Previous literature focusing on the relationship between throwing kinematics and kinetics are limited. Fleisig244 examined the relationship between humeral, torso, and lower body kinematic characteristics and shoulder kinetics during baseball pitching. Fortenbaugh and Fleisig105 attempted to determine the kinematic characteristics in pitchers with higher ball velocity and lower shoulder forces. No study has investigated the relationship between scapular kinematics and shoulder kinetics. As detailed in Specific Aim 7, a purpose of the current study was to identify the potential association between scapular kinematics and shoulder kinetics during maximum effort baseball throwing. In the current study, it was hypothesized that scapular
kinematics at SFC would be correlated to maximum shoulder anterior, posterior, superior, inferior, and compression forces. It was also hypothesized that scapular kinematics at the occurrence of maximum shoulder compression force would be correlated to maximum shoulder compression force.
No significant correlation was detected between scapular kinematics at SFC and maximum shoulder anterior, posterior, superior, and inferior forces. It should be noted that the correlation between scapular posterior tilt at SFC and maximum shoulder superior force only marginally failed to reach statistical significance (p=0.059). If this correlation had been significant, it would indicate that baseball players with more posteriorly tilted scapulae generated greater shoulder superior force. It is interesting to note that increased humeral external rotation at SFC has been found to be correlated to increased shoulder anterior force,244 considering the fact that scapular posterior tilted in coordination with humeral external rotation (Figure 11). Maximum shoulder superior and anterior forces occurred at approximately the same instance, right before MER.6 Maximum shoulder superior force calculated in the current study was 106.4±61.9N, lower than the data based on adult pitchers (250±80N).6 Shoulder superior force tend to move the humeral head upward, potentially placing stress on glenoid labrum structures superior to the humeral head reducing the subacromial space. Baseball pitchers with bicep tendonitis and rotator cuff bursitis typically experienced pain when approaching MER,2,21 where shoulder superior force reached the peak value.10 Further research is needed to investigate the existence of the correlation between scapular anterior/posterior tilt at SFC and maximum shoulder superior force.
At the occurrence of SFC there was a moderate negative correlation between scapular protraction and maximum shoulder compression force. At the occurrence of maximum shoulder
compression force, there was a strong negative correlation with scapular protraction. This was the strongest correlation identified in this study. At the same instance, maximum shoulder compression force was also moderately positively correlated to scapular posterior tilt. Among all the shoulder force components, shoulder compression force is of the greatest magnitude. Throwing motion generates a strong shoulder distraction force, which acts along the longitudinal axis of the humerus, tending to pull the humerus away from the glenoid fossa.33 Muscles surrounding the glenohumeral joint must be highly activated to generate a compression force to resist the shoulder distraction force, holding the humeral head within the glenoid fossa. Among the activated muscles, the rotator cuff muscles, triceps, biceps, pectoralis major, latissimus dorsi, and deltoid, the posterior shoulder muscles assume the major role.241,242 In the current study, maximum shoulder compression force calculated was 639.3±162.4N, lower than data based on adult baseball pitchers but comparable to the data based on high school pitchers (Appendix A.1).
As shoulder compression force increases the humeral head is pulled more forcefully into the glenoid fossa, potentially increasing the stability of the glenohumeral joint and preventing humeral head translation. However, the glenoid fossa and labrum also endure greater stress as the humeral head applies greater pressure to the glenoid. Compression and shear forces also create a resultant force pressing the glenoid rim. Moreover, shoulder compression force reached a peak value right after REL, about the same instance when shoulder internal rotation velocity reaches its maximum.6,10 This velocity typically can exceed 7,000°/s in baseball pitchers (Appendix A.1). The strong compression force and rapid humeral head rotation combined can create a grinding effect, tearing the glenoid labrum.10,245 Baseball players with greater shoulder compression force also place greater stress on the muscles listed above, resulting in higher chance of tissue damage and injuries. Accumulated tissue damage can further result in structure tensile failure, leading to
common shoulder injuries among baseball players such as rotator cuff tear.2,10,245 In addition, the greater demand to generating higher shoulder compression force potentially can also result in earlier fatigue during competition. When the fatigued muscles fail to generate a sufficient amount of compression force, the stability of the glenohumeral joint may decrease and injuries can occur. In an attempt to reduce maximum shoulder compression force, researchers have investigated various kinematic variables associated with the force during baseball pitching.9,33,244,246 However, none of the variables investigated was of scapula.
