what's here

  • intro
  • how propulsive forces work
    • two directions of every force
    • what is propulsive work?
    • recovery moves -- self-recovering push
    • four stages of a complete push
  • kinds of pushes
    • push against current resistance
    • build sideways kinetic energy for future push
    • build vertical potential energy for future push
  • "direct" push versus "reactive-force" move?
    • what's the difference in look + feel?
    • a difference of category or of degree?
  • directional components of push
    • forward-backward
    • up-down
    • side-side
  • more

see also

back to Top | more topics | Resources | Go by Skating index


intro

 

how propulsive forces work

two directions of every force

Every force in skating acts in two opposite directions, the action and the reaction.

 

what are we pushing against

 

inertia

resistance forces

gravity

   

current-advance versus energy-building moves

For human self-propulsion, the goal is to get all the parts of the skater's body to go some distance in some forward-travel direction relative to the ground. So in order for part of the skater's body to make a "current-advance-against-resistance" push move, the move must push some body parts in (more or less) the desired forward-travel direction, and must push the ground in the opposite direction.

Actually the body part (or parts) making this push move often is not directly connected with all those other body parts, and often is not directly connected with the ground. So the push-force must be transmitted through other body parts -- transmitted both to other body parts being pushed and to the ground. Transmitting the push-force has side-effects, many of them negative -- so the total force transmitted which ends up being effective for propulsion comes out to less than the total of the push-forces applied by different body parts.

This negative impact of transmission happens for all human-muscle-powered propulsive modes, not just skating.

There can also be an "energy-building propulsive" move, which pushes other body parts, but not in the desired forward-travel direction. Such a move is propulsive because it builds up energy ("kinetic" energy or "potential" energy) -- which can increase the force of future pushes which will be (more or less) in the desired forward direction. Many propulsive moves in expert skating are "energy-building".

Many skating moves are both "current-advance" and "energy-building". A single move all at the same time can: (a) receive energy from previous moves; (b) apply energy to push the skater to advance forward faster now; and (c) build energy which will be available to add force to future moves.

That's the magic of skating.

That's why mastering skating is fascinating, and that's why trying to understand how skating propulsion works is fascinating and complicated.

what is propulsive work + power?

Propulsion is about making things move some distance, so a push is not "propulsive" unless changes how something moves. Or builds energy which can readily be converted into a push that changes how something moves. It turns out that with energies for skating, building useful energy for the future normally requires changing how something moves now.

So propulsive work requires applying a force to an object while it moves some distance (for some time). Even if the object was stopped at the beginning of the move, applying the propulsive force gets it started moving, so then it has moved some distance.

Work is defined as Force times Distance.

Sounds simple, but it soon gets tricky. Because

  • usually in skating the direction of the distance is not exactly all in the desired forward-travel direction. Usually it's in some diagonal direction -- somewhat forward, but also somewhat off to the side. So only a portion of the work is "current-advance" propulsive. The remainder might be energy-building for the future, or (usually) some of it is just wasted and never can contribute to future skating propulsion.

  • Another tricky point is that the "sense" or "sign" of the distance moved might be backwards, not forward -- but it's can still be positive propulsive work if the force has a "sense" or "sign" toward contribution to forward propulsion.

So if a body part is first moving the "wrong" way, and a muscle-move resists it, slows it down to a stop, then starts it moving the "right" way -- then that muscle-move is already doing propulsive work while it is slowing down the early motion -- and then continues to do propulsive work while it is accelerating it in the new direction.

It's like as long as the muscle move is "trying" to move body parts in a propulsive direction, it counts as positive work, even if it is currently not succeeding.

It is definitely possible for a muscle-move to do negative propulsive work -- by acting at the wrong time. This actually hurts the achievement of the goal of forward-travel. But often in human motion it is necessary to make a negative-work muscle-move -- in order to prepare for a bigger positive-work move. The obvious case in skating is recovering the leg inward and forward after it finishes its big push outward and backward.

  • Transmitting without changing distance is not Work (in this technical physics meaning of "work").

Statically transmitting a propulsive force from other current muscle moves or from energy built previously is critically important -- but if there's currently no change in the distance between body parts or change in the angle of a joint connecting body parts which is affected by that muscle-move, then that muscle-move is not currently doing Work.

So in the discussion under the previous bullet, The only time when the muscle-move is not doing work is at the instant the other body part is stopped between the two senses of direction.

This is not just an "On or Off" thing: The more change, the more Work (provided the Force is the same magnitude). The less change, the less Work.

