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[ under construction ] 

 


intro

This is one phase in a detailed analysis of the sequence of moves for Leg-push motions of "normal  push" method of skating. For more context and an overview of all the phases of the sequence, see the summary of normal-push phases.

key points

  • Theme: Direct push with the two big sets of skating leg muscles, Side-of-leg-Out and Extension.

  • When in doubt, hold back on the starting and/or the magnitude of the Extension push force -- since there's a limit to its range-of-motion, and the more of it that can be saved for later, the larger its contribution to propulsive power.

  • Postpone the ankle-extension move as long as possible -- into Phase 3b. Transmission of push-force is more effective through the heel of the foot. And the ankle-extension move is better "aimed" for propulsion after the leg is close to straight.

  • Hold the Upper Body quiet. Wait for the better moment. Any upper body move which could be helpful in this phase could be done with bigger contribution to propulsive power either earlier in phase 1 or later in phase 3.

definition of this phase

??

I call this the "central" push phase because:

(a) It is the obvious push that most people think this push with the big hip-extension and knee-extension muscles (gluteus and quadriceps) as the main push move of skating;

(b) It comes in the center of this sequence of phases;

(c) The major leg muscles, especially for hip-extension and knee-extension, are pushing through the central section of their range-of-motion;

(d) It avoids getting too specific about the overlaps with moves identified more closely with other Phases.

?? This phase goes from (roughly) the completion of three "contractive" side-push moves (inward-knee-roll, ankle-pronation, ankle-flexion), until (roughly) the start of the "full leg-extension" moves (outward-knee-roll, ankle-supination, ankle-extension). ??

?? This phase typically overlaps with either or both of Phase 1 and Phase 3. Perhaps some skaters just "reverse" immediately from the "non-extension" side-pushes to the "leg-extension" pushes, so they have no distinct Phase 2 at all.

But the biomechanical geometry of the leg-push and the physics of transmission to the ground combine to make the Phase 1 moves most effective when they are completed with the leg nearly vertical, close underneath the skater's pushing hip, 

?? and the Phase 3 moves most effective if started when the leg is more slanted down, way out to the side.

So there might be more propulsive power if those moves are shoved out from the center of the total leg-push, which leaves the center open to focus on the hip-extension push.

drivers of propulsion

[ physics and biomechanics parameters that drive the amount of added propulsion work -- and the additional time it takes to perform that work. ]

?? [ to be added ]

 

direct push

?? [ to be added ]

   

reactive-force acceleration/deceleration effects

?? [ to be added ]

 

vertical effects

Typically the pushing hip reaches its highest distance above the ground around the start of phase 2, then starts falling slowly, then faster. The rate of acceleration of its downward motion increases as the leg-lean-angle away from vertical increase, and the pushing foot is further away from supporting the weight of the upper body.

Elite racers usually during phase 2 start raising the mass of the torso relative to the hip (by using back and abdominal muscles). So the net effect is that the skater's center-of-mass does not drop as far as the hips.

The reactive force from the stopping of the rise of the hips and the start of the (net) falling of the hips slightly reduces the propulsive force during this phase, but this is partly counteracted if the skater starts raising the torso relative to the hip.

Much larger is the positive contribution of the gravitational force from the skater's body weight to direct propulsion during this current leg-push. This contribution is proportional to the formula cos β sin β, where β is the angle of leg's leaning away from vertical. The contribution of this downward force to propulsion is zero when the leg is vertical (β = 0) and increases to its maximum when the leg is leaning at an angle of β = 45. As the skater's leg leans over beyond that angle, the contribution of body weight decreases. Few skaters lean their leg more than 45, except perhaps racers around a tight curve.

Thus in order for upper-body weight to contribute to propulsion, the pushing leg must lean away from vertical. But that results in the skater's upper-body falling. The more the leg is leaned, the greater the contribution to propulsion -- and the faster the hip and upper-body falls -- until the skater must set the other foot down to prevent it from falling all the way to the ground. So using the help of gravity inevitably results in losing its help -- until the other leg does the hard work of raising the mass of the skater's upper-body upward again.

