Rucking with a 20 kg pack, and why military researchers ration it to once a fortnight

In the 12 months to May 2025, sales of weighted vests in the United States rose by more than 50% to $27 million, according to the market research firm Circana, as reported by Axios. Rucking, walking briskly with a loaded backpack or vest, had moved from army training grounds to parks and pavements. This article looks at its heavier end: a fast walk with about 20 kilograms on your back.

For a 75-kilogram adult, 20 kilograms is 27% of body weight. A US Army public health review puts combat loads at 20% to 30% of body weight, while experts interviewed by Healio in August 2026 advised starting weighted vests at about 5% and usually staying below 10% to 15%. Almost everything known about carrying 20 kilograms for long distances comes from soldiers.

This article covers what a 20-kilogram pack does to energy use, heart rate and the body, where injuries concentrate, what reduces them, and what weighted-vest trials found. The conclusion is not to leave the pack at home. It is to build up to heavy loads slowly, carry them well and walk rather than run.

What a 20 kg pack does to the body

Carrying weight costs energy roughly in proportion to the total mass moved. The standard tool for estimating it is an equation published in 1977 by researchers working for the US Army, Kent Pandolf, Baruch Givoni and Ralph Goldman, which uses body mass, load, speed, gradient and terrain. For a 75-kilogram walker on a level road at 5.4 km/h, it predicts about 366 watts with no load and 447 watts with 20 kilograms, 22% more. On a 5% slope the loaded figure rises to about 696 watts.

Energy cost of a brisk walk with a pack (modelled)

Predicted metabolic power in watts for a 75 kg adult walking at 5.4 km/h on a treadmill or paved road, calculated with the Pandolf equation. Modelled, not measured.

20 kg pack, 5% uphill slope696 W

30 kg pack, level501 W

20 kg pack, level447 W

10 kg pack, level402 W

No pack, level366 W

Source: Pandolf, Givoni and Goldman (1977), equation as reproduced by Potter et al., PLoS ONE (2015); our calculation. A 2021 US Army analysis found the equation underestimates heavy-load costs by about 7%.

A 2021 US Army analysis of loads up to 66% of body mass found that it underestimated measured metabolic rate by about 0.46 watts per kilogram, roughly 7%. In a 1981 Israeli study, 15 men carried 20 kilograms up a 5% treadmill slope: oxygen use climbed twice as steeply with speed when they walked as when they ran, and above about 8.2 km/h walking became the more expensive gait.

Heart rate tells the same story. Eleven male soldiers carrying 26 kilograms at 4.8 km/h averaged 122 beats a minute on the level and 155 on a 5% slope. In a 2024 study, young adults running at 8 km/h averaged 142 beats a minute with nothing on their backs and 160 with 17 kilograms. A heavy pack on a hill can match a run.

Heart rate under load

Average heart rate in beats per minute. Different groups of young adults in two studies, so compare the pattern rather than single values.

Running 8 km/h with a 17.1 kg load160 bpm

Walking 4.8 km/h with 26 kg, 5% slope155 bpm

Running 8 km/h, no load142 bpm

Walking 4.8 km/h with 26 kg, 2.5% slope135 bpm

Walking 4.8 km/h with 26 kg, level122 bpm

Sources: Legg, Ramsey and Knowles, Ergonomics (1992), 11 male soldiers, backpack frame; Hagstrom, Conger and Steeves (2024), 11 civilians, rifle and pack.

Reviews of military and backpacking data list foot blisters, stress fractures, knee and back problems, pain under the ball of the foot, and nerve compression, including rucksack palsy, in which shoulder straps press on the brachial plexus and weaken the arm. Stress fractures were first described in Prussian soldiers after long marches, hence the old name march fracture. Palsies are uncommon, but recovery can take months.

Where the risk concentrates

Risk rises with load, speed and distance together, and it lands hardest on people who are not used to them. In 1992 US Army researchers followed 335 light infantry soldiers through a single maximal-effort 20-kilometre march carrying 46 kilograms. Within 12 days, 24% had at least one injury, every one of them to the legs, feet or back. Blisters made up 35% of the injuries and back problems 23%. Units without recent march training, the authors warned, should expect such numbers.

That load was more than twice 20 kilograms, but feet blister at lighter weights. In a UK study of relatively inexperienced soldiers on a two-hour, forced-pace treadmill march carrying 20 kilograms, about 60% developed blisters. Shoulder discomfort began almost as soon as the pack went on and grew steadily; foot pain appeared later but then rose faster. Early shoulder pain or a hot spot on the foot, the authors suggested, may predict worse trouble later in a long march.

