Can Gravity Propel Runners Forward?

[yoshason]

I am a competitive runner, who spends too much time on the Runner's World forums. From time to time questions arise involving the biomechanics of running, and the runners start debating running for and the effect gravity has on running. Specifically advocates of the POSE Running Method suggest that gravity can be used as the primary force for propelling you forward while running. Other of us physics lay people try and point out (possibly incorrectly) that gravity will just pull you toward the center of the Earth (straight down) but the POSE advocates point to the pogo stick as proof that gravity can indeed pull you forward.

A recent thread dealing with such issues is here. http://www.runnersworld.com/community/forums/runner-communities/elite-performance/still-want-argument-heel-striking/.14"
Mainly the last page or two are relevant. Seeing as no one commenting on the thread appears no all that much about physics, myself included I thought someone here might shed some light on the matter.

Thanks.

 

[Archosaur]

Wow, who knew running forums could be so heated? Haha.

The short answer is no. Because gravity is directed to the center of the earth, it does no "work" on an object that is moving along Earth's surface. To "do work" is another way of saying to "supply energy". Work is "force across distance" which means that, in order for work to be done, the force (or at least, a component of the force) must be directed along the path of motion.

That being said, running is essentially falling over and over again. During a fall, the contact force (which results from gravity) does have a component in the forward direction, however, the energy gained from this fall is lost when you pick yourself back up on your next stride.

So, ultimately, no. There is no net energy supplied to a runner by gravity.

 

[Danger]

Welcome to PF, Yoshason.
I'm afraid that I couldn't make it all the way through the first page of your linked thread; it was just too boring (except for the very nice picture). I don't know, therefore, whether this point was mentioned.
I submit that Earth-normal gravity is advantageous to running in a very indirect way. My reasoning is that our muscular-skeletal systems evolved to function efficiently in that environment and are optimized for it. I remember that Neil Armstrong and his peers found it more expedient to hop rather than walk on the moon in its 1/6^th g field.

 

[Archosaur]

I submit that Earth-normal gravity is advantageous to running in a very indirect way. My reasoning is that our muscular-skeletal systems evolved to function efficiently in that environment and are optimized for it.

I second this wholeheartedly. It reminds me of a commercial I saw today that says something to the effect of: "It's amazing how nature knows how to make food that's good for us".

 

[Danger]

"It's amazing how nature knows how to make food that's good for us".

I haven't seen that ad, but it certainly echoes the kind of crap that religious nuts try to use to "prove" Creationism.

 

[nasu]

During a fall, the contact force (which results from gravity) does have a component in the forward direction, however, the energy gained from this fall is lost when you pick yourself back up on your next stride.

The contact force is related to gravity but the forward component comes from interaction (friction type) with the contact surface. If one removes friction in conditions of unchanged gravity, walking and running become very difficult, if not impossible. The energy spent for walking or running on horizontal surfaces comes from the internal energy of the body. Even when walking on a horizontal surface the center of mass of the body goes up and down a little for every step. Part of the energy spent is used to lift the center of mass of the body against gravity, for every step. The energy does not go back into the system when the C of M is lowered (I mean into the chemical system "producing" mechanical energy in the organism).

 

[K^2]

In other words, you need normal force to run, and you need gravity to have normal force for more than one jump, but neither of these two are what pushes you forward.

 

[nazaretti]

Wow, who knew running forums could be so heated? Haha.

The short answer is no. Because gravity is directed to the center of the earth, it does no "work" on an object that is moving along Earth's surface. To "do work" is another way of saying to "supply energy". Work is "force across distance" which means that, in order for work to be done, the force (or at least, a component of the force) must be directed along the path of motion.

That being said, running is essentially falling over and over again. During a fall, the contact force (which results from gravity) does have a component in the forward direction, however, the energy gained from this fall is lost when you pick yourself back up on your next stride.

So, ultimately, no. There is no net energy supplied to a runner by gravity.

Archosaur,
So if you drop a pogo stick that is oriented at an angle to the vertical, will its rebound include a horizontal as well as a vertical component?

 

[stevenb]

... the runners start debating running for and the effect gravity has on running. Specifically advocates of the POSE Running Method suggest that gravity can be used as the primary force for propelling you forward while running. Other of us physics lay people try and point out (possibly incorrectly) that gravity will just pull you toward the center of the Earth (straight down) but the POSE advocates point to the pogo stick as proof that gravity can indeed pull you forward.

