Running on the Space Station compared with running on Earth

[Frodo]

TL;DR Summary

Does running on a treadmill on the Space Station require broadly the same effort as running on earth?

The ISS includes a very fancy running machine which astronauts use to maintain fitness. Several astronauts have run Marathons at the same time as the earthly race (London, Boston etc) and, while there have been many press reports of these runs, none has given any indication as to how hard it is compared with running on earth.

I wondered how hard it is.

My first thoughts are that running on a treadmill on the Space Station requires broadly the same effort as running on earth. Running requires the body's centre of gravity to be raised with each step.

On earth, the runner exerts an upwards force with her leg which accelerates her CoG so that her CoG rises.

On the Space Station the runner also needs to "raise" - actually move - her CoG by the same amount and has therefore has to accelerate her mass. I think that the impulse [force x time] will be the same in both cases.

Let's ignore second order effects like air resistance, the use of elastic straps to bring her down on the Space Station compared with gravity on earth, and assume no energy is recovered from the "fall" so as to keep things simple.

All thoughts will be welcomed.

 

[mfb]

The strength of the elastic straps sets an effective gravitational acceleration, that will influence the effort. Weaker -> easier to run.
As secondary effect: I could imagine that running with these straps is awkward and makes it more difficult to run.

 

[Frodo]

Thank you. I can now answer my own question.

If the straps are adjusted so they pull the astronaut towards to treadmill with a force equal to her weight on Earth then effectively it requires the same effort to run on the Space Station as it does to run on earth.

When a runner pushes off with her foot she gives her body an impulse (force x time) which creates an equivalent change of momentum (mass x velocity). Hence her CoG starts moving upwards with a velocity v.

On earth, she starts rising at v, and gravity exerts a force equal to her weight which decelerates her CoG until she stops rising.

On the Space Station, she also starts "rising" at v, and her elastic straps pull her "down" with a force equal to her Earth weight. Her straps therefore decelerate her CoG exactly the same as gravity decelerates her CoG on earth.

Hence there is no difference in effort required between running on the Space Station and running on earth.

Several thoughts arise:

1. The straps are presumably pre-tensioned so even when she is standing still they pull her with her weight on earth

2. Each astronaut needs a different strap strength

3. If you want to simulate running on the moon all you need to do is adjust the straps to provide a force equal to her weight on the moon - ie about 1/6 of her weight on earth.

4. If you are running on the Space Station and you want to cheat when "competing" in an Earth Marathon just get your straps set to less than your body weight on earth. You will run a very fast Marathon and set a Personal Best you are unlikely ever to reproduce on earth.

 

[sophiecentaur]

As secondary effect: I could imagine that running with these straps is awkward and makes it more difficult to run.

With a network of straps anchored 'below' the body CM and symmetrically placed, I'm sure the astronaut would soon develop the skills to wear themselves out quite effectively. A lot easier to learn that than to learn skateboarding and surfing, I'd bet.

 

[russ_watters]

Hence there is no difference in effort required between running on the Space Station and running on earth.

I'm not sure that's quite true. A certain amount of the effort is also spent lifting your back leg off the ground to move it forward. That effort is decreased. What fraction of the effort that is I'm not sure, but I do know shoe companies go to great lengths to make their shoes light and conversely people sometimes wear ankle weights to increase effort while training.

 

[Grinkle]

I expect it is a very different training experience when done mile after mile.

Elastic straps are springs:

F =kX which is not well approximated by a constant force when running.

Gravity is:

F = GmM/R-squared which is well approximated by a constant force when running.

I expect that since humans evolved to run when opposed by an essentially constant downward force, running against an elastic force would be more difficult to maintain and give rise to a different variety of over-use injuries than running vs gravity.

This is not a running forum, but I hope it is not off-topic to offer this anecdote:

When I train on a treadmill my pace is enforced to be uniform over short time periods (until I change the speed of the belt). When I run outdoors my pace varies quite a bit over short time periods. I get different over-use injuries on the treadmill than I do running outdoors - I have done a great deal of both.

