Recovery phase R of normal-push skating -- Roberts

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what's here

key points
definition of this phase
drivers of propulsion
details + hints
more

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: Prepare to add reactive force to Set-down phase, and perhaps currently add reactive side-force to the other leg's push.
The parts of the leg are brought inward further toward the other side (while in the air) than would be necessary to reach the landing position of the set-down. This adds propulsive work in this phase (for double-push stroking), or enables added propulsive work in the next Set-down phase (for normal-push stroking).
There may be propulsive work from bringing parts of the leg further backward (while in the air) than would be necessary to reach the landing position of the set-down phase -- but only if there are certain motion patterns in the push phases of the other leg.

[ normal-push: ] If using simple obvious push with a constant aim-angle throughout, there's little need to give attention to forward-backward leg motion in the Recovery and Set-down phases.

[ double-push: ] The "lite" style is better suited for extracting propulsion from forward-backward moves in Recovery and Set-down phases. So if using the "big" style of double-push, less need to give attention to such forward-backward leg motions.

definition of this phase

The Recovery phase starts when the pushing foot loses contact with the ground at the end of the previous push with this leg. I define it to end when its knee and foot have reached their farthest inward position while still in the air.

There are two required parts and one optional part to this phase:

(a) stopping any outward motion of the leg as "follow-through" from the leg-push move just ending, and starting it moving inward toward the other side, and accelerating it to maximum speed toward the other side;

(b) slowing and stopping the inward sideways motion accomplished in the first part;

drivers of propulsion

sideways moves

For double-push, these are the key physical parameters that determine the amount of propulsive work added from sideways components of moves in this phase:

(a) the maximum inward speed attained by the mass of each part of the leg (upper leg, lower leg, foot with attached skate or ski).

Secondary driver: The maximum speed is higher if the sideways finish-position for the move is farther inward (other things being equal).

(b) timing of how close to simultaneous the maximum speed instant of each sideways-inward move is with the Aim-switch phase ipA of the other leg.

This is easy to put it into words, but tricky to achieve in actual human performance.

For normal-push, no significant net propulsive work is added by sideways moves (because the other foot is pushing toward the same side throughout this phase), but the amount of propulsion added in the next Set-down phase 0 because of preparation enabled by sideways components of moves in this phase is partly determined by these physical parameters in this phase:

(a) how far inward is the position reached by the mass of each part of the leg (upper leg, lower leg, foot with attached skate or ski).

Note that this is very similar to the Secondary driver for driver (a) of Double-push above. So even though the magnitude and timing of the additional propulsive work is different, a key implication for performing the sideways moves in this phase is the same.

forward-backward moves

These are the key physical parameters that determine the amount of propulsive work added from forward-backward components of moves in this phase:

(a) how much smaller is the aim-angle of the skate or ski during the propulsion-negative part of these moves (starting of the backward component) than is the aim-angle during the propulsion-positive part of these moves (stopping of the backward component).

(b) the maximum speed of the backward component of these moves.

(c) timing coordination: how close to simultaneous the maximum speed instant of each backward component is to the aim-angle change time of the other leg.

The crucial point is to get parameter (a) to be significantly different from zero.

For normal-push, there will be a difference in aim-angle if: the initial Underneath push phase 1 of the other leg has its aiming-direction close to straight (during the starting of the backward component of the recovery moves), and the stopping of the backward recovery move comes during Central push phase 2 of the other leg, and the other foot has by then pivoted so that its aiming-direction if more out toward the side.

So if perform normal-push in the simple obvious way, with a constant aim-angle throughout the push, there's little need to worry about forward-backward leg motion during the Recovery and Set-down phases. Which is why there is usually little or no propulsive benefit to forward-backward leg motion in the Recovery phase with skis.

One obvious way to get propulsive benefit from the forward-backward leg motion components in Recovery and Set-down is to start the push with the aim-angle of the skate small during the early part of Phase 1 just after Set-down, then pivot the skate outward to a larger aim-angle before the start of the Central push of Phase 2.

For double-push, the best hope for a significant difference in aim-angle is if the starting of the backward component is coordinated to come during the in-push of the other leg, and the stopping to come during the main-push of the other leg -- and the aim-angle at set-down for in-push is small.

Small aim-angle on landing typically comes with the "lite" version of double-push.

For double-push, the timing-coordination driver (c) is based on timing the starting and the stopping to occur respectively before and after the Aim-switch phase ipA.

