Forcing Rotation: Exploring Lead-Leg Force Curves and Rotation in Hitters
“What do I need to do to become a better hitter?”
A common question asked by a lot of developing athletes. And the common answers are usually the same: fix your swing path and rotate faster.
Seems simple enough… but when you start to dive into the biomechanics of making these improvements, you see just how complex it can be. It’s like that Tiger Woods commercial “Golf’s Not Hard”, where he’s explaining how to swing the golf club. The deeper you go the more you will find can affect the end result. Things like:
- How much the rear shoulder raises when coiling
- The length of the stride
- How much flexion the lead knee has at foot plant and how quickly it extends
- The timing of lead elbow extension during the swing
Throw in the fact that they all interplay off of one another and have a time component and we come to a 100% agreed upon finding:
Hitting is Hard!
It’s like trying to solve an 18,000-piece puzzle (yes, those do exist); however, while it can be difficult and grueling at times, it’s still solvable. One piece of the puzzle that doesn’t get enough attention is the force applied during the swing, specifically in the lead leg.
Background
What is force exactly? Let’s start with a core physics lesson: Newton’s laws of motion.
1st Law: an object at rest remains at rest, and an object in motion continues to move at a constant velocity unless acted upon by an external force.
2nd Law: the net force acting on an object equals the object’s mass multiplied by its acceleration.
Force = mass * acceleration
3rd Law: for every action there is an equal and opposite reaction force exerted on the object
Basically, force is what drives and halts motion. You pushing into the ground to sprint or jump causes the ground to “push” back into you and allows your body to move through space. In the swing, every ounce of force a hitter generates starts at one place: the ground.
So far, the literature tells us that both legs are doing real work: rear-leg vertical force and medial-lateral impulse have been linked to bat speed, and lead-leg peak forces tell a similar story, correlating with bat speed, enhanced bat energy, and how much mechanical energy actually makes it into the torso during the swing.
As we continue to delve into the world of ground force production for hitting, we need to begin to ask ourselves, “what more can force tell us?”
One of my favorite shows growing up was Avatar: the Last Airbender. In season 2, Uncle Iroh provides some wisdom as it relates to the four elements, saying:
“It is important to draw wisdom from many different places. If we take it from only one place, it becomes rigid and stale.”
A powerful statement that can be applied to the sports realm. What sport could we possibly draw from that could match the complexities of hitting?
A highly rotational event, discus throwing is reliant on similar biomechanical features as hitting, such as hip-shoulder separation and the sequencing of pelvis and torso peak angular velocities. Discus throwers also require not only a significantly high amount of force to be produced during their event but also require that force to be produced in a rapid manner, aka rate of force development (RFD).
RFD has to deal with the slope of the force-time curve. The steeper the slope, the faster we are producing force. Think of it as “explosive” force. While peak forces tell us *how much*, RFDs can give us a better idea of how that force is being produced throughout a specific duration of the swing.
When looking at discus throwers, higher RFD in their plant leg seems to provide a brake to the angular momentum they’ve built during the wind-up and movement phases. Think back to Newton’s laws… sound familiar?
Lars Reidel
Here we can see Olympic champion and former world record holder, Lars Reidel, performing the discus throw frame-by-frame. Notice how his lead leg blocks out during his plant phase and everything upstream seems to follow, starting with the hips then the shoulders all the way through to the release.
If RFD matters that much in the discus, it’s worth asking whether the same principle applies in the batter’s box. Thus, the birth of our new question:
Are there specific windows during lead leg force production where greater/faster force production makes a difference on rotational capabilities in hitters?
Methods
The goal of this exploratory analysis is to determine if there might be any time windows where more force applied by the lead leg could explain faster rotation. Because of this, we cut our time window down from lead foot contact on the force plate all the way until bat-ball contact. We ended up looking at the force-time curves of 96 hitters spread between indy ball, the minors, and the majors. For each guy, we took their five fastest swings (via bat speed) from one session and averaged the force-time curves, peak pelvis angular velocity, peak torso angular velocity, and peak bat speeds.
Due to trial-by-trial differences in time windows, raw time output was converted to a percentage of the entire window, thus giving us a time% x-axis. We also wanted to account for the fact that heavier athletes are going to naturally produce more force, so we normalized the force output by body weight (body mass * gravitational acceleration constant):
Relative Force = Absolute Force/(Body Mass*g)
Another thing we wanted to consider is that force is a vector, meaning it has magnitude AND direction. Therefore, our analyses included 4 force vectors: the z-direction (vertical), the x-direction (toward/away from 2nd base), the y-direction (towards/away from the plate), and the combination of the overall vector (all three directions vectors combined).
