by Jordan Smothermon
As we look deeper into the physiological demands and effects of rucking, questions have guided us to interesting answers. As you might imagine, when questions are answered, they lead to more questions. The second and third generations of questions further focus our attention on narrower, more novel queries. In exploring these questions, here are five useful pieces of info you can use. These can be applied to loaded running as well.
- One pound on your feet equals five pounds on your back. (1 lb. feet = 5 lb. back)
- One pound on your feet equals five percent more energy expended. (lb. ft = 5% more energy)
- Every one percent of your body weight makes you six seconds slower per mile. (1% BW = +6 sec./mi.)
- Ten percent grade incline cuts your speed in half. (10% uphill = 1/2 speed)
- Going up slows you down twice as much as going down speeds you up. Every percent of incline adds 15 seconds to your mile pace. Every percent of decline takes 8 sec. off of your mile pace. (Uphill = +15 sec./mi.; Downhill = -8 sec./mi.)
This old adage from mountaineers holds true, according to a 1984 study from the U.S. Army Research Institute. They tested how much more energy was expended with different footwear (boots and shoes) and concluded that it take 4.7 to 6.4 times as much energy to move at a given pace when weight is carried on the shoe versus on the torso. Of course, this finding came forty years after the commonly accepted genesis of this nugget, Edmund Hilary’s 1953 ascent of Mount Everest.In practical terms, this means you could carry half a gallon more of water (a little over 4 pounds) if you buy boots that are a pound lighter, which isn’t hard to do; and that’s a lot of water.
2. One pound on your feet equals five percent more energy expended.
Heavier boots don’t just affect you because of their weight. They’re stiffer and less responsive as well. This reduces the efficiency of your body’s stretch reflex on hitting the ground.Five percent doesn’t sound like much, though, so how does five percent translate to run times? Well, five percent would slow your mile pace time down by 30 seconds, depending on how long you’re running. But, the faster you attempt to run, the more than five percent will affect your performance.
3. Every one percent of your body weight makes you six seconds slower per mile.
Carrying weight on your torso isn’t free of cost, though. As the thumb rule states, by adding 1% of extra load to your body weight makes you 6 seconds slower per mile. The average fighting load for a regiment is 35% (about 63 lbs.), which means a soldier’s mobility is hampered by 3.5 minutes per mile. If you work that backwards, that means a 12 second slower 100m sprint, which could double a fast soldier’s 100m time (of course, this math assumes a linear relationship mile times, which isn’t truly realistic; the broader point is that no matter how fast you’re moving, you’ll be moving slower. And that the faster you want to move, the more you’ll be impacted).
4. Ten percent grade incline cuts your speed in half.
Grade greatly affects speed. By “grade” we mean how much terrain incline or decline there is. Percent grade is a term that describes how much you go up versus how much you go forward. 10%, for example, means that for every 10 ft you travel forward, you’ll travel 1 ft. up. In terms of angles, 10% equals 5.74 degrees. A 5.74-degree angle doesn’t seem like much until you’re humping up it mile after mile. You’ll know how hard it is because you’ll move twice as slow over it than over flat ground with a given load. That last little part – with a given load – is important. A 10% grade will cut your speed in half no matter if you’re carrying 45 lbs. or 80 lbs. This, perhaps, has training implications. Let’s say you know you have to make a ruck for time over hilly terrain. You find the average grade of the ruck terrain is 10%, but don’t have the same terrain available to you – let’s say you live in Florida (flat) but have to ruck in the mountains of North Carolina. Given the load you’ll carry, you can find out what your pace must be over flat ground to roughly equal the time over the average grade. So, if you have to make 2.5 miles per hour over the 10% terrain, you could train at 5 miles per hour on flat terrain. Now, again, the math here assumes a linear relationship between grade and speed, which most likely isn’t true, but using the thumb rule will ensure your training will provide a proper time buffer.
5. Going up slows you down twice as much as going down speeds you up.
Don’t believe you’ll make time up on the other side of the hill. You won’t. You’ll only make half the time up. Famous running coach Jack Daniels provides numbers to this relationship, saying that uphills will add 12-15 seconds to your mile time and take off 8 seconds from your mile time on downhills. Another run guru, John Kellogg, says that for every 10 ft. of elevation change (up or down) changes your run time by 1.74 seconds, which, of course, is confusing as hell stated as an English sentence. Maybe it’s easier to think of it like this: if your uphill is 100 ft. uphill, you’ll run it 18 seconds slower than you would over flat ground. Why don’t you gain as much by running downhill as you lose running up? Braking forces. As you descend, you have to break your speed with your quads to keep yourself under control. The steeper the downhill, the more braking. This added load on your muscles further affects your uphill performance if you have repeated bouts of up and down work.
- The energy cost and heart-rate response of trained and untrained subjects walking and running in shoes and boots by Bruce H Jones, Michael M. Toner, William L. Daniels och Joseph J. Knapik. US Army Research Institute of Environmental Medicine, Natick, Massachusetts, USA. Publicerad i Ergonomics1984, vol. 27, No.8, 805-902.
- Energy cost of backpacking in heavy boots by S. J. Legg and A. Mahanty, Army Personnel Research Establishment, Farnborugh, Hants, England. Publicerad i Ergonomics, 1986. Vol. 29, No. 3.
- Paavolainen, L.; Nummela, A.; Rusko, H., Muscle power factors and VO2 max as determinants of horizontal and uphill running performance. Scandanavian Journal of Medicine & Science in Sports 2000, 10 (5), 286-291.
- 4Gottschall, J. S.; Kram, R., Ground reaction forces during downhill and uphill running. Journal of Biomechanics 2005, 38 (3), 445-452.
- The Modern Warrior’s Combat Load – Dismounted Operations in Afghanistan. U.S. Army Center for Army Lessons Learned. April – May 2003. pg. i-115.
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