Shoulder Stress

Written By Sherry L. Werner

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Shoulder Stress

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In 1996, I was lucky enough to lead a research team in collecting high-speed video of the pitchers participating at the first ever Olympic softball competition in Columbus, GA. From the high-speed (120 frames per second) video we were able to calculate kinematic (i.e. location, speed, acceleration) and kinetic (i.e. joint forces and torques) parameters of the pitches. In Biomechanics we use these parameters to better understand human movement. Sport Biomechanics strives to improve performance and reduce the risk of injury through such analyses. Not much quantitative information is available in softball, especially on highly-skilled pitchers, so this was an important study. This month I will share with you some of the relevant findings of our study.

From the video data we sought to better understand the mechanics and joint stresses in windmill pitching. Average ball speed for the riseballs thrown by the 24 pitchers we studied was 60 mph. We chose to study riseballs because all of the pitchers threw that particular pitch. If the ball was released 38 feet (it's probably more like 35 feet!) from home plate, the batter would have approximately 0.40 second (just under one half of a second) to react to the ball once it was released. The shortest time in which a human can react to a stimulus is 0.12 second!

The main emphasis of the Olympic study was to quantify joint stress. In particular, we were interested in elbow and shoulder loads in windmill pitching. A joint force is a representation of a “push” or “pull” on the joint. In Biomechanics we line these forces up with axes of the body to provide a more meaningful interpretation. For example, a force oriented along the upper arm tends to either push the upper arm into the shoulder joint (compression) or pull it away from the joint (distraction).

Forces are calculated as a percentage of body weight so that all pitchers, regardless of size, can be compared to one another. Maximum shoulder distraction force for the Olympic pitchers was 80% body weight. This corresponded to approximately 150 pounds of force acting to pull the upper arm away from the shoulder joint at ball release.

A distraction force was also found at the elbow near ball release. This force was directed along the forearm, and therefore acted to “pull” the forearm away from the elbow. Average elbow distraction force was 61% body weight. The average elbow angle at release was 30 degrees short of full extension.

Based on this data it seems that elbow and shoulder stresses are high during the windmill pitch. Over time, the loads that these pitchers are taking on their arms will certainly affect the muscles, tendons and ligaments of these joints. Although we need to carry out more research to be able to make more generalizable conclusions, it seems that pitching mechanics can affect these joint stresses. In particular, using the trunk and lower body to generate and transfer energy during the pitch to assist in propelling the ball can reduce the load on the arm. Proper follow through also aids in dissipating these loads after ball release.

For a long time it has been said that softball pitching is a “natural” motion and that it was much easier on the arm than overhand throwing. The joint stresses found for the Olympic pitchers do not support this contention. At this point we are just beginning to understand what goes on in pitching. Until we get more concrete conclusions it is important for young pitchers to learn pitching styles based on sound mechanical principles and to understand the importance of strengthening the arms, legs and trunk.

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Sherry Werner: PhD is currently a biomechanics consultant with TMI Sports Medicine, and Tulane Institute of Sports Medicine, and a pitching instructor at the Sherry Werner Fastpitch Academy. She has held research positions at the United States Olympic Training Center. She received a MS degree in Biomechanics from Indiana University, and a PhD in Biomechanics from The Pennsylvania State University.
Dr. Werner's research has focused on the effects of throwing motions at the shoulder, elbow and wrist. Past projects include data collection and analysis of elite softball pitchers during the 1996 Olympic Games. Sherry also released an instructional pitching DVD with Jennie Finch in 2011.

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