what's here

see also


This framework of understanding is very detailed and specific -- which is good if it's actually correct.  But it is not straightforward to verify if it's correct or not -- or which parts are correct how much.

So here I'm going to discuss why it's tricky, what's the real basis for me believing it, which leads to the point that you'd really have to know a certain area of basic physics very well in order to verify it's correctness. But for those without that physics, there are some approaches for "confirming" parts of it, without getting to full "proof".

why it's tricky to verify this

It's tricky to verify that the whole or a part of this understanding is correct or not, for several reasons.

  1. because it's about what sequence of moves physically and biomechanically possible to employ to add forward-propulsion power in skating, not what any particular skater does in a certain situation.

  2. because the framework is so complicated that it's difficult to analyze or simulate.

  3. because it's difficult for most of us non-elite skaters to coordinate performing so many moves when we try them.

  4. because for us non-elite skaters, some of our muscles don't have the speed / strength / endurance to add noticeable power even if we perform some of the moves correctly.

  5. because some of the moves are difficult for us to observe accurately or reliably when elite racers perform them.

basis in physics + biomechanics

The basis for the whole framework is that:

(a) the physics of how the ski transmits power to the ground for forward propulsion is understood, and that physics imposes contraints on what kinds of forces and motions can possibly be effective for propulsion.

e.g. A force whose direction is parallel to the aiming direction of the ski cannot add significantly to propulsive power (unless there's a lot of resistance in gliding on the ground, which is not a situation in consideration here).

e.g. A force whose direction against the ground does not include a backward-directed component cannot add to current forward-propulsion work (though it might cleverly add to propulsion work in a later phase of the skating stroke-cycle).

e.g. A sideways-directed component of force to add propulsion work must be toward the same side as the ski is currently aimed toward (off the skier's overall forward motion direction).

(b) the biomechanical geometry of the joints and bones of the human body allows some kinds of motions and not others, and can apply some kinds of forces and torques and not others. Sometimes a motion or force/torque is effective only from some starting configurations and not from others. For a given human-body-configuration, we could make a list of all the possible forces/torques.

(c) some moves have bad side-effects.

e.g. a force which (in addition to a useful force component) also includes a large component parallel to the aim of the ski might throw the skier way out of balance forward or backward, or at least leave the skier-relative-to-ski configuration in a state not effective for transmitting future forces.

e.g. there might be some sharp quick moves with the neck and head which could add propulsion power, but we think the effects on perception and balance are not worth it.

(d) If we put together the force-transmission constraints of skating physics in (a) with the geometrical constraints of the forces/torques and starting-configurations of human biomechanics of (b), and remove moves/configurations with bad side effects in (c), then we obtain a shorter list of force/torque moves which are both possible from a given body-configuration, and add significant positive forward-propulsion work to skating.

The moves on this shorter list will be different for different body-configurations -- both the configuration of joints and bones and muscles relative to each other -- and their configuration relative to the ski in contact with the ground. The starting velocity/momentum of various body parts in some configuration might also be relevant to which moves are effective for forward-propulsion, or for the amount of forward-propulsion work each move can add. All this is fully governed and understood by the laws and methods of physics.

(e) overall sequence and timing is important, because prior starting-configuration and momentum is important for each move. So which moves come before and after a given move is important. And to have a repeatable stroke-cycle, there must also be some "recovery" moves to get some of the joints and bones back again (and again) into a similar starting-configuration for each move. Some of these "recovery" moves might not add to forward-propulsion power, and some might even reduce it, so it's important to choose moves and a sequence which produce an overall net positive power over the entire repeatable stroke-cycle.

what claims are made

The claim here is that the listed set of moves done in the given sequence of phases is the set that complies with the constraints of (d) and (e) -- the full set of the effective moves available for a skater to select for effective forward-propulsion skating.

In more detail, the claims are:

  • if a skater executes any one these moves in its correct phase in the sequence, from an effective starting configuration (which might require some other preparatory move in a previous phase), and has developed sufficient specific-muscle strength-speed-endurance to execute the move with significant power -- then the skater's overall forward speed will be increased (other things being equal).

