Training Tactical Functional Movement Under Fatigue Using Work Capacity and Chassis Integrity Grinds

Above: A Special Forces candidate, from the U.S. Army John F. Kennedy Special Warfare Center and School, carries a weighted dufflebag during Special Forces Assessment and Selection (SFAS) at Camp Mackall, North Carolina September 5, 2024. Candidates who attended the three-week assessment and selection were evaluated on their ability to work individually and as a team. (U.S.Army photo by K. Kassens)

By Rob Shaul, Founder

Tactical athletes move under load, in awkward positions, while carrying weight, while thinking, while breathing hard, while fatigued.

I’ve been programming combined, extended (30+ minute) Work Capacity + Chassis Integrity “Grinds” across many MTI plans—both mountain and tactical—for over a year. They evolved from my own training. At 57, I avoid the intense, gym-based work capacity efforts I enjoyed and excelled at 20 years ago. It’s not the work I avoid—but the intensity—that I no longer want to do.

These efforts target and train functional movement patterns that directly transfer to tactical mission sets: downed officer drags, kit carriage, stair climbs, ruck running, prone-to-sprint transitions, movement under load, and awkward load carriage.

Traditional MTI base fitness programming for tactical athletes concurrently trains five fitness attributes: strength, short-duration work capacity (under 20 minutes), endurance (run/ruck), chassis integrity, and tactical agility. Often, these are trained in isolation. For example, a session may include a short, intense work capacity effort (like shuttle sprint repeats), followed by four rounds of a four-exercise chassis integrity circuit.

In contrast, these combined Work Capacity + Chassis Integrity Grinds train the two attributes not sequentially, but simultaneously.

Years ago, we experimented with what I called “gym-based endurance.” These were long (60+ minute) sessions that combined 2–4 exercises, one of which was a run or step-ups. They evolved from our even earlier “stamina” training experiments. Ultimately, we moved away from the concept. I had hoped these could replace outdoor endurance work—running, rucking, hiking uphill—but they didn’t. Aerobic endurance is mode-specific, and the best way to train rucking is to ruck. Our gym-based endurance efforts, while challenging, just made us better at gym-based endurance. They didn’t transfer to outside performance.

These Work Capacity + Chassis Integrity Grinds are different. They transfer. And they come with the added element of pacing and continuous movement under fatigue.

At MTI, “Grind” is a pacing directive. It means: complete the circuit again and again, for rounds or time. Pace yourself. Don’t stop to rest.

Our chassis methodology breaks midsection work into five categories: Flexion, Extension, Rotation, Anti-Rotation, and Total. “Total” exercises combine two or more of those patterns into a single movement. The sandbag get-up is a prime example—starting with flexion and rotation to rise off the ground, then combining anti-rotation and extension to reach standing. All exercises are completed from standing to reinforce integrity across the lower and upper body—that’s the transfer.

Here are several sandbag-based Total Chassis Integrity exercises I consistently use in these grinds:

• Sandbag Get-Ups (Flexion, Rotation, Anti-Rotation, Extension)

• Sandbag Cross Cleans (Extension, Rotation)

• Sandbag Twist & Toss (Extension, Rotation, Anti-Rotation)

• Sandbag Clean & Press (Extension, Anti-Rotation)

• Sandbag Get-Up + Run (Flexion, Rotation, Extension, Movement)

• Sandbag Keg Lift (Extension, Rotation)

• Sandbag Toss & Chase (Extension, Anti-Rotation + Sprint)

• Sandbag Mr. Spectacular (Extension, Anti-Rotation, Shoulder Press)

The work capacity element of these grinds comes from two sources. First, the inclusion of a single-mode movement task like a run, ruck, step-up, or sled push. Second, the overall demand of 30+ minutes of near-continuous, often loaded, movement.

Examples of these movement-based components include a 100m shuttle, a 200–400m run, step-ups, or a sled push—all chosen to elevate heart rate and challenge the lower body. I intentionally avoid squats or lunges with the sandbag in these efforts—these are garbage reps in the context of a work capacity circuit.

These sessions are grinds, not sprints. The objective is not to go “for time” or push at a frantic pace. The goal is to work steadily and deliberately, with no rest, for the entire session. You keep moving. You keep working. You don’t rush. You grind.

No more than four exercises per circuit. Two or three is ideal. Keep it simple.

Real-World Grind Examples from My Log

30-Minute Grind (wearing 20-lb plate carrier) …

• 10x Tire Flips

• 5x Sandbag Keg Lift @ 40/60#

• 100m Shuttle with Sandbag (50m down and back)

40-Minute Grind …

• 10x Sandbag Burpees @ 40/60#

• 50x Step-Ups @ 16” Box

60-Minute Grind …

• 5x Sandbag Twist & Toss @ 40/60#

• 200m Run

No warm-up is programmed. The first round or two serves that role. The sandbag load is light—40# for women, 60# for men—but don’t be fooled. A 60-pound sandbag isn’t a 60-pound barbell. It shifts, slips, sags, and forces constant correction. The longer the effort, the heavier it gets. And the cumulative work adds up fast when you can’t stop.

These long grinds deliver a full-body training effect, with the metabolic load driven primarily by the sandbag work. Unlike traditional core work that isolates the trunk, these movements activate the entire system. They blend hinge, press, carry, and rotation into a nonstop cycle of tension, breath control, and posture maintenance.

You might assume the moderate pace lowers the mental demand. That’s a mistake. These sessions grind on you. They don’t have the fast-burn pain of an AMRAP, but they ask for more patience, more pacing discipline, and more focus. Push through a 60-minute grind at a steady clip with a shifting sandbag and elevated heart rate, and you’ll catch yourself checking the clock—and hoping time moved faster. Want to train mental fitness? Try not to look at the clock. 

I haven’t run a formal study on these efforts, but they feel right. They make sense for tactical and mountain athletes with real-world movement and load carriage demands. They train not just strength and endurance, but the integration of both, under duration and fatigue.

I’m no longer a tactical athlete. But I had a reminder of what this looks like in the real world while backcountry hunting. I arrowed a bull elk with a grizzly sighted in the area. I had to bone the elk out, then move 200+ pounds of meat and antlers—alone—before dark. The terrain was black timber, deadfall, 10,000 feet. I had maybe 50 meters of visibility. I couldn’t risk losing track of my gear. So I moved everything—pack, gear, meat, antlers—in 50m shuttle intervals to a clearing 400 meters away.

One bag of meat went in the pack. My bow was strapped on. I zercher-carried a second meat bag. That was one load. After each 50m, I’d drop everything, sprint back, and repeat. I could only carry one bag at a time. Every 50 meters took four loaded trips. It took over 90 minutes to reach the meadow. It was a Work Capacity + Chassis Integrity Grind—real-world edition.

That’s the kind of transfer we’re programming for. There’s no substitute for the sandbag in this role. Dumbbells, barbells, even kettlebells—none of them replicate the chaos of a sagging, rotating, barely grippable object. Nothing else mimics the feel of a 50-pound meat bag over rough terrain.

I now integrate these extended grinds into all of MTI’s daily programming streams. They’re also great for travel or field use—any time you’re short on equipment but long on time. All you need is a sandbag and space.

If tactical athletes are going to move under fatigue in the real world, they need to train under fatigue in the gym. These grinds—built around total-body sandbag movements and nonstop pacing—do exactly that. They’re simple and mission-direct. 

Have questions?
Email me: rob@mtntactical.com