what's here
normal-push
stroking: the normal way that most people skate, with
alternating pushes by the two legs, each one aimed diagonally forward
toward its own side, each one pushing toward the outside on its own
side.
double-push
stroking: a special motion sequence where each leg makes
two pushes in a row without the other leg making any push in between
those. The first push (here called the in-push) is directed inward across toward the other leg's
side, then the second push (here called the
main-push) with the same leg is directed outward toward
its own side. Or the other way of looking at it is that there are two
pushes in succession toward the same side (but by different legs), then
two pushes in succession toward the other side (by different legs).
So the sequence of pushes is like: (L2) left
leg push outward toward the left side, (R1) right leg push inward across
toward the left side, (R2) right leg push outward toward the right side,
(L1) left leg push inward across toward the right side, (L2) left leg
push outward toward the left side, and keep repeating .
. .
As of 2005, this motion is used mainly by
expert speedskaters on inline skates, though it is straightforward to
perform it on ice using skates with a rockered blade. It is usually
rather difficult to perform effectively with skating skis.
"big" style
of double-push: double-push
stroking which is characterized by: (a) landing the skate or ski with a
significant non-zero aim-angle toward the inside; (b) followed by
significant travel of the foot toward the inside during the Under-push;
(c) push-force directed partly backward during the in-push.
"lite"
style of double-push:
double-push stroking which is characterized by: (a) landing the
skate or ski
with aim-angle nearly straight in the
forward-motion direction; (b)
push-force directed nearly all sideways during the in-push.
main-push:
a push by the leg directed diagonally back and outward toward its own side.
This is the only kind of leg push used in
normal-push stroking, and it's also
used in double-push stroking (where
it's the second push made by each leg without interruption by a push by
the other leg).
in-push:
a push by the leg directed inward across toward the other side (and
often also directed partly backward). This kind of push is used in
double-push stroking, where it is
the first push made by each leg without interruption by a push by the
other leg.
Speedskaters doing cross-over stroking around a curve
make a push inward across toward the other leg's side, often called the
"under-push" -- but that might
(or might not) be somewhat different than the in-push used for
double-push stroking.
forward-motion direction: the overall average direction of the
skater's center-of-mass while skating on the quickest path from one
place to another on uniform terrain. At any instant the skater's mass is
moving diagonally toward one side or the other, so in order to get a
consistent stable direction-vector, we use the overall average.
(symbol = x )
center-of-mass
("CoM"): the overall average position of all the mass of all the
body parts of the skater (including attached equipment and clothing).
(symbol = rcom)
Note that the center-of-mass (CoM) is not any
fixed point on or in the skater's body, such as the navel or the
pelvis-center. The CoM is a "virtual" position determined by a difficult
triple-integral calculation. Its relationship to any particular body
part changes as the skater's overall body configuration changes. In some
bent-over body configurations, the CoM is in the air somewhere out in
front of the skater -- not anywhere inside or on the surface of the
skater's body.
foot-ground-connection (or "fgc", or just "foot
position"): the position which is the effective center of the
transmission of forces between the skater's foot and the ground surface.
(symbol = rfgc
or nothing -- since its position is normally implicit as the Origin of
the positional cooordinate framework. )
aim-angle: the
angle between (a) the aiming direction of the skate or ski, and (b)
forward-motion direction of the
skater. So if the aim-angle is zero, then the ski or skate is pointed
straight forward, while if the aim-angle is 60-degrees, then skate or
ski is pointed way out toward the side. (symbol
= α )
out-of-balance slant angle (or
CoM-fgc slant angle or just "slant angle"):
the angle away from vertical of the line from the
center-of-mass to the
foot-ground-connection (but
excluding any component along the aiming-direction of the skate or ski).
Slant angle of zero means that the skater's foot is directly under the
skater's center-of-mass. Slant angle of 45 degrees means that the
skater's foot is way out to the side.
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positions + directions
origin of positional coordinate frame is
normally the foot-ground-connection ("fgc" or , unless otherwise stated.
rfgc
= position of foot-ground-connection
("fgc"), or just "foot position". Usually this symbol is not shown, the
foot-ground-connection is normally used implicitly as the origin of the
positional coordinate frame.
x = forward direction
unit-vector, in the skater's overall
forward-motion direction (parallel to the ground surface plane).
y = sideways direction
unit-vector, in the direction of the cross-product z
× x, which is normally toward the left
of the the forward-motion direction and parallel to the ground surface
plane).
z = upward direction
unit-vector; perpendicular to the ground surface plane. So if the ground
surface plane is tilted to represent climbing up a hill, then z is
not straight vertical. (or more formally, if
γ ≠
0, then z ∙ g
≠ 0).
a = aiming-direction
unit-vector, the line along which the skate or ski is currently gliding
or rolling.
n = pushing-direction
unit-vector, the intended direction of the propulsive push, which is in
the ground surface plane, perpendicular to the aiming-direction of the
skate or ski, generally in the sense of pointing backward rather than
forward.
rcom
= center-of-mass ("CoM") of the
skater's body (sometimes just r).
scalar quantities
α
= aim-angle of the ski or skate, away
from straight forward. [ α
= cos (x ∙ a) ]
β
= out-of-balance slant angle
(or CoM-foot-slant angle, or just "slant angle").
γ
= slope angle of hill being climbed. If
γ = 0, then the ground
surface is flat.
m = mass of a part of the skater's body,
which might include attached equipment or clothing.
M = total mass of skater's body
including attached equipment and clothing.
g = constant acceleration of gravity
at the surface of the Earth. (g = |g|)
forces
ffgc
= net total of all forces applied between the skater's foot and the
ground, at the "virtual" position rfgc
of the foot-ground-conection
("fgc"). [ ffgc = fn
+ fa + fz
]
fn
= pushing component of force through fgc; component in the intended
pushing-direction, which is the "currently propulsive" force. [ fn
= (ffgc
∙ n) n ]
fa
= aiming-direction component of force through fgc; component in the
aiming-direction a of the skate or ski.
fz
= upward component of force through fgc; component in the upward
direction z.
g = gravity vector. The
acceleration of gravity, vertically downward (from the surface of the
Earth). So the force of the skater's body weight = M g.
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