By Courtney Semkewyc (RPP Bio-mechanist Intern, PhD Candidate Biomedical Engineering)
In Part 1 of this article, we reviewed two of five topics related to hip-shoulder separation while reviewing motion capture data on Qualisys Mocap system. Here in Part 2, we are going to review the remaining three topics:
- Relationship between the Kinematic Sequence and Hip-Shoulder Separation
- Proper Kinematic Sequence
- Amount of Separation Time (the time difference between when the peak torso and peak pelvis angular velocities occur)
Before we begin with the remainder of this article, let’s quickly translate hip-shoulder separation terminology into angular velocities in the kinematic sequence:
- Hip becomes the pelvis
- Shoulder becomes the torso
Relationship between the Kinematic Sequence and Hip-Shoulder Separation
The kinematic sequence provides valuable information into the sequencing of segments and the energy transferred through the kinetic chain. In Part 1, we focused on hip-shoulder separation, which is related to the pelvis (red) and torso (green) lines in the kinematic sequence graph above.
As mentioned in Part 1, peak hip-shoulder separation occurs at the time point where the torso angular velocity intersects the pelvis angular velocity. This is due to the fact that once the torso begins to rotate faster than the pelvis it will begin to catch up to the pelvis thus reducing the space/angle between them. This can be seen in the figure below where the hip-shoulder separation graph and kinematic sequence graph were taken from the same pitcher.
Timing of When Peak Hip-shoulder Separation Occurs = Timing of When Torso Angular Velocity intersects the Pelvis Angular Velocity
Another important metric related to hip-shoulder separation is the Separation Time, the time difference between the peak torso angular velocity and peak pelvis angular velocity. It has been found to be linked to pitch velocity (Van der Graaf et al, 2018). It is a metric that can be increased to improve pitch velocity much like hip-shoulder separation, but Separation Time should only be examined after a pitcher has proper kinematic sequencing.
Proper Kinematic Sequencing
Just as with hip-shoulder separation, the timing of the peak pelvis and peak torso angular velocities are more important than the actual amount of Separation Time. So, the first thing to look for in the kinematic sequence is the timing of peak pelvis angular velocity. We want the peak pelvis angular velocity to occur after foot plant. If the pitcher achieves their peak pelvis angular velocity before foot plant, then the amount of Separation Time will not properly contribute to the pitching motion and shouldn’t be examined.
Many pitchers appear to have a good Separation Time, but this is due to the fact that they achieve peak pelvis angular velocity well before foot plant increasing the distance between the peak pelvis and torso velocities. This is not a true Separation Time, as for Separation Time to contribute to velocity the pitcher must be able to effectively use the ground reaction force at foot plant, while resisting torso rotation for as long as possible.
This combination of ground reaction force and delayed trunk rotation allows the pitcher to gather energy up the kinetic chain and provides additional time for the arm to get into layback.
The second thing to look for to maintain the order of the kinematic sequence is that they are not reaching their max elbow extension velocity before their torso reaches its maximum. This is more likely to be a factor for pitchers that are able to keep their trunk closed longer in their pitching motion. It can be a result of the pitcher starting their elbow extension early, or if they are unable to properly decelerate their torso as their elbow begins to extend.
It is especially important to maintain the kinematic sequence peak / deceleration order of pelvis, torso, and then elbow as maximizing the kinematic sequence increases efficiency and reduces the risk of injury. As such, the kinematic sequence order is more important than a large Separation Time and should be examined before looking at the Separation Time.
If the peak pelvis velocity is at or after foot plant, the peak torso velocity occurs after the peak pelvis velocity, and the peak torso velocity occurs before peak elbow extension velocity then the next step is to determine the Separation Time.
Amount of Separation Time
The amount of Separation Time (as defined above) is the time difference between the peak torso angular velocity and peak pelvis angular velocity. The larger the Separation Time the more efficiently the hip-shoulder separation is converted into energy through the angular velocity of the trunk, and the more time the arm has to get into the proper position for layback.
The larger the Separation Time the more potential for energy transfer up the kinetic chain. A study into Separation Time found that a pitcher increasing their Separation Time by 9.5ms would result in a 1 mph increase in their fingertip velocity (Van der Graaf et al, 2018). This means that if a pitcher is able to delay trunk rotation for a longer period of time, they can increase their Separation Time, and as a result their velocity. Generally, we look for 30-70ms of Separation Time when assessing a kinematic sequence, and on average we see a Separation Time of 30ms across our pitchers.
Gaining Separation Time should be done gradually as trying to make a large leap can result in timing problems between the torso and the shoulder, and result in additional stress on the body. It should also be noted that increasing Separation Time generally increases the time between foot plant and release, and as such the optimal Separation Time is one that fits with the pitcher’s natural sequence, timing, and anthropometric measurements.
Hip-shoulder separation is an important concept in pitching mechanics with its sequencing being of the highest importance. The sequence sets up the foundation for the amount of separation to build upon. A correct sequence allows for an efficient distribution of energy throughout the body, which can lead to an increase in velocity and reduction in the risk of injury.
The amount of hip-shoulder separation provides potential energy to be converted into the angular velocity of the torso with an increase in separation leading to an increase in stretch and potential energy. Increasing Separation Time provides the body more time to get into the proper position for both max external rotation and release. It also provides the torso more time to gather speed around a stable base, and thus more potential for energy to be transferred to the arm. Increasing these metrics has the ability to improve performance but should only be examined once the pitcher has a solid foundation in the form of a proper kinematic sequence.
- Source of energy
- Best to occur after foot plant for optimal transfer of energy
- Amount of hip-shoulder separation is dependent on an individual’s anthropometrics
- Proper kinematic sequencing is key
- Important component of efficient energy transfer
- Amount of timing is dependent on sequencing and anthropometric measurements
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