What a pack does to your body
Rucking is walking with weight on your back. That makes it simple, and it also means most of what we know comes from walking studies and from military research on load carriage. Here is what that research supports, and where it stops.
Energy: more weight, more work, and hills cost most
A pack raises the energy cost of walking roughly in line with the extra mass you carry. The classic model for this is the Pandolf equation from 1977, which the US Army used for decades. For an 80 kg person walking at 1.5 m/s on the flat, it predicts about 417 W without a pack and about 484 W with 16 kg — roughly 16 % more.
Hills cost far more than load. The same equation grows quickly with the grade, so a climb raises the cost much more than a few extra kilograms do.
For heavy loads the equations are known to be off: newer work from the US Army found that existing equations underestimate the cost of heavy backpacking. Treat any calorie number as an estimate.
Heart: the evidence comes from walking, not rucking
Rucking is walking, and regular walking is linked to better heart health. A meta-analysis of 42 walking-group studies (1,843 people) found lower systolic blood pressure and a higher VO2max.
| Measure | Change in walking groups |
|---|---|
| Systolic blood pressure | about −3.7 mmHg |
| VO2max | about +2.7 ml/kg/min |
These were walking groups, not rucking studies: no good trial has compared rucking with plain walking yet.
Bones: no good evidence that load builds them
There is no good evidence that walking with weight builds bone.
- The largest trial of weighted vests so far followed 150 older adults with obesity for a year. Hip bone density fell in every group, and wearing a vest for about 7 hours a day did not prevent it.
- A meta-analysis of walking programmes in postmenopausal women found no effect at the spine and at most a small one at the hip.
- Osteoporosis Canada sums it up: there is little to no evidence that a weighted vest during walking increases bone density.
If you have osteoporosis or a fracture history, ask your doctor before you add load.
Joints: the pack adds force to every step
Each step hits more softly than a running stride, but the pack adds force to every step: in a study of 15 soldiers, ground reaction forces rose in proportion to the load. Military research lists the usual problems of long loaded marches: foot blisters, stress fractures, back strain, pain under the forefoot, pressure on the shoulder nerves from the straps ("rucksack palsy") and knee pain. Hip belts help, because they take weight off the shoulders.
This article is general information, not medical advice. See Health & safety before your first loaded ruck.
Sources
- Pandolf KB, Givoni B, Goldman RF. Predicting energy expenditure with loads while standing or walking very slowly. J Appl Physiol 1977. (accessed 2026-10-20)
- Looney DP et al. Modeling the Metabolic Costs of Heavy Military Backpacking. Med Sci Sports Exerc 2022. (accessed 2026-10-20)
- Hanson S, Jones A. Is there evidence that walking groups have health benefits? Br J Sports Med 2015. (accessed 2026-10-20)
- Beavers KM et al. Weighted Vest Use or Resistance Exercise to Offset Weight Loss-Associated Bone Loss in Older Adults. JAMA Netw Open 2025. (accessed 2026-10-20)
- Martyn-St James M, Carroll S. Meta-analysis of walking for preservation of bone mineral density in postmenopausal women. Bone 2008. (accessed 2026-10-20)
- Osteoporosis Canada. Weighted vests — are they good for bone health? (accessed 2026-10-20)
- Knapik JJ, Reynolds KL, Harman E. Soldier load carriage. Mil Med 2004. (accessed 2026-10-20)
- Birrell SA, Hooper RH, Haslam RA. The effect of military load carriage on ground reaction forces. Gait Posture 2007. (accessed 2026-10-20)