Leading: When does a push become a shove

[David Franklin]

So other things being equal you need to push four times as hard to do a double spin on one beat vs. a single spin on one beat.

Of course, other things aren't equal - the standard advice is you lead a double spin earlier, and my own perception is that I certainly lead it over a much longer distance. (In fact, one of the things I'm often aware of leading double spins is that I 'roll' my fingers to get that extra bit of distance where I keep connection).

I also note I'm not sure how many followers can do a genuine 1 footed double spin (quite a few can, but I don't think it's the norm). If you look at strictlywestie.com, there's been quite a long discussion about multiple 1-footed turns in WCS, and the majority opinion is that they are very much an "exceptional" move that breaks the normal rules of WCS. Primarily because the leader really does provide the torque input for the move, when this is normally verbotten.

I'm not using a screwdriver to bang in nails. Trust me.

From the rec.arts.dance FAQ
Applying physics to ballroom dancing leads to the pushy F=MA view. The proper theoretical basis of ballroom dancing is signal analysis.

If you accept the signal analysis approach, then most quantitative bets are off. For example, a force that builds up from 0 to 10N over 200ms will be percieved quite differently from one that does so over 10ms (that's why we have a jolt term, after all). And the follower is reacting to her perceptions, so...

FWIW, I think there is actually a spectrum. Aerials are pretty much pure physics. Normal moves are nearly pure signal. For me, with the people I dance with, doubles are still usually 90%+ signal. Triples and above will involve physics.

[El Salsero Gringo]

Of course, other things aren't equal - the standard advice is you lead a double spin earlier, and my own perception is that I certainly lead it over a much longer distance. (In fact, one of the things I'm often aware of leading double spins is that I 'roll' my fingers to get that extra bit of distance where I keep connection).

If that's how you lead a double, I accept that. But by your own words there's a difference in the lead. The longer distance over which you maintain contact means you can put in more momentum without using four times the force. But you *are* putting in momentum and energy, and more for a double than a single.

I also note I'm not sure how many followers can do a genuine 1 footed double spin (quite a few can, but I don't think it's the norm). If you look at strictlywestie.com, there's been quite a long discussion about multiple 1-footed turns in WCS, and the majority opinion is that they are very much an "exceptional" move that breaks the normal rules of WCS. Primarily because the leader really does provide the torque input for the move, when this is normally verbotten.

You can break any analysis you like by saying that's not how the move's done, or moving to a different dance, like WCS. I totally accept that a full analysis of every single impulse in any move is going to be impossible. That doesn't mean that thinking about the physics involved can't be useful.

From the rec.arts.dance FAQ
Applying physics to ballroom dancing leads to the pushy F=MA view. The proper theoretical basis of ballroom dancing is signal analysis.

Possibly, I wouldn't know myself, or know which authority says so. But I'm not analysing ballroom. You don't lead ballroom spins with one hand, as far as I know.

If you accept the signal analysis approach, then most quantitative bets are off. For example, a force that builds up from 0 to 10N over 200ms will be percieved quite differently from one that does so over 10ms (that's why we have a jolt term, after all). And the follower is reacting to her perceptions, so...

Which is what makes partner dance so interesting. If you have any references to "signal analysis" in this context, can you post them or PM me? I'd like to follow them up.

[David Franklin]

But you *are* putting in momentum and energy, and more for a double than a single.

Something I'm unclear of, (and watch me make my own simplifying assumptions).

Suppose we have a car massing 1000kg, with a sensor on the back panel that causes the wheels to provide a horizontal traction of 999N when a force of 1N is applied to the back, so that me applying a force 1N to the back results in an acceleration of 1m/s. If I apply that 1N force for 1 second, do you consider that I have provided momentum and energy, and if so, how much momentum? Also, in a discussion of how the car gets from a to b, would you insist the momentum I provided be considered?

