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Biomechanics investigated
DISCUSSION
Ray Lloyd pioneered the research into counter-balanced load carrying systems. His research compared a backpack to a partially counter balanced load carrying system- a Bodypack with a greater proportion of the weight behind the wearer. Forward lean was significantly reduced with the Bodypack. The substantial benefits of the system are detailed in the charts below.
The amount of forward lean with a Bodypack is determined by the relative volume of the pack on the back of the wearer and the Balance Pockets in front of the wearer and the relative density of the items placed in each. By using the largest volume Balance pockets with a small to medium volume pack and packing your heavier items as far forward and as low down as possible, it is possible to create a body centered load carrying system. Then the center of gravity of the load perfectly matches the center of gravity of the body in both the horizontal and vertical directions. The addition of the load will not alter your natural posture or your balance and it will exert no leverage on your body during active movement.
A body centered load carrying system is the most efficent and strain free possible. Combined with our flow motion systems, it provides the greatest movement with the greatest stability for dynamic sports. When used with the most ergonomic poles available, Pacerpoles, we expect to see further gains in performance and energy efficency.
We are currently looking for sponsorship to support a new study under the supervision of Professor Stephen Legg, at the Center for Ergonomics, Massey University, NZ, to examine these premises. This is very exciting research with significant implications for the optimal experience of load carrying. If you would like to help fund this research, please contact us.
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MENU
SUMMARY OF RAY LLOYD’S RESEARCH
SUMMARY OF OTHER RESEARCH ON LOAD CARRYING SYSTEMS
PROPOSAL FOR A NEW STUDY
REFERENCES
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A SUMMARY OF RAY LLOYD’S RESEARCH
Your center of gravity with a load is measured by the position of the center of mass.
This graph compares the horizontal displacement of the center of mass of the Bodypack and the backpack to the unloaded condition. With the Bodypack it is very small (2 cm), showing that the center of gravity under load is very close to that without a pack on. In the Bodypack there was more weight in the pack than the front pockets. If the same weight was in front and behind, then there would have bene no displacement of the center of mass in the Bodypack. |
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Forward lean is determined by the slope of the terrain being covered and the center of mass. When standing still your posture is upright. Forward lean increases as you shift to walking downhill, it increases further when walking on level ground and is maximum when climbing.When you add a load, the forward lean in each situation is increased.
With a Bodypack the increase in forward lean is minimal: 4°-8° , due to a minimum displacement of the center of mass. This would have been eliminated if the load was fully counterbalanced.
With a backpack forward lean is substantial: between 14° and 26°. |
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The naturalness of your gait is indicatedby horizontal ground reaction forces.
The increased forward lean under load alters walking patterns. Horizontal ground reaction forces measures the forces on a ground plate as we step onto the plate (the braking phase), and as we step off it (the accelerating phase). Whilst there is not much difference between a Bodypack and a backpack in the braking phase, the Bodypack produces smaller forces than the backpack in the accelerating phase, closer to that of unloaded walking. This suggests that the gait is more natural with a Bodypack than with a backpack. |
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How load carrying affects the body is measured by physiological variables.
This graph summarizes the percentage differences between the Bodypack and the backpack downhill and uphill. The Bodypack comes out favourably throughout, indicating that the Bodypack minimises strain on the body under load. The minor exception is RPE walking uphill. RPE is a subjective measure and is the result of subjects noticing a greater contrast in exertion walking uphill with the Bodypack compared to level ground. Objectively, less energy is being used.
Minute ventilation: measures the volume of air moving through the lungs in one minute.
Tidal volume: measures the volume of air in each breath.
RPE: rating of perceived exertion. This is a subjective measure.
% VO2 max: measures energy use.
RER: respiratory exchange ratio measures the ratio of O2 consumed to CO2 expired.
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Energy expenditure.
This graph shows that the energy necessary to carry a given weight is consistently less when carrying a Bodypack than a backpack, both on level ground and uphill. This means that a Bodypack is a more efficient way to carry load on the human body, implying that the Bodypack is the optimum load carrying system.
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Comfort/Fit
This graph compares the number of subjects in the study who had a preference in terms of comfort and fit for either a Bodypack, a backpack, or no preference.
*Fit is a measure of how closely the harness matches your body shape.
*Comfort measures a combination of the fit, the amount of forward lean produced by the pack, and the ease of movement.
There is a striking preference for the Bodypack over the backpack for both comfort and fit.
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Balance
Balance is affected by the position of the center of mass and the resulting forward lean, by your ability to move freely to maintain balance, and to the fit and stability of the pack on your body.
Both downhill and uphill, the Bodypack was found to give far better balance than the backpack.This implies greater safety in rough and steep terrain with a Bodypack than a backpack. |
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Pack stability
Pack stability is determined by how close the center of gravity of the load is to your body, and by the design of the pack harness. The Bodypack brings the center of gravity of the load close to the middle of the body, and the Flow Motion harness systems allow free body movement. This results in maximum stability both uphill and downhill.
The backpack with its fixed harness is dramatically less stable—to move your body significantly you have to move the whole pack, which in turn affects your balance. |
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Body discomfort
This graph compares the % of subjects experiencing pain in different parts of the body under heavy loads of 22.5 kg. Discomfort and pain was significant in all major body areas with the backpack. Pain was dramatically reduced in the back with the Bodypack and eliminated in the neck, shoulders and thighs. This is a very important finding. It shows that virtually all the discomfort and pain associated with carrying a load in a backpack is due to the forward lean, and not the weight itself.
Balancing a load at the front and back in a Bodypack eliminates virtually all body pain, even with heavy loads. |
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A SUMMARY OF RESEARCH ON LOAD CARRYING SYSTEMS
A number of studies have compared energy use between different backpack designs:
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 . The energy requirement for the backpack with counterbalancing pockets was more than the unloaded state.
