A number of studies have compared energy use between different load carrying systems:
Kirk, J. and Schneider, D. (1992) compared energy use in female subjects using internal and external framed backpacks. Young, C. (2001) compared two backpacks of different shapes- a tall narrow pack and a shorter wider pack. No significant energy differences were found in either study.
Backpacks share the load between the shoulders and the hips, they restrict the free movement of shoulders and hips and they always cause a forward lean with a corresponding pulling back force on the shoulders.
Read, S.A., Whiteside, R.A. looked at lateral stiffness elements in the suspension system of a backpack and found that a means to bring the distribution of the weight forward on the hipbelt improved the comfort. This suggests that adding a loading component to the front of the hipbelt is advantageous compared to having the loading on the back of the hipbelt only.
Lloyd, R. and Cooke, C. (2000) compared the differences in oxygen consumption between a backpack, and a backpack with counter balancing pockets on the front. The counter balanced pack reduced forward lean significantly, reduced discomfort in the back, and eliminated discomfort in the neck, shoulders and thighs. It gave significant energy savings compared to the backpack alone. The energy savings were greatest for heavier loads and when working hardest, such as climbing. In this study a larger proportion of the load was carried in the backpack than in the front pockets so that there was still some induced forward lean.
Heglund N.C., Willems P.A., Penta M.C. & Cavarna G.A. (1995) showed that some African women carrying load on their head can carry a fifth of their weight without burning a single extra calorie; and although larger loads did require more
energy, the increase was only half of that needed by American soldiers carrying loads in backpacks. Some women could carry 70 percent of their weight. This shows that a load carrying system where the center of gravity of the load passes vertically through the center of gravity of the wearer can have very significant energy requirement advantages.
Carrying load on the head maintains an upright posture, places no load on the shoulders and hips, and the hips are free to swing freely. If the hands are not used to stabilise the load, then the arms can also swing freely. Because the load is unstable, this is not a practical way to carry load on uneven terrain, in windy conditions, in bush, or during dynamic movement.
Bastien G.J., Schepens B., Willems P.A. & Heglund N.C. (2005) Energetics of load carrying in Nepalese porters. Science, 308: 1755 showed that Nepalese porters carry loads greater than 20% bodyweight even more economically than African women. Female Nepalese porters, for example, carry on average loads that are 10% of their Mb heavier than the maximum loads carried by the African women, yet do so at a 25% smaller metabolic cost. Nepalese porters routinely carry head-supported loads equal to 100 to 200% of their body weight (Mb) for many days up and down steep mountain footpaths at high altitudes. They climb at a very slow pace with frequent rest stops.
The load carrying system used by the Nepalese porters is a trump line around the forehead, with all weight behind in a wedge shaped container. While the posture is stooped forward, (it is less so than a backpack due to the wedge shaped container), there is no loading on shoulders or hips, and the hips are free to swing freely. Generally the hands are used to stabilise the load. All the weight is transmitted directly down the spine and the porters have well developed muscles either side of the spine to support the spinal loading.Rome L.C., Flynn L., You T.D. (2006) Biomechanics: rubber bands reduce the cost of carrying loads.
Rome showed that allowing movement between the pack harness /hip belt and the pack load "provides the following ergonomic benefit: it reduces the peak forces on the body. If one wears a 50-lb backpack, the static force exerted on the body is equal to 50 lbs, however during walking or running the peak vertical forces acting on the body increase due to the necessary vertical acceleration of the added mass. During fast walking, the peak increases by 70% (e.g., to 85 lb); and during running, it triples to a 150 lbs. Rome invented the Suspended-load ergonomic backpack which greatly reduces the vertical movement of the load with respect to the ground. This in turn reduces the accelerative forces on the body by 82% during walking and 86% during running. Reducing accelerative forces reduces metabolic cost by 40 Watts and increased endurance, or alternately, it permits the carriage of 12 extra lbs for the same metabolic cost (i.e., for free ). Further, it permits running with heavy loads while avoiding orthopedic injury."