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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.
-
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.
??
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.
[ 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.
back to Top |
overview of phases | R 0
1 2 3 | d-p
phases | more Leg
??
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.
back to Top |
overview of phases | R 0
1 2 3 | d-p
phases | more Leg
??
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.)
Some coaches say the skater should continue to think of
driving the heel back during this phase.
-
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.
back to Top |
overview of phases | R 0
1 2 3 | d-p
phases | more Leg
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).
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