How We Interpret Biomechanics Reports


In our previous biomechanics post, we looked at the data we had collected over the past six months. We looked at the averages of key metrics and what metrics correlate to velocity and torques on the arm. But we didn’t really look at how we interpret a biomechanics report for our athletes.

When designing our report, we realized that there can be an overwhelming amount of information. It’s a lot to go through, even for someone with a trained eye. But an athlete understanding his biomechanics report is a vital piece in understanding what he needs to do to get better.

This means we spent a lot of time refining our report to be as readable and comprehensible as possible. To this day, we continue to improve our report as we learn more and more about the biomechanics of pitching.

The end result is a six-page report that looks at positional metrics, velocity metrics, sequencing, and joint kinetics—or forces and torques on the arm. From our internal database of motion-capture data, we have set normative ranges that we then use to assess and interpret what changes we need to make to an athlete’s mechanics.

In this post we examine an athlete’s biomechanics report and address what sort of changes we would like to make. We won’t look at every single metric; we will just run through the red flags and explain how our trainers would use that information to make specific training recommendations. 

Page 1: Title Page

This page is straightforward. Topical data, like the date of the capture and the athlete’s height and weight, are listed, and the three pictures shown are taken of the athlete at MER in Visual3D.

Definitions of the key events the report looks at are listed as well.

Page 2: Arm Action

The second page of the report looks at joint angles at key positions in the throwing arm, what we define as the arm action.

We’ve already examined one element of this page in a previous article, namely looking at shoulder abduction: the angle between the line of the shoulders and the upper arm. Ideally, we like to see the abduction path stay relatively close to 90 degrees, or neutral with the shoulders. Too high or too low could indicate arm drag or elbow climb.

Looking at the report here, we see that this athlete has a shoulder abduction at foot plant of 103 degrees and a maximum of 135 degrees, which is more than 40 degrees higher than our in-gym average! That means our guy has a drastic amount of elbow climb—not the most efficient arm path.

The next thing we look at is shoulder horizontal abduction, which is how we quantify scap retraction or scap load. A positive number indicates that the elbow is behind the line of his shoulders; a negative number indicates the elbow is in front. A good scap load looks something like this:

Looking back at our athlete’s report, we see he has 3.76 degrees of scap retraction at foot plant (average is roughly 40 degrees), and only 4.83 degrees maximum (average is roughly 57 degrees), which are both far below average.

So we’re seeing excessive elbow climb and a lack of scap retraction. To put it another way, instead of the elbow travel backwards into scap retraction, it travels upwards as the elbow hikes up. This gives us a more precise measurement of how a pitcher is moving, which provides our pitching trainers with more specific information to use to help make a change.

So how do we fix this?

We go to our bread and butter arm action drill: the Pivot Pickoff. More specifically, we have the athlete focus on driving the elbow backwards. With this particular athlete, we also added in Scap-Retraction Throws to really focus on cuing the elbow backwards to improve that scap retraction.

Page 3: Midsection/ Lower Body Positions

The third page of the report focuses on midsection and lower body positions.

Here we note things like if the athlete stays closed and stacked, if the athlete generates good hip/shoulder separation, or if the athlete has a good lead-leg block.

For our athlete, everything looks pretty good. A negative trunk angle and negative forward trunk tilt at foot plant indicates that he stays stacked and closed, and he generates nearly 48 degrees of hip/shoulder separation, which some might call elite.

We do see 48 degrees of lateral trunk tilt at ball release, which in our last biomechanics article we found could be linked with higher torques on the arm. In this case, we can explore what the root cause of the excessive trunk tilt is.

When lateral trunk tilt occurs early in the delivery, there are several factors that may contribute. It’s worth further assessing hip mobility and strength, as a lack of either of these can result in postural shifts that affect trunk positioning. From our mobility screening, we can see if hip mobility or hip strength could be a limiting factor. Or maybe the athlete just isn’t strong enough to hold himself in position. All these factors need to be considered when making a mechanical change—it isn’t just as simple as forcing an athlete to move differently.

