Chassis Integrity: MTI’s Functional Core Training Theory with Mission-Direct Transfer

By Rob Shaul, Founder

At MTI, all that matters is outside performance. Every programming decision is filtered through a single question: does the fitness training transfer to the real thing outside the gym?

For the athletes we serve—military operators, firefighters, law enforcement officers, and mountain professionals—a strong midsection is a fundamental fitness demand. I envision the midsection as a “barrel” of flesh and muscle that starts deep at the spine, wraps around the torso, and ties back into the spine on the other side.

It’s built of layered muscle, fascia, and connective tissue, and its job is to transmit and resist force in every direction. It has to flex, extend, and rotate when the task demands it, but also lock down like steal when the job is to stay rigid against torque and load.

In the real world, it’s this “muscle barrel” that keeps a soldiers spine safe under a 90-pound ruck, allows a fireman to drive through a casualty drag without folding, and allowes a mountain guide to stop a client’s fall when belaying an alpine climb. 

If this barrel of muscle weak or poorly conditioned, force leaks out of it. If it’s strong and well-trained, you can carry, lift, and move more — and do it longer — without the chassis collapsing.

Importantly, not only must this muscle have max effort strength for a single effort like lifting a fallen colleage to your shoulders for a fireman’s carry to safety, but it must also have a high level of strength endurance – for an extended effort, like a 3-hour, 6 mile, 100 pound meat packout after a successful backcountry hunt. 

The “Combat Chassis”

I first heard ‘chassis’ used to describe the midsection from Marines classmates during a shooting course on Camp Pendleton. The Marines spoke of the “Combat Chassis” as the area on the body between the knees and the shoulders. A strong Combat Chassis was key, they argued, to combat load carriage and tactical performance.

“Chassis Integrity” is the name I eventually developed to train that muscle barrel.  But I didn’t start there. Prior to 2015, MTI’s core training philosophy deployed common midsection exercise in movement based circuits. I called these “FIRE” circuits – as each included a Flexion, Isometric, Rotational and Extension core exercise. The movement-focus was solid, but I learned that the ground-based, often isolated core exercises, didn’t transfer. 

That year I discovered firsthand that a strong core in isolation and strong legs in isolation don’t mean much if they can’t work together. At the time, I’d stepped away from tactical programming to focus on distance running and bodyweight work. I hammered my “core” with sit-ups, planks, GHD sit-ups, bridges—typcial “core’ training exercises at that time. I completed more volume and variety than I programmed for my athletes.

I assumed I’d built something formidable. But the first time I got under a barbell for heavy front squats, and my midsection folded. My legs were strong, my midsection was strong in isolation, but the coordination and unified strength between them was missing. I lacked “Chassis Integrity.” 

That moment forced me to scrap the core exercise we were using—the ground-based, and isolation-focused core exercises—and rethink midsection training the ground up. The result was a new theory that trained the system as a whole rather than its parts.

“Chassis Integrity”

Chassis Integrity works from the understanding that the midsection’s job in the real world isn’t to crunch forward for reps. It’s to stabilize against rotational forces, to extend the hips and spine under load, to rotate powerfully and safely, and to connect the lower body to the upper body in a way that transfers directly to real-world tasks. Weak Chassis Integrity leaks force and breaks position. Strong Chassis Integrity transfers power between the lower and upper body efficiently, protects the spine, and keeps the athlete effective deep into fatigue.

The movement pattern approach to exercise categories remained, but expanded: Total, Flexion, Rotation, Extenion, and anti-rotation/carries. I define “Total” as a Chassis Integrity exercise that includes two or more of the movements. For example, the Sandbag Getup includes flextion, rotation, extension and anti-rotation.

What did change was the exercises themselves. Isolated, ground-based (laying down), core exercises were set aside in favor of Chassis Integrity exercises completed standing or kneeling. It was clear that to train “integity” the whole system needed to be trained together – no isolation. Exercises were developed and chosen to train the “muscle barrel” in it’s fuctional mode – as the link between upper and lower body. Click HERE for an exercise menu by movement pattern. 

