Leg-push from ankle out in front

rec.skiing.nordic thread on "Grip physics" in March-April 2006 includes a couple of messages with some interesting discussion of starting the leg-push from out in front versus from more underneath -- especially one of Zach Caldwell's posts.

Ken Roberts reply on 2006 April 16

Subject: pushing from out front in classic striding?

Zach Caldwell wrote:

" . . . A skier who is trying to initiate a forceful kick with the kicking foot well in front of the center of mass will struggle compared to a skier who initiates a kick with the center of mass forward over the kicking foot."

This seems very helpful -- it's going to change how I ski -- but also rather complicated. Thanks a lot for sharing it, Zach.

It's complicated because: (a) a key bottleneck which limits power in classic diagonal striding (compared with skating) is the short _length_ of the leg-push while the grip zone is stopped against the snow, so I've long thought that by lengthening the leg-push out in front should add power. (b) elite racers (including Kris Freeman) in side-view videos do move their ankle joint significantly out in _front_ of both the hip joint and the knee joint before starting their leg-push. (c) "center of mass" is a "virtual" concept. Though precisely defined in physics, it's not located in any specific location of the body -- its virtual position moves with different body configurations, might even be outside the body -- pretty difficult to tell where the skier's center-of-mass is located from video analysis.

Here's my response to those complexities: (a) starting the propulsive leg-push with ankle-joint out in front is usually not effective for elite racers on moderate terrain -- because they've got another more important limiting bottleneck to deal with, namely reducing the "dead spot" in time (thus increasing stroke turnover frequency). (a) non-racers who don't want to try to "rush" their turnover frequency might benefit from pushing with the ankle joint out in front. (a) serious racers in special terrain or snow situations might sometimes push from out in front (b) elite racers have two physical reasons for first moving the ankle joint out front before they push, even though they do not start their actual propulsive push until the ankle joint is closer to underneath their hip joint. (c) "center of mass" is not very important to the key physical arguments here, so let's talk instead about locations relative to the hip joint of the currently-pushing leg, which _is_ readily observable in side-view video.

Details . . . 

(a) power bottlenecks for serious racers.

I'm thinking now that the reason elite racers do not push with the ankle joint out in front is because they would first have to wait for their whole body to stop relative to the snow before they started their leg-push. That's because in order for the grip zone of a classic ski to grip with static friction to transmit pushing into the snow, the ski must be stopped relative to the snow. So that's what some (many?) non-racers do: after making the previous leg-push, then Wait and slow down until the ski roughly stops, so they can push again with the other leg, then Wait . . .

The "wait" part doesn't work for winning elite racers, because speed depends on power, and physical Power can be thought of as having three key physical drivers: pushing Force multipled by pushing Distance divided by Time. So the racers need to manage the Time aspect, which implies looking for ways to reduce the Wait. The trick is to start the ski moving backward relative to the hip while the rest of the skier's body mass is still moving forward -- so the net resulting speed of the ski relative to the ground goes to zero -- and static friction happens and the skier can start the propulsive leg-push which transmits force into the ground.

(b) why elite racers move the ankle joint out in front

Getting the ski moving backward requires some distance, and that's the first reason that elite racers move their ankle joint (and ski) out in front of the knee joint. If they start accelerating the mass of the ski and lower leg backward from out there, then by around the time the ankle joint passes underneath the knee joint and/or hip joint, the ski will have achieved enough relative backward speed to be temporarily stopped against the snow, in a good configuration to get lots of propulsive work from the main leg-push.

Of course elite racers get very good at unconsciously sensing and controlling the instant and leg-joint-configuration when the ski stops and the neural impulses for the main leg-push muscle-push starts.

And there's trade-offs between the Force - Distance - Time drivers of physical Power: If a racer got in the habit of starting the main push with the ankle joint back farther behind the hip joint, the backward ankle speed would be faster, so the wait Time could be cut further -- but the pushing Distance driver in the numerator of Power would be reduced, so the net impact might be to reduce overall stroke-cycle Power (and thus overall forward skiing speed). Each racer learns to make their own trade-offs based on their own specific muscular capabilities and different terrain and snow conditions.

The second physical reason racers and many other skiers can add power by moving the ankle joint out in front of the knee joint is more fun in the skiing, but takes too much time to explain here (unbalancing the forward-backward reactive force pair in the leg-recovery move).

I think it's still valuable for speed to move the ankle-joint way out front, and valuable for fun to "kick" the foot explosively forward -- even though in most situations like Zach says that's not where the propulsive push should start. What I want to learn now is how to start my leg moving back _early_, and learning how to "sense" the transition to static friction -- without the obvious signal of waiting for my body to stop.

more . . .
  • I think elite ski racers can train their leg muscles (notably the knee-flexor "hamstrings") to push strongly with the ankle-joint starting out in front of the knee-joint. (Especially if they apply some help from the pole-push.)

  • When doing hill-bounding up steep hills on snow, I've seen videos where elite racers actually do make their main leg-push with their ankle-joint in front of the knee-joint. They've learned to employ a different trade-off of Time versus Distance than on moderate terrain, because the Time cannot be reduced much anyway in bounding, because once both skis are up off the ground, there's no choice but to wait for gravity to do its thing. And unlike runners, skiers (and "nordic walkers") can use their poles to help their leg muscles push through the ankle-joint out front phase.

  • Has anyone put force-sensors in cross-country ski bindings, with radio digital output synchronized with side-view video frames, so we could actually measure from what ankle - knee - hip configurations different skiers stop the ski against the ground and start their main leg-push? And measure how the starting leg-joint configuration might change in different snow and hill situations -- including steep hill-bounding? How is it different if the skiers do not use poles?

Thanks for the insight, Zach. It will definitely change how I ski classic.


Keep the heel down 

NENSA Dryland Classic Drills web page 

emphasis on keeping the heel in a down position to the end of of the leg-push (but nothing about "pressing" it or "driving" it or "focusing" pressure through it). 

"Classic Technique" thread, April 2002 on rec.skiing.nordic: 

[ archive copy of thread

Message 9 describes the "stronger" version of this tip -- of "driving the heel down and back" with "weight" on it.  And it gives an interesting two-person dry land exercise for objectively verifying that the heel is kept in the down position. 

back to Topic

Conflict:  Press the Toe versus Drive the Heel Down?

"Classic Technique" thread, April 2002 on rec.skiing.nordic: 

[ archive copy of thread

Discussion of "press the toe" versus "drive the heel down and back" starts at message 9.