Bro, this is next-level nerdiness, love it! I suggest snowboarders check this out too, you will need to imagine some of the aspects a bit differently visually, but most of it (if not all) translates really well to snowboarding-especially if you know how or want to learn how to use your edges when turning, instead of skidded turns
Glad you enjoyed it and got something out of it from a snowboard perspective. The whole joystick analogy, at its root, was actually inspired in part by working with and listening to snowboard trainers which gave me new perspectives on how to think about skiing. The physics of how we get our planks to perform is exactly the same but we come at it from such different perspectives sometimes and stand to learn a lot from each other.
Biophysicst here (though kinematics are not my field). Anyways: Nicely done! I really liked the way you showed how foot inversion and eversion propagates up the kinetic chain. That was *very* well done. What's now clear to me is that when you roll the feet into the turn, it also rotates the tibias into the turn. I already knew, because the ankle is a triplanar joint, that when you invert the inside foot, the foot adducts (turns inward). But what I realized from your visualization that, if the foot’s direction is fixed, as it will be when you are on skis, instead of the foot turning inward, the inside tibia will rotate laterally (external rotation), and thus into the direction of the turn. And the opposite for the outside foot, where the tibia will rotate medially as the foot everts, and thus also into the turn. I also really liked your embed with the cut-out boot showing the ankle motion! And your statement that “The hardest part of skiing is not getting on your edges. It’s getting off of them.” is great. Some thoughts/questions: 1) I think you could have more explicitly connected the discussion of foot tipping with the discussion of extension vs. flexion at transition. Here I'll use inward and outward to to be relative to the direction of the turn. Specifically: As discussed above, the inward rolling of the feet causes the tibias to rotate into the turn. But here's the difference: When you are extended at transition, this will cause your femurs to likewise rotate inward. But when you are flexed at transition, this will cause your femurs to *point* inward. You can test this yourself by rollling your feet when standing vs. seated. Question: What is the significance of this difference for turn mechanics? 2) If you're interested in the kinematics of skiing, you should definitely check out Robert Reid's 2010 PhD thesis, "A kinematic and kinetic study of alpine skiing technique in slalom." I'd recomend downloading both the main thesis and its appendix. You can find the thesis by going to Google Scholar and searching for "robert c. reid skiing". Sorry, I forget where I found the appendix. 3) I disagree with what you wrote about tipping wide skis vs. narrow skis. I'm a narrow ski fan myself, and they are easier to tip, and put less lateral stress on the leg when up on edge. But the difference between the two is a matter of degree. You were instead saying the difference is qualitative--when you have a ski wider than the boot, the moment arm precludes tipping. That's not the case, because when you tip you aren't only applying downward pressure on the inside (turn side) of the ski; you are also (because you are fixed to the skis with bindings) able to pull *up* on the outside of the ski. Thus you can tip regardless. You might be interested in this detailed treatement of the effect of ski width on knee kinematics: Zorko M, Nemec B, Babič J, Lešnik B, Supej M. The Waist Width of Skis Influences the Kinematics of the Knee Joint in Alpine Skiing. J Sports Sci Med. 2015 Aug 11;14(3):606-19. PMID: 26336348; PMCID: PMC4541126. 4) One topic that would be fun to cover would be how, in a properly excecuted WC turn, the divergence between the direction of travel of the skis, and the skier's COM, as the skier approaches the switch, causes a force imbalance, and that good skiers use that to create the float at the transition, and how flexion is used to manage those forces: More flexion => dissipate more of those forces => less float (desireable for slalom); less flexion => dissipate less of those forces => more float.