Also, the amount of propulsion added in future phases because of preparation enabled by forward-backward components of moves in this phase is partly determined by these physical parameters in this phase:

(a) how far backward is the position reached by the mass of each part of the leg (upper leg, lower leg, foot with attached skate or ski).

muscle moves

normal-push: the propulsive impact of almost all Leg moves in the Recovery phase is self-cancelling, because both the acceleration and deceleration take place with the ski or skate configured to transmit force toward the same side, toward the side away from the recovering leg. Therefore,

No moves net positive for propulsion with Normal-push stroking.

double-push: both the starting and stopping of sideways leg recovery moves can be propulsive for Double-push, because the starting acceleration takes place while the other pushing ski or skate is aimed to transmit force toward the side of the recovering leg, while the stopping deceleration takes place while the pushing ski or skate is aimed to transmit force away from the side of the recovering leg.

hip adduction - [ see more ] -- to add inward speed to mass of the entire leg, in early + middle parts of this phase (which adds reactive side-force to the other leg's in-push).
hip abduction - [ see more ] -- to subtract inward speed from mass of the entire leg, slowing then stopping the inward motion in the later part of this phase (which adds reactive side-force to the other leg's main-push).

The next two pairs of moves are available only with inline or ice skates, not with skis -- because most skating skis are so long that the tip or tail of the ski would hit the ground if the first move of each pair were attempted.

[ inline + ice: ] outward-hip-leg-rotation - [ see more ]) -- in a bent-knee configuration, this move adds inward speed to mass of the lower leg, in the early part (a) of this phase (which adds reactive side-force to the other leg's in-push).
[ inline + ice: ] inward-hip-leg-rotation (in deceleration) - [ ?? see more ]) -- in a bent-knee configuration, this move subtracts inward speed from mass of the lower leg, in the later part (b) of this phase (which adds reactive side-force to the other leg's main-push).
[ inline + ice: ] inward-ankle-rotation - [ ?? see more ]) -- to add inward speed to mass of the front of the foot and attached skate, in the early part (a) of this phase (which adds reactive side-force to the other leg's in-push). The contribution of this move to propulsion is small (but the cost is smaller).
[ inline + ice: ] outward-ankle-rotation (in deceleration) - [ ?? see more ]) -- to subtract inward speed from mass of the front of the foot and attached skate, in the later part (b) of this phase (which adds reactive side-force to the other leg's main-push).

The next pair of moves is available to ski-skating as well as for inline + ice, if preceded by an outward-leg-hip-rotation move to get the tip of the ski away from hitting the ground. Not needed for ice + inline, because the previous two pairs of moves already provide as much reactive side-force benefit, and also position the knee better for propulsion in Phase 1.

knee-flexion - [ see more ] -- in an outward-leg-hip-rotation configuration, this move adds inward speed to mass of the lower leg, in the early part (a) of this phase (which adds reactive side-force to the other leg's in-push).
knee-extension (in deceleration) - [ ?? see more ]) -- in an outward-leg-hip-rotation configuration, this move subtracts inward speed from mass of the lower leg, in the later part (b) of this phase (which adds reactive side-force to the other leg's main-push).

The knee-flexion move leaves the leg in a less favorable position for main-push set-down phase 0 and underneath side-push phase 1 so it is not likely to be helpful for normal-push stroking.

But the resulting leg configuration is more favorable for in-push set-down phase ip0 and underneath side-push phase ip1, so knee-flexion recovery might be helpful for double-push stroking.

details + hints

Bring each part of the leg (upper leg, lower leg, foot) inward further toward the other side than necessary.

To position these parts to generate greater propulsive reactive side-force in acceleration in starting the Set-down move of Phase 0. The further they go inward in Recovery phase R, the more distance is available to accelerate to higher maximum speeds outward in Set-down phase 0. Reactive-force work is proportional to the maximum speed.

The basic leg-recovery move for all kinds of equipment is hip-adduction - [ ?? see more ], which moves the entire leg inward.

[ inline + ice: ] On skates, if the knee is flexed early in the recovery to bring the foot up and behind (which on skates often feels natural after finishing the main push), then an outward-hip-leg-rotation move will move the mass of the lower leg and foot and attached skate further inward, even "hooked" around behind the other leg.

If there's also some hip-abduction combined with the outward-hip-leg-rotation move, then the knee moves up and out, and from that prerequisite configuration, a further knee-extension move can send the mass of the lower leg and foot even further toward the inside behind the other leg. Having the knee in a more upward and outside position at the end of the recovery phase R can be helpful for double-push stroking, because then the leg has more space available to attain maximum speed when it is driven inward in phase ip0 to add power to the tranverse-hip-adduction move in in-push phase ip1, and the lower leg and foot have more space available to attain maximum speed when they are driven down and inward in phase ip0 to add power to the inward-hip-leg-rotation move in in-push phase ip1.

Having the knee more outside is counter-productive the the phase 0 set-down of normal-push, so this knee-flexion approach for increasing recovery side-motion is typically not used for normal-push stroking on skates, though for some reason it is used often on skis (see below).

