NEW INSIGHTS INTO THE BIOMECHANICS OF WALKING AND STEPS IN THE SKI TURN


Comments made by S.S. Komissarov in his paper, Dynamics of carving runs in alpine skiing. The centrifugal pendulum in conjunction with a critical examination of the biomechanics of the walking cycle and subjective on-snow experiments I did last ski seaon has given me insights into the mechanism that enables fluid dynamic skiing with directional control.

A telling statement by Komissarov is that in the fast skiing typical of FIS WC racers’ rhythmic carving turns are still possible but balanced carving turns are no longer possible. Komissarov further states that during rhythmic carving turns a skier is never in balance. I would modify this statement to posit that during rhythmic carving turns a skier is only in what can be described as a state of dynamic balance wherein neurobiomechanical processes effect tight control over variances in the orientation of the transverse plane of the base of the edged outside ski as it pertains to the alignment of the vector of opposing applied and reaction forces for a few milliseconds.

These insights explain why static balance exercises done on one foot, lateral side to side jumps where a subject lands and balances on one foot and even one ski turn exercises don’t equate with the dynamic mechanism responsible for the fluid movement of dynamic skiers.

A critical examination of the walking (aka gait) cycle raised issues that as far as I know may never have been explored. These issues have potential implications for the role of steering in the alignment of the pendulum vector of COM with the transverse aspect of the outside ski as it pertains to the edge angle in carving and the stiffening of the outside foot and leg that enables powerful carving forces to applied to the outside ski that when released act can act as catapult mechanism to propel the skier into the next turn.

Pelvic rotation appears to be a key component in the dynamic processes of both walking and skiing.

In my next post I will start to explain how I believe pelvic rotation in walking relates to pelvic rotation in a ski turn and what the differences are.

 

10 comments

  1. Thank you for continuing this Platform Angle discussion.

    I checked out Komissarov’s paper last night and was a little baffled by his balance comments stated above. I did not see a definition of balance in my quick scan of the paper (way above my pay grade). How can one not be in a state of balance, and not fall or be in danger of falling? Is balance in Physics’ terms different than the common concept? Smooth fluid movement seems to require a continuous process of automatic adjustment and compensation, (“i.e. balancing), for the constantly changing variables of the surface we are on whether walking,running, skating, skiing, etc. as you describe in your modification. Why only for a few milliseconds?

    Also, are you and he saying that smooth fluid skiing can only happen if one is an elite skier? I do not consider myself to be elite level, but I have often experienced smooth fluid skiing. Is speed the only difference? Or is it the high edge angles? Or both?

    Could you also define what you mean by “dynamic”?

    Thanks

    1. “Smooth fluid movement seems to require a continuous process of automatic adjustment and compensation, (“i.e. balancing), for the constantly changing variables of the surface we are on whether walking, running, skating, skiing, etc. as you describe in your modification. Why only for a few milliseconds?” = Dynamic.

      You are correct that even walking does not involve a freeze frame point in the progression of stance where it can be claimed that the person is ‘in balance’ on the stance leg. Walking is a progressive dynamic, seamless, interactive process.

      1. Hi David,

        So, I looked up some basic physics terms and learned that in physics “balance” is a stationary state of being, and “dynamic” means moving. It seems that if one is in “balance ” for a few milliseconds he/she would have to stop for a few milliseconds which means he/she is no longer “dynamic” during those milliseconds. This does not occur during smooth fluid movement, skiing, walking, etc.
        So, are you referring to the movement of the body of the skier relative to the skis, i.e. the “inverted pendulum” movement which I understand as the movement of center of mass back and forth from side to side as the skier alternates from right/left to left/right and where the skis are the pivot or axle of the pendulum?

      2. Your are correct. Since walking and skiing involve motion the textbook physics definition of balance is a static or stationary position or state; typically a fixed alignment in 2 planes of equal and opposite applied and reaction forces, applies.

        As Komissorov points on in his paper the term ‘balance’ is loosely used in skiing to imply a skier is not in danger of falling. With a definition of balance in skiing objective discussions are not possible.

