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Skate
what's here
- intro
- overall stroke-cycle
- finish Recovery instant
- landing of foot instant
- motion just after landing
- motion in final section of push
- lift-off of foot instant
- more - - see also
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[ under construction ]
?? [ more to be added ]
video checkpoints
units of measurement
It's difficult to measure absolute units like cm off a
video. And those might not be the most helpful units for analyzing
performance.
Often it is easier (and perhaps more useful) to scale
the measurements in terms of the skater's own body.
For front-view observations, often I use "hip-width" as
the unit: the distance from the outside of one hip to the outside of the
other.
For side-view observations, often I use
"hip-knee-length" as the unit: the distance from the hip joint to the
knee joint.
observation notes
-
absolute sideways positions are often easiest to
reliably measure relative to a distant object behind the skater.
-
hip horizontal sideways position or distance is
often easiest to reliably observe and measure in video by the
position of the crotch.
force + feel checkpoints
learning stages and stroke techniques
Sometimes the checkpoints and measures are different for different
learning stages and stroke techniques. Here are some categories used
below for distinguishing these:
- 1 - focus on heel-push + lower-body fundamentals
- 2 - basic total stroke for normal-push
- 3 - advanced points for normal-push
- dp2 - basic double-push stroking
- dp3 - advanced points for double-push
overall stroke-cycle
measures in front-view
(A) difference in pushing-ankle / pushing-hip horizontal relative
sideways signed-distance.
Why this measure? These A measures are
fundamental for the sideways component of range-of-motion of the
main-push and the in-push.
(A1) for double-push: from Landing to Aim-switch.
(A2) for double-push: from Aim-switch to Lift-up. for
normal-push: from set-down to lift-up.
(A3) total of A1 and A2. (so for normal-push it's the same as A2)
(B) difference in pushing-hip horizontal absolute sideways position.
Why this measure? These B measures are of
interest for helping to interpret the A measures: How much is motion of
the hips + pelvis, versus how much is creative tracking of the foot.
(B1) for double-push: from Landing to Aim-switch.
(B2) for double-push: from Aim-switch to Lift-up. for
normal-push: from set-down to lift-up.
(B3) total of B2 for current leg and B1 for next leg. (so for
normal-push it's the same as B2)
(C) maximum difference in pushing-hip horizontal absolute sideways
position over the whole stroke-cycle.
Why this measure? C is a measure of overall
effectiveness of lower-body side-weight shift.
(D) ratio of the vertical hip-to-ankle distance to the horizontal
hip-to-ankle distance:
This ratio is the tangent function of the
vertical slant angle (not including toe-push or ankle-pronation). So the
vertical slant angle can be found by taking the arc-tangent function of
this ratio.
(D1) for normal-push, this ratio at Landing. for double-push,
this ratio at end of in-push, just before Aim-switch.
(D2) this ratio at Lift-up.
Why this measure? D2 is a key measure of
leg-extension.
(E) difference in hip / shoulder horizontal relative sideways
signed-distance.
only movement opposite to the leg-push
is positive. Movement of shoulders toward the direction of the current
leg-push is negative.
Why this measure? The E measures are
fundamental for upper-body side-weight-shift.
(E1) during in-push for double-push.
(E2) during normal-push.
(E3) total of E1 and E2.
(F) maximum difference in horizontal absolute sideways shoulder position over
the whole stroke-cycle.
measures in side-view
(K) maximum difference in absolute vertical position of hip joint
relative to ground from set-down to phase 1 or phase 2.
(L) difference in backward-forward relative position of next hip
joint versus previous hip joint, from set-down to lift-up.
checkpoints
inline learning stage 1: lower-body heel-push
side-weight-shift: side-to-side distance of hips = ??
(absolute in hip-widths)
side-to-side distance from landing to lift-up of foot =
?? (absolute in hip-widths)
single-ski balance: time spent on one ski = ??
(absolute in seconds)
hip motion side-to-side = aggressive
arm+shoulder motion = gentle (not aggressive)
slant-angle of leg at lift-up instant = ?? (vertical
height of hip versus horizontal hip / heel distance)
force + feel
inline learning stage 2: basic total stroke
side-weight-shift: side-to-side speed of hips =
?? (measure distance + cadence)
side-to-side distance from landing to lift-up of foot =
?? (absolute in hip-widths)
single-ski balance: time spent on one ski = ??
(absolute in seconds)
hip motion side-to-side = aggressive
arm+shoulder motion = gentle (not aggressive)
force + feel
-
pushing throught the heel in phase 0, 1, 2, shift
to ball + toe only in phase 3.
-
pushing directly out toward the side in phase 1, 2,
shift to push more backward in phase 3.
inline learning stage 3: advanced refinements
?? [ more to be added ]
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finish Recovery instant
end of Recovery phase R
landing of foot instant
end of Set-down phase
motion just after landing
phase 1 (possibly early phase 2)
motion in final section of push
phase 3
lift-up of foot instant
end of phase 3, start of phase R
?? [ more to be added ]
back to Top |
overview of phases | d-p
phases | definitions | more Leg
back to Top |
overview of phases | d-p
phases | definitions | more Leg
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