 

[DaveC426913]

Don't forget that humans have a strong tendency for optimizing their movements to expend minimum energy.
Astronauts have been running for 40 years before arriving on the space station. The movements are down pat.
Running with elastic bands will be awkward, and that will throw off their muscle rhythm. That's bound to add a goodly percentage of effort.

Example, as Russ points out: it takes no effort to lift their legs. It also take no effort to lift their arms. This may counter-intuitively take more effort, since they'll be awkward.

 

[berkeman]

Elastic straps are springs:

F =kX which is not well approximated by a constant force when running.

Gravity is:

F = GmM/R-squared which is well approximated by a constant force when running

Precisely. Elastic straps are a very poor approximation for running in gravity, IMO. You would think that rocket scientists (or student competitions) could come up with a better setup that does not add much weight to carry to orbit...

 

[mfb]

A certain amount of the effort is also spent lifting your back leg off the ground to move it forward. That effort is decreased.

On the other hand there is additional effort getting the legs back to the ground.

Elastic straps are a very poor approximation for running in gravity, IMO.

If the straps don't change their length much (and based on videos, they don't seem to do) that should not matter much.

 

[256bits]

When I train on a treadmill my pace is enforced to be uniform over short time periods (until I change the speed of the belt). When I run outdoors my pace varies quite a bit over short time periods. I get different over-use injuries on the treadmill than I do running outdoors - I have done a great deal of both.

When a person walks or runs on the Earth surface, they lean from the vertical.
Would you happen to know if when exercising on a motorized treadmill, does the body do a lean also?

I kindof of think that since the track is moving of its own accord for a motorized treadmill, one could be in a vertical position and just move their feet -

 

[A.T.]

I expect it is a very different training experience when done mile after mile.

Elastic straps are springs:

F =kX which is not well approximated by a constant force when running.

Gravity is:

F = GmM/R-squared which is well approximated by a constant force when running.

I expect that since humans evolved to run when opposed by an essentially constant downward force, running against an elastic force would be more difficult to maintain and give rise to a different variety of over-use injuries than running vs gravity.

The main difference is that gravity acts uniformly on the entire volume of the body, not just on some point or area.

 

[sophiecentaur]

The main difference is that gravity acts uniformly on the entire volume of the body, not just on some point or area.

That's amazing. The quote I lifted reads differently from what I pasted - "much bigger" changed to "main" in five seconds.
It's true that lifting your legs is a relevant force in the normal exercise of running so an authentic simulation would have to involve very stiff trousers, perhaps. Starched jeans?
But the cardio vascular system will still be exercised by the total work done and most of the power is used in lifting the body. In any case, what else can they do? A big hamster wheel like the one in Kubrick's 2001 film would be a possibility but why not the whole ship etc. etc.

 

[A.T.]

In any case, what else can they do?

I don't think they have to emulate exactly the same exercises as on Earth, because those are sometimes not perfect either. The question by the OP is about the differences, not which is better.

 

[Grinkle]

that should not matter much.

Not matter much with respect to what? EG:

add 'vs running with the same belt speed on an earthbound treadmill' to each of the 1-4 below -

1. Perceived effort during the run
2. Calories burnt per hour
3. Calories burnt per mile of belt movement
4. The runner thinking their overall training experience and outcome is the same

Other?

While I think 1 and 4 are going to be different, I'd be very interested to hear what an astronaut has to say on the matter. The OP's question might mean any of the above.

2 and 3 I think depend on the spring constant of the bands.

If the straps don't change their length much

I'm not sure about that. The ratio between the change in elastic force vs the change in gravitational force is essentially a division by zero, so the force change experienced by a runner in straps (per stride) is many orders of magnitude more than the force change experienced by a runner on the ground (per stride).