Hey, thank you! I appreciate all of the kind words and feedback. This webinar should have been broken into two parts but by the time I realized what I should have done, I had already committed to the topic and didn't have enough time to adjust so I definitely glazed over some details and oversimplified in others for the sake of not turning this into an even longer webinar than it already was. To your points: 1) The simple version of the way I think about femur rotation as it relates to extension vs flexion in transition is that the more perpendicular the femur is with the ski, the more femur rotation results in ski rotation/steering. The more parallel the femur is with the ski, the more that femur rotation becomes an edging movement. So if you're standing up with the knee and hip extended so that your femur is essentially perpendicular with your foot (or the ski if we were on snow), the femur rotation rotates your foot internally, assuming fixed tibia rotation. If you sit down so that your femurs are parallel with your foot and the floor, that same femur rotation will start to lift the feet off the ground and starts rolling the feet side to side. Any kind of femur rotation in between fully extended and being flexed to that parallel femur orientation starts to blend the two movements and when combined with tibial rotation can result in a lot of options between edging and ski steering. 2) Always interested in more reading material on the subject; we don't have a lot of in depth *scientific* material specific to the ski instruction world. The second sentence of the abstract about "our scientific understanding of how the underlying mechanics of alpine ski racing technique relate to performance is surprisingly weak" is so true. Actual scientific approaches to understanding this sport are definitely something I want to see more of; way too much "because that's what my trainer told me and now I tell everyone as if it's undeniable truth" dogma that may or may not be true out there. 3) I'm actually planning on doing another video more focused on this specific topic soon and appreciate your arguments here. I was very time limited in this presentation. I actually don't disagree with you on this point, but another point that I raised in the video but may not have done a great job of communicating is that the muscles involved in the lifting the outside edge of the ski up and the muscles involved in pulling in the lower leg in general are not particularly strong. These are muscles I actively train in the gym to strengthen for skiing, but I load somewhere in the range of 30 lbs for 10-15 reps at a time. Admittedly, there's a lot of anecdotal evidence behind my argument, however, my intuition when considering that we are relying on these small tipping muscles of the foot/ankle to lift a ski that I am balancing 200 pounds of human (for myself) over with the addition of turn forces on top of that suggests to me that there could be strength limitations, especially in the vast majority of the general population that probably don't specifically strength train these muscles. Especially when considering the hundreds to thousands of repetitions of this movement a skier may do in an average day, I would venture to think that the muscles would not be able to reliably and accurately repeat the movements at the level expected in high-end, technical skiing. And in my own, anecdotal experimentation, my logic seems to transfer on snow experimenting on both myself and when observing ski instructors I train seeing fundamental differences in their movement patterns simply by changing ski width. There may be some nuances in terms of moment arms and leverages that I'm failing to account for in my logic, however, my main point is not that it can't be done, it's that I question if it's the best, most repeatable, and/or most efficient way. To me, it's significantly easier and more accurately repeatable especially during a long day of skiing to use physics and forces to my advantage by using a narrower ski rather than have to actively and significantly strain a weaker muscle with a wider ski in less appropriate (i.e. firmer) conditions for that type of ski. I definitely left out some nuances in this webinar due to the aforementioned lack of time and will revisit it soon in a RU-vid-specific more refined video. I've heard of your cited study and will read it more in-depth, but I did actually purposefully choose to leave the knee health arguments out. While too wide of skis on firm snow do actually cause issues with my own knee pain thanks to some past injuries, it seems to be an argument that falls on deaf ears with a huge portion of the ski population. My brief skimming of the study though seems to suggest more forces being transferred through the knee and into the thigh muscles on a wider ski, which to me further represents my point that pulling the outside edge of the ski up is not enough to overcome the forces required to tip the ski on edge. I would be super happy to be wrong on this topic quite honestly; as I said before, we need more measured, scientific, and objective looks at these topics by people far smarter than I 😅 4) Cool topic idea! I will keep that in mind. I actually have some other perspectives on that topic to go along with it as well. Thanks again for watching the video and taking as much time as I'm sure it did to write your comment.