The absence of current positive Work is not necessarily mean that this muscle-move is "doing a bad job" or "not trying hard". Often it just means that the good propulsive results from trying hard are not currently "visible". The goal of propulsion is change in position: from where we started to where we want to get to. So it's not surprising that absence of change doesn't get "counted" as propulsive Work (even if it's better than some of the alternatives, such as "losing position"). Often a segment of low Work output in the middle of a muscle-move is a necessary stage on the way to the next segment of high Work output.

Actually human muscles engaged in propulsive moves are almost never in a static position. Each currently-active muscle-move is usually either gaining or losing in position.

Though many human muscles are fairly good at transmitting large force without changing position -- called "isometric" contraction. They cannot handle as much force if they move very much. So often they don't move much. But trying hard to move positively is good, and the more the muscle-move succeeds the better it gets.

Unless the muscle-move is not yet in a configuration where its own Work will not be transmitted strongly and positively to the ground-contact -- then it might be better to hold back the motion, to save it for a bigger payoff.

  • A third point is that most individual human muscle moves cause other body parts to move in a section of a circle, not a straight line. The only way to make body parts move in anything like a straight line is to coordinate multiple human muscles moves to work together, to sort of blend their individual circles.

Power is the Rate of doing Work.

[ more to be added ]

 

recovery moves

 

repeatable moves

a stroke "cycle"

 

self-recovering push

 

 

four stages of a complete push "cycle" sequence

 

  1. acceleration of primary propulsive push

  2. deceleration of primary propulsive push

  3. acceleration of recovery move

  4. deceleration of recovery move

 

Cost-Benefit analysis of some complete push cycle

leg Extension

primary move:

  • makes large contribution to advance-against-resistance propulsive work

  • makes large contribution to side-weight-shift propulsive work

recovery move:

  • has significantly lower work cost than primary move's benefits.

  • adds small contribution to side-weight-shift propulsive work.

Adding it all up: Obvious winner.

advance-next-hip-rotation

primary move:

  • advances some body parts farther forward.

  • higher forward velocity of those parts increases resistive force from air-resistance.

  • engages additional muscles to deliver additional propulsive force to counteract the increased resistive force, and thus sustain the higher overall forward speed.

recovery move:

  • the next primary move for the other side is virtually the whole recovery move required to complete the pushing-cycle.

  • so the recovery move is equally propulsive as the primary.

Adding it all up: Winner.

forward arm-swing

primary move:

  • advances some body parts farther forward.

  • higher forward velocity of those parts increases resistive force from air-resistance.

  • engages additional muscles to deliver additional propulsive force to counteract the increased resistive force, and thus sustain the higher overall forward speed.

recovery move:

  • moves some body parts backward.

  • normally cancels all the benefits of the primary move.

Adding it all up: No point. (assuming simple normal-push stroking with continuous leg-push)

Unless:

  • unless the "deceleration of primary propulsive push" stage comes at a time when ground-contact is transmitting much lower percentage of force than during the preceding "acceleration of primary push" stage (e.g. deceleration during the in-push phase of double-push stroking)

  • unless the "acceleration of recovery move" stage comes at a time when ground-contact is transmitting much lower percentage of force than during the subsequent "deceleration of recovery move" stage (e.g. acceleration during the in-push phase of double-push stroking)

  • unless the arm is holding a ski pole -- then after the forward arm-swing is complete the pole tip is planted down for ground-contact, and the arm pushes backward to generate propulsive work against resistive force. So the backward-pole-push stages of the cycle are even more positive for propulsion than the forward swing.

side-swing of arm

primary move:

  • makes some positive contribution to side-weight-shift propulsive work.

  • makes a small contribution of work to counteracting the resistive force of air-resistance.

  • on the other hand, much of this air-resistance would not have been encountered if the arms had instead just been tucked behind the back. Unlike for the advance-next-hip-rotation move, the additional air resistance is not a natural result of advancing more body further forward at a higher velocity.

  • engages additional muscles to contribute propulsive work.

recovery move:

  • the next primary move for the other side is virtually the whole recovery move required to complete the pushing-cycle.

  • so the recovery move is equally propulsive as the primary.

Adding it all up:  Adds propulsive power if accurately timed.  Increases velocity if the benefit of additional power is greater than the cost of additional air resistance.

kinds of pushes

 

push against current resistance

 

build kinetic energy for future push

 

build potential energy for future push

 

"direct" push versus "reactive-force" move?

Another possible way to divide up kinds of moves might be between "direct" push versus "reactive-force"moves.

What is this difference?

Are they two fundamentally different categories? Or rather the same underlying physical drivers, with large quantitative differences?

How they look and feel different

Some moves look and feel very different from others. Here's two examples:

  • "Direct" push look + feel:  Using the big hip-extensor and knee-extensor muscles to push from the hip through the leg and foot against the ground.