Now, for the interesting part: how do you actually analyze a force-time curve?
Our first instinct was to do what most research does: pick a specific time window (say, 0-90ms after foot contact) and run correlations from there. The problem? Swing timing isn’t the same across athletes. Taller, longer-limbed hitters have naturally different timing than shorter ones, so pre-defining a window ends up comparing apples to oranges.
That’s where Statistical Parametric Mapping (SPM) comes in.
Instead of us telling the data where to look, SPM lets the force-time curve tell us where something meaningful is happening. It works by running a regression at every single time point across the swing, from foot contact to ball contact, and flagging the regions where force production significantly predicts our outcome variables. Think of it like a highlighter that automatically marks the parts of the curve that actually matter, rather than us guessing beforehand.
So… what did we find?
Findings
We ended up analyzing the FT curves for each of the four vectors as they related to peak pelvis rotational velo, peak torso rotational velo, and max bat speed, giving 12 different combinations for SPM regression.
Significant relationships emerged in the anterior-posterior (X) and medial-lateral (Y) directions for both peak pelvis and peak torso angular velocity, and the timing of those windows tells an interesting story. Notice how most of the highlighted windows occur early in the time window…
For peak pelvis angular velocity, a significant cluster appeared between roughly 11% and 37% of the swing phase. Importantly, this window aligns with the RFD phase, not peak force. In other words, it wasn’t how much force was being produced that mattered here, it was how quickly it got there.
For peak torso angular velocity, two windows emerged in the X direction: one overlapping with the pelvis window around 12-37%, and a second larger window spanning 38% to 92% of the swing phase, covering much of the analysis window including the peak force phase. The Y direction also showed significance early, from 8% to 25%.
Put simply, it’s like cracking a whip… the handle stops sharply, and that sudden stop sends energy forward, making the tip snap fast. Your lead leg works the same way: it ‘brakes’ hard and early, and that sudden stop is associated with faster hip and torso rotation.
Early and rapidly developed horizontal force in the lead leg, particularly in the x-direction, is significantly associated with how fast both the pelvis and torso are rotating at their peak. The fact that the pelvis window sits squarely in the RFD phase is particularly interesting. It suggests that the rate at which the lead leg builds horizontal force early in the swing may be setting the table for everything that happens rotationally upstream.
Now for what we didn’t find, because null findings are just as important as positive ones.
No significant windows were found for vertical GRF or overall force magnitude in relation to any outcome variable. More notably, no significant windows were found for GRF in *any* direction predicting bat speed. Why is this?
One possible explanation is related to arm mechanics. By the time energy travels up the kinetic chain and reaches the arms, things like elbow tuck, flexion, and extension timing can either amplify or dampen how much of that energy actually makes it to the bat. The lead leg may be doing everything right, but if the arms aren’t efficiently transferring that energy, bat speed becomes its own story.
A second potential explanation is that anthropometric differences between hitters, things like arm and leg length, could alter how force travels up the kinetic chain and ultimately influences bat speed. Two hitters producing identical lead leg force profiles may still arrive at very different bat speeds simply due to how their individual proportions affect energy transfer along the way. Since we didn’t account for these differences in the model, it’s possible that any underlying relationship between lead leg force and bat speed was obscured in the aggregate.
Conclusion
We started with a simple question: what does it take to become a better hitter?
And while we still can’t hand you a magic checklist, what we can say is that the lead leg is doing a lot more than just holding you up at foot contact. The rate at which it builds horizontal force early in the swing appears to be meaningfully tied to how fast the pelvis and torso are rotating at their peak, and that’s not a small thing.
We still need to understand the mechanism more clearly, figure out how to actually train it, and dig deeper into how individual differences between hitters might be shaping the relationship between ground force and bat speed.
But that’s what makes this exciting. Every answer in sports science tends to uncover three more questions, and this is no different. The lead leg has been an underappreciated piece of the hitting puzzle for a long time, and we’re just starting to understand what it’s actually capable of.
The 18,000-piece puzzle isn’t finished, but with each new piece, we’re starting to see how force fits into the swing, and how close we are to the full picture.
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