  • if a move is not in this set, and a skater tried to execute the move, then it will not repeatedly or sustainably add to overall motion speed, or it will have significant undesirable side effects on balance, perception, or on other propulsive moves.

  • the overall phase sequence is more effective for forward-propulsion speed for healthy human skaters than other possible overall sequences. If some additional move is discovered, then it can be incorporated into this phase sequence with at most minor adjustments.

methods to verify (or falsify)

Since the moves and sequence are about what's physically and biomechanically possible, it's hard to imagine a way to verify them without a very sound working understanding of Newtonian mechanics like is typically taught in a first American-college-level physics course. (But my suspicion is that many American students do not emerge from such a course with a "very sound working understanding", so simply finding somebody who did well in the course is not sufficient.)

  • The main way to verify is to carefully analyze the physics and biomechanics of each move listed.

Little or no physics is needed to falsify or contradict it:

  • Find a new move which repeatably increases the sustainable speed and your buddies (with careful measurement), and which is not on the list.

  • Find a new overall sequence which produces higher sustainable speed.

Falsification has actually been done by elite racers on inline skates -- because they effectively use at least one additional propulsive move, and use one or two additional phases not included in the sequence given here.

If you find some new moves that work, I'd love to hear about them.

If you find a whole new overall sequence which is substantally different, that would be wonderful. I'd love to try it out (even if it's not actually faster or otherwise more effective).

Some ways to support or confirm the value of this framework, without getting to the level of proof:

  • feel it personally:  For some of the moves, when I do them in the right sequence and configuration, I find that I can simply feel the force, the additional force being applied between my body and the ground.

The main problem here is that some of the individual moves only work after there's a basic foundation of sound skating in place. Also that some moves require a high degree of neuromuscular coordination which might take weeks of careful (well-coached?) practice to achieve.

  • race videos:  I think that the World Cup winners are using most of these moves in most of their competitive skating in actual races.

The main problem with this is that many of the key moves are subtle and difficult to detect in actual race videos with unknown camera angles and distances -- probably need to understand a lot about physics and the overall sequence and the camera-angle process, in order to get reliable specific observations.

  • technique-demonstration videos:  Observations in controlled environments have the potential to be more accurate and helpful than actual race videos -- with clothing marked for easy observation, carefully-selected camera angles (and best of all together with force-sensors in the boot-binding-ski interface).

A key test of this framework is: Can it be made operational in a controlled lab environment with video + position-velocity sensors + force-torque sensors? Is it straightforward to accurately determine when and where each identified move starts and finishes?

The main problem here is that racers may skate differently in a controlled situation than in an actual race. My special concern is that racers in a technique-demonstration video try to follow a simplistic theory of movement taught by their coaches that fits into the limitations of the conscious rational mind. So we miss out on the more complex movements that really work for winning -- which can only be controlled by the full computational power of the unconscious neuro-muscular control systems of the winning racers in the heat of real competition.

My favorite approach:  Try out the feelings of many different movements -- have lots of fun.

go beyond this understanding

I think the most important ways to go beyond this framework is to get quantitative.

  • The starting questions are like: How many hundredths of second does each move (or phase) take? How does this change in different situations with different skiers and equipment and different performance objectives?

  • More interesting is: How much work is done by each move (or phase)? How much of this work is effective for forward propulsion?

Since physical work is force times distance, one way to estimate this is by using force sensors and position sensors in a controlled lab environment with a human skater. Since work can also be defined as the difference between energy states, sometimes it can be estimated by having ways to measure and calculate kinetic and potential energy.

  • Which leads to yet more interesting questions like:

What's the proportional contribution of each move to total work per stroke-cycle?

How does the contribution of this move interact with other moves to reach a calculation of the average rate of power over the whole stroke-cycle?

Which segments of each move deliver the most or least propulsively-effective work?

How does these proportions change in different performance situations?

Which moves or segments to eliminate if trade-offs must be made?

Which moves and muscles to give priority to in training development?

  • How to construct a theoretical quantitative simulation model of human skating motions -- which could be calibrated to predict how much time and propulsive work would be done by each major movement subset in certain simple situations?

Very difficult, I expect.

back to main Leg motion skate page