My answers would be "yes, one newton-second, and no". I'm sure you'd answer "yes" to the first bit, but I honestly don't know what your answers would be to the 2nd and 3rd parts.

You can break any analysis you like by saying that's not how the move's done, or moving to a different dance, like WCS.

So what is someone who disagrees with you supposed to do? If you made the "classical" assumption of a spherical follower, then pretty much everything you've said would obviously be true. But I'd be inclined to point out followers aren't spherical.

In this context, I would agree with you that in a multiple turn where the follower stays on one foot, the leader very definitely provides the majority of the torque. But I very rarely see it done that way MJ, to the point where a couple doing it will stand out to my eye. I find it is difficult to lead, and I'm not the only one, as I've seen various couples working on it with varying degrees of success. There is extensive discussion of that move on another forum (albeit a WCS one) where they agree it's a "trick" move that works differently from normal moves. So I do not think it unreasonable of me to suggest it is not the usual way a follower executes the move.

[Ghost]

Ok I've highlighted the important bit in red. The rest is stuff I'm interested in discussing, but also kinda distracts from the point.

No, look, this is what you're not getting.

Ok first of all thanks for still responding at this point . I appreciate how frustrating it can be trying to convey a concept that someone just can't seem to grasp, particularly in text.

There's no *fixed* amount of force that I need to use to lead a triple. There's no threshold that's *needed*.

Ok I need to think about this for a while

If DavidB really wants his follower to do a double spin he's going to have to lead it faster than he would lead a single. If he's going to lead it faster, he's going to have to lead it firmer.

My one and only problem with this is that if I've understood him correctly, DavidB is saying that he doesn't do this. So either he's right and it's not necessary to do it the way you suggest, or he's wrong / mistaken and it is.

At this point, I would really be interested if you were to meet DavidB at a venue (you're both London based right?) and let him lead you.

Actually that leads to a simple question. FoxyLady - can DavidB lead you through triples and double-time moves without extra force?

The communcation is in the lead. The only communication is in the lead.

Really tempted to discuss this further, maybe later

The difference in the lead between a single and a double is speed and force applied.I'm not using a screwdriver to bang in nails. Trust me.

Ok how about figure of 8 into Cerocspin vs Slow comb into Cerocspin. The first will more easily lead a triple without extra force because the momentum's already high, whereas the second's trickier and more naturally implies a single. (Slow comb's not the greatest example I'll admit)

I'm thinking of how to include some load cells into a glove so I can measure the forces involved in different moves, as lead by different people, and the same people with different followers. I think it would be fascinating.

If you actually do this I'd be interested in the results

Be Well,
Christopher

[El Salsero Gringo]

Something I'm unclear of, (and watch me make my own simplifying assumptions).

Suppose we have a car massing 1000kg, with a sensor on the back panel that causes the wheels to provide a horizontal traction of 999N when a force of 1N is applied to the back, so that me applying a force 1N to the back results in an acceleration of 1m/s. If I apply that 1N force for 1 second, do you consider that I have provided momentum and energy, and if so, how much momentum? Also, in a discussion of how the car gets from a to b, would you insist the momentum I provided be considered?

I think that's pretty much how power steering works, and I think it's a fair analogy for how a follower feels. You've provided 1Ns of momentum. The car will have accelerated at 1ms^-2, and travelled (if starting from rest) a total of 0.5m. You will have provided 0.5 joules of energy. In the next second the car will travel a further 1.5m, and you will provide in that second second (sic) 1.5 J of energy (note that even though your push remains constant, the energy you need to put in to maintain the push goes up with time as the car speeds up). But I know you know all this, David.

You may be blessed with dancing always with followers who 'amplify' by a factor of 1000. I don't, not all the time. There are some people I dance with where I might judge the 'amplification' to be only 2, or 1, or less. I don't think they're unsatisfied with the way we dance together - it looks good, it feels good and they keep coming back for more.

