Durability: The Hidden Variable in Rucking

Alastair Hunt
2 days ago
7 min read

Updated: 7 hours ago

Every experienced rucker knows the feeling. The first half of a long event goes to plan: pace feels controlled, load feels manageable, breathing is steady. Then something shifts. The pack feels heavier. Each step costs more. The pace that felt sustainable an hour ago now feels like it belongs to a different day.

This is not a fuelling miscalculation or a fitness deficit. It is the expression of poor durability and understanding it is one of the most important things a serious rucker can do to improve performance over longer distances.

Durability describes how well you maintain performance output as fatigue accumulates during prolonged exercise. More precisely, it reflects the time of onset and the magnitude of deterioration in your key physiological markers during sustained effort. In rucking terms: it is the difference between the athlete who finishes a 30km event at close to their opening pace, and the one who is survival-marching the final ten kilometres.

Traditional endurance models describe performance through three pillars: your aerobic ceiling (VO2max), the fraction of that ceiling you can sustain at threshold, and your movement economy: how efficiently you convert oxygen into forward motion. These variables are all measured in a rested state. What happens to them after 90 minutes or more of sustained effort under load is a different question entirely and the answer is what durability science addresses.

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 all studies / resources at bottom of page.

What the research shows

Two recent studies examined how the physiological pillars of endurance performance change during prolonged effort, with findings that translate directly to rucking.

The first, Zanini et al, tested fourteen well-trained marathon runners before and after 90- and 120-minute runs at marathon-intensity effort. After two hours of sustained running:

Aerobic capacity dropped by 7.1%, accelerating in a nonlinear pattern, most of the decline occurring in the final 30 minutes.
Movement efficiency deteriorated by 5.8%, reflecting a progressive shift toward less metabolically efficient muscle fibres and altered biomechanics.
Speed at lactate threshold fell by 6.6%, meaning the pace sustainable before tipping into unsustainable metabolic territory dropped substantially.
The fraction of remaining aerobic capacity being used at a fixed pace rose by 12%, the same effort becomes a progressively higher percentage of a shrinking ceiling

The second study (Hunter & Muniz-Pumares, 2025), conducted with London Marathon participants, found a direct association between durability and race performance. Runners who showed smaller deteriorations in lactate threshold speed following a 90-minute fatiguing run recorded faster marathon times. Durability was not merely correlated with performance; it was one of its meaningful predictors.

Critically, movement economy showed no significant deterioration over 90 minutes in that second study, suggesting economy may be more robust than peak aerobic capacity or threshold speed, particularly in athletes with adequate training volume. Durability is not a single property but a composite of how well each physiological subsystem holds up under fatigue.

Why it matters for ruckers

Running studies provide the closest laboratory proxy for what happens physiologically during rucking. Both activities are weight-bearing, bipedal, and sustained over prolonged durations. But rucking introduces a variable that makes durability even more critical: external load.

A weighted pack changes the metabolic cost of every step. It increases ground reaction forces, increases oxygen cost at any given pace, and accelerates the degradation of neuromuscular coordination as fatigue sets in. The biomechanical shifts observed in running research, where fatigued athletes transfer work from efficient ankle plantar flexors to less efficient muscles of the knee and hip, are likely amplified in rucking due to the constant downward pressure of the pack altering posture and gait mechanics.

This means the threshold at which performance deteriorates may arrive earlier in rucking than in unloaded running. An athlete whose movement economy collapses under fatigue is now doing so while carrying weight, compounding the metabolic penalty with every step.

Research also shows that central fatigue, the reduction in neural drive from brain to working muscles, plays a significant role in late-stage performance decline. Muscles become slower to fire, coordination degrades, and the body recruits less efficient motor units to maintain output. For a rucker, this manifests as form breakdown: forward lean collapses, foot strike patterns change, and the mechanical efficiency of the opening hours is replaced by a costly, grinding forward motion.

The mechanisms

Central fatigue Reduced signalling between brain and muscles causes slower firing, degraded coordination, and form breakdown. The rucker's posture collapses, stride efficiency drops, and the pack's weight is increasingly borne by passive structures rather than active musculature.

Peripheral fatigue Micro-damage in muscle tissue and depletion of type I fibres forces recruitment of less efficient type II fibres, increasing the metabolic cost of every step. The same pace that felt sustainable at hour one now demands significantly more physiological output.

Metabolic shift As carbohydrate stores deplete, the body shifts toward fat oxidation, a less efficient fuel source requiring more oxygen per unit of energy. Under load, this shift accelerates the rise in perceived effort, particularly over events exceeding three to four hours.

Threshold drift The pace at which lactate accumulates falls, meaning the intensity that felt manageable crosses into the heavy-to-severe zone earlier than expected. The rucker is working harder than the pace suggests, and fatigue accelerates accordingly.

How to train durability

Aerobic volume is the foundation The most consistent finding across durability research is that sustained aerobic volume is the primary driver of improved fatigue resistance. High weekly training volume builds mitochondrial density, improves fat oxidation efficiency, and delays the metabolic shift that accelerates physiological unravelling.

This means consistent long sessions at controlled effort, 60 to 90 minutes at a conversational pace, done frequently. Volume also appears to be the primary determinant of movement economy durability: athletes with higher training loads show smaller economy decrements over prolonged effort.