How often loaded marching hurts

Share of soldiers affected, from three military studies with different loads, durations and definitions.

Blisters after a 2-hour treadmill march with 20 kg60%

At least one load carriage injury in service (survey of 338)34%

Injured after one 20 km march with 46 kg (335 soldiers)24%

Blisters, as a share of those injuries35%

Back problems, as a share of those injuries23%

Sources: Birrell and Hooper (2007); Orr et al. (2017), Australian Army; Knapik et al., Military Medicine 157(2):64 (1992).

Load also reaches the bones. In a 2025 Korean study of ten soldiers and veterans walking in trainers, peak bending on the shinbone rose 15% with 15 kilograms and 33% with 30. A 2025 American study found a 20.4-kilogram load raised stress behind the kneecap more in women than in men, even after allowing for size and thigh strength.

Rucking is often sold as low-impact, and peak forces partly agree. With a 20-kilogram pack and a hip belt, nine men walking at their own pace met the ground with about 1.5 times their body weight, against 2 to 2.9 times in running. A per-step force is not the whole story, though; as our article on running injuries showed, running itself is not the joint-wrecker it is often taken for.

Peak force per step

Peak vertical ground reaction force in multiples of body weight. Different studies and people; the bar for loaded walking includes the pack.

Running, fastest speed tested (6 m/s)2.9x

Running, slowest speed tested (1.5 m/s)2.0x

Walking with 25 kg pack, hip belt1.56x

Walking with 20 kg pack, hip belt1.48x

Walking with 15 kg pack, hip belt1.43x

Walking without a load (1.0x to 1.5x by speed)1.2x

Sources: Nilsson and Thorstensson, Acta Physiologica Scandinavica (1989); Oberhofer et al., Applied Bionics and Biomechanics (2018); Keller et al., Clinical Biomechanics (1996).

The comparison with running cuts the other way when distance is counted. In a survey of 831 US infantry soldiers, the risk of injury per mile of road marching was 1.8 times the risk per mile of running, and soldiers who covered more distance with more weight had more injuries. Military data are an imperfect guide: soldiers march to orders, at set paces, often with heavier loads. As one Atlanta physiotherapist told Axios, much rucking research compares ruckers with sedentary people, not with walkers or weightlifters.

What has been measured to reduce the harm

In a 1987 report on 18 army trainees who developed brachial plexus injuries on road marches, the injury rate was 7.4 times higher among those carrying rucksacks without a frame. A widely cited review of soldier load carriage advises using hip belts whenever possible, because they shift weight off the shoulders, and keeping the load close to the body’s centre of mass, which gives the lowest energy cost.

How often you ruck matters as much as how much you carry. A 2021 international review by military researchers found load carriage performance improved with one dedicated session every seven to 14 days, while more than four a month brought more injuries without further gains. It recommends no more than one session every 10 to 14 days, alongside strength and aerobic training, and notes that neuromuscular function can take 48 to 72 hours to recover after a loaded march.

To progress, the review suggests changing one thing at a time: speed, gradient, terrain or load. Its example raises a load from 15 to 20 kilograms while slowing from 5.0 to 4.5 km/h, so the effort stays similar. Civilian advice starts far lower. In the Healio feature, one expert recommended about 5% of body weight, worn two or three times a week for two to three weeks without pain before adding more. Most of the measured benefits, though, come from trials of vests rather than packs.

What weighted-vest trials found

Percentage change after training with a vest, from small randomised trials, mostly in older women. Vest loads were up to 15% of body weight, far below 20 kg.

Ankle push-off strength, 12 weeks of vest training40%

Chair stands, aerobic dance with a 7% vest, 8 weeks38.1%

Chair stands, the same dance without a vest11.3%

Bone breakdown marker in blood, 12 weeks of vest training−14.5%

Hip bone density, a year of weight loss, all groups incl. vest (−1.2% to −1.9%)−1.9%

Sources: Klentrou et al., Journal of Aging and Physical Activity 15(3):287 (2007); Sukjit et al., Physical Activity and Nutrition (2026); Beavers et al., JAMA Network Open (2025), PMID 40540267.

The vest trials are small and short. In a 2003 trial, nine women in their late sixties who walked, climbed stairs and lifted weights in vests for 32 weeks improved bone density at the femoral neck compared with nine who did not exercise. A 2026 trial of 31 older women with low bone mass found T-scores rose by 0.08 with a vest and fell by 0.18 without one over eight weeks. Another 2026 trial of 57 women found strength gains but no change in bone density, and no training injuries.