@@BrandonBock Thanks so much for taking the time to write that detailed reply!. I'd planned to respond sooner, but wanted to give what you wrote the consideration it deserved-lots of good stuff there!-so I put it aside for a few days, which then turned into a month :D. Anyways: 1) You wrote: " So if you're standing up with the knee and hip extended so that your femur is essentially perpendicular with your foot (or the ski if we were on snow), the femur rotation rotates your foot internally, assuming fixed tibia rotation." My thinking (and this may be the same as yours) is that, when you tip when standing extended on skis, the feet don't rotate relative to the ground, since they are constrained by the skis. Instead, your feet stay pointed in the same direction, so your tibias and femurs rotate into the turn. When I do this standing, I find this causes my pelvis to also rotate into the turn, which would result in a loss of counteracting (CA). [As expected, this doesn't happen if tipping when flexed.] If I do the tipping aggressively, I can actually impart an inward rotational momentum to the pelvis, which could result in a rotary action. But I suspect that people who use rotary aren't tipping so agressively to start with, and thus to the extent you see rotary initiation, it's due to a whole-body movement rather than from tipping when extended. In summary, the main issue I see with tipping when extended isn't that it directly creates rotary initiation, but rather that you lose the angle creation, and that it causes loss of CA. Your thoughts? Is there something I'm missing about how tipping when extended could create rotary initiation? This still leaves me wondering about the next part of the kinetic chain, which is the pelvis. I think we have a clear picture of how tipping when flexed can create knee angulation. But how does this then propagate to the pelvis to create hip angulation? And is there a difference between tipping when extended vs. flexed for this? 2) Agreed! 3) OK, got it. And I agree with what you wrote here. I didn't understand what you were thinking of, which is that ankle fatigue can result from repeated tipping of wide skis on flat snow, until I read your response above. That's probably because I don't use wide skis on flat snow, so this never occurred to me! Instead, that's something I'd encounter only intermittently on my pow skis, when I end up on firm patches. I can feel ankle fatigue in those situations, but it's more acute than chronic, e.g., if they're on edge at the bottom of a turn on a firm patch on a steep slope. So that's an isometric fatigue. And I can feel actual ankle discomfort if the skis break loose and then suddenly grab. So I think it's just a matter of presentation--making it clear that the above is what you mean, rather than that it's impossible to tip a ski when its edge is outside your boot sole. I.e., you'll want to rephrase statements like this (at ~1:07:30) : "...this is this is only possible...this tipping motion because my forefoot and the base of my boot are wider than the width of my ski and that allows me to direct my pressure past the edge of my ski...". I see now that you instead meant: "...this *easy* tippping motion is only possible...." 4) Another way to think about this is in terms of torque (cribbed from something I wrote elsewhere): When you ski, you create a ground reaction force (GRF), which results from your resistance against the force due to gravity plus the centripetal force from the turn. We can describe that force using a vector whose origin is at your base of support (BoS; let’s assume it’s somewhere near your outside boot), and that angles inward (towards the inside of the turn) and upward. Now imagine another vector that goes from your BoS to your CoM (note this is a location vector, not a force vector). In the middle of the turn, those two vectors typically point in nearly (but not exactly), the same direction. Their mild divergence generates a mild torque that you use to increase or decrease your inclination. However, as we approach the switch, and move our CoM closer to our skis, the direction of the CoM vector deviates strongly upward from the GRF vector. This divergence creates a substantial push + torque against our legs that, unless dissipated through flexion, creates the float. [As you know, if you push on an object anywhere other than towards its CoM, you'll create a torque.]
@@alfonsoeae thanks Alfonso! Love what I’ve seen you putting on Instagram as well. I was actually thinking about messaging you in the future to get some insight on how you’re doing your 3D anatomy visuals.
@@BrandonBock whenever you want... The last models are really amazing, I can show almost anything I want from a skeleton, muscular, body ski to a skier in ski suit... Audio done movements into the boots... I studied physics at university and I've enjoyed very much this webinar...
@@alfonsoeae Awesome, I appreciate it! I've had aspirations of doing some similar 3d anatomical visuals but I haven't had the time to figure out how I'd do it yet. I'll reach out on Instagram soon!
I am an engineer by training, I am interested in disassembling complex systems into simple understandable parts, you have a similar approach. I want to translate your video into Russian, dub it and post it on my channel if you give me permission to do this work. This December I will be studying to become a ski instructor and will try your method on myself.
It's on my radar but not yet, unfortunately. I'll have to get on some of our snowboard trainers to help put something together this winter! I'm primarily a skier but it was actually snowboarding that inspired some of the analogies in this webinar.
@@BrandonBock Я инженер по образованию, мне интересно разложить сложные системы, на простые понятные детали, у вас аналогичный поход. Я хочу перевести ваше видео на русский язык, озвучить его и выложить на своем канале, если вы даете мне разрешение на данную работу. В декабре этого года я буду проходить обучение на инструктора по горным лыжам и проверю вашу методику на себе. ))
