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Is a wide stride for higher top speed misleading?

Evidence has shown that players who achieve the highest top speeds have the greatest stride width.  Stride length is the more common measure discussed by hockey coaches so stride with may be a foreign concept.  Stride width is the lateral reach of the legs as they extend away from the body during the forward stride.   High speed paired with a wide stride is a correlation, or a situation where two phenomenon are typically seen together whenever they are seen.  As has been said many times over the history of science, "correlation does not necessarily imply causation".  When two things occur together on a consistent basis, there are a few possibilities about what is happening; the first thing could cause the second, the second could cause the first, or a third phenomenon could be causing both.  To understand how these things are related, we'll have to decipher this riddle of causation.   Before we get into that, we should look at this evidence referred to above.  The most convenient source of evidence for the correlation between stride width and speed can be found website of Dr. Michael Bracko who has executed many studies on hockey skating (View Dr. Bracko's Website Here).  If you surf around on that site to the areas that focus on his research and look at some of the published studies, you will find a a handful of results that show the very correlation we are talking about between stride width and top speed.  So some evidence is there (and if that evidence isn't enough, we can rely on physics to explain not only that we should expect this correlation, but also, as we will see, why there is a correlation).   (I have referred to Dr. Bracko's website in a previous article.  Find all of our previous articles at this link:  Previous Hockey Skating and Physics Articles)   What stride width do sprinters (runners) execute?       To illuminate our subject, we can take a look at the methods used by sprinters.  Viewing skating and sprinting together can tell us a lot, usually not by comparison, but by contrast.  Stride width is an area where it is 100% contrast.   In skating, stride width is an asset that leads to a higher top speed. In running stride width is basically absent and common sense says it would be counter productive.  So, what is it about skating that makes stride width necessary?   Part of the answer comes from the design of the ice skating skate blade.  The blade is designed to take advantage of friction when we need it and eliminate it when we don't.  Friction, in practical terms, speaks to the grip that exists between one surface and another.  In hockey skating, the surfaces that come in contact are cold steel and ice.  For all practical purposes in the study of hockey skating technique, there is zero friction between them.  This is what allows the skate to slide along the ice in the forward or backward direction.   But we need some way to push our body in the direction we want to go.  To do this we sharpen the two edges of the skate blade.  This allows the blade to sink into a depth into the ice, creating a ridge that we can push against.  This ability to push against the ridge isn't truly friction in the traditional sense. However, by creating this ridge, our skate, in essence, creates perfect friction (grip) in the direction perpendicular to the blade's edges.  So any time we want to push ourselves, we must turn our blades perpendicular to the direction we intend to push.  Any time we want to glide, we want those edges parallel to the direction that we will be gliding.   With "perfect" friction in the lateral directions, shouldn't we turn our skates perpendicular to our direction of motion during skating?   In forward skating, we have moments where one skate is pushing and one is gliding.  Shouldn't these skates be perpendicular?  This would be a pretty reasonable approach based on the information provided so far.  Plus, in some sense, this would match our understanding of the sprinting stride from running but with the additional benefit of being able to glide with the skate that isn't pushing.   The following is what the forward skating stride would look like if we based it on the design of the skate blade and little other information (as hinted above)... To skate straight ahead we could turn our feet perfectly sideways with the toes pointed away from the other foot during the "pushing" part of the stride.  During the glide phase, our toes would point straight forward in the direction of motion.  We would ignore for now that it would be very difficult turn our feet quickly when switching from the glide position to the push position when we switched phases (from glide to push).   Just as in running, we would be pushing straight to the rear and would have no stride width.  As long as we can produce enough force at high speed with our foot turned out (toe pointed away from the other foot) as we can with our foot in normal position (forward facing position) we should be able to achieve amazing speed.    Contraction Force vs. Contraction Velocity   But this does not work.  We have accounted for the skate blade and how it interacts with the ice, but we have not accounted for the human body.  One simple fact makes the rearward push with the toe straight sideways a poor choice for the forward stride.  