Ruck Speed, Not Just Load, Is the Biggest Driver of Energy Cost
- Alastair Hunt
- 1 day ago
- 4 min read
A study published in Nutrients set out to answer a question directly relevant to anyone who straps on a weighted pack:
How much does speed, versus load, actually change the energy cost of moving?
Researchers put 24 Polish special forces operators through a carefully controlled lab protocol, testing them at four different speeds, both with and without a 20 kg tactical backpack.
As ever, please talk to your doctor or medical practitioner most familiar with your medical history before implementing any changes in diet, exercise or lifestyle, especially if you are under treatment. Links to studies at bottom of page.
The Testing Protocol
This was a genuinely rigorous setup. Testing spanned three consecutive days. On day one, soldiers underwent medical clearance, body composition measurement, and a shuttle run test to establish individual VO2max and maximum heart rate, which were later used to express workload as a percentage of each soldier's personal ceiling rather than an absolute number. This matters because it lets researchers compare relative effort fairly across soldiers of different fitness levels.
On the following days, soldiers moved at four fixed speeds, each held for 6 minutes:
4.5 km/h - a leisurely walk
6.5 km/h - a brisk walk
8.5 km/h - a jog
10.5 km/h - a run
These speeds were deliberately chosen to mirror the natural progression from walking to running that soldiers experience during real tasks, from patrolling to closing distance under pressure.
Each soldier completed the full speed sequence twice: once unloaded, then again a day later wearing an evenly packed 20 kg backpack, individually fitted to each person, worn with standard tactical boots and field uniform.
What They Found
The results were striking. At the slowest walking pace, energy cost was 5.01 kcal/min unloaded and 6.35 kcal/min loaded. At a run, that jumped to 14.77 kcal/min unloaded and 17.70 kcal/min loaded, nearly a threefold increase.
Speed turned out to be the dominant factor overall, with very large statistical effect sizes across nearly every physiological measure, while the backpack added a consistent, significant burden at every pace, roughly a 10% rise in heart rate and a 20% rise in oxygen uptake.
Heart rate responses were remarkably consistent regardless of load, climbing from under half of max heart rate at the slowest pace to nearly 90% of max heart rate at the fastest loaded pace. Oxygen uptake told a similar story, rising from around a quarter of VO2max unloaded and walking slowly, to over 80% of VO2max when running loaded, edging close to territory that's difficult to sustain for long. Notably, the gap between loaded and unloaded oxygen cost widened as speed increased, meaning the backpack's penalty wasn't fixed, it compounded as pace picked up.
Perhaps the most practically important finding involved fuel source. At slower speeds, fat was the dominant energy substrate. But from jogging pace onward, and especially once the backpack was added, the body shifted decisively toward burning carbohydrate. This shift reflects a well established physiological principle: as effort intensity rises, the body increasingly relies on faster-access carbohydrate stores over slower-mobilising fat stores, since carbohydrate metabolism can keep pace with higher energy demand.
What This Means for Us
This has a very practical implication for anyone training for a ruck: pace matters enormously, arguably more than the weight on your back.
Pushing your speed, especially into a jog or run with load, disproportionately increases energy cost and shifts your body toward burning through glycogen (stored carbohydrate) rather than fat.
That's exactly the fuel source you're most likely to run low on during a long event, and depletion is closely linked to the dreaded "wall" ruckers and endurance athletes experience.
If you are training for the PARAS'10, a Norwegian Foot March, or just to get fast and fit, our self-paced online course"Built to Ruck The Science and Practice of Rucking" covers everything the research tells us about load carriage training, pacing, progression, heat management, hydration, fueling and more. It is built for ruckers at every level of experience, and draws directly on the peer-reviewed military and sports science research.
Final Thoughts
If you're planning nutrition for a ruck, especially one with faster segments, time pressure, or a pass/fail pace standard like the Norwegian Foot March or PARA'S10, prioritise carbohydrate availability before and during the effort, not just total calories.
Slow, steady walking paces are comparatively fat-fuelled and metabolically forgiving, which is one reason patient, conservative pacing is such a reliable strategy for long events. But the moment you pick up the pace under load, your body's fuel demands change in character, not just in intensity, and your fuelling strategy should reflect that shift.
One caveat worth flagging: this was a controlled lab test in an athletics hall, without full tactical gear or real terrain, so actual field energy costs are likely somewhat higher once you factor in hills, uneven ground and weather.
Wherever you are: train safe and enjoy the process!
Alastair
Key Studies and Resources
Zadarko E, Marszałek P, Zadarko-Domaradzka M, Penar-Zadarko B, Przednowek K. Energy Expenditure of Special Forces Soldiers in Relation to Equipment Load and Movement Speed. Nutrients. 2025 Dec 20;18(1):27. doi: 10.3390/nu18010027. PMID: 41515145; PMCID: PMC12788011.
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