Dynamic compression with ankle bend

The secret sauce of XC skiing by Frank Feist

with CSU skiers

Part 2: Skate

Overview: 5 phases of skate V2

Pole Plant Compression Kick Glide Swing-in

2 frames (0.06s)

6 frames (0.2s)

10 frames (0.33s)

4 frames (0.13s)

9 frames (0.3s)

Efficiency and performance of V2 determined by ankle

bend

Pole Plant & Compression

P1 P2 C1 C2

C3 C4 C5 C6

• Description – Starts with pole plant (P1), compression starts 1 or 2 frames after that (depends on speed, terrain) – Compression slower than classic (no “freefall“), because V2 double polling force slows downward motion

(single stick classic pole is already in a phase where much of its force is directed backwards, rather than downwards)

– Ends when swing leg starts moving away from kick leg (C6, hard to see from side) – Compress ankle, and to lesser extend knee

• Look for

– Forward lean – Compression angels

Pole Plant & Compression

Kick

K1 K2 K3 K4

K6 K7 K8 K9

K5

K10

• Description – Onset of kick further compresses ankle and knee, swing legs starts moving away from kick leg (K1) – Ends one frame before ski lifts off ground (K10)

• Look for

– Deep position at full compression – Full execution of kick – Sideways motion (no back kicking) – Weight equally distributed heel and balls; no tippy toeing – Buttocks remain forward of horizontall from heel

Kick

Glide

• Description

– a

• Look for

– Hold kicking leg out during glide to provide couter balance to shoulder which leans over the gliding ski

– Slowing this motion is what allows to reduce overall cadence, and reduces intensity

G1 G2 G3 G4 G5

Swing-in

S1 S2 S3 S4

S6 S7 S8 S9

S5

• Description

– As ski swings in, center of gravity shifts and causes the skier to tilt into the kicking motion (ski edges)

• Look for

– Slow and controlled execution, no „rushing in“ but controlled glide during swing-in motion

Swing-in

Biomechanics – skate V2

• Relax lower leg muscels to allow gravity to compress leg by simultaneously bending ankle (α to β) and knee (δ to ε) joints

• The speed of compression is slower than in classic (no “free fall“ because double poling force slows downward motion)

• The ratio of ankle bend to knee bend determines kick efficiency:

– Small ankle bend requires larger knee bend for same compression -> causes “thigh burn“ and “sitting back“

– Small compression causes weak kick

• “Fireing your quads“, actively compresses (bends) ankle and knee joints some more (to δ and λ)

• You generate kick force by explosively straightening ankle and knee joints from their lowest position

• Distance d that your leg straightens from its lowest position and leg strength F determine work you do during the kick: W=F*d

– d = shin length * (1 – sin δ) + thigh length * (1 – sin λ)

– For shin=40cm, thigh=38cm, δ=50°, λ=45°, d =20cm

• The faster you straighten those angles, the more power you put on the ski: P=W/t = F*d/t (get t by counting video frames)

• Your hip must remain forward of your heel so your power goes into the center of the ski (if you sit back from the dotted line, part of your power gets waisted)

Compression Kick

C6 C1 K6

α = 85°

ε = 85°

β = 60°

θ = 60°

δ = 50°

λ = 45°

Frank

C1

FRA

ME

C6

K6

δ=50°, λ=45°, d=20cm

α = 85°

ε = 85°

β = 60°

θ = 60°

δ = 50°

λ = 45°

Alex Jordan Rion

C1

C4

K5

C1

C3

K4

C1

C3

K6

δ=..°, λ=..°, d= δ=..°, λ=..°, d= δ=..°, λ=..°, d=

Frank Andy Max Rebecca

C1

FRA

ME

C6

K6

C1

C4

K7

P1

C8

K7

C1

C4

K4

α = 85°

ε = 85°

β = 60°

θ = 60°

δ = 50°

λ = 45°

δ=50°, λ=45°, d=20cm δ=..°, λ=..°, d= δ=..°, λ=..°, d= δ=..°, λ=..°, d=

Frank Bob B John L

C1

FRA

ME

C6

K6

C1

C7

K13!

P1

C5

K8

α = 85°

ε = 85°

β = 60°

θ = 60°

δ = 50°

λ = 45°

δ=50°, λ=45°, d=20cm δ=..°, λ=..°, d= δ=..°, λ=..°, d=