We will now consider that contraction force diminishes with increased contraction velocity.   This graph is intended to give a qualitative idea of how contraction force diminishes with increased contraction velocity.   The contractions we are talking about are human muscle contractions.  The fact that contraction force drops off the faster we have to move our muscle during contraction means that, in running, the faster we are going, the less forward force we can produce.  The same is true in the skating stride proposed above. If we push straight back, our foot has to move backward at the same speed that we are going forward.  The faster the skater moves, the less force he can produce if he is pushing straight back.   But the design of the skate gives us options.  Because the skate can glide in the forward direction, we need not push against a foot that is stationary relative to the ground (ice).  Instead we can push and glide at the same time on the same foot.  In order to do this, we must turn our toe not straight forward or straight sideways but somewhere in between.  This allows the foot to move forward down the ice as the leg extends (this extension of the leg is what pushes our body forward).  If we turn it only slightly forward, it will move only slightly forward (down the ice) as it moves to the outside and our leg extends. If we turn our foot almost straight forward it will move much further forward as our leg extends.   Which option leads to a lower contraction velocity?  To answer this we need to understand that we will be moving down the ice at a somewhat constant speed.  Therefore if one option lets us move further down the ice while extending our leg, that option gives us more time to do that. More time to complete extension clearly demands a lower contraction velocity allowing us to produce more force.   Turning the toe of the foot forward gives us the most force, but this still isn't the whole answer.   If we turn that foot forward we can produce more force, but what direction are we pushing?  In order to take advantaged of the lateral "friction" of the skate blade, we would push perpendicular to the blade.  This means we are pushing very sideways if the toe of the foot is pointed nearly forward.  Now, finally, we are getting a picture of a "wide" stride as pushing laterally leads to the feet going out to the side.   But pushing laterally pushes our body laterally... not in the direction of motion which is forward.  So by turning the toe forward we run into a trade off.  On the one hand the reduced contraction velocity allows us to produce more force.  On the other, the bulk of that force is in the lateral direction.  In order to achieve the highest possible speed, we need the most force in the forward direction.  This means finding the perfect angle to turn the toe that leads to the maximum value when multiplying the total force produced by the percentage that is in the forward direction.   These demonstrate the forward force production we can expect when we turn our skates either too wide or not wide enough.  Note, the wider stride (left) leads to greater force produced (longer blue line pushing to the side) but no increase in forward component force relative to the narrower stride (right).  The large arrow represents the direction of skating by the athlete. Here the toes of the skate are turned out perfectly, maximizing forward force.  The balance is achieved between producing force and keeping that force in the forward direction.   This is complicated.  It is moreso when you ask how this perfect angle that leads to maximum forward force changes depending on the speed you are traveling.  Without getting into the math, the important thing to understand is that, as we travel faster, we need to have the toes of our skates pointed more forward and we need to push more laterally.  In other words, at higher speeds we will need to have a wider stride.   The Chicken or The Egg Research has shown that faster skaters tend to have a wider stride.  Now we have seen how a higher speed demands a wider stride.  So, which causes which?  It would be simple to say that it is the speed that forces the skater to have a wider stride.  On the other hand, skaters need to produce the most forward force they can in order to achieve the highest speeds, so one could certainly say that the fastest skaters must have a precise knowledge of the correct stride width for all speed ranges.  So, indeed fast skaters need to know how to produce a wide stride and therefore the idea of the having a wide stride for higher top speed does have merit. Bringing it back to the game...   But when playing hockey, we spend much more time in the lower speed ranges than near top speed.  In fact, it is more important to get through these stages in order to have the opportunity to get to the higher speed ranges. And, in starts from a stop, we should indeed turn our toes straight sideways and push straight back for 2 or 3 strides, making a wide stride very counter productive at low speed.   In the end, a wide stride for higher top speed is accurate.  It is however misleading as a training guide.  If one was to train only for a wide stride they would suffer greatly in the low and mid speed ranges that dominate the majority of the game.  Instead, players should train for the narrow stride during starts, a medium stride width for the "middle gears", and the wide stride to pull away when speed is at a premium.

