Plyo Velocity, Weighted Balls, and Replication

The benefit of science is that it continues to grow and develop. More weighted-ball studies have been published, and more will come out. So what we know about baseball training (weighted ball or not) in three, five, and ten years from now will be far more than what we know now.

One thing that we don’t have a great current understanding of is how heavy-overload balls affect athletes, both in regards to biomechanics and how they may or may not change mechanics over time.

We wanted to start off our research looking into heavy-overload balls by trying to answer two questions:

  1. How stressful are plyo velo’s in relation to pitching off a mound?
  2. What is the difference between throwing heavy plyo balls at high intent versus low intent?

We decided to look at this with the motus sleeve by comparing our plyo velo to hybrid B and pitching off a mound.

This means we compared throwing our plyoballs from high intent to medium intent to pitching stress.

In case you don’t know, plyo velocities are thrown once a week in the off-season at high intent. These are the same drills that athletes use nearly every day at low intent.

The plyo velocities are meant to replicate in-game stress. Similar to pulldowns, these two high-intent days are intended to push athletes to meet or exceed what they would do in a game.

ASMI had previously compared crow hop throws to pitching with weighted balls. They found that crow hop throws produced slightly higher stresses on the arm, just like we hypothesized.

We wrote an in-depth post on the study itself and then replicated the study comparing pitching 5- oz balls to 5- oz pulldowns. We ended up finding similar results as ASMI.

Before we get to the numbers from our own study, let’s recap a quick history of weighted balls and heavy-overload training.

A Brief History of Weighted Balls

Weighted balls have been around for a long time, with most of the baseball studies looking at over- and underload baseballs near the weight of a baseball.

Below are 11 studies that have looked at performance effects of weighted balls, and they found that weighted balls can have a positive effect on throwing velocity.

Screenshot (137)

As we mentioned previously, there has also been an ASMI study looking at the biomechanical comparison of 4–7-oz balls that found similar stresses between the 4- and 5-oz balls, whereas they found that the 6- and 7-oz balls result in less torque.

We see that we can use 3–7-oz weighted balls for velocity development and the 9- and 11-oz balls to aid in warming-up.

We use our PlyoCare balls (measured in grams) to work on mechanics and use them nearly every day throwing at sub-max intent.

The heavy-ball training originated with Dr. Mike Marshall. Dr. Marshall won the Cy Young as a reliever in 1974 for the LA Dodgers, and he also holds a PhD in exercise physiology. After he retired from baseball, he was motivated to try and figure out how to keep pitchers healthy, and he later moved on to training pitchers.

Now Dr. Marshall uses very heavy balls (6+ pounds) in his training for a few reasons. Some of those reasons are mechanical while others have not fully been studied, such as using overload balls to aid in bone density, ligament, and tendon strength.

We do believe that there are good mechanical reasons for using heavy-weighted balls, but we use less-weighted balls and different exercises than Dr. Marshall did in his training.

We also throw heavy-weighted balls with the expectation that they should come close to replicating the kinetics of throwing a baseball while showing decreased kinematics.

This means that elbow stress should be similar throughout the weighted balls, but the arm speed should be different. So, the overload balls cause slower arm speed, which should be obvious by the lower velocities that occur when throwing overload balls.

Currently we know that over- and underload balls similar in weight to a baseball can be effective at increasing velocity. We’ve also seen evidence (from the ASMI) study that overload balls can result in less torque than 5-oz balls. What isn’t understood is the biomechanical data on heavier overload implements.

Our hypothesis is that the heavy-overload balls result is less than or similar stresses compared to a baseball.

The Drills

We have five constraint drills that we use heavy-overload and underload balls: Reverse Throws, Pivot Pickoffs, Roll-Ins, Rockers, and Walking Windups. Three of these drills are used for velocity training: Roll-Ins, Rockers, and Walking Windups.

For the Roll-In Throws, athletes throw the green and blue plyo ball. For the Rockers and Walking Windups, the athletes throw the blue, red, yellow, and gray plyo balls. Athletes only velo the Rocker and Walking Windup for two throws with each ball.

So this small data set is comparing the two highest velocity throws from a velocity day to the two highest velocity throws from a hybrid B day, or two fastest pitches off a mound.

The First Study: Plyo Velo to Mound Comparison

We were able to get 18 athletes that threw a plyo velo and off the mound. We’re comparing the hardest two throws at each ball weight.

The data for these 18 athletes can be found here. This study was started before tagging was available in grams, so these ball weights were tagged in the closest ounce weight. We don’t believe that this significantly changes the results.

It looks like pitching is most similar to throwing the blue plyo ball. The green ball is more stressful, while the red, yellow, and gray balls are all less stressful than pitching.

These are certainly interesting findings considering a baseball is most similar in weight to a yellow plyo, but both the velocities and stress were much less.

There are two main ways to look at the stress numbers at high intent. One is thinking that everything more stressful than a baseball is bad, so we should stay away from heavier weighted balls; two, we want to train at or similar to game stress.

The second is what we mentioned above. Athletes should have scheduled time in the offseason to match or push past the stress that they would see in games.

We are looking to replicate or go above game stress, and it looks like plyo velocities currently do that.

The Second Study: High-Intent versus Low-Intent Plyo Throws

This second study included the 18 athletes that threw above, plus an additional 7 for a total of 25 athletes. We collected data from one of their plyo-velo days and from one of their hybrid B days (a low-intensity day).