The biomechanical rationale behind the identified correlation remains unclear. Stride foot contact can be viewed as the “ready position” of a baseball thrower. It is the end of throwing preparation and the moment to initiate the most explosive part of throwing. From this moment, energy is transferred from the lower body to the upper body. Stride foot contact also serves as a “checkpoint” used by baseball players and coaches, as it is easier to evaluate and change throwing mechanics at this time as the movement is relatively slower.8 It has been proposed that good kinematics at SFC can lead to good kinematics throughout a throw.7,244 It is likely that a more protracted scapular position at SFC improve a thrower’s readiness, or is a sign of improved readiness, for the following explosive phases. The term “readiness” here refers to a state that a thrower’s joints and body segments are in appropriate positions to efficiently and effectively initiate the kinetic chain from bottom up. Throwers with better readiness may be able to generate the high ball velocity with decreased joint forces.105
The current results also suggested that baseball players with more protracted and more anteriorly tilted scapulae at the occurrence of maximum shoulder compression force generated a decreased maximum shoulder compression force. Unlike SFC, the occurrence of maximum shoulder compression force is a kinetic event instead of kinematic event. A kinetic event is not
intuitive for a kinematic “checkpoint”. Since the peak value of shoulder compression force occurs right after REL,6 the current results may be loosely interpreted that baseball players with more protracted and more anteriorly tilted scapulae when releasing the ball produced less shoulder compression force. Increased scapular protraction and anterior tilt at resting position has been identified in healthy overhead athletes in the dominate shoulder compared to the non- dominate side.53 At 90° arm elevation and above, asymptomatic baseball players demonstrated increased scapular protraction compared to healthy non-throwers.54 It is likely that increased scapular protraction and anterior tilt are normal adaptation occurred due to repetitive throwing. Further research involving injured shoulders should be conducted to assess if such adaptation protective to baseball players.
Since correlations do not necessarily indicate causal relationships, it is not appropriate to conclude that baseball players should have their scapula more protracted and anteriorly tilted to reduce shoulder forces. Identifying scapular kinematics that can reduce shoulder loads should be a topic of future studies. Interestingly, the current findings are, to some degree, relevant to baseball coaching. Coaches may encourage players to release the ball in front of the body, which naturally leads to a more internally rotated humerus at REL and therefore a more anteriorly tilted scapula, as well as increased scapular protraction. Failure to do so results in early ball release, and such delivery is often described as “jerky” and considered harmful to players’ throwing shoulder. Early release indicates shorter path and time of both arm acceleration and deceleration. It is plausible that shoulder muscles must work harder to reach the same ball velocity and then decelerate the throwing arm, and therefore creating higher shoulder forces.
On the other hand, one may question if a more protracted and anteriorly tilted scapula can increase the risk of shoulder injury through other mechanisms. For example, more anteriorly
tilted scapula has been identified in subacromial impingement patients at 90° arm elevation or above.42,43 Current evidence, however, is not sufficient to support that the suggested changes poses greater risk of subacromial impingement in baseball players. Increased scapular posterior tilt was also found in subacromial impingement patients.41 With the mechanisms causing subacromial impingement remaining debatable, the identified characteristics of patients cannot be concluded to be the result of injury or the cause of injury.39 The mechanisms of shoulder injury can be complex, with multiple biomechanical factors involved in. For example, without increased humeral horizontal adduction, a more protracted and anteriorly tilted scapula may result in anterior shift of the humeral head and stretched anterior glenohumeral capsule.247 Increased contact between the humerus and the posterior rim of the glenoid as well as entrapment of the rotator cuff muscles can also occur, resulting in internal impingement.47 Such risk cannot be assessed by solely reviewing the scapular kinematics without looking at the humerus simultaneously. The risk of shoulder injury of a baseball player should be therefore evaluated on a case-by-case basis, with multiple biomechanical variables of the individual taken into consideration.