In the previous segment we looked at a skating technique adjustment that led to more speed, a slightly shorter stride length, and a greater stride rate.  While extolling the virtues of this adjustment we knocked stride length off of its metaphorical pedestal as the key metric in excellent skating technique.  While, the rumors of stride length's demise may have been greatly exaggerated (as stride length is still a hugely important concept that skaters should value in many ways), dethroning it does leave room for the true king of hockey skating concepts.  All hail knee bend!  Long live the king.

 

Readers of last week's article would remember discussion about how, in the forward stride, forward horizontal force production is what it all boils down to.  Wouldn't that make that the king?  It would... if we were looking to focus on the result that we are trying to produce.  But, what we are talking about here is a question of how we can generate that result (how we can produce maximum forward horizontal force).  Knee bend is the single most important variable that helps us achieve that goal.  And knee bend is so important, it makes the abbreviated recovery strategy from the previous "Hockey Skating and Physics" segment seem downright trivial.

 

(See the May 11, 2011 article below)

  

Increased knee bend has an effect on two key stride metrics which lead to speed.  This dual effect is what makes deep knee bend such an important component of the high-performance skating stride.  We will delve into both the increased extension length and flatter extension force direction that deeper knee bend leads to. 

 

To understand extension length, it helps to use the skater's body as a reference point from which to take our measurements.   

 

When we measure stride length we are talking about the distance down the ice that a player moves during each stride.  To take this measurement, it requires us to use the rink to establish a baseline from which we take our measurements.  In other words we are measuring relative to the rink.  But using this, it is hard to learn much about the effects of knee bend.  For that we should use the skater's body as the central point that we measure from even though the skater would be moving around the rink.

 

Using a perspective that treats the body as stationary, we can talk about extension length in the simplest terms.   Extension length is the distance along the skating surface that the player pushes the skate away from the body.  At the beginning of this extension, the skater's leg is at a maximally bent position and his skate is very nearly underneath the body.  At the end of extension, the leg is (ideally) straight and the skate is out away from the body.  In between, as the skate moves away from the body during extension, the skater's muscles are applying the propulsive force that we so covet to push the body forward.  Extension is the part of the stride where we push ourselves forward so it stands to reason that more extension length would lead to more force and more speed. 

 

What does deeper knee bend mean for extension length?  

 

Imagine a person standing straight up with both knees locked straight.  How far to the side along the ground can that person reach with one leg with all of their weight on the other leg and neither knee bent?  If you don't allow the pointing of the toe, the answer is zero distance.  The only place that foot can touch the ground would be right next to the other foot because with both legs straight, bringing one leg to the side means you can no longer touch the ground with that foot.  This ultimately means that with zero knee bend you can achieve zero extension length.

 

Now, if we allow the knee to bend on the support leg and then reach out to the side with the other leg, how far out can we tap the ground with the leg that is reaching out?  We assume that the leg that is reaching out is locked straight as this gives the maximum outward reach.  Now the answer is, "it depends".  You may ask, "it depends on what?".

 

It depends on the height of the hips.  The closer the hips are to the ground, the further out along the ground the foot on the extended leg reaches.  This spot along the ground where the foot touches is the point of maximum extension and the bigger that maximum the greater extension length we can have.  So one huge benefit of greater knee bend (and the lowering of the hip joints that it creates) is an increased extension length.

This figure offers more detail than this article. Please don't hesitate to ask any questions that the extra detail may bring up.  Note that in the figure we are talking about the propulsive phase of the stride.  For the purposes of the article, the "propulsive phase" can be taken to mean "extension phase".