The data can be found here. This study was started before tagging was available in grams, so these ball weights were tagged in the closest ounce weight. We don’t believe that this significantly changes the results.

You can pretty clearly see some big differences between the high- and low-intent throws, which would not support the idea that throwing heavier balls regardless of intensity would result in similar biomechanical loads.

The idea that they would be the same is somewhat confusing considering it’s largely accepted that throwing a baseball at a lower velocity and intent level is “safer” than throwing at max intent. This is easily realized when you listen to arguments about pitching and injuries that ultimately mention that pitchers throw too hard.

The big takeaway from this for athletes: when you aren’t scheduled to throw high intent, don’t throw high intent.

It’s not uncommon for athletes to want to “let a few loose” when they feel good, regardless of what intensity level they are suppose to be throwing at. This data supports the idea that you shouldn’t do that. Low-intent days need to be taken seriously by sticking to throwing at lower intent.

Notes on External Rotation

There are a few things that we have seen in research about external rotation and pitching:

  1. More external rotation when throwing is linked to higher velocities (when comparing low- to high-velocity groups)
  2. More external rotation when throwing is linked to higher elbow stress 

It is currently generally assumed that the heavier weighted balls increase range of motion in a negative way. But that claim seems more confusing when looking closer as the above numbers.

The motus data above shows that shoulder rotation (motus’s measurement of external rotation) changes, but not in the the straightforward way we are often told.

First, you can see that shoulder rotation changes per throwing drill. There has been research suggesting that different trunk movements and timings can affect arm stress and external rotation. This would align with the idea that constraint drills result in slightly different stresses and forces depending on each drills setup and execution.

Second, you can see that for the high-intent throws, the ball with the most shoulder rotation was the lightest ball in the walking windup drill. Also, shoulder rotation did not seem to move linearly one way or another and varied depending on drill, ball weight, and intent level.

There are also differences between pitching and plyo throws but the motus sleeve measures shoulder rotation from the ground. So comparing throwing off a slope to a flat surface isn’t a great comparison.

The only other study we are aware of that has looked at external rotation with weighted balls was a thesis titled The Effect of Throwing Under- and Over-weight baseballs on the pitching motion.

Now, this paper did not use a standard marker-based system but instead used a velcro strap for markers on the arm, which tends to have a higher margin of error because it can move. It also used a tiny sample of 6 college pitches.

However, that study also saw a variety of changes in external rotation between different ball weights, but the 3 oz underload ball was seen to have the highest ER.

It’s clear that there should be more detailed looks into the biomechanical effects of throwing weighted balls and external rotation.

Underloads Are Less Stress?

Both the high-intent and low-intent throws saw the stress for the underload plyoball to be lower than the stress of the 5-oz ball. ASMI had seen similar stress levels between baseballs and 4-oz balls, and, generally speaking, knowledge of 4- and even 3-oz balls still isn’t very well understood.

ASMI has also looked into throwing underload balls in youth athletes and found that the 4-oz balls resulted in less stress than the 5-oz. This finding was not replicated in the more recent biomechanical comparison of weighted balls that we mentioned above.

This should be a good reminder that research changes as it grows over time and that age differences can be a possible reason for differences in the results of replications.


Research and science are messy. This is also why we avoid saying that one research paper “proves” anything. One paper, or blog post, can’t prove anything. At best, they simply provide evidence for or against different theories.

The data in this article suggests that the blue plyo may be the best ball that replicates pitching stress when thrown at high intent. But the exacts reasons for why that is are unknown.

This data also suggests that throwing plyo balls at low intent is less stressful than throwing plyo balls at high intent.

But the biggest takeaway is that this data should be replicated to see if we find the same results of elbow stress and external rotation.

As we hinted at earlier, reading the methods section of a study is also vitally important. Some differences in measurements can simply be the result in different technology being used. Different kinds or number of biomechanics cameras, different amounts of markers used, or whether wearable tech was used should all be factors in why findings may vary.

This is also why we insist on replication as such a vital (but often missing) piece of sports science. Theories get stronger based on additional evidence provided, and if we don’t replicate certain findings under different technologies (such as throwing plyo balls in a marker-based lab vs. wearing a motus sleeve) or under different participants (high school vs. college vs. pros), we are leaving gaps.

We continue to move forward with research on all weighted balls in our lab, and we look forward to releasing what we find.

This article was written by Research Associate Michael O’Connell. Kyle Lindley and Anthony Brady assisted in the data collection.

Ricky Norton

I asked motus to add the tag so we could look at the towel drill.(which some organizations still swear by, still thought to be arm saving. We did our case study with 20 high school guys and got mixed results. Trunk angle and other biomechanics lead to different results. Was less stress for some guys and very high stress for others. Goes to show you cant assume things are ‘good’ for every guy. Would love to see you guys repeat that stidy with your pro population.

Comparison of Elbow Torques Between Pulldowns and Pitching - Driveline Baseball

[…] If you’re curious how plyo velocity stress relates to pitching, you can find that study here. […]

Comparing Overload vs. Underload Weighted Ball Stress on the Arm - Driveline Baseball

[…] We looked into the stress of Plyo Care velocities, hybrid B days, and how they compare to pitching stress here. […]

Leave a reply