 

[Grinkle]

I kindof of think that since the track is moving of its own accord for a motorized treadmill, one could be in a vertical position and just move their feet -

I don't think that is a sound mental picture. The track moving relative to the floor is not relevant. To develop an intuition for this, I suggest you imagine a very long treadmill, like one sees at airports (remember them? ;-) ) with the moving sidewalks. Running on one of those is no different that running next to one of those with respect to the biomechanics. Any change in biomechanics comes from the need not to fall off the back of the belt and hence to regulate ones pace more strictly on a short treadmill than on a long treadmill. You might think of the Earth's latitudinal direction as a very long treadmill, if that is a helpful mental image for you. Its no easier to run west than east. One does not need to just moves ones feet in one direction but actually run in the other direction.

I lean just as much on a treadmill as I do not on a treadmill - I don't know how much that is.

Edit - it strikes me this is getting a bit OT. If you want to PM me I can point you to some good treadmill threads on these forums.

 

[Frodo]

May I pick up several points.

1. You will see that in Post #3 I reasoned that the elastic straps must be pre-tensioned to be equal to the runner's weight on earth. If the runner's CoG rises by, say, a few cm, the restraining force exerted by the elastic strap, which is about 1m long, will be constant to within a few percent.

2 Several have said "it takes no effort to lift their legs / arms". That is clearly wrong as, although the leg (arm) is weightless, it has a mass. The leg (arm) must be accelerated and decelerated both of which will require effort.

3. My question was "Does running on a treadmill on the Space Station require broadly the same effort as running on earth? ... Let's ignore second order effects".

The answer is "Yes - running on a treadmill on the Space Station requires broadly the same effort as running on Earth if the restraining straps are set to exert a force equal to the runner's weight on earth". I doubt that I am the only person to find that rather counter intuitive!

And, never forget, (and going off topic) that the strength of the Earth's gravity at the altitude of about 250 miles at which the Space Station orbits is 88% of the strength at the Earth's surface, or about 8.6m/sec^2. They are not weightless because they are a long way from Earth - they are weightless because they are in free fall. Gravity, like centrifugal force, is a Virtual Force which can be made to disappear by selecting an appropriate frame of reference. If you select your frame as one falling freely in a gravitational field you do not experience that gravitational field.

 

[Frodo]

The main difference is that gravity acts uniformly on the entire volume of the body, not just on some point or area

While that is correct if one delves into great detail Newton's Superb Theorem proved that for a solid spherical body in a uniform gravitational field the entire mass can be considered to be located at the CoG and all analyses will be correct.

So, in general, ignoring second order effects, I think we can conduct the analysis on the basis that the forces act on the CoG.

 

[DaveC426913]

While that is correct if one delves into great detail Newton's Superb Theorem proved that for a solid spherical body in a uniform gravitational field the entire mass can be considered to be located at the CoG and all analyses will be correct.

So, in general, ignoring second order effects, I think we can conduct the analysis on the basis that the forces act on the CoG.

I think you're missing the point. It's not about CoG, it's about apparent weight.

On the ISS, even though you're held to the floor by straps, your arms and legs have zero weight. It requires no effort to lift them.

 

[jbriggs444]

I think you're missing the point. It's not about CoG, it's about apparent weight.

On the ISS, even though you're held to the floor by straps, your arms and legs have zero weight. It requires no effort to lift them.

Reduced effort, not no effort. The pace imposes acceleration requirements.

 

[Frodo]

I think you're missing the point. It's not about CoG, it's about apparent weight.

On the ISS, even though you're held to the floor by straps, your arms and legs have zero weight. It requires no effort to lift them.

I completely disagree.

First, I agree that on the ISS your arms and legs have no weight - they are completely weightless.

But I completely disagree that "It requires no effort to lift them" (where lift should be in quotes as there is no up or down on the ISS).

Your leg (and arm) has mass and to move it, you must accelerate it, and to accelerate a mass you must apply a force as in F = ma. So it does require an effort to move, say, your leg, as you first accelerate it with a force, then decelerate it with a force to a stop, then accelerate it again and either decelerate it or wait till it hits the treadmill. Each acceleration requires a force act and hence an effort.

 

[A.T.]

I think we can conduct the analysis on the basis that the forces act on the CoG.

The forces of the straps? That's not even a good approximation for a rigid body, let alone for a dynamically moving human. If you want to analyse the differences in the training effect, you have look into biomechanics.

