be seen that this athlete has some growth between the peaks of the pelvis and torso, a large amount of growth to the shoulder, but a massive loss of energy during the transfer to the hand (yellow). This loss of energy could be the result of many factors including timing, muscle engagement and anatomical positioning (strength and/or mobility).
The kinematic sequence in Figure B (on the right) on the other hand has an improved differential between the torso and pelvis peaks corresponding to more energy transfer, and better transfer from the shoulder to the hand. Here the shoulder and hand have about the same angular velocity demonstrating that there isn’t much energy loss, but also no energy gained.
Ideally, much like cracking a whip, a pitcher will continue to increase their angular velocity from the largest muscles (the pelvis) to the smallest (the hand), which creates the most transfer and gain of energy leading to more velocity and less stress on the body.
3. Shape of the Curves (accel/decel)
The third factor to examine is the shape of the curve. This means to look at the slope, how fast is the curve accelerating (increasing) and how fast is it decelerating (decreasing). We want the curve to have steep slopes, to increase fast and decrease fast.
This is due to the fact that when the proximal segment decelerates it frees up muscles to be used by the distal segment to accelerate forward thus increasing the angular velocity of the distal segment and the transfer of energy. As a result, quicker deceleration corresponds to better energy transfer to the next segment, so we like to see the proximal segment decelerate as the distal segment accelerates leading to sharp peaks. A lack of acceleration or deceleration of a segment could be due to timing, physical limitations, or the influence of another segment.
A common problem often associated with a slow deceleration is poor segment timing. An example of this can be seen in Figure A (left) below. Red arrow indicates the point of peak pelvis velocity.
This graph shows that this pitcher’s pelvis has a very slow deceleration. Since this pitcher is unable to quickly decelerate their pelvis, they are forced to start both their pelvis rotation and their deceleration sooner since they take longer to reduce their speed. Starting their pelvis rotation earlier in the pitching motion when combined with poor deceleration leads to the peak pelvis velocity occurring before foot plant (blue line) instead of after it.
Since this occurs before contact with the ground it also demonstrates that they are not efficiently transferring the ground reaction force through their pelvis and into the torso. These reasons could explain why the peak pelvis velocity of this pitcher is 552 º/s, which is 103 º/s (15.7%) slower than the average pelvis velocity (655 º/s).
In the Figure B (right), you can observe an improved pelvis deceleration. Here the pitcher can take longer to accumulate their velocity as they are able to quickly decelerate. This pitcher has a peak pelvis velocity of 875 º/s that is 220 º/s (34%) faster than the average pelvis velocity, and more importantly occurs after foot plant demonstrating a good transfer of the ground reaction force into rotational energy.
The kinematic sequence provides excellent insight into the transfer of energy throughout the pitching motion. Pitchers with a more efficient kinematic sequence are able to minimize energy loss through their body positioning, muscle engagement, and proper sequencing. Maximizing energy transfer corresponds to an increase in velocity, and a reduction in anatomical stress when maintaining a constant velocity. Consequently, it is important to not only be able to assess, but also understand the kinematic sequence and its relation to both biomechanics and the pitching motion.
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