Ultralight Load Carrying Theory

Last year I made an ultralight pack based loosely on a GoLite Jam. Although it was lighter and although it had internal stays it didn't carry all that well. In fact carrying it day-in-day-out proved to be quite wearing. This was a bit of a puzzle. My peak load was under 20lbs and was more typically 15lbs. That's not a lot of weight and is less than 10% of my body-mass. Why was it so uncomfortable? Why was it so tiring? Why did it throw me off balance?

I began to wonder if I had been riding the wrong horse. What if the real issue wasn't weight but weight distribution? What if the perfect backpack was not a backpack at all?

I started by looking at how loads are carried around the world. Many Africans carry large loads on their heads. Many soldiers don't wear backpacks for short sorties - and they are still carrying more than 20lbs BEFORE body armour. I began to look at how loads were distributed and what effect this might have on centre of gravity.

Fundamentally, I began to wonder if a lot of problems with load-carrying were in fact that my centre of gravity (CoG) was being thrown too far out from it's natural place. I thought that maybe the need for boots and occasionally a trekking pole was compensating for a lack of balance.

I started to look at how soldiers moved when carrying their loads - did it affect how they moved across terrain. I looked at Sherpas and others - how did their load distribution affect things.

I thought that maybe the perfect way to carry load would leave my CoG unchanged. My theory was that if the CoG was unchanged by carrying a load then my feet, ankles and in fact the rest of my body would continue to work efficiently.

I was very interested in adapting my load-carrying in such a way that my body and in particular legs and ankles were working as if a load was not being carried.

In order to understand all of this I looked at how my pack was typically loaded and what the alternatives might be.

Before

My UL pack is a typical pear-shape with two pouches. I looked in detail at what was carried where for a total weight of 9Kg.

The sleeping bag was at the bottom - which was good and light. The two pockets each had a 1L water bottle in it - 1Kg each. The rear pocket - a bullfrog tended to be stuffed with all sorts of things - food, GPS, coat and so on. Even more weight.

In fact with the shape of the pack - a bloated pear - what was happening was lots of heavy things were ending up at the bottom of the pack and very little weight was above my physical CoG. More critically most of the weight was torquing my hips and pushing my pelvis out of alignment which in turn was changing the weight transfer through my ankles and causing my feet to slide forward slightly in my boots - I was off-balance.

Experimentation

I looked at the classic rigid-framed army packs like this one

I bought one from the Army Surplus for £5 and looked at how it distributed and transferred weight. It was radically different in how it was loaded and how that load was transferred through the body.

I was so surprised by how well it carried an UL load that I had to stop and question what I knew about load carrying. I decided to go back a step - to review the biomechanics of how load was being transferred.

I built an experimental kit - some bathroom towels, some free weights (water and steel) and pieces of string.

The first thing I did was tie the weights onto different parts of my body. If I hung 10lbs from my hip belt how did it feel? What about from the shoulders?

I also tried hanging weights inside a pack at different positions - near to the top, near to the bottom, near the back, far away from the back. I also tried the same thing with a frameless pack.

I was very interested in distributing the weight so that it felt 'light'. I was also interested in how the weight was transferred into my body. Finally, of course, I wanted to distribute the weight in such a way that it had no effect on my CoG and no effect on how load was transferred through my feet. I figured that the lightest way to carry a given weight would require me to not compensate for carrying it.

The Science Bit

Let's look at a few things, centre of gravity, torque, and load transfer.

Centre of Gravity

On average a typical man's CoG is just below the navel somewhere around the stomach. All the movement of your body is defined by how this CoG moves around. The human body is not very stable in some directions - you don't have to lean too far back before you topple over. A martial art such as Judo relies on CoG manipulation to throw people. If you try Judo you'll soon discover how critical maintaining balance is and how easy it is to impact it.

Wearing a pack can have a big impact on where this CoG moves to and your body has to compensate for it. If it doesn't you fall over.

Consider the typical heavy backpacker. They are bent over double with a huge pack on their back. They have to shift their CoG to be back over their feet

Torque

This is one that you never read about but has a huge impact on your musculature. Wherever a pack or any other load pushes against your body your body will push back. This requires the muscles to work and requires energy. If the load transfer is pushing you out of balance your body HAS to compensate - otherwise you fall over!!!!

Here is an experiment - go into the kitchen and grab a tray. Place a [plastic] litre bottle of water on the tray - firstly close to your body and then far away from your body. Each time hold the tray very near your body.

You soon discover that of course having a bottle of water near to your body feels a lot lighter than if it is far away from your body.

The Human body is relatively thin front-to-back. Even though I'm not the thinnest of guys I'm still only around one foot thick. So, my centre-of-gravity is probably about 6 inches inside my body at most. In fact it's probably just behind my stomach and close to my spine.

This picture (not me) shows how a typical backpack is quite wide when compared with the human body. You can see that even the small 20L pack worn on the front is about the same thickness as the bloke wearing it.

If you look at how a typical frameless pack provides a frame it's by shoving a thick foam mat down the back. This might be stiff but it shifts the weight away from your body by maybe 3-4 inches. That is quite a lot if you think that your centre of gravity only started around 3-4 inches inside of your body (from the view of your back). Although the weight hasn't changed the perceived weight - and the torque it applies has increased significantly. It feels heavier and your body works harder.