Heglund N.C., Willems P.A., Penta M.C. & Cavarna G.A. (1995) showed that 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 has 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.
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PROPOSAL FOR A NEW STUDY
The purpose of this study is to examine the premise that for a load carrying system to be the most comfortable, pain free, energy efficient, and stable, it must have the following characteristics; it will not alter the natural posture or the balance of the wearer, it will minimally disturb the movement of the wearer's body and it will spread the load primarily onto the hips. A carrying system with these characteristics will be compared with a backpack with the same load and volume and to the unloaded state. The parameters considered are energy consumption; discomfort in neck, shoulders, back and thighs; freedom of movement of shoulders, hips, waist and back; pack stability during these movements; and balance.
In this study a new load carrying system which we will call a Flowmo Bodypack will be used with the following characteristics:
A pair of pockets in front of the wearer is attached to the hipbelt and shoulder straps of a special backpack. The gap between the pockets allows the wearers to see their feet as they walk. The connection of the pockets to the shoulder straps is a sliding connection so that the weight in the pockets is transmitted to the front of the hipbelt via a frame inside each pocket. The weicht in the pockets thus balances the weight on the back of the hipbelt from the pack. The majority of load will be carried on the hips rather than the shoulders. The pockets will be slightly smaller in volume than the backpack but heavier items will be placed in the front pockets so that there is no forward lean induced by the load. In addition, the heavier items will be placed low so that the center of gravity of the load can be matched to the center of gravity of the wearer's body in both the horizontal and vertical planes. Thus the wearer's posture and balance will not be altered by the load and this will be considered to be a body centered load carrying system. The harness system of the backpack will consist of interlinked parts that slide or pivot so that the wearer can move shoulders, hips and waist without restriction. The attachment system of the pockets will not hinder body movement but will stabilize the pockets against their own unnecessary movement. The front pockets allow access to items without stopping and the Flowmo Bodypack is considered a practical way to carry load on uneven terrain, in windy conditions, in bush, or during dynamic movement.
The backpack will be a highly rated design with a similar capacity. The weight of the pack may be lighter than the Flowmo Bodypack and any weight differences will be taken into account in energy use measurements. The same items and weight will be loaded in the backpack and the Flowmo Bodypack for the comparison. The weight of the subjects will be measured and the weight of the load will be a fixed proportion of their bodyweight and no more than 20%.
The aim of the study will be to further substantiate the directions indicated by previous research on a counter balanced load carrying system, this time using a body centered load carrying system. A summary is given below of the main principles (navy), the system components (red) and findings and benefits (grey).
1. BALANCE THE LOAD FRONT AND BACK
Free vision dual compartment framed body centered load carrying system.
* Eliminates virtually all body pain even with heavy loads
* 100% weight-free shoulders possible
* Less energy needed to carry a given weight
* Balance unaffected by the load
* Instant access to items on the move
2. ALLOW FREE-FLOWING BODY MOVEMENT
U, V, X, Free, Multi, Flexi & Omni Flow systems
* Performance in high motion sports not compromised by the pack
* Unrestricted agility of the body
* Unprecedented stability of the load
* Enhanced ability to maintain balance
3. MAKE CONTACT PARTS A MIRROR-IMAGE OF BODY SIZE AND SHAPE
Auto Flex, U Lite & Custom Mould Frames; Auto Mould shoulder straps; Dual Adjust Wave-form Hipbelt
* Perfect fit
* No pressure points
4. LIGHTEN THE WEIGHT
Minimalist design without compromising biomechanics
* Lighten the pack and its contents
* When load is balanced front and back energy needed is proportional
to weight carried
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REFERENCE
Basnyat B. & Schepens B. The burden of the Himalayan porter (2001) High altitude medecine & biology 2: 315-316
Bastien G., Heglund P.A., Schepens B. & Heglund N.C. (2001) No free load for porters in Nepal Arch. Physiol. and Biochem, 109: S121
Bastien G.J., Schepens B., Willems P.A. & Heglund N.C. (2005) Energetics of load carrying in Nepalese porters. Science, 308: 1755
Bastien G.J., Willems P.A., Schepens B. & Heglund N.C. (2005) Effect of load and speed on the energetic cost of human walking. Eur J Appl. Physiol., 94: 76-83
Cavagna G.A., Willems P.A., Legramandai M.A. & Heglund N.C. (2002) Pendular energy transduction within the step in human walking. J. exp. Biol. 205: 3413-3422
Heglund N.C., Willems P.A., Penta M.C. & Cavagna G.A. (1995) Energy-saving gait mechanics with head-supported loads. Nature, 375: 52-55
Kirk, J. and Schneider, D. (1992).” Physiological and perceptual responses to load carrying in female subjects using internal and external frame backpacks.”Ergonomics, 35, 445-455.
Lloyd, R. (1997).“ A comparison of load carriage using two different rucksac designs with unloaded walking”. MSc Thesis, Leeds Metropolitan University, UK.
Lloyd, R. and Cooke, C. (2000). “The oxygen consumption associated with unloaded walking and
load carriage with two different backpack designs”. European Journal of Applied Physiology, 81, 486-492
Read, S.A., Whiteside, R.A. ”Biomechanical assessment of lateral stiffness elements in the suspension system of a rucksack”. Ergonomics Research Group, Queens University, Kingston, Ontario, Canada
Schepens B., Basnyat B., Willems P.A. & Heglund N.C. (2001) Measurement of the loads carried by porters in Nepal Arch. Physiol. and Biochem, 109: S72
Young, C. (2001). “A comparison between two backpacks with various loads on energy expenditure of experienced male backpackers” MSc. Thesis, University of Wisconsin, La Crosse, USA.
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