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Turnover frequency is a key component of the physics of
propulsive Power. Many of the the tough problems in maximizing Power are
the compromises between improving range-of-motion Distance while
decreasing turnover rate. Video analysis tends to give priority to
making improvements in range-of-motion distance, because the positions
are readily observed by observing the video with the pause-frame
function, while Turnover requires some careful counting and calculating.
Turnover frequency is defined as the number of complete
stroke-cycles (with pushing by both legs) per minute. So if a skater
makes 92 leg-pushes in a time duration of one minute, 46 with the Right
leg and 46 with the Left leg, that's a turnover frequency of 46 rpm
("repetitions per minute").
Typically turnover frequency is lower when skating down
a hill or cruising for long time on flat or gentle terrain, and higher
frequency stroking when climbing up a steep hill, or doing a short
intense sprint.
Often it's valuable to explicitly train muscle
quickness and neural coordination for higher turnover. One aspect that
holds back turnover is slowness of leg-recovery moves. The muscles of
leg-recovery otherwise don't get much attention in a training program.
Sample turnover frequency observations:
?? [ more to be added ]
priority: A
It's easy to overlook this sort of thing because it's
not a specific "move". But it's important because one of the key
determiners of the amount of Power (in Watts) from every positive
propulsive Work move is turnover frequency. So anything that slows down
frequency has a bigger impact than you'd expect -- and so does anything
that speeds up frequency.
Changing the timing from "one foot and then the other"
to overlapping the pushing by the two feet deserves strong
consideration for any skater looking for more power + more forward
speed.
The other reason for "overlap timing" between Finish
and Set-down is that if the foot newly set down helps support the weight
of the upper body, then the other leg can spend more time making its
Extension push from a lower hip position. In a lower-hip configuration,
a higher proportion of the force from the big hip-extension and
knee-extension muscles goes directly into propulsive Work.
This "weight-support" role can be emphasized
more (and with more precise control of angular configuration and timing)
if the foot is set down at first aiming straight in the direction of the
skater's overall motion, then pivoted to aim diagonally.
standard-form perceptual check
The other foot is already set down on
the ground before this leg lifts off. The Set-down of the next foot should
occur before the Finish of the current foot's pushing: "overlap" timing.
A stronger form of perceptual check is to have the
other foot already set down on the ground when the main pushing leg
reaches its Midway point (midway between vertically underneath hip and
full extention at lift up).
simple Normal-push
Usually the other foot should be already set down on
the ground before this leg lifts off. The Set-down of the next foot should
occur before the Finish of the current foot's pushing: "overlap" timing.
This (usually) increases propulsive Power by increasing
the turnover frequency all the skater's propulsive moves.
exception
The only exception I've seen is in very high-force
sprinting situations: e.g. from a standing start in a short
time-trial or race, or sprinting up a short very steep hill (on a
cross-country snow-skating course) -- often both of the skater's feet
are seen up in the air briefly between leg-pushes.
Seems like the skater is pushing so with such a strong
fast delivered so quickly that there's no place left for it to go but
up. There's always an upward-downward component of the main
leg-extension push in skating, and it vertically raises the mass of the
skater's upper body during the early phases of the leg-push -- but
normally the vertical rise of the upper body is not so high that it
pulls the feet up off the ground with it.
The obvious interpretation of how this exception works
is that the increased Force of the leg-pushes adds more propulsive Work
+ Power than the time gap between pushes subtracts. There are always
trade-offs among the main components that determine the amount of Power.
The other interpretation is that sprinting skaters
would have even more Power (in Watts) if both feet did not "go airborne"
and they eliminated the gaps between strokes -- but time delays in their
neural-muscular control networks do not permit this level of precision
in ending such a strong push.
I remember seeing a study with force sensors
on bicycle pedals which indicated that sprinting cyclists would often
"overshoot" on the timing of their main leg-extension push, which
hindered the phase of their stroke -- but the additional positive work
from the extension push was more than the loss in the next phase, so the
"overshoot" in timing was worth it.
Double-push
The other foot is already set down on
the ground before this leg making its outward main push lifts up off the
ground. The Set-down of the next foot should
occur before the Finish of the current foot's pushing: "overlap" timing.
This "overlap" timing applies even to the
"hopped" Double-push (a.k.a. "W skate") done on skis for snow-skating.
The hop is performed as the Aim-switch move between the inward and
outward pushes by the same leg. But when the other leg is Set-down for
its inward push, it overlaps on the ground with the Finish of the
previous leg.
Sometimes the Set-down of the other foot is as early as
Midway through the main outward push. Initially the other foot does not
push much. It mostly helps support the weight of the upper body so the
main pushing leg's hip can stay lower longer during the Extension phase.
There may (should?) also be an overlap of pushing by the two
legs, but that overlap has a smaller time duration than the overlap of
both legs down gliding on the ground.
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