It's well known that pitchers get injured at a higher rate than position players, and it’s also known that they incur a tremendous amount of stress during the delivery of a single pitch, to say nothing of the course of a start or a season. Theories abound as to why pitchers are hurt more frequently, but their tendency to break down is generally assumed to be a combination of fatigue, altered biomechanics, improper strength and conditioning, and sheer repetition. With a slow few days behind us on the injury front, we're going to devote another special edition of Collateral Damage to two key points of the pitching motion where injuries are likely to result.

The act of pitching can be broken down into six major phases: windup, stride, arm cocking, arm acceleration, arm deceleration, and follow-through, which you can see in this video of Justin Verlander.

Each phase is unique and has a set starting and ending point:

  1. Windup starts the moment the back foot hits the rubber and ends when the hands first begin to separate from each other. This also tends to—or should—correspond to the moment when the front foot is at the maximal height off the ground.
  2. The stride phase is when the leg begins to move forward and strides (surprise, surprise) toward the plate as the arms swing apart from each other. As soon as the front foot makes contact with the mound, the stride phase ends.
  3. The armcocking phase is where the body gathers up all of the potential energy stored during the windup and stride phases and prepares to transform it into to kinetic energy. The hips and trunk twist to face the plate, and the arm cocks back to the point of maximal external rotation, at which point the pitcher transitions into the acceleration phase.
  4. The acceleration phase lasts from the moment the shoulder reaches maximal external rotation until the second the ball is released toward the plate.
  5. Ball release initiates the arm-deceleration phase, which continues until the shoulder is no longer able to internally rotate, at which time the follow-through phase begins.
  6. The follow-through phase continues until the pitcher regains his balance and moves into a fielding-ready position.

During the windup phase and stride phase, the arm is not significantly stressed, but that changes very quickly. During the stride phase, the arm is brought backward, and during the arm-cocking phase, the body stores potential energy in preparation for the acceleration phase. Once the foot hits the ground at the end of the stride phase, the arm-cocking phase begins. Both arms should be in line with the shoulders, but the dominant arm is positioned behind the body. This places stress on the front of the glenohumeral joint and approaches the position where the shoulder is inherently unstable.

Many different muscles and soft tissues act to protect the shoulder while still allowing it to be highly functional. At the end of the cocking phase, we reach the first critical instant where significant potential for injury occurs, when the shoulder is at its most vulnerable position of maximum external rotation, horizontal extension, and abduction.

Numerous studies going back many years have discussed the anatomy and injuries associated with pitching, but a terrific study by Fleisig, et al. really started to bring attention to the magnitude of the forces exerted at this critical moment in healthy, highly-skilled, adult pitchers, including 56 pounds of superior shear force, 70 pounds of anterior force, and 107 pounds of joint compressive force. These forces not only stress the muscles but also stress the ligaments, capsule, bones, and labrum.

Evidence is growing by the day that at this critical instant, the shoulder can suffer from internal impingement where the postero-superior labrum is pinched between the rotator cuff and the head of the humerus. Combine this with the normal forces mentioned above, and numerous injuries are possible, including but not limited to labral tears, slow-forming under-surface tears of the rotator cuff, or injuries to the head of the humerus. Rotator cuff weakness and tightness of the posterior joint capsule are often major contributing factors, especially in the setting of anterior shoulder instability.

At the same point in the delivery, the pectoralis major, subscapularis (the sole rotator cuff muscle that is an internal rotator), latissimus dorsi, and teres major are under the maximal eccentric stress. They are stretched to the maximum before forcefully contracting to accelerate the ball. We have seen an increase in diagnosed latissimus dorsi strains over the last several years, likely due to improved accuracy of diagnosis. The latissimus dorsi is at risk for strains beginning at this point.

The next critical instant occurs immediately after ball release in the arm-deceleration phase, when rotator cuff activity is extremely high, providing dynamic resistance to the distraction force on the arm. The muscles surrounding the shoulder must produce a force roughly equivalent to or greater than the pitcher’s body weight—245 pounds, according to Fleisig's study—to prevent his arm from essentially being ripped out of its socket. The posterior capsule and rotator cuff are stressed by this effort and can lose their flexibility as a protective mechanism from the repeated stress. This can lead to problems with internal impingement, as mentioned above, and could touch off a devastating cycle that is difficult to escape.

Injuries have been proven to occur at other points during the pitching motion, but currently the majority of injuries to pitchers involve shoulders and/or arms. This is certainly not an exhaustive description of the forces and muscle activity encountered during pitching, but it provides us with a little more insight into why pitchers become injured.

Flesh Wounds: Updates from Monday's Collateral DamageShin-Soo Choo did have surgery yesterday to stabilize his left thumb fracture. Fractures can take at least six weeks to heal, and displaced fractures can easily take two months or more… Ryan Madson's hand did not improve quickly enough to satisfy the Phillies, and he was placed on the disabled list retroactive to June 19th… Jose Tabata's MRI came back clean except for a strained left quad, and he was placed on the disabled list… Delmon Young reportedly does not have any ligament damage but does have bone bruising and soft tissue swelling. Did we just confuse Young with Ike Davis? Based on Young's mechanism of injury and his complete unwillingness to put any weight on the ankle, don't be surprised if there is something else going on that would cause him to miss more than the minimum… Grant Balfour’s right oblique strain landed himon the disabled list retroactive to June 22nd…

New items…The Red Sox activated Bobby Jenks and Franklin Morales and are slowly getting healthier, as are the Cardinals, who welcomed back David Freese and Nick PuntoTorii Hunter returned to action on Monday after missing several days with bruised ribs following his collision with the wall… Kevin Hart will have to shut down throwing for a month to focus on strengthening, per Dr. Andrews’ recommendations… Ryan Hanigan is day-to-day with a back strain… Darwin Barney is expected to be activated and placed in the lineup today… Clay Buchholz still feels soreness and tightness in his lower back and is hesitant to return before he’s 100 percent, since doing so altered his mechanics the last time he tried it… Things do not look good for Jonathan Broxton,who will be shut down for the next three weeks. There is a chance that he won't make it back this season, according to manager Don MattinglyJosh Johnson will have his shoulder examined today by Dr. Andrews, because it is not responding as well as Johnson and the Marlins had hoped. The inflammation he suffered at the end of last year is looking more and more likely to be related to his current problems.

Thank you for reading

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Guys, this is a great piece. Watching a professional pitch - especially one like Verlander, whose mechanics are sound - makes it look very easy and stress-free. Thank you very much for describing what is going on that we CAN'T see.