Lastly, looking at front knee flexion, we see that the athlete doesn’t collapse on the lead leg and is very slightly extending it by about three degrees. But looking at the graph, we see that after foot plant, the athlete sinks into the lead leg a bit before starting to push back, indicating a slight inefficiency in the lead leg. Ideally, we would like to see the knee continually extend after foot plant, indicating a clear sign for improvement.

Page 4: Kinematic Velocities

The fourth page of the report looks at the kinematic velocities of the thrower.

We note that the athlete has above average pelvis and torso rotation speeds as well as average arm speeds (elbow-extension angular velocity and shoulder internal rotation velocity).

We also see that his maximum lead-knee extension velocity (how fast the angle of the knee changes) is 242 deg/s which is below our target of ~350 deg/s. This fact, coupled with the notes above, indicate that our athlete has a subpar lead-leg block—which we noted in our previous biomechanics article has a significant correlation to throwing velocity.

Page 5: Kinematic Sequencing

The fifth page of the report looks at the kinematic sequencing of the athlete.

Although there is still very little we know about sequencing and timing, we do have a couple of things we can glean from this page.

The timing of peak-pelvis angular velocity to peak-torso angular velocity has been shown to be significantly correlated to pitching velocity. Our athlete has an average of 0.0389 seconds, which is pretty good.

Other studies have examined the time from foot plant to max external rotation and into ball release, but we have yet to see any significant correlations to velocity with those timings.

This carries into the second thing we can examine: the graph. The colored arrows correspond to when the maximum values of each metric occur. As has been talked about before, we like to see those peaks happen from the ground up: Pelvis → Torso → Elbow → Shoulder.

Speeds of each segment: red is minimum speed, green is maximum speed

Page 6: Elbow and Shoulder Kinetics

The last page of the report looks at elbow shoulder kinetics—namely elbow varus torque and shoulder internal rotation torque.

We look at three sets of torques: total torque (Nm), torque normalized for height and weight (%), and miles per hour per normalized torque (mph/ %).

We note that across the board our thrower is worse than our average thrower. He has more torque on his shoulder and elbow, even when normalizing for height and weight, and gets fewer miles per hour per unit of stress than our average thrower. Not ideal.

Now does that mean that we need to shut our thrower down because he could be more prone to injury? Absolutely not.

The calculated loads on the elbow and shoulder are the total loads on those joints, but there is no insight into the underlying muscles and how they work to protect a pitcher. Simply put, there is no insight into the actual torque on the UCL or any other ligament. Just because somebody has higher torques doesn’t necessarily mean he is more likely to get injured. There are a lot of factors at play.

Perhaps the most useful thing we can take from this page is the ability to track torques and velocity-torque efficiency over time. We can retest an athlete after he has made mechanical improvements and track any changes to the resultant forces on the elbow and shoulder. We can test an athlete before and after a season to see what kinematic and kinetic changes have occurred over the course of the year.

These changes are largely unexplored for us, and as we collect more longitudinal data, we can hopefully start to answer the questions surrounding what mechanical changes actually result in less stress and more efficient mechanics.

Putting the Report into Action

With notes in hand, we sent our athlete away to train. Our trainer will be able to look at this information, along with an athlete’s strength and mobility assessment, to put together a plan.

After a couple of months, we were able to do a retest in the biomechanics lab, where we saw significant changes to his arm action—all for the good. There was less elbow climb and more scap retraction, and we even saw slight reductions in the joint forces on his elbow and shoulder, despite throwing the same ball velocity.

This is where the real value of motion capture lies, especially for elite throwers where the mechanical discrepancies can be tiny. By routinely taking thumbprints of our athletes, we can test and retest to see what training improvements contribute to performance gains and what stimuli can hurt them, like the accumulated fatigue over the course of a 162-game season.

Going forward, as we collect more and more biomechanics captures and are able to cross-reference data with training info, we can continue to improve our understanding of what markers contribute to success, and which are associated with backslides in performance.

We are only at the beginning of tapping into the ability of the biomechanics lab.

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