 

[Frodo]

The forces of the straps? That's not even a good approximation for a rigid body, let alone for a dynamically moving human. If you want to analyse the differences in the training effect, you have look into biomechanics.

Please remember the original question is "Does running on a treadmill on the Space Station require broadly the same effort as running on earth? ... Let's ignore second order effects" .

When her leg pushes on the treadmill she causes her body's CoG to "rise". That is the analysis where we can calculate on the basis of how high did her CoG "rise".

Similarly, when the straps apply a force to pull her down we can assume that that force causes her CoG to move and we can calculate on the basis of how far it moves and at what rate it accelerates.

 

[DaveC426913]

Of course I didn't literally mean 'no effort at all' - I simply meant 'only as much effort as is required to move a limb in micro-g'. I thought that might have gone without saying.

While of course your limbs have mass - they have no weight. The point remains - while you CoG is held to the floor via elastic bands, your limbs are free to move in micro-g.

OK, you're dismissing these as second-order effects. Granted.

 

[hutchphd]

In any case, what else can they do? A big hamster wheel like the one in Kubrick's 2001 film would be a possibility but why not the whole ship etc. etc.

Let us not forget the bounding hamsters on skylab...that looked like a bunch of fun...I wonder if they got any decent workout. Surely there is actual data for those activities. Of course they had more space than they knew what to do with but some activity requiring constant changes of velocity in free fall would be a lot more inviting (the oft-referenced "rubber room") . I guess hand injuries might be an issue.

Here's Pete Conrad:

 

[Frodo]

The point remains - while you[r] CoG is held to the floor via elastic bands ...

This may be just poor use of words but the runner's CoG is not held to the floor.

The whole point is that the runner's CoG goes up and down and, to a first approximation, that defines how much effort they expend.

The straps do not "hold your CoG to the floor". The straps apply a force "downwards" equal to your weight on earth. The force is constant (to a few percent) because they are pre-stretched to apply a force equal to the runner's weight on earth. That is they apply a force just like gravity does.

So, as you cause your CoG to rise you do work against the force of Earth's gravity on earth; and do work against the force of the straps pull (which is set to be equal to your Earth weight) in space.

Hence the work done is, broadly, exactly the same.

See The Physics Of Running which states

"As the runner runs along, his center of mass follows a parabolic arc ..."
"The force that the runner pushes off the ground with serves as an initial launch force which causes his center of mass to follow a parabolic arc, as predicted by Newton's second law ..."

So, I think it fair (in our first approximation) to use motion of the body's CoG to do our analysis.

 

[Frodo]

I thought I would search NASA (I searched with treadmill) and came across Biomechanics of Treadmill Locomotion on the International Space Station: Does gravity influence running biomechanics? as a starter.

Biomechanics of Treadmill Locomotion on the International Space Station: Does gravity influence running biomechanics? is also very interesting and says

Exercise prescriptions completed by International Space Station (ISS) crewmembers are typically based upon evidence obtained during ground-based investigations, with the assumption that the results of long-term training in weightlessness will be similar to that attained in normal gravity. Coupled with this supposition are the assumptions that exercise motions and external loading are also similar between gravitational environments ...

Cross-correlation analyses between gravitational conditions revealed highly consistent movement patterns at each joint. Peak correlation coefficients occurred at 0% phase, indicating there were no lags in movement timing. Joint ranges of motion were similar between gravitational conditions, with some slight differences between subjects. Motion patterns in weightlessness were highly consistent at a given speed with those occurring in 1G, indicating that despite differing sensory input, subjects maintain running kinematics.

It also says

The data suggest that individuals are capable of compensating for loss of limb weight when creating movement strategies.

which I presume means that runners were able to compensate for the fact that their arms and legs were not individually pulled "downwards" on the ISS in the way they are pulled downward by gravity on earth.

It seems there is very little difference between running on the ISS treadmill and running on earth.

Speaking as someone who has trained for a Marathon I am most impressed that an astronaught on the ISS is able to do the training there required to maintain fitness well enough to run 26 miles.

 

[A.T.]