This picture shows three typical packs - an alpine pack, an alpine pack that's further out from the body because of a foam-mat frame and finally a traditional fat pack.

This picture assumes that the pack is rigid. Very few packs are. Moving the pack away from the body is just the same as moving a bottle around on a tray - it increases the torque on your body and increases the perceived weight. The increase is in proportion to the distance moved.

Now let's look at the difference between framed and frameless packs in terms of torque

On the left we have 3 bookshelves on a piece of string. When the string is hung from the top you can see the shelves torque and push to the left - pushing your spine out of alignment. Next we have a hip belt scenario - the bottom shelf is resting on something. Here you can see that the effect is now that the top and bottom are working against you. The hip belt is pushing the pelvis quite strongly out of alignment and the holders are being pulled back. All of these things have to be compensated for.

Finally there is the picture on the far right - 3 bookshelves are done as racking. Here is the big difference - there is no torque being applied anywhere but at the top. More critically, this torque will be minimal and evenly distributed.

Imagine that you then put those three weights onto your back - firstly as three sacks of sugar tied together and then as three bags of sugar taped to a plank - the torque applied to your body would be totally different even though the weight is the same.

If this ideal picture is now transferred into the real world of frameless and framed packs then the main difference becomes apparent. The main difference between framed and frameless packs is how they torque the body. The framed pack applies very little torque.

This seems to be where the difference lies as far as comfort goes. The frame puts very little stress on your body. If you put a 20lb weight into the bottom of a framed pack it doesn't feel all that different to if you'd put it at the top - the torque is more or less the same. If you do the same with a frameless pack there is a massive difference - at the top it's pulling on your shoulders and at the bottom it's pulling on your pelvis.

So, even though you have two packs - one a light frameless pack and one a heavy framed pack it is in fact the framed pack that puts the least stress on your body.

Of course, we've almost all grown up with frameless packs. We have built compensatory musculature that help to overcome the difference. It cannot however disguise how it is affecting your balance.

This brings us onto the next aspect.

Weight Distribution

For a frameless pack changing the weight distribution has two effects - the one is torque and the other is the pack's CoG. This is in fact less critical than torque.

The common designs for a frameless pack are alpine and teardrop and the typical way to load it is weight-in-centre. That's the theory. In practice water often goes in side-pockets - shifting weight down, or in a bladder. Frameless packs tend to be narrow and thick. This shifts the weight away from the body, increasing the torque, increasing the perceived weight and increasing the shift of the CoG. All of these are bad things.

In contrast, a common design for a framed pack is a wedge of cheese on a stick - most of the weight is carried high and close to the body. The effect of this is that the pack's Cog ends up near the middle of your back and close to your own CoG.

This diagram shows how the shape of the pack and its position has an impact on your centre of gravity. Remember that unless your new CoG is roughly over where it was before you will be off-balance and may well fall - backwards usually.

From the pictures you can see that the big thing about the Alpine pack and the Framed pack is that although the CoG moves up it doesn't move out much - and so you need to lean forward by only a small amount to compensate.

The Teardrop pack shows a big problem - it shifts the CoG away from the body. This may seem surprising but it's quite difficult to pack a Teardrop shape to prevent this.

It's worth looking quickly at non-pack alternatives.- as used by many armed forces around the world. The options are things like a Load-bearing Vest and a load-bearing hip belt.

You can try them for yourselves with a fishing vest and some free weights and a good leather belt and some free weights tied onto it.

A load-bearing vest works very well for small heavy things - but an UL load consists of bulky things. A load-bearing belt has similar problems. I made one and even with just a 20L capacity it was already difficult to find space to swing my arms and there were issues with dynamic loading that made me think again.

There is another interesting twist on the Load-bearing Vest - it doesn't take much weight on the front of your chest to have a big effect on your CoG and the load transfer through your feet. This suggests that the CoG is quite close to the spine and so weight on the chest is providing a disproportionate shift of CoG.

If you think of a large and small kid on a See-saw you'll know how sensitive the CoG can be..

Shock-Loading / Dynamic Loading

This is the last significant thing and it is really interesting. When you are walking you are constantly moving your CoG and are constantly shock-loading your body. Every step that you take is putting a shock-load on your body as you stop yourself falling over. If you are wearing any extra weight - such as a pack then that weight is falling and then stopped continually. That's adding shock-loading to your body.

Let's look at some common scenarios:

1. LBV - Body Armour with pockets
2. Diver's belt - or Yoke and Load-bearing-belt (LBB)
3. Frameless backpack
4. Framed backpack
5. Front and backpacks together.

1. Body Armour with pockets

The body Armour is rigid and so the bulk of the weight is carried by the shoulders. Most of the shock loading is transferred through the shoulders and the spine. This actually is very comfortable. In Afghanistan this seems to be the most popular way to carry compact weight - much to the annoyance of the guys who designed the body-armour to be NOT load-bearing.

