Long Distance Hiking Injuries, Part 2: Pack Weight

Be master of your petty annoyances and conserve your energies for the big, worthwhile things. It isn’t the mountain ahead that wears you out – it’s the grain of sand in your shoe.  -Robert W. Service (1874-1958)

There are a ton of claims floating around the long distance hiking community about how to stay injury-free on the trail and most make both theoretical and common sense.  Taking time to build up mileage should allow the body to adjust to long distances gradually.  Going through a warm-up routine at the start of the day is supposed to ready muscles and nerves for the day ahead.

Perhaps the most universal recommendation for injury prevention is the most basic and  common-sense of them all:  reduce pack weight.  Adherents to ultra-light backpacking techniques claim that lowering pack weight reduces the risk of injury, arguing that less weight in the pack equals less stress and strain on the body.  While this makes theoretical sense, is it actually true?

To date, very few studies have established a strong relationship between pack weight and hiker injury.   A 2011 study by Matthew Hamonko, MD and colleagues, published in the journal Wilderness and Environmental Medicine, examined over one thousand NOLS participants and found no relationship between pack weight and injury.  Unfortunately, however, the study adopted a narrow definition of injury.  To be counted in this study, an injury needed be classified as “reportable” by NOLS, which strictly limited the scope as follows:

“To be considered a reportable incident, an injury or illness must require more than simple first aid, need follow-up care or the use of prescription medications, interfere with active participation for 12 hours beginning the next day (lost-day case), require evacuation, or be considered a near miss (the incident could have resulted in an injury or illness but did not, eg, a “close call”).”

20 of the 26 injured individuals in the NOLS study required evacuation and the most common specific mechanism of injury was a fall.  The average pack weight of study participants was 49 pounds, which included 1L of water and a full supply of food.  Because the definition of injury in the study was limited and the NOLS participants’ average pack weight seemed excessive for a modern-day AT or PCT long distance hiker, the results can’t readily be generalized outside of the NOLS environment.

This post is the second of a three part-series about injuries to long distance hikers on the AT and PCT.  Part one described hiker injuries in general.  It established that the overall injury rate on the AT and PCT is 58.8% where injury is defined as “injury or more than a few days of aches and pains.”  Proportionally, AT hikers experience a greater number of knee injuries where PCT hikers experience a greater number of foot injuries, though foot and knee injuries are the top two injury locations for both trails.  Also, the data suggested that taking time off the trail to heal an injury may reduce the risk of having a long-lasting injury on a thru-hike.

This post looks exclusively at pack weight, hopefully shedding a little more light on the question “Does lowering pack weight actually reduce the risk of injury to long distance hikers?”  The question will be addressed utilizing the data from the 2013-14 AT and PCT long distance hiker survey completed by distancehiking.com.  Before diving into the data, a few definitions to take care of:

Injury means an actual injury or more than a few days of aches and pains.  (The rationale for this definition was covered in part one.)  Hikers were asked to identify the location of their most significant injury for this survey.

Pack Weight means the weight of a backpack including all gear, but without food and water (i.e., base weight).  Survey participants were asked to characterize their average base pack weight from a menu of options:  <10lb, 10-14lb, 15-19lb, 20-24lb, and >25lb.  Base weight was chosen in order to provide some semblance of “apples to apples” comparison; it tends to be a more known and consistent quantity amongst long distance hikers.  Although average actual pack weight would technically be a more accurate indicator, the ever-changing quantities of food and water carried on a day to day basis make even close estimation of this weight an impossibility for such a large, survey-based study.

AT and PCT data will be separated for this analysis.  The two trails are different distances and are just too dissimilar in terms of elevation gain, terrain, and climate.  The data includes 466 AT and 316 PCT thru hikers

Appalachian Trail

Injuries by Pack Weight: AT

 Injury = NoInjury = Yes
< 10lb711
10-14lb2842
15-19lb5885
20-24lb5291
>25lb3555

The table above contains the raw data from the survey.  At first glance, it appears as though there isn’t a strong case for base pack weight having any sort of association with overall injury to AT hikers.  Statistical analysis confirms this observation (chi square, p=0.965).  AT hikers across all base pack weight categories experienced injury at similar rates.  Here’s another way of visualizing the data:

Injury by Average base pack weight- AT

For those AT hikers who did experience injury, it’s interesting to see if pack weight had any relationship with injury location.  Looking at the top four injury locations: feet, ankles, lower leg, and knees, there are some interesting results.