Sometimes disagreements result from poorly defined terms and poorly defined starting assumptions. If you don't define the conditions and if you don't define what you mean by "being propelled forward" then it's easy to get into disagreements.

Gravity can pull you forward, if you think about a ball rolling down a hill. Gravity can also pull you backward, if you think about trying to roll a ball up a hill. Even though gravity acts toward the center of the earth, the direction can be changed to forward or backward by objects that get in the way. In effect, the object and gravity create a net resultant force that is no longer completely down, but partially forward or backward. Even on flat ground, if you stand in place and lean forward, then you do fall forward due to gravity and friction. How that helps in running efficiency is probably something you can answer better from experience. As someone said above, walking/running is the state of falling forward, and as someone else said, the person still exerts a horizontal force against friction, otherwise one just slips and falls straight down. So, the case of flat ground is difficult to say without a clear definition of "propel", but a runner should easily be able to learn from experience whether their efficiency increases by trying to fall forward, and really it's efficiency that a runner cares about. Clearly, no energy is gained from gravity on flat ground, but efficiency might be gained in trying to fall forward due to optimizing the way the body interacts with the environment.

So, if you run on flat ground, you will get no energy boost from gravity and all supplied energy comes from the runner (i.e. food). In principle, no additional energy is required to keep moving forward on flat ground once you have used energy to accellerate up to pace, but of course there is outside friction force and internal energy loss in the body which implies that considerable energy is still required to run at constant speed on flat ground. Clearly, from experience, and basic physics, we know that more energy is required to run uphill, and less is required to run downhill ( within limits because too steep down can require extra effort to not get hurt).

Good runners more or less know (either consciously or unconsciously) how to nearly maximize their efficiency to help maximize speed and minimize energy consumption while running. Clearly, understanding how the body should best interact with the running surface and gravity is important. Also, the rules are bound to change if you change the strength of gravity (running on Earth, Mars or the moon etc), the angle of gravity (up- or down-hill) and the running surface (road, dirt, sand, puddles, ice etc).

Years ago I used to run 5 miles per day and was in very good physical condition as far as strength and endurance. Yet, I was not a great runner (6 min mile pace was the best I could do) because of my body type and my lack of interest in being efficient (it was just excercise and I wasn't competitive about it). However, I could keep pace with some very good runners if there was a very slight downgrade in the road. I would in fact lean slightly forward and could get into a very efficient running groove dispite my build (that was stockier than ideal for endurance running). So I do believe there is some benefit to the idea of using gravity as an aid. In principle, if you go downhill, there is some gravitational energy that can be captured, and we all know how much energy get's sapped running up hill. No amount of technique, or even mechanisms (such as bycycles, roller skates, skateboards etc.) can eliminate the need for a person to supply the energy to offset the potential energy gained by going uphill. This is why a good (in-shape) runner can easiliy pass an out of shape bicyclist going up a steep hill, and yet my grandma (God rest her) on a bike with training wheels would pass an olympic runner going down a steep hill. So, the idea that a human might be able to capture some small amount of gravitational energy (despite the fact that we have legs and not wheels) is not outlandish.

So my guess is as follows.

1. Uphill, you can't get propelled forward (meaning energy gain) by any means.

2. Downhill, you can possibly get propelled forward (meaning energy gain), depending on the conditions, if correct technique is used.

3. Flatground, you don't really get propelled forward (and there is not energy gain), but you might gain efficiency, and you might acutally feel as if you are getting propelled forward.

As far as your comment, "the POSE Running Method suggests that gravity can be used as the primary force for propelling you forward while running", I'm not familiar with the method and I'm not going to look it up, so I'm not responsible for the accuracy of the quote, but it's clear that gravity can not be the primary driving force in running, if "force" is meant to mean "the source of energy". Food is definitely the primary source of energy and muscle provides the drive force. As far as forces in the physics sense, there are 4 fundamental forces in nature and generally humans only interact with two of them; electromagnetic force, which is friction and human muscular force in this case, and gravitational force. So I would say that friction force, muscle force and gravity force are the primary forces of running, and all are equally important because you can't run if you are missing anyone of them. The primary energy source is the chemical energy of food, but some energy can be captured from gravity during periods of running downhill, at least in principle.