2. Diver's Belt or Yoke and Load-Bearing Belt

A pure diver's belt with all the load carried on the pelvis is actually quite hard work. if you strap on a 10Kg diver's belt you'll soon realise. The shock is transferred straight to your pelvis and there is nothing to absorb the shock.

If you go for a LBB with a Yoke then things improve greatly. The Yoke carries some of the weight but most importantly it carries most of the shock-loading and transfers it through the shoulders and into the body. So the belt carries a high percentage of the weight and the shoulders most of the stress. This works reasonably well. It's a very compact way of carrying weight.

Both 1. and 2. tend to be used for tactical reasons - typically for urban warfare where the need to move in and out of tight spaces is vital. In Northern Ireland the British Army designed a belt,chest harness and yoke system just for urban warfare and in particular so that soldiers could jump out of APCs ready for action.

The only thing is, it's not very comfortable....

3. Frameless Pack

This is not very good for shock loading. If the pack has no hip-belt then the weight keeps slamming into your holders and your back. The problem is that it gets to shake a little as it does it. So there is quite a lot of shock. Try walking with three weights tied to a piece of string.....

If you are wearing a hip-belt then there is a double-shock load. First onto the rear of the pelvis, then onto the shoulders and finally onto the pelvis again - with the bounce. The loading onto the pelvis tends to be torquing the pelvis outwards.

The result is that a hip-belted frameless pack induces multiple shocks into your body. If you don't believe me go for a walk and pay very careful attention. If you double the weight of your pack it will become more obvious.

4. Framed backpack

Framed backpacks are pretty universal in how weight is transferred to the body - through a shoulder-blade pad, a hip pad and the shoulder straps. Where framed packs have hip belts that is often more about load stabilisation than load-transfer. For UL framed packs a hip-belt adds very little.

With a framed pack most of the shock-loading is through the shoulder-blades and the shoulders in that order. The shoulder-blades are a nice big muscle-group and transfer load through the spine.

In fact a framed backpack with soft pads provides very little shock-loading of the body.

5. Front and Backpacks together

This tends to transfer most of the shock-load to the shoulders. The problem is that there can be quite a bit of bounce front-to-back.

Putting it all together

Remember, even though we are only carrying light loads we are trying to minimise the stress on the body. So, we are trying to minimise the CoG changes, the torque on the body, and the shock-loading. If we minimise the impact on the body then this will mean that we will be doing the minimum amount of work and will be able to walk for longer - and that's what it's all about. Of course we also don't want a pack that weighs 5lbs....

CoG

To minimise CoG changes the weight needs to be as close to the body as possible. To retain balance any shift in CoG caused by the pack has to be compensated for - by leaning - so that the new CoG is at very least over the old one - since this is in turn over your feet.

Torque

A frame minimises torque. It doesn't matter what the frame is made of or whether it is inside or outside of the pack. The main thing is that it is stiff. I've made two frames - the first one was 100g and the second one was 80g. Both were made out of cheap stock aluminium and plastic. Nothing fancy, nothing heavy. Several backpack manufacturers use carbon stays but of course they are unbent.

The main thing that a frame does (and all external frames do) is transfer load onto the shoulder-blades and shoulders before the hips.

Weight Distribution

The pack should have most of it's weight near the shoulder blades - hanging off them with very little weight around the hips. The reason for this is so that the shoulder-blades and shoulders take a lot of weight and the weight has minimal impact on the hips. It also fits in nicely with the density of the body

The Pack

All of this research and experimentation was for a purpose - to work out what was good about an Army Surplus pack and how I could transfer it to an UL pack and packing strategy.

This is my latest pack. You'll notice that it has almost no hip-belt - just 25mm webbing. All the webbing does is stabilise the pack and keep it close to the body, This helps with shock-loading and CoG.

Working from first principles the pack has ended up being packed just like external framed packs have always been designed to carry weight. You will note how similar it looks to the army pack above.

It works incredibly well. The pack weighs only 500g and that includes some heavy features and fabric from features I didn't use. The internal contoured frame is a mere 90g and yet it has a massive impact on how the pack works.

The pack is wide and narrow. The base is 10cmx35cm and the top is 35cmx25cm. The pack is designed to be thin and tall. I tend to pack the pack in a certain way (below) and use a small chest pouch.

The one thing that is significant is that the pack has no effect on my sense of balance. I can wear trail-running shoes comfortably, I don't need a trekking pole and of course it doesn't feel like I'm carrying lots of weight. In fact it's always a surprise when I pick up the pack.

This picture shows how the pack is worn and how it affects CoG etc.

The pack is thin - even at its widest point. It's packed in such a way that all the heavy stuff is really close to the spine and near the top. This minimises the perceived weight of the pack and the torque applied to the body - in particular the hips.

The chest pouch carries navigation tools and snacks - no more than 500g typically. This weight has a disproportionate affect on the CoG because the human CoG is near the spine and so it is further away from this than the pack is.

Summary

Load-Carrying seems simple until you look at it in detail. In the UL community there is a strong focus on weight. This focus can lead to other factors being ignored. If your 20lb pack carries as if it were a 30lb pack then it is a waste of time. If however your 20lb pack carries as if it was a 10lb pack then a lot of benefit may be had.