Flattening our Extension Force  

 

When we flatten something, one way to define that is to say that we minimize the difference in height from its highest point to its lowest point.  Our extension force by nature will always have a vertical component and a horizontal component to it.  This means that it is directed upwards at an angle.  To flatten this force would be to diminish the vertical component and increase the horizontal, or get it closer to the horizontal.  Greater knee bend accomplishes this for us.  Lets look at how it does this.

 

As we extend our leg and produce force to drive ourselves forward, our leg straightens out (which is obvious).  This results in a force that is on a line starting at the point where our skate blade edge interfaces with the ice and goes through the hip of the same leg.  If we extend our leg when our foot is directly below our body, this force is essentially all vertical.  Do this with enough explosion, and you will jump.  But jumping does nothing for us in terms of building speed in hockey.  It is horizontal, not vertical, motion that we need. 

 

Consider the final moments of extension (just before the leg gets fully straight during a hockey stride).  In these moments, a violent push of the skate into the ice would drive the player's body away from that push (as was the case before).  But, since the foot is now way out to the side of the body, and since the force of such a push would go on a line from the place where the blade edge interfaces with the ice through the hip joint of the same leg, this force would drive the body not straight vertical, not straight horizontal, but a diagonal in between.

 

Now consider this same final moments of extension with greater knee bend.  The hips would be lower and this force would be directed closer to the horizontal.  Since we are looking to maximize horizontal force this is huge for our ability to achieve speed.  Within the same body, if we get in the habit of skating with deeper knee bend, we can produce the same force and yet achieve greater horizontal force.   

 

Combine the two effects of knee bend to understand its importance.

  

With greater knee bend we can increase our extension length AND improve our horizontal force production even without training the body to produce more force.  By combining those two effects we see that it is simply a huge factor in our ability to produce horizontal force and to get from place to place on the ice more quickly, efficiently, and effectively.

 

 

 

So far we have looked at the impact of knee bend in terms of the forward stride and horizontal force production.  In crossovers, increased extension length and horizontal force add to speed and lateral acceleration which are the goals there.  In the backward stride, increased knee bend helps us produce more rearward horizontal force.  Knee bend also helps with performance on stops, starts, and tight turns.  All of these effects of knee bend in all these areas of skating are related to one or both of the effects discussed above (increased extension length and/or flattening of the extension force).  The effects and benefits of knee bend in high tempo skating remain present regardless of what on-ice maneuver or stride technique we are attempting.  And if it wasn't so critical in all facets of skating... well then it wouldn't be the king.

When physics nerds take an interest in sports it is pretty common for them to analyze the actions on the field, court, or rink from a physics perspective.  I am certainly a hockey nut and I would put myself in the physics nerd category, yet that thought hadn't crossed my mind until 2005.  This is the story of the event that led me to do just that specific to hockey skating technique.  

 

During that spring of 2005, I had the opportunity to observe the front lines of a discussion among the world's hockey skating experts about the optimal technique for the forward stride.  It took place in a banquet hall at a Sheraton Hotel in Novi, Michigan during the International Hockey Skating Symposium.  It was two and a half day event featuring numerous strength coaches, skating instructors, and researchers.

 

For the bulk of the event, skating instructors preached the the tenets of stride length plus a recovery (return of the foot underneath the body after a stride push) to the outside edge directly underneath the center of gravity of the skater.  Then, in the final presentation of the event, Dr. Michael Bracko (Dr. Bracko's Website) presented evidence that the fastest skaters among us actually have a slightly shorter stride length than those just slower than them.  His evidence also showed that these fastest skaters recover not to the outside edge underneath the skater's center of gravity but to the flat of the blade under the hip of the recovering leg. 

 

Before I get into what makes this interesting, I should mention that these instructors who were preaching stride length were doing what they should be doing.  This is because the technique adjustment described above is a pretty minute detail that an instructor may bring to bear with a student who already has tons of very good things going for them with their stride.  Plus, preaching stride length is a great way to help players understand many of the things that will improve their stride.