 

[Archosaur]

Archosaur,
So if you drop a pogo stick that is oriented at an angle to the vertical, will its rebound include a horizontal as well as a vertical component?

Absolutely. But, the key word is "drop". You dropped the pogo stick. The energy supplied by gravity is the force of gravity, multiplied by the distance of the fall. If you want to receive more energy from gravity, you have to pick it back up, which is to work against gravity, i.e. lose energy.

As a rule of thumb, you cannot have lost or gained net energy from gravity if your altitude has not changed.

A runner, running on flat ground, can't gain energy from gravity.

 

[nazaretti]

Absolutely. But, the key word is "drop". You dropped the pogo stick. The energy supplied by gravity is the force of gravity, multiplied by the distance of the fall. If you want to receive more energy from gravity, you have to pick it back up, which is to work against gravity, i.e. lose energy.

As a rule of thumb, you cannot have lost or gained net energy from gravity if your altitude has not changed.

A runner, running on flat ground, can't gain energy from gravity.

Thanks for the response.

Understood. If you look at the initial condition and final condition, no change in distance from the center of the Earth means no change in gravitational potential, no net gain of energy from gravity.

When you run, your center of mass oscillates up and down. It appears that your legs act as springs, capturing some of the energy supplied by gravity and returning it (at less than 100% efficiency) during the next rise phase of the stride. If your legs are efficient springs, I'd expect them to return a higher fraction of the gravitational energy than if they were inefficient springs. It seems like there is a net loss of gravitational energy in both cases, but less of a loss with the efficient spring.

As for the slanted pogo stick, runners are interested in moving horizontally. The slanted pogo stick moves (in part) horizontally, while the one dropped in a vertical orientation does not. While there is no net gain in energy from gravity, there does seem to be a transformation of the vertical force of gravity into a horizontal movement of the stick.

 

[Archosaur]

no change in distance from the center of the Earth means no change in gravitational potential, no net gain of energy from gravity.

Bingo

there does seem to be a transformation of the vertical force of gravity into a horizontal movement of the stick.

There is a force in the horizontal direction. You could say that it "comes from" gravity, but it's important (at least in this conversation) to note that it isn't a component gravity. It's friction, which is directly proportional to the force with which the stick is pressed into the ground.

To prove it's friction and not gravity, try pogo-sticking at an angle on a sheet of ice. :)

If it's weird to think of friction as a propelling force, a car's tires might be an easier example. How well does a car fair on a sheet of ice? Cars are ultimately propelled by the friction at the point of contact between the tires and the road.

 

[Dr Lots-o'watts]

Well clearly, there is energy to be gained if you don't oscillate up and down while running. One could claim this gain to be from gravity without being completely false, but it seems to me to be a bit misleading. Just keep your center of gravity at a constant distance from the floor, that is certainly crucial for performance.

Perhaps regular running is more like jumping and landing (up then down), while POSE running is more like falling forward than pushing yourself back up (down then up). Somewhere between the two, there should be a technique where you don't go neither up nor down, but straight ahead. This would seem to be the most efficient energy-wise, since no work is ever done against gravity - i.e. no vertical oscillation.

 

[nazaretti]

Bingo


There is a force in the horizontal direction. You could say that it "comes from" gravity, but it's important (at least in this conversation) to note that it isn't a component gravity. It's friction, which is directly proportional to the force with which the stick is pressed into the ground.

To prove it's friction and not gravity, try pogo-sticking at an angle on a sheet of ice. :)

If it's weird to think of friction as a propelling force, a car's tires might be an easier example. How well does a car fair on a sheet of ice? Cars are ultimately propelled by the friction at the point of contact between the tires and the road.

Friction, then, and gravity plays a vital role in creating the friction. Thanks for helping me understand all this!
By the way, you might enjoy this video of running on ice:

 

[sophiecentaur]

Friction, then, and gravity plays a vital role in creating the friction. Thanks for helping me understand all this!
By the way, you might enjoy this video of running on ice:

A good movie which shows the different technique needed when traction is very limited. He's all 'up and down' and not leaning forward as he would on a gripping surface. I wonder what was on the soles of his shoes. There must have been some grip or he wouldn't have been able to get going - he managed to stop pretty quickly at the end.