Completing exercises from standing or kneeling automatically forces link function and transferable fitness training. We still deploy some ground-based core exercises such as Eos, weighted situps and face down back extension – but few of them. 

Chassis Integrity circuits are usually for time, not rounds, and often 20-minutes long. Twenty minutes straight of a chassis integrity circuit is no joke, and trains max effort midsection strength, and becuase of the duration, strength endurance. We continue to use movement-based circuits. For example, here is a 20-Minute “TRE” circut where “T” is a Total Body Chassis Integrity Exercise, “R” is a rotational exercise, and “E” is an extension exercise.

20 Minute “Grind”

• 5x Sandbag Getup @ 60# (Alternate shoulders each round)
• 5x Sandbag Keg Lift @ 60#
• 10x Sandbag Good Morning @ 60#

“Grind” = work steadily, not frantically through the exercises in this circuit for the prescribed tiem. 

Sandbags are central. No other tool is as awkward, as hard to grip, as naturally unbalanced. Over the years, I’ve yet to find a better tool to train Chassis Integrity. 20+ minute long Long grinds test the athlete’s ability to keep bracing and moving under load while fatigue sets in – so strength endurance is also trained.

Early on we found that these Chassis Integrity circuits, in addition to training midsection strength and strength endurance, can also be killer work capacity efforts. Sometimes we combine chassis integrity exercises and running, rucking or step ups, into an extended work capacity or gym-based endurance circuit. For example,

40 minute Grind …
• 5x Sandbag Getups @ 60#
• Run 200m with the sandbag
•Run200m unlooaded. 

20+ minute long Long grinds test the athlete’s ability to keep bracing and moving under load while fatigue sets in. The movements are chosen for their transfer, not novelty.—and therefore as honest in exposing weaknesses.

In the field, MTI atheltes lift ore move something heavy, bulky, and unbalanced. They’re doing it standing or kneeling, not lying on their backs. Often they are doing it for minutes or hours, not seconds. Chassis Integrity is deployed to prepare them for that. It makes athletes stronger in the moment and more resilient when fatigue makes mistakes and injuries more likely.

A Chassis Integrity Assessment

For all its proven value in programming, Chassis Integrity still lacks a single, validated assessment. We’ve tested one. In a recent mini-study, four athletes performed a four-part test—sandbag sit-ups, keg lifts, good mornings, and a weighted bridge hold—each for max reps or time in ninety seconds. Every athlete improved over a five-week cycle, but the test itself revealed serious problems. It took more than an hour to run, loading wasn’t consistent across athletes, and movement standards were hard to enforce. The short, fast efforts skewed the results toward speed and away from the blend of strength and endurance that defines Chassis Integrity.

After this study we tried another – focused just on the Sandbag Getup – thinking that a single ten-minute sandbag get-up could be an effective Chassis Integrity assessment.  The SBGU is my favorite Chassis Integrity exercise and  integrates flexion, extension, rotation, and anti-rotation under load, is easy to set up, simple to standardize, and brutally honest. However, a follow up mini study found that increased Sandbag Getup fitness didn’t transfer to improvement in other chassis integrity exercises. So, we’ve got work to do. 

The end state is outside performance: an athlete whose midsection is a strong, mobile, static-when-needed barrel of muscle, built to hold the chassis together and perform without weak linkage in on the battlefield, firegrounds, urban streets or mountains.

Chassis Integrity, like all MTI methodology, is a living concept. It started as a reaction to a testing my own programming. It has become one of MTI’s most transferable and field-proven programming elements. The work now is to keep refining it, to keep testing, to strip away what doesn’t work and double down on what does. That process—deploy, assess, iterate—continues at MTI.

Questions/Feedback? Email rob@mtntactical.com