For over a decade, the baseball world has been fixated on ground force. Coaches, biomechanists, and researchers have drilled into the load and ride phase of the pitching delivery in a relentless search for optimal force production and biomechanical efficiency. At The Florida Baseball ARMory, we've led the charge—unearthing key metrics and coaching insights that have changed the game for thousands of athletes seeking more velocity, sharper command, and devastating secondary stuff.
But while we've been digging in on ground force, another vital link in the kinetic chain has remained elusive—a biomechanical enigma we’ve only scratched the surface of:
The T
runk.
The trunk is the essential bridge between the lower body and the throwing arm. It’s the power conduit, the energy converter, the timing mechanism. Yet, for all its importance, the trunk has remained a virtual black box—difficult to define, tougher to measure, and nearly impossible to optimize using traditional models.
Hip-to-Shoulder Separation: A Flawed but Familiar Start
The most common attempt to quantify trunk efficiency is the now-famous metric “hip-to-shoulder separation.” It’s a well-intentioned concept rooted in the idea that rotational separation between the pelvis and trunk enhances velocity by storing and transferring elastic energy.
The problem? The way we measure it.
Most systems—markered or markerless motion capture—track this separation by calculating the linear distance between a marker on the front hip and one on the back shoulder, typically in the frontal plane. But the real picture is three-dimensional. True separation includes anterior-posterior tilt (sagittal), lateral tilt (frontal), and rotation (transverse) of both the pelvis and the trunk. When we reduce a complex 3D system to a 2D measurement, we risk missing the forest for the trees.
To make matters more complicated, current motion capture systems—even the best available—have significant limitations in accurately measuring key contributors like pelvic tilt. They rely on Inertial Measurement Units (IMUs)—tiny devices made up of accelerometers, gyroscopes, and magnetometers. These devices tell us what the sensors are doing, and researchers infer that the body must be doing the same. That’s a big assumption.
Take elbow torque, for instance. A motion capture system might report 70 Nm of torque. That value reflects what the sensor interprets via Newtonian physics. But it doesn’t account for well-timed, well-synchronized muscle synergies that the body uses to absorb, dissipate, and redirect that force. The same applies to trunk movement. A raw measurement can’t tell us how efficiently or effectively the trunk is functioning within the total system.
More Data, More Error
Researchers often attempt to overcome this by collecting massive sample sizes—hundreds of thousands of pitches—believing that the law of large numbers will average out the noise. But as Van Orden, Holden, and Turvey remind us, if your assumptions are flawed, adding more data only amplifies the error. It's the same fallacy that plagued early cartographers: local maps were accurate, but as they scaled up, their inaccuracies compounded. More maps didn’t fix the problem—they made it worse.
Paul Glazier drives the point home: “If markerless motion capture is to be useful in a profiling or monitoring capacity, the amount of measurement error needs to be smaller than the amount of movement variability over iterative performance trials.” Right now, it isn’t. Not even close.
So what do we do? Abandon motion capture altogether? No.
We recognize it for what it is: a tool—not a truth. The insights we gain from hip-to-shoulder separation and similar metrics can be incredibly useful when interpreted with care and combined with clinical observation, athlete feedback, and anatomical understanding.
But we must always guard against the McNamara Fallacy—named for former U.S. Secretary of Defense Robert McNamara—who said, “If you can’t measure what’s important, you make what you can measure important.”
Just because we can measure separation doesn’t mean it’s the most important or useful metric.
The Real Key: Muscle Synergy and Slack Regulation
If we want to unlock the trunk’s role in high-performance pitching, we have to move beyond the data and into the deeper science of movement. That means understanding muscle synergy and slack regulation.
Muscles don’t come pre-loaded. They hang off bones like ropes. Before they can generate force, the slack must be removed. Think of towing a car with a rope—until the rope is taut, the car won’t move. Same with the body.
And here's the kicker: synergy between muscle groups—trunk, pelvis, scapula, arm—requires a precise, time-sensitive regulation of slack. Too much slack, and the force dissipates. Too little, and the movement becomes rigid and ineffective.
The idea isn’t to eliminate slack. It's to regulate it—finding the Goldilocks zone where everything comes together with just the right amount, at just the right time. And here’s where things get interesting…
Slack regulation is individual.
One athlete might find success with more pelvic tilt and a delayed trunk rotation. Another might require more lateral trunk flexion and earlier thoracic extension engagement. The permutations are virtually infinite.
In fact, if we consider the three planes of motion (frontal, sagittal, transverse) for both the pelvis and the trunk, we can easily calculate over 700 discrete permutations of how these segments might move. But when we acknowledge that each movement occurs along a continuum—not as strict 1s, 2s, or 3s—the number of possibilities becomes infinite.
And that’s the truth about human movement: It’s not about finding the solution. It’s about helping each athlete discover a family of solutions that work under varying task, environmental, and organismic constraints.
The Body Organizes Around Attractors
The key to guiding self-organization in the trunk lies in understanding that the body doesn’t operate on a blank slate—it organizes around universal anatomical attractors of agility. These are recurring movement solutions grounded in anatomy and physics that appear across high-level performers.
In the case of slack regulation in the trunk, two attractors are especially relevant:
- Hip Lock
- Chest Out When Rotating
Instead of chasing some mystical “ideal” amount of transverse or sagittal trunk motion by actively trying to separate the pelvis from the trunk, we should guide athletes toward developing a stable Hip Lock at back leg toe-off. This simple act—momentarily stopping the back of the pelvis from rotating forward—anchors the pelvis and allows the athlete to find just the right amount of abdominal stretch in both the transverse and frontal planes.
At that same moment, if the athlete also expresses the Chest Out When Rotating attractor, they gain optimal thoracic extension. This adds sagittal plane length to the abdominals and sets the stage for an explosive, efficient trunk rotation with precise slack regulation.
Training strategies that encourage athletes to stabilize these two attractors simultaneously—like our Hinge to Final Connection Drill.
Provide the fertile environment for individualized self-organization. They don’t dictate “how” to move, but instead point the athlete’s body toward natural, efficient movement solutions that produce results.
The Bottom Line
The trunk may never give us the clean, black-and-white answers the baseball world so desperately craves. But that’s okay.
Because at The ARMory, we’ve learned that the body doesn’t need perfection—it needs permission. Permission to explore. Permission to self-organize. Permission to move in ways that respect its unique anatomy and leverage its natural attractors.
We don’t chase averages. We chase outliers. And we do it with a blend of science, skill acquisition theory, and anatomical truth that you won’t find anywhere else.
So if you’re a parent, coach, or player ready to go beyond conventional wisdom and unlock the real potential of the trunk—velocity, command, secondary stuff, and arm health all included—then come train with us.
The Florida Baseball ARMory’s SAVAGE Summer Training Program is open now.
Five days a week. Four hours a day. Two to ten weeks of relentless, individualized, movement-first training.
Let’s stop guessing. Let’s stop chasing generic movement patterns. Let’s train smarter.
Let’s unlock your trunk.
Sign up for our SAVAGE Summer Training Program now.
Spots are limited. Results are unlimited.
Randy Sullivan, MPT, CSCS CEO, Florida Baseball ARMory