And let me point out also that even if the force you need to apply is small, it's small linearly not quadratically (I think you know what I mean by that) - you can't "tend it to zero". You still need to quadruple it, or increase it at any rate, to make the move go faster.

Finally, the "pushing the car" analogy breaks down a bit if you consider a move where the best place for the lady to get the momentum to do the move is by bracing against your lead - do a first move for example, stopping on each beat (to minimise the 'help' she gets from stepping in) and examine what happens when you twist the lady to the side. The easiest place for her to push to turn out against is against your hand. (I know when you do the move faster, to time, smoothly, there are dynamics involved: that changes the amount of help you need to give her to twist out but not the basic point). She can still provide the energy - or you can - but you've still got to push against each other.

But I'd be inclined to point out followers aren't spherical.

I agree completely. But as the joke goes, considering even the spherical horse enabled the mathematician to put his money on the right nag.

In this context, I would agree with you that in a multiple turn where the follower stays on one foot, the leader very definitely provides the majority of the torque. But I very rarely see it done that way MJ, to the point where a couple doing it will stand out to my eye. I find it is difficult to lead, and I'm not the only one, as I've seen various couples working on it with varying degrees of success. There is extensive discussion of that move on another forum (albeit a WCS one) where they agree it's a "trick" move that works differently from normal moves. So I do not think it unreasonable of me to suggest it is not the usual way a follower executes the move.

Well, I didn't realise that there was a difference of opinion about double spins in MJ, I confess that I thought that they are done on one foot - which is how I do mine. Maybe I should have been more prescriptive and said "to lead a double spin on one foot" compared to "a single spin on one foot" etc.

[El Salsero Gringo]

Ok I need to think about this for a while

Don't think too hard - I mean only that comparatives like 'harder' or 'faster' apply only to leading singles/doubles on the same follower. One person might fly into a double with half the lead that someone else needs for a single - you can only compare the two on the same person (same floor, same shoes etc)

My one and only problem with this is that if I've understood him correctly, DavidB is saying that he doesn't do this. So either he's right and it's not necessary to do it the way you suggest, or he's wrong / mistaken and it is.

Difficult to speak for others, especially for one so eloquent as David. I gather from Foxy that we lead very differently anyway. I'd like him to show me how he would lead a double spin, and I'll ask him to do just that when I next see him.

[Ghost]

Don't think too hard - I mean only that comparatives like 'harder' or 'faster' apply only to leading singles/doubles on the same follower. One person might fly into a double with half the lead that someone else needs for a single - you can only compare the two on the same person (same floor, same shoes etc)

Ah ok I get you now - thanks. I use the preceeding move to set it up eg the figure 8 example, plus musicality, plus floorcraft, but ultimately it's their choice what to do with the energy so there's not an actual difference in the force of my lead whether I'm leading a single or a triple with the same person.

Difficult to speak for others, especially for one so eloquent as David. I gather from Foxy that we lead very differently anyway. I'd like him to show me how he would lead a double spin, and I'll ask him to do just that when I next see him.

Cool. If you could either post on this thread the results (or PM me) that'd be appreciated

Be Well,
Christopher

[El Salsero Gringo]

Ah ok I get you now - thanks. I use the preceeding move to set it up eg the figure 8 example, plus musicality, plus floorcraft, but ultimately it's their choice what to do with the energy so there's not an actual difference in the force of my lead whether I'm leading a single or a triple with the same person.

It is indeed her choice whether to comply or not - but if there's no actual difference in the force of your lead, then you're not telling/suggesting/implying/asking/differentiating between leading single or triple - and you just aren't leading that aspect of the dance at all.

Cool. If you could either post on this thread the results (or PM me) that'd be appreciated

Be Well,
Christopher

Will do.

[Ghost]

It is indeed her choice whether to comply or not - but if there's no actual difference in the force of your lead, then you're not telling/suggesting/implying/asking/differentiating between leading single or triple - and you just aren't leading that aspect of the dance at all.