  • "Reactive-force" move look + feel:  Swinging the arms from side-to-side or back and forth -- nowhere near the ground, not seeming to push against anything substantial.

Let's see what factors tend to go together with each of these styles of move:

  • "Direct" push style  versus  "Reactive-force" move style.

  • pushing a large mass away from ground-contact  versus  pushing a small mass.

  • bigger force  versus  smaller force.

  • Required  versus  Optional.

  • Slower  versus  Quicker.

  • obvious Simple timing  versus  Tricky timing (hold back, then go quick).

  • No worry about Recovery move  versus  Careful Consideration of recovery move cost and timing and path.

Is there a fundamental distinction?

Is there a fundamental "categorical" distinction in the underlying physics which explains why these two move styles look and feel so different?

My answer is No, the differences are only quantitative. All the observable characteristics and underlying physical drivers are a matter of degree, not of fundamentally different category of move.

What we've really got is a spectrum of moves -- from "more direct closer to current ground-contact" to "less direct farther from current ground-contact".

And a spectrum of moves from "more focused on advance-against-resistive-force and less on reactive-force" to "more focused on reactive-force and less on advance-against-resistance".

For how this "spectrum" explains the "look and feel" differences, see below.

For consideration of some other attempts to find a fundamental "categorical" distinction, see further below under "other attempts".

Those who want to find the best approach for making each move for propulsion will need to carefully analyze all the specifics of each push-move, not make arbritrary simplistic distinctions between moves. Identifying which "style" will suggest tendencies and priorities for analysis, but usually not give useful specific answers.

Analysis to explain the differences

"Direct" push moves tend to be those made by parts close to ground contact -- with few other parts as "links" between the pushing parts and ground-contact.

(a) So these "direct" pushing-parts tend to have a small total mass between them and the ground, and a large total mass of body parts beyond them which they must be push away from ground-contact. This larger mass has larger inertial force if its motion is changed (which is what propulsive push is supposed to do), also a larger gravitational force if the skater is going up a hill.

(b) The larger number of parts "beyond" them (away from ground-contact) tends to have a larger surface area, and thus larger resisting-force from air-resistance which must be opposed just in order to maintain their forward speed, and this resisting-force gets even larger when try to move them forward relatively faster -- which is what usually happens in a propulsive push.

(c) If other parts "beyond" them (away from ground-contact) are also making current-advance propulsive pushes themselves, then the additional forces toward ground-contract must be transmitted through these parts -- so those get added to the propulsive force generated by these closer parts.

(d) Normally there is sideways kinetic energy (from previous pushes) in those other parts "beyond". Since there are more such parts with more mass, they have more kinetic energy which must be "caught" by decelerating the sideways motion. This also gets added to the force of the "current advance" push of the closer parts, more than for parts farther from the ground.

Therefore:

  • Larger forces:  Pushing parts closer to ground-contact usually have larger forces for three reasons: (a), (b), (c), (d).

  • Required:  The loss of a "closer-to-ground-contact" push has more impact because it's contribution of work tends to be so much larger. The "farther-from-ground-contact" moves seem dispensable because they're so much smaller.

  • Optional:  Actually in a very-low-friction situation, a skater could move forward with only arm-swing and torso-shoulder side-swing moves -- no leg-push. So the "direct" push moves are also optional.

  • Slower motion:  The force available to move those other parts "beyond" them (away from ground-contact) tends to be proportionally smaller relative the mass of those parts, because of reasons (c) and (d) -- a larger percentage of muscle capacity is getting diverted to transmitting other push-forces from those other parts beyond and from previous pushes.

  • Simple timing:  "Direct" moves just have fewer options for timing, because they tend to be slower. And because their contribution to propulsive power is so much larger, so they tend to be the primary drivers of stroke turnover-frequency and key transition points of the stroke-cycle phases. The timing of the stroke-cycle and its phases is normally "naturally" optimized for maximum exploitation of the big-contribution moves. Timing seems "simple" for them because everything is being accommodated to them.

  • Tricky timing (hold back, then go quick):  Actually timing is also important for extracting maximum propulsive power out of "direct" push moves. Examples:

the leg Extension moves are mostly held back during phase 1, and then made as much as possible during phase 3, because later gives a much more favorable push-direction angle.

the Side-of-leg-Out moves are made move quickly in phase 1, and finished before phase 3, because earlier gives more favorable push-direction angle.

