While I concur with Cecil, it got me wondering why this question should have come up in the first place. [substitute “you” with "the original poster’s wife]
My guess is that when you’re walking downhill, you tend to let yourself “drop” for most of the step, turning gravitational potential energy into kinetic energy. When your foot lands on the next step down all this kinetic energy has to go somewhere, quite quickly (what isn’t wasted as heat/sound energy will be transferred to elastic potential energy in the tendons/ligaments*).
Although you will have to build up energy in the muscles to walk up a step, my guess is that this occurs over a somewhat longer time, so maybe you don’t feel it as much.
This effect would probably be more noticeable over short flights of steps - over longer flights, the increased energy requirement of climbing would make itself elevated in increased heart rate, breathing rate and muscle fatigue.
*Just to clarify, this energy won’t hang around long (it would be transformed to heat, most likely within the muscles) , so it is not like you’d actually be energized by walking downstairs (duh) - but it is used when running to conserve as much energy as possible by storing some and using it in the next stride.
This is probably an obvious point, but Cecil’s explanation hinges on the fact that there is a total energy gain in going upstairs i.e mass x distance x gravity.from which he concludes that it MUST use more energy to go up than down.
This is not necessarily so (although, admittedly, it’s probably so) It all depends on the efficiency of the body in performing the two activities. Muscles are less efficient at relaxing under load than contracting and their efficiency varies in inverse proportion to load.
If you compare two other situations - Cycling up a very gradual incline (say 100:1 gradient) there’s a potential anergy gain but it’s not too tiring. Now try running back down the slope in the same period of time. You’ll use far more energy because the body is so much less efficient than the body/bicycle system.
So it all depends on whether the potential energy gain involved in climbing the stairs, is enough to offset the less efficient performance of a human body walking down stairs.
I’ve hiked through the Grand Canyon many times, usually with a 30 lb pack. Most of the trails are between 8 and 15 miles top to bottom.
I prefer climbing out to descending. That jarring of going down is really tough by the end of a day. Climbing up is preferred by me as just a steady paced exercise.
It certainly seems more dangerous to go downhill, and it feels less comfortable because you’re spending your energy on maintaining your balance and such instead of moving yourself against gravity.
Sorry Dex, your column misses the mark almost completely.
The amount of energy it takes to raise a mass one foot is precisely the same as the amount of energy it takes to stop a mass that has dropped one foot. Your first para is bunk. You mock the “absorb the energy” phrase but it is precisely correct. Going down the stairs you need to absorb the energy you are gaining from descent. And your rubber ball analogy is a complete fail because you compare apples with oranges; your analogy leaves the ball pushed down the stairs moving rapidly, with no consideration of the energy required to stop it. Leaving aside elastic losses and air resistance, the amount of energy required to throw the ball up the stairs is precisely the amount of energy required to stop a ball moving fast as a consequence of falling down the stairs.
Your final paragraph again gets it wrong. You start out OK in that the questioner’s SO is (correctly) arguing that you have to exert force to prevent you from falling down the stairs with a splat. But then you go off the rails (or over the bannisters?). That force is quite obviously not the same as the force you need just to stand on your feet. To stand on your feet you just need to exert a force equivalent to your weight. To step down one step (starting and ending at rest) you have to exert a force equivalent to your weight and a force sufficient to decelerate you from the speed you gain from stepping down.
Your column is a solid F-.
The correct answer is that the questioner’s SO is wrong; assuming you start and finish at rest, it takes exactly the same amount of energy to go up stairs as down. However, going up the energy comes entirely from your muscles so it’s tiring. Going down exactly the same amount of energy has to be absorbed but it can be absorbed significantly by the inefficient elasticity of your soft tissue and bones, and not all by your muscles.
Think about this: step down from say a chair very slowly. You will really feel the burn in your muscles. What does that burn come from? Your muscles working hard to slow you down. You are clearly absorbing energy. Now get up onto the chair and jump down as rigidly as you can without breaking your knees. No muscle burn. Why not? The energy has been absorbed into the spring-like features of your legs.
Your logic here is solid, but what accounts, then, for the difference in stored potential energy at the top of the stairs versus the bottom? In order for the potential at the top to be greater, doesn’t the climber have to put in more energy?
Powers &8^]
I have never met a hiker who didn’t agree that coming down the mountain is much harder physically than going up the mountain. This is probably the reason for the original question - the mistaken belief that since coming down is harder, then it must expend more energy.
One reason descending is harder was alluded to in campp’s post - the effect of repetitive impact on various leg joints, particularly the knee. However, that’s just part of the problem.
