laden or unladen?
Steel plate armor seemed like a great idea to protect soldiers in the days before gun powder, but like anything else there are trade-offs between mobility and protection. An escalating race between armor and weapons brought about heavy suits that weighed thirty to fifty kilograms by the fifteen century. Breeding programs for larger cavalry horses became necessary, but finding much stronger people is problematic ... not to mention the user experience issues associated with trying to work in one of these contraptions.
Researchers at the University of Leeds were interested in precisely how much effort was required to move around in all of that steel kit and partnered with the Royal Armouries to study the impact using real people on an instrumented treadmill. (the paper is behind a paywall)
They found the cost of motion - the work required to move - of a person with a 110 pound metal suit increased by about 2.3 times over an unladen person. This turns out to be worse than the cost of motion increase for someone wearing a modern backpack of the same weight due to the distribution of the suit’s weight. The leg protection amounted to about 20 pounds and it was that swinging weight that contributed to the extra cost.
The studies were done on smooth treadmills at room temperature. Real world conditions that often involved poor terrain would make matters much worse. This led the authors to speculate on one of the more remarkable military upsets in history - the Battle of Agincourt - where Henry V with a small army of men mostly without armor defeated a much larger French army of heavily armored soldiers.
It happened the fields in the area had just been plowed for Winter wheat and heavy rain showers turned them into mud. The French had to move across the mud burning huge amounts of energy and tiring in the process - the cost of motion may have been well over four times as high as an unladen person moving through similar terrain. The asymmetrical use of heavy armor may have been the decisive factor in what should have been a huge loss for Henry. Shakespeare maintained Henry was a great orator who moved his troops to greatness, but such is the privilege of the storyteller.
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The cost of motion has been measured for numerous human activities. A nice way to think about this is to consider the power you need to perform an activity above your normal metabolic needs remembering that power is just the energy expended per unit time. The average person needs about 2,400 watt-hours of energy a day in the form of food (a bit shy of 2,100 Calories). This corresponds to an average power of 100 watts - you can think of yourself as an incandescent lightbulb equivalent. We aren’t terribly efficient at converting food into energy our bodies we can use, so we give off much of that in the form of heat.
Exercise adds to our input power requirement. I'm far from an athlete, but I row as a form of aerobic exercise. Rowing happens to be a nice way to illustrate power at a human level, so if you are willing suffer along I'll detail my exercise story.
My rowing machine is an ancient Concept2 Model B ergometer that measures and displays my real power output at a flywheel.1 When I started to get serious about rowing three years ago I was out of shape and could only manage about 100 watts for ten minutes or so, but with the help of an athlete friend, I've gone to well over an hour at an average power of 150 watts. This has been a big achievement for me, but is somewhat depressing when you think of the power requirements for stoves, hair dryers, furnace fans and air conditioners. The scale of human performance differs from the requirements of our modern lifestyles by about two orders of magnitude.
A rower in good shape turns food into the mechanical work with an efficiency of about 20 percent. At 150 output I need 750 watts coming from food. Since energy is conserved I'm generating a large amount of waste heat. Even in the Winter, when the basement temperature is under fifty Fahrenheit, I'm in a tshirt and cutoffs and go through a large amount of water.
If you are trying to lose weight human inefficiency is a good thing, although it isn't as good as some would like. A normal Snickers bar only supplies about about 325 watt-hours of energy. I need a bit more than two Snickers bars worth of energy for an hour’s worth of rowing. It is easy to scarf down a few Snickers, but difficult to do that much exercise. Exercise is good for you, but you have to do a large amount to lose weight.
This chart of my rowing is shows the mechanical work from rowing sessions over a month's time. The units of work are on the vertical axis and require a bit of explanation. I'll describe their somewhat messy history in the footnote as it illustrates how creating your own unit system can be ugly and how they can evolve over time.2 Use whatever tracking and motivation mechanism works for you.
It takes a lot of time and energy to exercise heavily for long periods. I rarely go over 90 minutes a day and can only justify the time by using my iPod to time shift some entertainment and news consumption. When Colleen was training she required north of 5,000 Calories, nearly 6,000 watt-hours of food, a day. Six kilowatt-hours of electricity goes for less than a dollar in most parts of the country. Sadly energy in the form of food is much more expensive, although a fine meal is much more pleasant than plugging your body into a wall socket.3
Colleen has a lot of explosive power and can easily develop 300 watts for periods of 15 minutes. Two hundred watts for ten minutes is very difficult for me and 220 is currently impossible. For very short periods she can burst to much higher levels probably over 700 watts (nearly a horsepower). She's entirely anaerobic doing this but her fast twitch muscles are not as good as my slow twitch for long periods at lower output levels. Our genetics can dictate the physical activities we are most likely to survive at intense levels. Most of us never approach limits so it usually doesn't matter, but it is important for elite athletes.
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1 The Concept2 is a serious piece of kit. Mine is over 20 years old and still is in great shape with a beautifully smooth and natural motion that give a great user experience. The brand is used by most NCAA Division I rowing programs as well as Olympic training centers. A home user is likely to have one for a lifetime and the company stocks spares for all of their older models. Buy a piece of high quality gear and you're more likely to use it. The current models are quieter than mine, but the basic mechanism is similar, so if you buy one it is likely to be useful for the rest of your life. The cost per year over a few decades is very low. A great company with a great product - I give them my highest recommendation.
2 I was thinking about putting a mark on a stud in the basement noting my progress - a nice visual goal I would see every session and a not so subtle hint. It occurred to us that a goal could be visualized by equating one minute of rowing at 100 watts of work with an inch. An hour of rowing would represent a five foot high mark and would be nicely visible on the beam. I met that goal and new goals changed to our respective heights - one minute of rowing at a 100 watt power level was still an inch. Slowly I began to regularly achieve the goals at a hundred watts, but I would be running out of room as the basement ceiling is only seven feet from the floor. It was also time to work on the power level of the workouts as that is a lot of time for an exercise session and she thought she could increase my power output. We added a literally off the basement chart's goal for days when the workout is going exceptionally well that represents her standing reach (both feet flat on the ground) of 101 inches. It is good to have something that always seems out of reach when you start, but there are times when you manage to make it.
The new power goals were increased to average to 125 watts and finally to 150 watts. Each "minute" now represents 150 watt-minutes of work by this crazy goal-driven history (hey - whatever works to help you visualize a goal). Rowing for 80 minutes represents 12,000 watt-minutes or 200 watt-hours of work . Divide that by my efficiency (about 0.2) and you get the food energy I "burned" - 1000 watt-hours or a kilowatt-hour in this case.
3 A curious thing to think about is the power transfer rate of an electric outlet. Most 110 volt outlets in the US are limited to about 1,800 watts of power. Transferring 2,400 watt-hours takes about 80 minutes at the limit the line can supply. We can eat the food equivalent in under ten minutes if we want. The chemical energy in our food is a relatively dense energy store. This is a serious problem for electric vehicles. The last mile of the infrastructure is not well matched to high densities of electric cars in residential areas.
I'm a runner and it is amazing the difference weight at the end of your legs makes in effort. Not to mention you are more likely to injure yourself.
Posted by: Jheri | 10/03/2011 at 07:31 AM