I am, I'm just using the other stuff I mentioned.

Put it another way - I appear to be really lucky. When I want ladies to do triples, 99% of the time they do triples. When I want them to do singles they do singles. (I suspect they're telepathic )

Will do.

Thanks,
Christopher

[El Salsero Gringo]

Put it another way - I appear to be really lucky. When I want ladies to do triples, 99% of the time they do triples. When I want them to do singles they do singles. (I suspect they're telepathic )

hehehe - isn't it amazing how much telepathy you can squeeze down a left hand to right hand hold?

[Ghost]

hehehe - isn't it amazing how much telepathy you can squeeze down a left hand to right hand hold?

[David Franklin]

I think that's pretty much how power steering works, and I think it's a fair analogy for how a follower feels. You've provided 1Ns of momentum. ~snip~

Without wanting to belabour the point, I note that you haven't answered the last part of the question. If the car provides 99.9% of the energy, is it reasonable to say (as you did in another context):

you *are* putting in momentum and energy

My position would be it is technically true, but practically meaningless, particularly when the main starting point of this whole debate was you disagreeing with:

For normal vertical dancing (ie no drops, leans or aerials) then the force comes from the lady's legs, not the mans arms.

Any lady who says you need to be stronger should stop being lazy and move herself. Laziness is the man's job.

Only putting in 0.1% of the energy seem plenty lazy enough to me! (I grant I may not always be so lucky).

You may be blessed with dancing always with followers who 'amplify' by a factor of 1000. I don't, not all the time. There are some people I dance with where I might judge the 'amplification' to be only 2, or 1, or less. I don't think they're unsatisfied with the way we dance together - it looks good, it feels good and they keep coming back for more.

I chose 1000 for convenience, I'd agree it's not terribly realistic. My belief is that for translational movements (e.g. whip, catapult), the gain is far far higher than unity, however. I would grant you that it is often lower for twist type movements; I think this is both a failure of leading (because when I get it right I can lead such twists with very little force indeed), and a reflection that it takes relatively little force to literally "force" the follower to twist, so higher gain is not required.

And let me point out also that even if the force you need to apply is small, it's small linearly not quadratically (I think you know what I mean by that) - you can't "tend it to zero". You still need to quadruple it, or increase it at any rate, to make the move go faster.

Yes, but it's fairly clear at such gain levels that the change in momentum all happens "in the car", and so that is really a question of signal processing, rather than anything else. It would be perfectly possible to produce a system which took into acount the jolt, so you could actually move faster using less force, albeit applied more suddenly. And something like this actually happens in dancing - if the follow starts a leverage pose, and I slowly increase the tension, she can take advantage of that to lean away from me (-ve gain!). But a sudden increase tells her to come back towards me.

Well, I didn't realise that there was a difference of opinion about double spins in MJ, I confess that I thought that they are done on one foot - which is how I do mine.

Interesting you say this. My understanding is that you do your spins by yourself, and you find it difficult to be lead to do a double spin. I didn't particularly want to confuse matters in the previous post, but my impression is most followers who do spin on one foot are actually in a similar boat. That is, they can do a double spin on one foot "on their own" - the lead may indicate, and aid balance, but he doesn't provide signficant torque. I appreciate this is something on which we may need to differ.

As a side note, in the previous post I was actually discussing multiple turns. That wasn't intended to be as a "trick" to change the subject. The reason I was talking about turns is that with a continuous multiple turn (e.g. the 30 odd turns Jordan did with Sarah van Drake in a video clip somewhere) it is very easy to say "OK, whatever initial force the girl provided, I don't believe it is making any difference at turn 10", without having to argue about the ratio of force provided by lead and follow at the start. And again, it's providing the torque without sending the follow off balance that is difficult.