It seems there is very little difference between running on the ISS treadmill and running on earth.

This is not quite what your links say. What they claim to be similar is merely the kinematics (movement pattern). But the ground reaction force and loading rate are different (per first link).

Note that the motion capture method used on the ISS was rather limited (a single video camera), and thus not able to capture kinematics in the frontal plane, and not as accurate the 3D motion capture used on the ground.

Also note that even with the same kinematics and ground reaction forces, the muscle activation can be very different, because of the redundancy of the muscles and antagonist muscle co-contraction.

Speaking as someone who has trained for a Marathon I am most impressed that an astronaught on the ISS is able to do the training there required to maintain fitness well enough to run 26 miles.

As I said before, the 0G training doesn't have to be identical to 1G training, in order to be effective.

 

[sophiecentaur]

That is they apply a force just like gravity does.

And don't we say that gravity holds you down on the ground? I think you may have been nit picking a bit.

 

[sophiecentaur]

I think you're missing the point. It's not about CoG, it's about apparent weight.

It's very dependent upon where the CM is. If the force doesn't act through the cm the person will be unbalanced and fall over. Balance is essential so, for stability, the force on the body needs to act at a point 'below' the person's cm.The acceleration of limbs that's required would be significant but you'd need to have lead boots and gloves to get the full benefit, I think.

The bounding hamsters video is entertaining but I have a feeling that the exercise is not well matched to the human body for actual useful exercise. The centripetal acceleration would be v²/ r and that would involve running faster than your little legs could carry you, if you want anything like g. Think of the speed that the Rotor fairground ride has to go - or cycles on the Wall of Death. They do need a magnitude of acceleration that's bit more than g but you cannot run around the wall of death - even with sticky shoes and a crazy angle of leaning.

 

[jbriggs444]

It's very dependent upon where the CM is. If the force doesn't act through the cm the person will be unbalanced and fall over.

On Earth, that is certainly so. Neglecting air resistance, you have to keep your feet under your center of gravity.

On a treadmill in space, you need to arrange for your contact force to be on a line that intersects with the anchor for the straps. Instead of moving the contact point relative to the center of gravity, you'll want to move the contact point relative to the strap anchor.

 

[sophiecentaur]

Neglecting air resistance, you have to keep your feet under your center of gravity.

I imagine that sort of skill is easy enough to acquire if you already have the Right Stuff. I quite recently used a treadmill for the first time (on Earth) and I had to hang on for a while before being sure of myself.

 

[jbriggs444]

I imagine that sort of skill is easy enough to acquire if you already have the Right Stuff. I quite recently used a treadmill for the first time (on Earth) and I had to hang on for a while before being sure of myself.

My brother, my sister and myself were allowed to play pretty much as we saw fit in the backyard using an array of stuff such as horizontal ladders, ropes, swings, spools, barrels, crates and tall trees. My parents believed, correctly, that we were aware of our limitations. One evening my dad had a bunch of twelve year olds over from the school where he worked. He had to put away the rope swing because we (several years younger than they) were all performing stunts too dangerous for them to safely imitate.

Sadly, age steals youthful competence.

 

[256bits]

On Earth, that is certainly so. Neglecting air resistance, you have to keep your feet under your center of gravity.

On a treadmill in space, you need to arrange for your contact force to be on a line that intersects with the anchor for the straps. Instead of moving the contact point relative to the center of gravity, you'll want to move the contact point relative to the strap anchor.

It would/should be possible to simulate running uphill / downhill on the space treadmill by the angle of lean of the anchor strap.
Something not possible on Earth except by raising or lowering the bed ( or perhaps that could be done with a horizontal tether attached to your hips attached to the front or rear of the machine depending ).
Something I was to expand on in my previous post.
Apologies to @Grinkle for being too terse and certainly not explanatory.

 

[sophiecentaur]

Sadly, age steals youthful competence.

The worst thing is the memory. You can work around lack of strength and agility by taking your time but . . . now where was I??

 

[A.T.]

Fun side note: There is also the exact opposite of the NASA treadmill, one that reduces the effective weight.