[ inline + ice: ] On skates, there is a way to bring the mass of the front of the foot and attached skate even a little further inward: by making an inward-ankle-rotation move, which point the toe of the foot inward. The addition this makes to propulsive work is small, but the cost is smaller. For an example of an elite racer using it, see the video of Andrea Gonzales winning the 300-meter time trial in the 2004 Inline World Championships. (Actually most elite speedskaters do a partial inward-ankle-rotation move, since typically the foot gets pointed outward as the main leg-push finishes, and the elite racers rotate it during recovery to point straight forward at the start of Set-down phase 0. But Andrea Gonzales takes the move further, and gets more propulsive benefit.

[ ski: ] For ski-skating, the danger of the tip of the ski hitting the ground contrains the kinds of moves that can be used -- different from inline + ice skates, and the amount of side-motion possible. So to achieve additional inward motion with a long ski, must perform an outward-hip-leg-rotation move to get the hip up from the ground and the whole ski more horizontal, then a knee-flexion move to bring the mass of the lower leg and foot and attached boot and ski further inward.

[ ski: ] The disadvantage for Normal-push of this knee-flexion configuration at the end of Recovery phase, is that it tends to lead to more inward momentum and less outward momentum in different parts of the leg during Set-down phase. But the only push in Normal-push stroking is directed outward, so it's better to have more outward momentum of the mass of the leg in the Set-Down moves. Perhaps this is another reason why elite racers in ski-skating tend to make an inward-knee-roll move in Phase 1.

For Normal-push stroking, the maximum speed of the recovery moves is not important, as long as each part reaches its position for Set-down soon enough to not delay something else. Because the reactive side-forces from accelerating and from decelerating the sideways recovery moves in this phase is self-cancelling, because the aim-angle of the other foot is for transmitting the force toward the same side throughout this Recovery phase R. But double-push is different . . .

For Double-push, even better if bring each part of the leg further inward quicker than necessary.

In order to add more propulsive reactive side-force work of acceleration to the in-push of the other leg, deceleration -- then in decelerating to add more reactive side-force work to the main-push of the other leg.

Double-push can win both ways, because the aim-angle of the other foot for in-push is configured to transmit force inward during the acceleration part of the recovery moves in part (a) of this Phase, and the aim-angle of the other foot for main-push is configured to transmit force outward during the deceleration part of the recovery moves in part (b) of this Phase.

The higher the maximum velocity of the moves sideways to the inside, the more propulsive work is added. Since the range-of-motion available is limited, sometimes the start of the inward-sideways moves must be delayed, or the finish must be hurried and then hold in a waiting position for the next following move.

But the timing coordination must be accurate. The key principle for maximum propulsive benefit for double-push recovery phase is to attain maximum speed of the inward-sideways moves at the instant of the Aim-switch phase ipA of the other leg. Which leads to these timing points for double-push stroking: (a) can delay the start of the inward-sideways moves a little, but must start them during the in-push of the other leg, before the Aim-switch; (b) finish the inward-sideways quickly during the main-push of the other leg, after attaining maximum speed during the Aim-switch of the other leg, then hold the recovering leg stable until start of Set-down phase ip0.

For a slightly different discussion of these details, see above under drivers of propulsion.

[ ice + inline: ] Lifting the foot a ways up off the ground (by flexing the knee to bring the foot behind and up) enables larger inward motion of the leg in recovery.

[ inline: Some of the world's fastest skaters let their feet go fairly high in recovery, so it seems like it doesn't do any great harm, and probably getting into this configuration makes it easier to add propulsive work in Phase R and Phase 0. ]

[ ski: ] No need to lift the foot higher than necessary.

[ ski: Especially in soft snow, it's good to set the ski down gently from a low position, to minimize the friction from digging deeper into the snow. But sometimes in icy conditions it might help to lift the ski higher, then drive it down sharply into the hard snow to "dig" the edge in for better grip. ]

The foot can be moved further backward than necessary.

This can deliver additional net propulsion only if the timing of the starting and stopping of this backward move can be worked cleverly, or the timing of the starting and stopping of the subsequent forward move of the leg (in set-down phase) can be timed cleverly with changes in the aiming-angle of the other ski or skate. For more details on managing this opportunity, see above under drivers of propulsion.

This extra backward move is used mainly with normal-push (also the "lite" style of double-push) on skates at higher speeds, because the stopping of the set-down forward move naturally occurs while the skate is aimed close to straight forward early in phase 1 just after landing, which minimizes the negative effect on propulsion. At lower speeds the skate is set-down aimed more out toward the side already in Phase 1, so there is less benefit in this extra backward-forward part of recovery.

[ ski: These extra backward-forward moves are unlikely to add net propulsion on skis, since it's difficult to change the aim-angle of the ski, so usually the negative and positive propulsion effects just cancel each other. ]