Mechanical energy and propulsion mechanics in roller-skiing double-poling at increasing speeds
Jorgen Danielsen, Oyvind Sandbakk, David McGhie, and Gertjan Ettema
(July 28, 2021) This is fresh off the press!
Introduction
For having a thesis focused on double pole research, it took me longer than I’d hoped to accurately analyze, fully digest, and hopefully make sense of the latest double pole research coming from Danielsen, Sandbakk, and the whole crew over in Scandinavia. Receiving email notifications from Pubmed is almost identical to reaching into your Christmas stocking and discovering an extra candy cane. The way I open these tidings is to gad from the abstract to the discussion, skip over the charts and skim the conclusion, and then, befuddled and disoriented, shamefully traipse back to the top of the page and labor diligently to investigate each section sequentially. I have come to expect skilled scientific writing from NTNU; if you pay attention, and look up unknown terms, each premise connects to the next.
The disciplined pen of this prose doesn’t attract the same volume of customers on WordPress, which is why I am not compelled to invest the sweat equity required to produce it. On this note, I am compelled to share a humorous anecdote. Dr. Sandbakk, a philanthropist to the world of ski research as evidenced by his shepherding of lost sheep in America like myself, has happily lended his time and expertise in my own thesis research development. His generosity in guidance and support has been invaluable. After donating time – which he undoubtedly does not have – towards the effort of reading my 100+ page review of the literature, he gave me some advice. Basically, the path to whittling down my unnecessarily exhaustive commentary on past research was to add a pinch more “scientific journal style” and go easy on the “Seder-Skier.com” (my summation, not his exact words). In my thesis proposal presentation, I of course welded my own mold, injecting a dynamic video clip Johannes Klaebo getting sniped at the line (to show the progression of the double pole….not something done in jest to rib my Norwegian friend), complete with my finest Chad Salmela impersonation. His appreciation showed up in the comments, as he noted both my enthusiasm for the content and my clear revealing of my true calling as the host of a ski podcast. All of this is to hopefully point to a clear appreciation of his involvement in my ski journey; I am honored to have him provide counsel and steer me in the right direction when questions arise.
I guess the real point of all that is to say I hope to provide a writing style which merges the people who have subscriptions to Sports Illustrated (a fair chunk of people) with the ones who receive the European Journal of Applied Physiology in their mailbox (probably less).
The goal here is to be able to summarize the findings in laymen’s terms and give the citizen racer something they can use. Just remember: These guys know how to study ski-runners, and I know how to run my mouth. I don’t mean to trumpet a sort of faux humility, but rather, accurately establish where I’m coming from – I like this stuff, but I may arrive at a few false conclusions. I apologize. If we’re lucky, maybe one of the distinguished authors will clarify, correct, or further the discussion in the comments.
Main Idea
The focus of this research was to look at the effect of speed on mechanical energy fluctuations and propulsion mechanics in the double pole (DP). In other words, what is the relative trunk+leg power contribution and arm power contribution to DP on the flats as speed increases.
The findings: as speed increases, the relative power contribution between those two entities changes very little. At 15 km/hr, the contribution to poling external power was 63% from the arm and 37% from the trunk+legs. At 21 and 27 km/hr, it changed to 66% arms and 34% trunk + legs.
This might leave you thinking:
What is the role of the legs in double poling?
Four words – body mechanical energy transfer.
“With minimal option for the legs to contribute directly to poling power during the poling phase, the only energy source left that can contribute to doing poling work is body mechanical energy transfer.”
Danielsen et al., 2021
This quote from the article is a fancy way to say that while your legs contribute to pole force production, you don’t strap your poles onto your boots (although, maybe Liebner and USSPC is looking to develop a space for this type of a ski pole technology… I’ll keep you posted) If ‘body mechanical energy transfer’ almost scared you away – stick around – we aren’t going to conjure up dark memories from middle school Algebra….we want you to feel comfortable here. I don’t see any hands that are up, but that’s probably because you forgot deodorant. Unlike your teacher, I’ll try and answer the questions which aren’t getting asked.
Basically, during the swing phase, you generate power in your legs as you extend your body and poles into a heightened position. When you ‘fall’ onto your poles during the initial engagement of the poling phase, this potential energy decreases. A lot of that energy turns into mechanical energy in the poles. However, some of that is absorbed by the legs.
Interestingly, it appears that incline determines how much of that potential energy goes into the poles and how much of it has to be absorbed by the legs. Prior research from 2019 indicated that more body energy was was transferred to pole power at steep inclines compared to moderate inclines.
“Essentially, at steeper inclines, the poles can take more body weight than while at moderate inclines, where more of the downward momentum generated by lowering the body must be counteracted by the legs, leading to greater energy absorption.”
Danielson et al., 2021, talking about the research from 2019
A side note – my thesis is on the impact of strength/weight relationships and double pole performance at incline and flats. For this reason, I found the above quote to perhaps provide further validity to my research. If you know of any elite 20-30 year old skiers who want to participate, have them look me up.
how to apply to your cross country skiing and double pole performance
- The heightening-lowering of the body is essential to double pole technique at all inclines, but especially on the flats, where an increase in speed is going to hinge mostly on exceptional technique, not superior lower body conditioning (or upper body conditioning, for that matter). Since the relative contributions from the upper and lower body don’t change as you speed up, you can’t get away with ‘relying more on your arms’ or ‘relying more on your legs’ to speed up. You need to know how to get into a good, advantageous, high and forward position. This is probably the first place where the smart dudes should either correct, clarify, or comment, because I could be wrong here…
- Moderate inclines might be my baby….If it is true that moderate inclines are where the legs have to absorb a lot of the body’s potential energy in addition to working to repositioning the body for every cycle, I imagine the endurance runner or biker might thrive on those moderate climbs more so than the very steep climbs. This fits my skill set and background. On the very steep climbs, more force CAN be absorbed by the poles, which might be why the only people who can force themselves over really steep sections with the double pole have to have a certain level of upper body power or even upper body mass…just speculation….smart dudes, where are you?
- Generally, focus on athletic, optimal movement of the COM. In this study, better skiers had less vertical displacement of the center of mass (COM) throughout the cycle, which led to a lower energetic cost of double pole (DP). However, the present study seems to be arguing that vertical COM fluctuations and active leg work are essential to DP. Well, which is it? It seems like in the Zoppirolli study, the excess movement occurred near the end of the pole phase, when lowering the body. If you go too low to follow through on your double pole, you are going to be absorbing more energy in your legs, which isn’t good. If you are going to dial in on anything, focus on the repositioning of the body during the swing phase and work to improve your lower body explosive power, forward hip-thrust, and high hands position at the top of the cycle.
recommendations for improving your double pole technique for cross country ski performance
Get out and DP…. alot.
Watch video of beautiful double-polers and imprint it in your mind as a visual to shoot for.
Video yourself if possible, from various angles.
Finally, DP on a variety of terrains, especially the ultra steep stuff. I think it can help your strength, your technique, and your guts.
Seder-Skier.com 