The science behind the impact a heavy pack has on your lower back. 

Last summer while walking the South Island section of Te Araroa, I hit some hurdles. I have lower back issues sustained from an injury as a teenager. Apparently, I’m not alone ‒ more than 80 per cent of us experience lower back pain at some time in our lives. So Te Araroa was always going to be a challenge, but one I thought was manageable with careful planning, strength training, and good gear selection.

I had already walked the North Island with a pack weighing 10–12kg, and it went well, so after a short break I started the South Island. It felt good to be walking again with trail fitness from the North Island.

In the Richmond Range, to keep my pack as light as possible I opted for a food pick-up on day three of the nine-day section. With the extra load on board my pack weight ramped up to 15–16kg, which was a bit heavier than I’d intended but still manageable ‒ or so I thought. 

But by the time I exited at St Arnaud six days later, the familiar and unwelcome grumblings of back strain had appeared and I was forced off the trail to rehabilitate.

It was disappointing, and made me question whether I could handle the heavier packs needed for longer sections. 

During the downtime, armed with a digital scale, I took a critical look at the contents of my pack. I replaced some gear with lighter options, picked lower-weight dehydrated meals, and committed to carrying only enough water to get to the next hut or river rather than a standard two litres. My revised six-day pack (including food and water) now weighed 12kg rather than 15kg+, and with this I was able to complete the six-day Twizel to Lake Hāwea section without any problems.

Why was 3–4kg the difference between success and failure? I got curious and searched for answers in the published research on pack weight.

What I learned is that as your pack weight increases above 10% of bodyweight, there is a disproportionate increase in the compression of your lumbosacral spine; this effect intensifies when your pack weighs more than 20% of body weight (a range in which many trampers operate).

Two studies showed that, compared with no pack, a light backpack (10% of bodyweight) causes only a small (7%) increase in peak lumbosacral compression. This increases to 31% with a 20% bodyweight pack, and jumps to 64% with a 30% bodyweight pack. 

The reason for this disproportionate increase is that our spines are S-shaped rather than straight, so the downward load from our packs is not borne uniformly. The weight-bearing lower spine cops most of the load and mechanical stresses from our pack.

Crunching the numbers for a 65kg tramper like me was a eureka moment ‒ 20% of my bodyweight equals 13kg; 30% is 19.5kg. The 15–16kg pack that caused problems in the Richmond Range sits squarely in the danger zone where the spinal load starts to ramp up, whereas a 12kg pack causes less compression. 

The bottom line is that every extra kilogramme has implications for your lower back. Personally, I need to keep my pack below 20% bodyweight to avoid problems.

For those who are injury-free this may not matter too much. But if, like me, you wonder how much weight is too much for your next multi-day tramp, these figures may help with packing.