The other issue is the fact that when climbing your quadriceps are undergoing concentric contractions - the muscles are getting shorter as they contract. When descending, your quadriceps are undergoing eccentric contractions - the muscles are lengthening as they contract. A better explanation (cite):
Eccentric contraction is much more likely to result in delayed onset muscle soreness, the pain you experience a day or two after hard exercise.
Yes. But when the climber descends he gets all that energy back and unless he somehow disperses that energy, at the bottom of the stairs he will have an amount of kinetic energy equal to that he put in to get up the stairs. He has to get rid of that energy somehow.
If you think about it as, say, a simple upward throw of a rock. Throwing the rock up takes energy. Now the rock is at the top of its arc and has (as you say) greater potential energy. Now the rock falls. When it reaches the point from which you threw it, it has lost all that potential energy, but (friction losses aside) has the same amount of kinetic energy as it had potential energy a few moments before.
No, there is no energy source to make it go up. Never said otherwise. However, your jab is instructive as to why I’m right. We are considering a situation where one is at the top of the stairs and what occurs when you go down.
A slinky on stairs is too hard to model because of its mechanical complexity. But to model the basic energy flows, consider a ball on a ramp with some sort of C shaped wall at the bottom so that if you start the ball at the top it will roll down the ramp, get turned around by the C shaped wall and go back up the ramp. Ignore friction for the moment.
Imagine you start your ball at the top. It will now roll down, and then back up. Where did the energy to get back up come from? It must have had that energy at the bottom, right? Where did that energy come from? From the fact it started at the top.
By Princhester’s logic, a ball should be as likely to go up a slope as down it, if it has started at the top first, such that it still has the energy it gained from coming down.
By **Gary Kumquat’s **logic, that energy magically disappears.
There’s several reasons I think. I agree with the reasons you give. But there’s at least two other things. Firstly, coming down is jarring, because (as I said above) you are absorbing energy in part using the inefficient elasticity of your soft tissue and bones, which is a technical way of saying you are whacking them repeatedly with a planet, which tends to hurt.
Secondly it’s mentally much harder work on rough ground, and it does actually take energy to keep your brain going. When you go up your eyes are perfectly positioned to see where you need to put your feet, your feet (with toes forward) are perfectly shaped for stepping onto flat spots etc. and if you trip or slip you will fall a short distance forward onto your hands. When you are coming down every single one of those advantages is turned to disadvantage. It’s harder to see where you are stepping, you have to find grip with your heels, and if you trip you’ll fall a long way forward and if you slip you will fall backward which can lead to a broken wrist.
Only if you consider negative numbers to be precisely the same as positive numbers. You do not expend energy at all to stop a mass. You absorb it, and absorbing energy is the opposite of expending it.
Agreed, I was lax with my terminology in that and a few other sentences. But given that immediately after the sentence you quote I said “You mock the “absorb the energy” phrase but it is precisely correct. Going down the stairs you need to absorb the energy…” I think it’s clear what I meant.
No, by my logic our tendons, ligaments and muscles all have a degree of elasticity that helps to absorb energy as we walk downhill, and in the same manner also allows to transform some of that absorbed energy into longitudinal energy.
Do you reckon you can run faster downhill or uphill? If the former, please advise why. If the latter, please try it out for yourself.
Yes, going down stairs you need to absorb energy, but so what? Absorbing energy does not itself require energy. Anything can do it-- Not just living things with muscles, but things like a stone, too.
If you absorb the energy by falling down the stairs, sure you aren’t expending energy (other than screaming). But if you are using your muscles to control your descent you are certainly expending energy - muscles don’t contract for free…
For that matter, your muscles expend energy just sitting in a chair. But it’s by no means the same amount of energy in each case. In all cases, your muscles are expending energy on basic metabolic processes to stay alive. When you’re walking on stairs, they’re also expending energy to keep you balanced and so forth. When you’re walking up stairs, they’re also expending energy to raise your mass.
When you are walking down the stairs, they are expending energy to lower your mass in a controlled fashion.
Consider a biceps curl. Clearly as you raise the weight your bicep is expending energy as it contracts. As you lower the weight slowly, your bicep is expending substantial energy keeping the weight from just dropping - muscle fibers are still contracting, even as the muscle elongates. This goes back to concentric vs. eccentric muscle contractions.
The amount of energy expended is possibly less, but it is still orders of magnitude greater than basal metabolism. Add the fact that eccentric contractions (quadriceps going downhill, biceps lowering the weight) are much more likely to result in delayed onset muscle soreness as well as the jarring on the body and the additional degree of concentration required when descending and it’s no wonder that someone could think that it takes more energy going down stairs than up.
I’m not saying that they are right, just that they are confusing higher strain on the body with a higher expenditure of energy.