Why the answer is not to put the pack away

The largest trial tempers expectations. In the INVEST trial at Wake Forest University, 150 older adults with obesity lost about 10% of their weight over a year, alone, with resistance training or while wearing a vest refilled to replace lost weight, for about seven hours a day. Hip bone density fell by 1.2% to 1.9% in every group, and the vest did no better than weight loss alone.

A 2026 follow-up of the same trial, in Frontiers in Aging, suggested why. In the vest group, more time spent standing or stepping was linked to better bone density, while with weight loss alone it was linked to losses. “A vest can be a great tool,” the lead author, Jason Fanning, told Wake Forest News, “but, like any tool, it’s not going to do the work for you.”

Loaded walking is also trainable. Weeks of loaded walking reduce the energy cost of carrying, and in a 2019 Australian study, 15 young men who combined resistance training with weighted walking for ten weeks made task-specific gains on a five-kilometre march at 5.5 km/h carrying a 23-kilogram vest. For many people, a pack adds strength work to a walk they already enjoy.

Some people should ask a doctor or physiotherapist first: anyone with disc disease or back pain, hip or knee arthritis, osteoporosis, heart disease or high blood pressure, and anyone who is pregnant. Experts interviewed by Healio advised supervision for people with osteoporosis, because heavy or badly distributed weight raises the risk of spinal compression fractures and falls. Stop for new pain, chest discomfort, dizziness, or numbness or tingling in the arms or legs. And walk heavy loads; do not run them.

Questions people ask

Is 20 kg too heavy for rucking?

For a beginner, usually yes. It is 27% of body weight for a 75 kg adult, close to a military combat load. Experts quoted in 2026 suggested starting near 5% and usually staying below 10% to 15% for vests; trained ruckers reach 20 kg gradually.

Is rucking easier on the joints than running?

Per step, yes: walking with 20 kg produced peak forces of about 1.5 times body weight against 2 to 2.9 in running. Per mile, soldiers’ injury risk was higher on loaded marches than on runs.

Can I run with a weighted pack?

It is better not to with heavy loads. At 8 km/h, a 17 kg load raised heart rate from about 142 to 160 beats a minute, and 20.4 kg raised peak force on the shinbone.

Does rucking build bone?

Small trials of weighted vests found gains in strength and some bone measures, mostly in older women. The largest trial found a vest did not prevent hip bone loss during weight loss.

The short version

• A 20 kg pack is 27% of body weight for a 75 kg adult, close to a military combat load and well above the 10% to 15% experts suggest for weighted vests.
• Modelled with the Pandolf equation, a brisk level walk costs about 22% more energy with 20 kg; uphill, heart rate can match that of a run.
• Blisters, back pain, stress fractures and strap-related nerve palsy are the classic injuries; about 60% of novices blistered in a two-hour 20 kg march.
• Frames, hip belts, gradual progression and no more than one heavy session every 10 to 14 days are the measures with military evidence behind them.
• Weighted-vest trials show modest gains in strength, mixed results for bone, and work best when you keep moving.

This is a summary of published research, not medical advice. If you have back, joint or heart problems, osteoporosis or high blood pressure, are pregnant, or develop pain, numbness or tingling while rucking, talk to your doctor or a physiotherapist before carrying heavy loads.

Further reading: Orr et al., International Journal of Environmental Research and Public Health 18(8):4010 (2021), on conditioning soldiers for load carriage. Knapik, Reynolds and Harman, Military Medicine 169(1):45 (2004), on the history and biomechanics of soldier loads. Beavers et al., JAMA Network Open (2025), the INVEST weighted-vest trial.