As far as the prevalence of women[*] spinning on one foot, Hannah is frequently described as one of the best spinners in MJ, and I've been at a workshop where Mick and Hannah were teaching spinning. As soon as Mick started putting any significant power in, Hannah was very definitely using two feet. And in fact, when Mick was really putting in the power, Hannah was skittering all over the place on both feet (she also did about 12 turns!) Apparently there is some research implying that it's not so much that excellent spinners have superb balance, but that they have superb ability to correct (they did things like push them off balance half way through a spin) compared with us "mortals". Having seen Hannah control those spins I can well believe it.

[*] Men seem to spin on one foot much more often than women. I've spent a few minutes trying to find an clip with a woman spinning on one foot, and come up empty. While finding about 5 examples of men doing so. Given the amount men spin v.s. women, this is interesting. Of course, two of those clips had Kevin St Laurent and Ben Morris dancing, so...

[El Salsero Gringo]

Without wanting to belabour the point, I note that you haven't answered the last part of the question. If the car provides 99.9% of the energy, is it reasonable to say

Yes, it is reasonable to say that given your figures, but not applicable to the case in point (dancing) because ...

I chose 1000 for convenience, I'd agree it's not terribly realistic.

Yes, but it's fairly clear at such gain levels that the change in momentum all happens "in the car", and so that is really a question of signal processing, rather than anything else. It would be perfectly possible to produce a system which took into acount the jolt, so you could actually move faster using less force, albeit applied more suddenly. And something like this actually happens in dancing - if the follow starts a leverage pose, and I slowly increase the tension, she can take advantage of that to lean away from me (-ve gain!). But a sudden increase tells her to come back towards me.

What you're saying is that the follower isn't a simple force amplifier and I accept that. You can do a 'snap' lead (keep it small or it's going to be unpleasantly forceful) or all sorts of things. But to do a faster lead, other things being equal - you're going to need more force. You're moving the same mass (even if it's just her hand) but faster. Note the emphasis. I obviously don't think that lead-and-follow is pure action/reaction or else I'd have as much fun dancing with a chair. To apply your more sudden lead you have to use more force - otherwise it simply wouldn't be more sudden. Once the lady cottons on and changes the force-reaction to your lead *because* it was more sudden, then you're no longer comparing like with like, and the bets are off.

The bit of David's post that I disagree with most strongly (thanks for making me think about it again) is that he's mixing up energy ("lazy..." ... "should move herself") with force. What I've been trying to say (not withstanding the differences in the way that it seems David and I lead, according to FoxyLady) is that is possible, and desirable sometimes, for the man to provide strong force in his leading hand for the lady to push against yet for the lady (by pushing back against that force) still to move herself. He being strong isn't (always) an invitation for her to be lazy. He can be strong so she can be energetic.

If DavidB is saying that in no cases does the lady ever push against the man during the course of any (non-aerial, regular, lead-and-follow) move, then, respectfully, I disagree.

[DavidB]

The bit of David's post that I disagree with most strongly (thanks for making me think about it again) is that he's mixing up energy ("lazy..." ... "should move herself") with force. What I've been trying to say (not withstanding the differences in the way that it seems David and I lead, according to FoxyLady) is that is possible, and desirable sometimes, for the man to provide strong force in his leading hand for the lady to push against yet for the lady (by pushing back against that force) still to move herself. He being strong isn't (always) an invitation for her to be lazy. He can be strong so she can be energetic.

If DavidB is saying that in no cases does the lady ever push against the man during the course of any (non-aerial, regular, lead-and-follow) move, then, respectfully, I disagree.

And now you are confusing leading & following with connection. They are not the same thing.

[El Salsero Gringo]

And now you are confusing leading & following with connection. They are not the same thing.

I don't see that helps us out any. If you say that

For normal vertical dancing (ie no drops, leans or aerials) then the force comes from the lady's legs, not the mans arms.

then you're saying - well - no force comes from the man's arms. That's the bit I disagree with most.

[DavidB]

I don't see that helps us out any.
If you say thatthen you're saying - well - no force comes from the man's arms. That's the bit I disagree with most.

