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
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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.
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
-
acceleration of primary propulsive push
-
deceleration of primary propulsive push
-
acceleration of recovery move
-
deceleration of recovery move
Cost-Benefit analysis of some complete push cycle
leg Extension
primary move:
recovery move:
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:
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:
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:
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.
push against current resistance
build kinetic energy for future push
build potential energy for future push
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.
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.
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.
forward-backward
up-down
side-side
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