Sources

1. Mallenbaum C. Rucking: Why more women are doing weighted walks. Axios, 25 June 2025 (Circana sales figures).
2. Bascom E, Tingle C. Not just a fad: The impact of weighted vests on bone health. Healio, Orthopedics Today, 5 August 2026.
3. Hauschild VD, Roy TC, Grier T, Schuh A, Jones BH. Foot marching, load carriage, and injury risk. US Army Public Health Center (2016), DTIC AD1010939.
4. Pandolf KB, Givoni B, Goldman RF. Journal of Applied Physiology (1977); equation as given in Potter AW, González JA, Karis AJ, Xu X. PLoS ONE (2015), doi:10.1371/journal.pone.0132698.
5. Looney DP, Lavoie EM, Vangala S, et al. Modeling the metabolic costs of heavy military backpacking (2021), OSTI 1905049.
6. Keren G, Epstein Y, Magazanik A, Sohar E. The energy cost of walking and running with and without a backpack load. European Journal of Applied Physiology (1981), doi:10.1007/BF00423407.
7. Legg SJ, Ramsey T, Knowles DJ. The metabolic cost of backpack and shoulder load carriage. Ergonomics (1992), doi:10.1080/00140139208967382.
8. Hagstrom S, Conger SA, Steeves JA. The impact of load mass and distribution on heart rate, perceived exertion, and accelerometer measured physical activity during running (2024), doi:10.70252/cvkt2704.
9. Knapik JJ, Reynolds K. Load carriage-related injury mechanisms, risk factors, and prevention (2015), doi:10.1007/8415_2014_182.
10. Orr RM, Pope R, Lopes TJA, et al. Soldier load carriage, injuries, rehabilitation and physical conditioning: an international approach. International Journal of Environmental Research and Public Health 18(8):4010 (2021).
11. Knapik JJ, Reynolds KL, Staab JS, Vogel JA, Jones BH. Injuries associated with strenuous road marching. Military Medicine 157(2):64 (1992).
12. Birrell SA, Hooper RH. Initial subjective load carriage injury data collected with interviews and questionnaires (2007), University of Warwick repository.
13. Orr RM, Coyle JA, Johnston V, Pope R. Self-reported load carriage injuries of military soldiers (2017), Bond University repository.
14. Han S, Park J, Lee J, et al. Footwear has a modifying effect on tibial loading during military weight carriage. Scientific Reports 15:26531 (2025).
15. Willy RW, Simon JE, Hanser B, et al. Females exhibit greater peak and cumulative patellofemoral joint stress with moderate and heavy load carriage compared with males. European Journal of Sport Science (2025), doi:10.1002/ejsc.70046.
16. Oberhofer K, Wettenschwiler PD, Singh NB, et al. The influence of backpack weight and hip belt tension on movement and loading in the pelvis and lower limbs during walking. Applied Bionics and Biomechanics (2018), article 4671956.
17. Nilsson J, Thorstensson A. Ground reaction forces at different speeds of human walking and running. Acta Physiologica Scandinavica (1989), doi:10.1111/j.1748-1716.1989.tb08655.x; Keller TS, Weisberger AM, Ray JL, et al. Clinical Biomechanics (1996), doi:10.1016/0268-0033(95)00068-2.
18. Schuh-Renner A, Grier T, Canham-Chervak M, et al. Risk factors for injury associated with low, moderate, and high mileage road marching in a U.S. Army infantry brigade. Journal of Science and Medicine in Sport (2017), doi:10.1016/j.jsams.2017.07.027.
19. Bessen RJ, Belcher VW, Franklin RJ. Rucksack paralysis with and without rucksack frames. Military Medicine 152(7):372 (1987).
20. Knapik JJ, Reynolds KL, Harman E. Soldier load carriage: historical, physiological, biomechanical, and medical aspects. Military Medicine 169(1):45 (2004); Knapik J, Harman E, Reynolds K. Applied Ergonomics (1996), doi:10.1016/0003-6870(96)00013-0.
21. Klentrou P, Slack J, Roy B, Ladouceur M. Effects of exercise training with weighted vests on bone turnover and isokinetic strength in postmenopausal women. Journal of Aging and Physical Activity 15(3):287 (2007).
22. Sukjit S, Choktaweepanich B, Thuwakum W. Efficacy of a weighted-vest aerobic dance intervention for enhancing lower extremity strength and mobility in older adults. Physical Activity and Nutrition (2026).
23. Jessup JV, Horne C, Vishen RK, Wheeler D. Effects of exercise on bone density, balance, and self-efficacy in older women. Biological Research for Nursing (2003), doi:10.1177/1099800402239628.
24. Tantanasest J, Manimmanakorn A, Tunkamnerdthai O, et al. Comparative effects of weighted vest and whole-body vibration training on bone and muscle health in osteopenia. Life 16(2):229 (2026).
25. Beavers KM, Lynch SD, Fanning JT, et al. Weighted vest use or resistance exercise to offset weight loss–associated bone loss in older adults. JAMA Network Open (2025), PMID 40540267; results as summarised by Epocrates, 26 June 2025.
26. Roberts A. Wearing a weighted vest to strengthen bones? Make sure you’re moving. Wake Forest News, 2 March 2026 (Fanning et al., Frontiers in Aging).
27. Wills JA, Saxby DJ, Glassbrook DJ, Doyle TLA. Load-carriage conditioning elicits task-specific physical and psychophysical improvements in males. Journal of Strength and Conditioning Research (2019), doi:10.1519/JSC.0000000000003243.
28. Willy RW, Christensen K, Hanser B, Plemmons M, Ruby BC. Journal of Sports Sciences (2025), doi:10.1080/02640414.2025.2567781.