Stacking: training on tired legs One of the most effective and underused durability tools is the back-to-back session. A moderate ruck on day one followed by a longer ruck on day two forces the body to begin the second effort in a pre-fatigued state, directly training the systems that govern late-stage performance. Over time, the body adapts to perform more competently in this degraded state. No amount of single-session work replicates this stimulus.

Strength training Strength training is perhaps the most evidence-backed modifiable variable for improving durability in load-bearing endurance athletes. A 2024 systematic review and meta-analysis (Llanos-Lagos et al.) covering 38 studies and 894 middle and long-distance runners found that high-load strength training produced a significant moderate improvement in running performance, and that combined strength methods (heavy resistance plus plyometric work) produced a large effect. Crucially, these gains occurred without changes to VO2max or metabolic threshold, meaning improvements were driven by neuromuscular adaptations: improved rate of force development, enhanced musculotendinous stiffness and more efficient motor unit recruitment.

For ruckers specifically, a stronger athlete recruits less of their total available neuromuscular capacity to maintain pace under load. The same pack weight represents a smaller relative demand. Fatigue still arrives but later, and with less severe consequences.

Practical prescription:

Heavy compound lifts
Plyometric work
Loaded carries
Combined methods, of the above

Fuelling as a durability strategy The metabolic shift from carbohydrate to fat oxidation is both inevitable and consequential. The London Marathon study noted that the absence of carbohydrate supplementation during a fatiguing 90-minute run may have contributed to the magnitude of physiological decline observed, even at that relatively short duration. For rucks exceeding 90 minutes, carbohydrate intake is a direct durability intervention, not an afterthought.

Pacing, durability as strategy The research consistently shows that physiological deterioration is nonlinear, most of the decline in aerobic capacity and threshold speed accelerates after 90 minutes. Aggressive early pacing advances the arrival of that tipping point and amplifies its severity. A conservative first half, preserving glycogen and neuromuscular integrity, pays disproportionate dividends in the second half.

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.

A note on the evidence base

The durability research cited in this article draws primarily from running science: the closest available laboratory proxy for the physiological demands of rucking. Direct rucking-specific durability studies do not currently exist in the published literature. However, the military field research used elsewhere in this course provides meaningful real-world support for the mechanistic framework the running studies describe.

A Swiss Armed Forces 34km field march demonstrated that less fit soldiers worked at significantly higher heart rates throughout, 133bpm versus 125bpm in the fittest group, a direct expression of the aerobic ceiling deterioration and threshold drift the Zanini and Hunter studies document in runners. Dropout rates of 21 to 28% in the lower fitness groups reflect durability failure under sustained load in a real-world event.

A Singapore 72km tropical march, pushing well beyond the 90-minute threshold at which nonlinear physiological deterioration begins, further supports the finding that a higher aerobic ceiling slows the rate of relative deterioration under prolonged effort. The mechanisms are the same; the load on the back makes their consequences more pronounced.

What This Means for Us

Most ruck training optimises for the first half of an event. Durability training is preparation for everything after that point, ensuring that the degradation arrives later, progresses more slowly and leaves more capacity intact for the finish.

The science is unambiguous: the deterioration of aerobic ceiling, movement economy, and sustainable threshold pace is trainable. Strength training slows its progression. Volume delays its onset. Strategic fuelling pushes back the metabolic tipping point. Back-to-back sessions teach the body to function in the degraded state a long event inevitably produces.

The ruckers who finish strong are not necessarily the most aerobically gifted. They are the ones who have trained for what the second half of the session actually demands.

Wherever you are: train safe and enjoy the process!

Alastair

Read about mental durability here.

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Key Studies

Zanini M, Folland JP, Blagrove RC. The Effect of 90 and 120 Min of Running on the Determinants of Endurance Performance in Well-Trained Male Marathon Runners. Scand J Med Sci Sports. 2025 May;35(5):e70076. doi: 10.1111/sms.70076. PMID: 40375575; PMCID: PMC12082016.

Hunter B, Muniz-Pumares D. Durability of Parameters Associated With Endurance Running in Marathoners. Eur J Sport Sci. 2025 Nov;25(11):e70073. doi: 10.1002/ejsc.70073. PMID: 41128270; PMCID: PMC12547624.

Llanos-Lagos C, Ramirez-Campillo R, Moran J, Sáez de Villarreal E. The Effect of Strength Training Methods on Middle-Distance and Long-Distance Runners' Athletic Performance: A Systematic Review with Meta-analysis. Sports Med. 2024 Jul;54(7):1801-1833. doi: 10.1007/s40279-024-02018-z. Epub 2024 Apr 17. PMID: 38627351; PMCID: PMC11258194.

Oeschger R, Roos L, Wyss T, Buller MJ, Veenstra BJ, Gilgen-Ammann R. Influence of Soldiers' Cardiorespiratory Fitness on Physiological Responses and Dropouts During a Loaded Long-distance March. Mil Med. 2023 Jul 22;188(7-8):e1903–9. doi: 10.1093/milmed/usab540. Epub 2022 Jan 7. PMID: 35015894; PMCID: PMC10363014.

Poon BH, Prakaash S, Teo YS, Fan PW, Wei Lee JK. Thermal strain and fluid balance during a 72-km military route march in a field setting. Singapore Med J. 2022 Sep;63(9):497-502. doi: 10.11622/smedj.2021053. Epub 2021 Apr 19. PMID: 34005849; PMCID: PMC9678143.