Normalized stride length and ball velocity or arm joint kinetics

Caught Looking: Articles of the Week — September 24


Here are three studies the R&D department at Driveline read this week:

Influence of stride length on upper extremity joint moments and ball velocity in collegiate baseball pitchers

Solomito et al. recently published this study investigating the relationships between stride length and a few different biomechanical parameters from an impressive dataset (99 collegiate level pitchers). Stride length is an aspect of pitching mechanics that is often manipulated by players and coaches to achieve desired mechanical and performance outcomes, and many attempts at quantitatively determining its effects have resulted in mixed conclusions. 

This experiment tested the association between two different measurements of normalized stride length (stride length divided by body height, stride length divided by lower limb length) to mid-section mechanics, throwing arm joint kinetics (torques and forces on the shoulder and elbow), and ball velocity.

Stride length should be normalized to account for differences in height between pitchers. A taller pitcher will be able to stride farther due to having longer limbs than a shorter pitcher. If we compared the two pitchers and their stride length, we wouldn’t be sure any differences in biomechanics were from the difference in stride length or their difference in height. 

The results showed that normalized stride length was associated with a more open mid-section. This means that for throwers who strode farther down the mound relative to their height, they also were more rotated open toward the plate (body more square with home plate and the back stop), and the torso was more tilted forward (chest closer to home plate) at foot plant (when the front foot plants into the ground.

However, a longer normalized stride length was not associated with a change in ball velocity or a change in throwing arm joint kinetics. These are very interesting results that add to a growing number of analyses contributing to our understanding of the stride in the pitching delivery. Samples this large are not very common, which is a significant strength to this experiment. 

Normalized stride length and ball velocity or arm joint kinetics

The study found no significant relationship between normalized stride length and ball velocity or arm joint kinetics

The authors hypothesize that the increases in mid-section rotation and forward tilt associated with increased stride length may contribute to more joint load based on relationships in previously published research. I am not very confident in this association, however, because there was no direct statistical relationship in the present study with an extremely large sample. Nonetheless, the relationship between stride length and mid-section mechanics is still noteworthy. Coaches and players can use it as a potential fix when pitchers are too open or too closed at foot plant.

Applying the brakes in tennis: How entry speed affects the movement and hitting kinematics of professional tennis players

In more outsider fashion, we reviewed a tennis biomechanics study.

Giles et al. at the University of Western Australia investigated the biomechanical differences in forehand tennis strokes with varying ‘entry speed’—defined as how fast the athlete approached the position where they struck the ball. The reason this was interesting to us was that this environment is similar to what a position player in baseball might be experiencing: having to make a lateral athletic move toward a batted ball, fielding it, and throwing the ball to a target.

This experiment resulted in some statistically significant relationships between entry speed and biomechanics. Those were:

  • Female drive leg “loading kinematics” were increased with higher entry speed (more hip flexion, knee flexion, etc.)
  • Female participants trunk rotation was decreased with higher entry speed
  • Participants of both sexes increased stride length and backswing with higher entry speed
  • Female racquet speed was decreased with higher entry speed; male entry speed was unchanged

Entry Speed tennis

In summary, it seems that participants executed a more significant load during the high entry speed trials. Without being able to apply these findings directly to baseball, my main takeaway is that tennis stroke mechanics (very similar to baseball throwing mechanics) are affected by the way an athlete approaches the throwing cycle.

In baseball, whether that is crow hopping from the outfield, fielding a back-hand ground ball and throwing a runner out at first, doing a run-and-gun workout, or many other potential constraints on the field, mechanics will change. Maybe normal, flat ground training won’t address the particular needs of these situations in the field. 

On another note, a constraints-led training approach uses an exploratory approach for the athlete to find movement solutions to a particular task by destabilizing their current learned pattern/solution.

If there are mechanical changes that we aim for in the pitch that also present themselves in a more athletic throw—like fielding a ground ball or throwing off-balance (such as more loaded lower half as seen in this study)—perhaps we can use more creative solutions like this one to teach desired mechanics. A pitcher fielding back-hand ground balls at high speeds for example—food for thought.

For a look into how we approach breaking down mechanics for athletes who train here in Kent, take a look at this blog from last spring.

Biceps Tendon Changes and Pitching Mechanics in Youth Softball Pitchers

The Auburn University biomechanics lab is back at it again with another interesting study on softball pitchers.

Dr. Gretchen Oliver et al. experimented with bicep tendon thickness and compared how it changes following a pitching appearance to how kinematics and kinetics change throughout that appearance.

The study basically tells us, “Does bicep tendon thickness change from before to after a pitching workout” and “if so, what mechanical changes are associated with this change in tendon properties.” These ideas could help us better understand potential injury risk, what training we can do to minimize these adverse mechanical changes, and what fatigue indicators we can use to determine when to take a pitcher out of the game.

From this experiment, they found that bicep tendon longitudinal thickness did change from pre- to post-simulated game. Additionally, after separating the participants into a group who experienced a change in tendon properties and a group of those who didn’t, it was found that trunk rotation and trunk flexion changed more from pre- to the post-simulated game in the group that experienced tendon changes. This suggests that certain trunk mechanics may contribute (or are at least associated with) physical properties of the bicep tendon. 

Longitudinal thickness is the thickness of the tendon in the direction along the length of the humerus (upper arm)

These are interesting results and ones that will hopefully be built upon in the coming years to better understand the material and functional effects of fatigue from throwing. Injuries are all too common in baseball and softball pitching.

An important note to make when interpreting these results is that the association between tendon property changes and injury rates is unknown. We cannot be sure that the bicep changes measured in this study indicate an increase in injury risk, so the results need to be taken with a grain of salt and cannot be generalized as a solution to the injury problem in baseball softball. However, this is exciting research and excellent work coming out of the Auburn University biomechanics lab, as usual. 

By Kyle Lindley (@kyle_lindley_


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