# Stride Length Impacts on Sagittal Knee Biomechanics in Flat Ground Baseball Pitching
## 1 Introduction  
Lower extremity biomechanics are known to influence the kinetic chain in baseball pitching [1; 2; 3; 4]. Coordinated kinetic chain movements progress up from the feet and lower extremities through the trunk and throwing arm and culminate in the ball release. Efficient distal (lower extremity)-proximal (trunk)-distal (throwing arm) energy transfer depends on how effective momentum is developed and transferred between the segments, beginning with the larger lower body distal segments that achieve peak linear and angular velocities first followed by the lighter distal throwing arm segments that reach peak linear and angular velocities in succession [5; 6; 7; 8]. Changes in the lower extremity joint power are known to affect the throwing arm kinematics and kinetics, which are reportedly associated with altered trunk-throwing arm momentum sequencing without affecting the ball velocity [9; 10; 11; 12; 13; 14]. An increased lead leg knee extension concomitant with a longer stride length have reportedly been associated with higher fastball velocities among less adeptyouth pitchers [15; 16] whereas proficient pitchers maintain the ball speed despite altered ground reaction force profiles resulting from stride length changes [17; 18]. The evidence suggests that longer strides have a detrimental physiological cost [19] yet may be beneficial mechanically in mitigating throwing arm stress [20; 21]. Conversely, shorter strides reduce timing in the generation phase and prolong the brace-transfer, which evoke inefficient pitching mechanics in preparing the throwing arm for a maximal external shoulder rotation [22]. Increased throwing arm lag prior to the acceleration phase with shortened strides, concomitant with a greater trunk transverse angular momentum, may increase the strain on the shoulder and elbow [21] and exacerbate dominant forearm fatigue [23], which has been associated with a 36 X increase risk for shoulder and elbow injuries [24].  
To explore further, this paper is a continuation of our comprehensive single cohort research study examining whether altering the stride length influences the overhand baseball throwing mechanics [19] with significant changes in temporal, physiologic, ground reaction, momentum, grip strength, and performance profiles having already been reported [17; 18; 19; 20; 21; 22; 23]. Whether the stride length adaptations affect the bilateral knee coordination and sagittal link-segment biomechanics without influencing the ball velocity in baseball pitching remains conjecture. Therefore, a secondary analysis of the sagittal plane stride (lead) and drive (trail) leg knee joint dynamics (kinematics and kinetics) in response to altered stride lengths [18] was used to determine whether the knee joint moments and powers are sensitive to the stride length changes and alter the pitching delivery that may predispose players to an increased risk of lower body and throwing arm injuries.  
In the context of this investigation, it was hypothesized that closed kinetic chain lower extremity biomechanics were altered in response to changes in the stride length. Greater drive knee extension dynamics (kinematics, kinetics, and joint powers) were expected with increased strides during the generation phase of pitching (peak knee height to stride foot contact). Likewise, a greater stride leg knee flexion was expected to occur earlier in the pitching cycle (closer in proximity to the stride foot contact) followed by greater stride knee extension dynamics during the acceleration phase (maximal external rotation to the ball release) for shorter strides.  
Our findings emphasize that stride length is an important biomechanical parameter that impacts on drive and stride knee dynamics during the pitching delivery, from which the power flow up the link-segment model regulates the angular joint mechanics through to the trunk, shoulder, elbow, wrist, and hand [2; 25; 26; 27; 28; 29].