Another portion of the gravitational force from the skater's body weight contributes to propulsion in the next leg-push with the other leg toward the opposite side -- since the motion of the "falling" of the skater's upper body goes partly sideways toward the other side. The sideways component of motion must be stopped during the next leg push, and some of the force of decelerating goes gets transmitted through the muscles and joints and tendons of the other leg into propulsive force in the next leg push toward the other side. But some of that gets lost along the way, so better to focus on the direct contribution to the current leg-push.

muscle moves

??

for forward propulsion:

also continuation of

  • forward-pelvis-rotation - [ see more ]
  • and perhaps some other moves from phase 1.

upper body interactions

side-side moves:  Elite racers typically have the mass of the torso and/or arms moving toward the side of the leg-push at the start of phase 2. During phase 2 they decelerate and stop this motion, then start and accelerate the mass of the torso and/or arms moving sideways toward the other side.

Both the deceleration and acceleration contribute to the propulsive force of this leg-push (by Newton's Third Law), to the extent that the skater's upper-body and leg muscles and structures transmit this force through the skater's foot to the ground, instead of absorbing it or applying it to something else. There is always some loss in transmission and/or negative impact on transmission of other force -- but the overall effect on propulsion is positive. Elite racers try to train their body to improve the transmission and increase the positive.

up-down moves: Elite racers typically start to raise the mass of their torso during phase 2, by using their back and abdominal muscles and related complex structures. This upward acceleration of causes a downward force through the legs, and a portion of this contributes to propulsion in the current leg-push. Once the upper-body is moving upward at a steady speed, this "reactive" downward force ends, so the timing of its starting is significant.

The contribution is partly proportional to how far the leg is leaning, by the formula cos β sin β, where β is the angle of leg's leaning away from vertical. The contribution of this downward force to propulsion is zero when the leg is vertical (β = 0) and increases to its maximum when the leg is leaning at an angle of β = 45.

So it could make sense to delay start of this torso-raise move until later in phase 2 when the leg-lean angle is closer to . On the other hand, the later you wait to start, the less total vertical raising gets done (or the quicker and harder the muscles must work in order to achieve the same amount of raising) -- and the total amount of raising has benefits for the next leg-push. There's a trade-off to be managed.

details + hints

??

  • Main push is with the hip-extension and knee-extension muscles, working together. And some overlap with propulsive moves from other phases.

  • Main push is both sideways and backwards.

Sideways component is more important on the flat. Backwards component gets more important for climbing up a steep hill.

Some coaches say that learning skaters should think only about the sideways component of the push -- because the backward component feels so "natural" that we could never forget it -- and that consciously thinking about the backward component will make us forget the sideways component. (My thought is that's likely good advice for those coming from a background of running, but that sometimes people who do lots of flat skating need to be reminded about the possibility of pushing backward more when they get on a steep hill.)

  • Push through the heel for maximum efficient transmission of force through the skate or ski to the ground..

Some coaches say the skater should continue to think of driving the heel back during this phase.

  • Continue the forward-pelvis-rotation move.

  • Perhaps could continue the ankle-pronation and inward-hip-leg-rotation moves, though they get less effective as the foot moves further out to the side, and at some leg-to-ground lean angle they even become counter-productive.

more . . .

other moves

It might be thought that these moves could prepare for additional propulsive range-of-motion in the second half of phase 2 and in phase 3:

  • outward-hip-leg-rotation (or perhaps "outward-knee-roll") - [ see more ]
  • ankle supination - [ see more ]

because they extend the length of the leg -- provided there were opposite inward-knee-roll and ankle-pronation moves in phase 1 or phase 2.

But the direction of force from these moves is negative for propulsion in the leg-configuration and the start of phase 3. They do extend propulsive range-of-motion (and slow the fall of the skier's body mass and the fall of the hip) -- but at the cost of reducing the propulsive force.

There might be some large angle of the leg leaning away from vertical where adding these moves is a net positive for propulsive work, but I doubt at any leg-lean angle normally used for human skating (except possibly around a very tight curve?). Since it slows the stroke slightly, even if it is positive for propulsive work, it still might not be positive propulsive for propulsive power (the rate of work).

see also

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