No force (other than enough to move the lady's hand) should be used for leading.
A force can be applied in response to the lady's following. That is not leading, that is maintaining the connection.

This is not just me being lazy. Applying force while leading is dangerous to the lady. It is unlikely to result in a dislocated joint, but it is very likely to lead to repetitive strain injuries after a few years, or aggravate an existing injury. Just watch any couple on the floor, and see the amount of movement of the lady's shoulder, elbow and wrist. It is unlike anything they do outside of dancing.

However if the lady is in a position to apply some force herself, then presumably her joints/muscles/tendons etc are lined up in a way that is not uncomfortable. I can then match the force she is applying. The problem is that she can injure me doing this. She needs to know when to do it, and I need to know when to be prepared. I don't do this with anyone except Lily.

[El Salsero Gringo]

No force (other than enough to move the lady's hand) should be used for leading.
A force can be applied in response to the lady's following. That is not leading, that is maintaining the connection.

This is not just me being lazy. Applying force while leading is dangerous to the lady. It is unlikely to result in a dislocated joint, but it is very likely to lead to repetitive strain injuries after a few years, or aggravate an existing injury. Just watch any couple on the floor, and see the amount of movement of the lady's shoulder, elbow and wrist. It is unlike anything they do outside of dancing.

However if the lady is in a position to apply some force herself, then presumably her joints/muscles/tendons etc are lined up in a way that is not uncomfortable. I can then match the force she is applying. The problem is that she can injure me doing this. She needs to know when to do it, and I need to know when to be prepared. I don't do this with anyone except Lily.

I'd be interested to have, for instance, Andreas's opinion on the forces involved in leading Salsa. Nothing I've ever lead in Ceroc is more forceful than in Salsa, or in the little ballroom rumba, cha cha, waltz, lindy or WCS that I've learnt (such as in a basic sugar-push). Yes - forces are involved, and in a dynamic dance I don't distinguish between providing them as part of connecting back to the lady, or leading her. It's a smooth continuous flow, and such forces, light as they may be, don't come as any surprise to her.

[foxylady]

Actually that leads to a simple question. FoxyLady - can DavidB lead you through triples and double-time moves without extra force?

Hmm, am loath to get back into the fray here, but for what its worth, DavidB wouldn't 'lead' a double/triple spin, he would put me into a 'musical' position that suggested a double or triple was appropriate, and it would be up to me to take him up on it... and I can quite easily do a double/triple by myself, but possibly not double time...

If 'led' by ESG, he is more forceful, and he would probably give me that extra momentum to do it double time if I chose to do it, but thats only because I can't currently do it on my own; if I could I wouldn't need him to....

Some men do shove.... As Lory says a shove is when more force is used than is necessary. The amount of force needed will vary from woman to woman, which is why a 'led' dance is a very different one to one that relys mostly on equal and opposite connection.... I believe that the longer one dances and the more one gets used to adjusting to the lead or the follow one find oneself with the more one dances connectively..... (or perhaps I'm just talking a load of b*ll*cks)


Foxy

[LMC]

The amount of force needed will vary from woman to woman, which is why a 'led' dance is a very different one to one that relys mostly on equal and opposite connection....

- and size/weight* is not important in this instance.

*of the follower. Honestly.

I believe that the longer one dances and the more one gets used to adjusting to the lead or the follow one find oneself with the more one dances connectively..... (or perhaps I'm just talking a load of b*ll*cks)

Variety of dance partners definitely helps IMO.

But the more I think about all this, the more I realise that what Franck calls the 'internal connection' is key to getting and maintaining a good connection with your partner. Must get back to those yoga classes.

[MartinHarper]

Witterings of an intermediate, past midnight, etc. Caveat lector.