        Dynamic ‘balance’ in the context of skiing and skating is really tight neuromuscular control of transverse torque oscillations of the platform under the outside foot in relation to the pivot axis created by inside edge of the outside ski. Momentum acts as a stabilizer of multi-plane pendulum oscillation of COM.

        Walking involves similar counter torque mechanisms that come into play as COM advances on the path of travel.

        A central issue in skiing is that critical issues like ‘balance’ are subject to personal opinions which can and do vary greatly.

  2. …and I also wonder, in my own Skiing, I definitely need to devote a high level of concentration/mindfulness to produce those moves in the right sequence and timming…which would seem is a clear indication of “executive control” as opposed to the laisser-faire neuromuscular attitude both you and Mr. Komissarov advocate…sea of doubts….

    1. As a quick comment until a skier has experienced fluid skiing they have no way of knowing what they are missing. Decades ago when I started asking elite fluid skiers to explain how they could ski so smoothly they had a hard time explaining it When they tried to explain what they were doing it was usually recited from a technical manual and had nothing to do with what they were actually doing.

      Many factors like mis-aligned feet (usually toed out which literally means ‘toes pointing to the outside’ or compromised biomechanical function can stand in the way of fluid skiing. Getting there took far longer and was far harder than I could ever have imagined. The only thing that kept me going was knowledge and signs that I was progressing.

  3. @…the role of steering…enables powerful carving forces to applied to the outside ski that when released act can act as catapult mechanism to propel the skier into the next turn.
    Pelvic rotation appears to be a key component in the dynamic processes of both walking and Skiing@.

    No doubt that steering the stance leg while in closed-chain greatly facilitates rotating the pelvis towards the outside of the turn…. AND doing so, greatly contributes to both stretching core muscles and increasing stance ski bending, which produces stored energy that, when released at the end of transition (after releasing the old stance ski) contributes greatly to reversing the body/skis/boots into the new turn (some call it holding counteraction until the initiation of the new turn), thus greatly facilitating the execution of the transition into the new turn.

    However, I have felt in my Skiing that (and I wonder if this is somehow related to your post about the Austrian move) it is the old inside (uphill) ski/edge being held against the slope that greatly contributes to this effect….it feels like if the role/intent of the inside ski changes after the fall line and especially at the end of the turn, so that when the old stance ski is about to be released, the focus of the old inside ski is trying to hold onto the mountain till the very end of current transition while helping to maintain the counteracting of the previous turn…eventually releasing the uphill ski releases the stored energy…and this is also why flexing to release, as opposed to extending, is a more effective and efficient way to release…however, all of this explanation, I believe, is in contradiction to my understanding on your very interesting posts on releasing by inside leg extension and flat ski two phase second rocker and so on….I would greatly appreciate your help to clarify these issues, when and if you feel appropriate.

    1. just a brief clarification note: when I refer in my previous comment to the Austrian move in your Post of the same name, I mean the one movement in the series of pictures in which Marcel Hirscher makes a subtle divergent move with his inside ski.

      1. It is common for racers at the WC level to use the inside leg/ski in GS turns to achieve the inclination angle needed to oppose the force of gravity before they bring their outside ski into position to orient COM in alignment with the ball of the boot with the fall (rise or gravity) line. It is common to see the inside ski at a divergent angle with outside ski; inside ski more directed towards the gate that the outside ski.

    2. The neurobiomechanics of the transition from one stance leg to the other is extremely complex even in walking. The role of the inside leg after the fall line meaning in the lower part of the C referred to as the bottom of the turn is critical to the ability to the creation of a dynamic platform under the outside foot. I am currently trying to come to terms with new insights on walking and the role of the counter torques of passive pivoting of the pelvis in walking and pronation in late stance. This issue does not appear to have been explored and has implications for steering in what is called ‘counter’ in the bottom of a ski turn in what I call the ‘load phase’.

      Given the static official positions on issues such as skier balance and the extreme complexity of the issues I expect that it will take a lengthy interaction to arrive at point where issues like you raise become clear. It will be a one step at a time process that will take a long time. Keep the questions coming.

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