Graph of injury location by pack weightA few numbers seem to pop out here.  Hikers who carried less than 10 pounds seemed to have proportionately more ankle injuries than any other pack weight category.  36% of of all injuries to hikers carrying <10 pounds were to the ankle.  Additionally, hikers carrying <10 pounds seemed to have proportionately fewer knee injuries.  Injured AT hikers in all the other pack weight categories were in the mid 20% – mid 30% range for the knee.  Only 9% of injuries for hikers carrying less than 10 pounds were to the knee.

Of these two observations, the only one that met statistical significance was the ankle injury observation.  The distribution of ankle injury incidence across pack weights for injured hikers was unevenly distributed (chi square, p=.032) and we can see that the under 10 pounds group had a much higher proportion.

Pacific Crest Trail

On the PCT, the injury vs. pack weight figures seem to be more skewed than on the AT.  The hikers carrying under 20 pounds of base weight seemed to fare better in the injury department.

Injuries by Pack Weight: PCT

 Injury = NoInjury = Yes
<10lb149
10-14lb4952
15-19lb5463
20-24lb1536
>25lb813

Although more skewed than the AT number, the PCT numbers were not quite skewed enough to reach statistical significance, just barely missing the mark at p=0.081 (chi square).

Graph of injury by pack weight PCTHowever, when the upper pack weight categories of 20-24lb and more than 25lb are combined into a single category, statistical significance is achieved at p=0.049.  This means that the injury distribution in the graph below represents an actual relationship between pack weight and injury that cannot be explained simply by chance.

graph of Injury by Average base pack weight PCT combinedWhat about the influence of pack weight on injury location for injured PCT hikers?

Injury Location by pack weight PCTUnlike with the AT, there were no specific areas for injured PCT hikers that were significantly influenced by pack weight.  This may be because foot injures so dominated all categories of pack weights that the smaller numbers in the other categories were not big enough to show a statistically significant association with pack weight.

Summary

It seems as though trail matters.  On the AT, there was not association between base pack weight and injury rates.  For those that did get injured, hikers carrying less than 10 pounds experienced higher ankle injury rates than hikers in other base pack weight categories.

On the PCT, there was a significant relationship between pack weight and injury rate when the upper pack weight categories were combined.  68% of hikers carrying 20 pounds and up experienced at least one injury, whereas hikers carrying 15-19 pounds were at 53.8%, 10-14 pounds at 51.4%, and hikers carrying <10 pounds were at 39.4%.

Now why would this be?  Why would pack weight matter on one trail, but not the other?  It’s speculation from this point, but the key, I believe, is either in the geologic and meteorologic differences between trails, the people who hike them, or both.  Both trails certainly have unique challenges, but the AT treadway is much more uneven and the climbs are steeper, so the physical challenges to the body as a whole may outweigh the influence of pack weight on injury on the AT.  Also, the AT is wetter, which can lead to more slip-and-fall-style accidents which could take place regardless of pack weight.  (Are heavier packers more likely to fall on slippery rocks?  Who knows!)

But here’s another thing to think about:  We’re mostly sure that the AT has more newcomers to long distance hiking than the PCT.  So what we’re seeing in the data might be the effect of inexperience, which might mask any effect of pack weight for AT hikers.  This would explain the equal injury rates amongst pack weight categories.  On the PCT, we actually might be closer to seeing the actual effect of pack weight, since hikers on this trail have more prior experience and so presumptively more know-how about injury prevention within their own style of hiking.  Could this be true?  Is there a difference in injury rate between experienced and inexperienced hikers?  Are there really more inexperienced hikers on the AT?

Here’s one more bit to chew on.  A subset of 29 would-be thru-hikers who dropped off the PCT due to injury (after mile 454) were asked to identify their average base pack weight.  Here’s how they were distributed:

Now this seems weird.  We just established that there is an association between pack weight and injury rate for PCT hikers.   Why were most of the injured hikers who dropped out due to injury in lower pack weight categories?  Would it make sense if I told you that 23 of these 29 hikers (79%) had no prior experience with long distance hiking?

I’ll try to wrap this all up in the third installation in this series of posts, coming as soon as I can sum up the right combination of coffee and lunacy to ignore the Maine summer outside my window and sit at the keyboard.

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