  • No worry about Recovery move for "Direct" push:  Since the Recovery move is made when the pushing-part is not closely connected to a ground-contact point, what determines which body parts do most of the moving is driven primarily by which set has the lower total mass. Thus the primary "direct" Push moves a large mass of body parts away from ground-contact, with large surface area and resisting-force, but its corresponding Recovery move mostly involves a smaller mass of body parts (e.g. the upper and lower leg) with smaller surface area and resisting-force.

For the "Direct" push, the work cost of the Recovery move is much smaller than the propulsive work benefit from the Push move -- so it's "No worry".

For the "Reactive-force" move, the primary move pushes a smaller mass of body parts away from ground-contact, with smaller surface area and resisting-force. Then the Recovery move pushes roughly the same smaller mass with about the same smaller surface area and resisting force. Thus the work cost of the Recovery move tends to be similar in magnitude to the work benefit from the primary move, so it requires careful consideration.

  • Careful Consideration of recovery move: Actually the recovery move for a "Direct" push also calls for careful consideration of its timing and path. Often there are clever ways to decrease its work cost, or even derive positive propulsive work benefit from it.

Elite inline speedskating give careful attention to the path of the Leg-recovery move. For a grand elaboration of the power of the Recovery move, analyze a video of Chad Hedrick doing double-push stroking. 

other attempts to find a distinction

Here's some other possible principles for trying to find an sharp underlying "categorical" distinction in the physics:

(A) "Direct" push is "advance-against-resistance" work

"Direct" push is "advance-against-resistance" work, not side-weight-shift work. "Reactive-force" moves are not about "advance-against-resistance" work.

My reply:

Actually all propulsive moves in skating are "reactive force" moves: They push in one direction toward the current ground-contact point, and they push in the other direction against some body parts with some mass. When the push is finished, the motion of the mass of those body parts has been changed so that it's moving more away from the ground-contact point (than it would have been if the push had not been made). Some portion of the propulsive force in the opposite direction through the ground-contact is due to the "equal and opposite" reaction to the change in the motion of the mass -- i.e., some portion is "reactive" force.

In actual competent skating at higher speeds, it is obvious the leg Extension push is directed substantially out toward the side, and one of its obvious results is that it slows and stops the skater's total body mass from moving toward the current pushing foot, and starts and sends it sideways the other way. So for the biggest "Direct" push move,  it's not just reactive force, but reactive side-force.

Actually most "non-Direct" propulsive moves must push against resistive force also.

Even moves focused mainly on sideways motion, e.g. arm side-swing, must also apply a forward component of force against air-resistance. Otherwise they would move diagonally sideways-and-backward (not purely side-to-side), and they would still have to fight extra air-resistance force in recovering for their next swing move.

A forward-backward reactive-force move (e.g. forward arm-swing) must advance against resisting-force, not just inertial reactive-force.

(B) "Direct" push get its forward advance "locked in"

"Direct" push moves get their forward advance "locked in" when the other foot sets down. "Reactive-force" moves do not.

For skating with no poles, there are two ground-contact points, namely the skater's two feet.  The transmission linkage paths to each ground-contact point connect at the hips and pelvis. Therefore a push-move made by a body part at or below the hips is completely different from a push-move made by a body part above the hips.

The argument might be made that for moves at or below the hips, the recovery move is in the forward direction, so it adds force to the next leg-push on the other side.

My reply:

The hips are indeed very important for skating propulsion, but . . .

The argument does not work because while the acceleration stage of forward motion is positive for the next leg-push, the subsequent required deceleration stage of forward-motion is negative for propulsion.

Actually there are simple explanations for why some moves "work" better -- explanations which have nothing to do with "lock-in": 

Leg Extension and Side-of-leg-Out moves work because each moves a big mass in its primary move and a small mass in its recovery move -- so the positive from the first outweighs the negative from the second.

Advance-next-hip-rotation works because it is self-recovering -- so there is no "cost" from its recovery move, only more benefit.

Other self-recovering moves work also (e.g. arm side-swing) even if they are completely above the hips and pelvis.

Also, if poles are used to enable the arms to help push, then each time a pole tip is planted down on the ground, that establishes a new ground-contact point. So then what's the "connection" of the linkages between the pole-arm ground-contact point and the foot ground-contact point?

Seems like it would be the entire body except for the neck and head. So this approach seems unlikely to be a helpful general principle for analyzing human propulsion.

Anyway, what does "lock-in" of forward advance really mean for skating?

"Lock-in" sounds like a static (or "quasi-static") concept. "Lock-in" might perhaps be a helpful concept for walking or running -- but skating is fundamentally dynamic.

directional components of push

 

forward-backward

 

up-down

 

side-side

 

more . . .

 

see also

back to Top | more topics | Resources | Go by Skating index