Dynamic stability is a phrase used to describe some modern aeroplanes. Simply, this means that they're naturally unstable and prone to flipping upside down and falling out of the sky. The thing that prevents them from doing this is clever software that continually applies very small changes in thrust and whatnot in order to keep them airborne. Without the software, they'd go barrelling headlong into the nearest mountain range.
People are also dynamically stable, which is why we can't fall asleep standing up. When we stand up, we're continually making tiny (normally subconscious) muscle movements to keep ourselves upright. I hope that comparing followers to fighter jets is marginally more flattering than comparing them to shopping trolleys and/or perfect spheres.

A follower has her weight on just one foot at a time, often the ball of one foot. This makes her less stable - she's still stable, but only just. A very small push on her back is required to disrupt her dynamic stability, and cause her to move forwards. She resolves this disruption by taking a step forwards onto her other foot. Again, this is pretty instinctive stuff, or at least learned at an early age.
The trick is the size of the step. Normal folks take quite a large step, end up with their weight distributed between both feet, and so don't go very far. A follower will take a smaller step, so her weight ends up completely over her other foot. Ideally, this leaves her with some residual forward momentum, which causes her to again lose dynamic balance, and take another forward step, and another. Thus, when a follower is lead into forward motion, she remains in motion until acted upon by another force. She is behaving similarly to an object with low mass and low friction.

The leading force is significant, at least in this model of leading a simple forwards walk. A very small force will unbalance the follower slowly, causing her movement to be delayed and slow. A slightly larger (but still small) force will unbalance her more quickly, causing faster movement. A shove would have broken the delicate balance of dynamic stability, probably causing her to spread her weight between both legs and thus become temporarilly unleadable. In this simple scenario, the upper limit of the force you can apply on a follower is determined primarilly by her ability to remain stable and balanced when pushed around. There are other limits, but they are not relevant. The force required to unbalance someone on one foot will not tire out the leader, nor will the pressure be sufficient to cause pain (assuming one doesn't lead with a pin). Because we are leading the torso directly, frame tension is not relevant, though it would be in other situations.

The majority of the energy comes from the follower for this walk - both starting the motion and remaining in motion. However, she may not recognise this as such, just as we do not recognise the energy we expend to remain standing up. This gives rise to the sensation of being whisked effortlessly around the floor. It's an illusion, but it can feel very real, to both leader and follower.

-------
Rotation + Math.

What about rotation? Well I'd really like to do accurate calculations on that from a F=ma view first, but my rotational mechanics is all rusty. Still, I'll give it a shot and let more scientifically literate folks call me out. Leader is doing work, applying a force over a distance. On a Ceroc spin, a reasonable amount might be 10N over one meter. Energy expended = 10J.
Moment of Inertia of a follower, theorised as a solid cylinder (better than spheres, anyway). I'll go with DF's estimate of a mass of 60kg, and further assume that our follower has a circumference of 0.6m, so a radius of 0.1m. That means a moment of inertia of 0.3 kgm².
We want to get our follower spinning in time to do one spin in two ceroc counts = 4 beats. For a 240bpm song (fast), that's 1 revolution per second, or about 6 rad/s. That means we need to give her rotational energy of around 5J. If we wanted her to do a double time spin, we'd need to give her rotational energy of four times that: 20J. If we were leading a woman weighing 120kg, we'd need to give her twice the energy: 10J.

The energy input versus energy output is the same order of magnitude, which is promising. I'm going to stick my neck out and say that when a follower is put into a static single spin, a decent proportion of the energy can come from the lead pushing on her and/or her pushing on the lead, without this causing serious injury. Typically a chunk of energy will come from the follower stepping onto her spinning foot, as part of the "staying upright" energy described earlier, but this is not required.

Predictions from this theorydancing:
1. Heavy folks will be able to follow movements back and forth as lightly as others (or lighter), but will feel weightier in some spins.
2. Double-time spins require around four times the force, and are thus significantly harder to lead than single-time spins.
3. Leading a follow to step onto a foot and spin on it is lighter than leading her to spin on the foot she's already on.