 

Yet, this debate among hockey skating instructors is interesting because the instructors weren't preaching stride length as a teaching tool that helps skaters better understand how to produce an efficient stride, they were talking about it as the key concept in the stride they have their students ultimately work toward.  The experts were consistent with this message throughout the event until that last presenter showed evidence that provided strong resistance to that idea.  In other words, many of the luminaries of the hockey skating instruction world spent the event consistently stating something that appeared to be wrong according to the evidence provided at the end.  

 

 

Lets look at the evidence that was presented.  It was the top few percentile skaters who stood out.  For almost all of the other skaters, stride length was in direct correlation with speed (meaning that more speed was accompanied by a longer stride in most all cases other than at the very top) and this matches common sense as more distance down the ice per second (speed) would logically lead to more distance down the ice per stride (stride length). 

 

Why is it that it is only with the very fastest skaters where increased speed and increased stride length didn't show a strong correlation in the data? 

 

Among the fastest skaters something different is happening. 

 

A simple equation will help us understand this phenomenon...

Stride Length * Stride Rate = Speed

The truth is, we cannot go faster without producing more force, so this equation can be deceptive if used incorrectly.  With that said, if we want to increase speed based on this equation, we have a few options; increase stride length or increase stride rate (or both).  One question that could come to mind is, "which is preferred?"  Traditional hockey skating instruction will tell you stride length is the answer.

 

Compared to Stride Length, Stride Rate is under-appreciated 

 

If you see a skater accelerate, what happens to their stride rate?  It almost always increases.  When we need to accelerate, we increase our stride rate... interesting.  That is what these fastest couple of percentage points of skaters were doing.  They did have a slightly shorter stride length than the skaters just slower than them, but they had a greater stride rate.  They were executing more stride thrusts per minute, and this led to greater overall forward horizontal force produced on average over time.  You do that and you will be faster.  Forward horizontal force production is the key to forward speed (and forward acceleration).

So, if they have a shorter stride length, were they not reaching full extension?  No!  Full extension is still key.  In order to gain anything from an increased stride rate, you must not shorten up your extension (except with the possible exception of the first few strides during a start).  Without shortening up the extension, are there any other technique based strategies we can use to increase our stride rate?  There is one and it has already been alluded to above.  I call it "abbreviated recovery".

 

For a player with a given athletic foot-speed, to increase stride rate, preserve full extension and shorten recovery slightly.  

 

These skaters were shortening the recovery phase of the stride by recovering to the point underneath the hip of the recovering leg.  This also shortens the extension phase of their stride, but not the all-important final moments of the extension phase where maximum horizontal force is produced.  Instead, they were shortening the beginning of the extension phase where almost no forward force can be produced.  By eliminating this time where the skater is not producing forward force they increased the percentage of time where they were producing stride thrusts creating more forward horizontal force on average over time paving the way for more speed and acceleration.

 

There are a couple of things to note about this abbreviated recovery strategy.  First, this is not a "Railroad" stride where recovery is abbreviated too much.  Second, to do abbreviated recovery correctly requires tremendous skill and athleticism.  Only polished skaters with high athleticism and skill are ready to make this adjustment (though some do learn this even without instructor input through the course of learning to skate). 

 

Abbreviated recovery is one speed enhancing strategy that decreases stride length.  Consider that there are many speed-increasing technique strategies which lead to longer stride lengths.  This is why instructors should use stride length as a teaching tool.  In most cases it is a key concept for skaters to focus on to improve their stride technique and habits.  

 

While our habits can never be greater than our capabilities, it is our habits, not our capabilities that we use in a game.   

 

Should a player be thinking about these technique issues during a game?  Of course not.  A player in as complex of a multitasking environment as the game of hockey has no room in their mind even for a concept as simple as lengthening their stride.  The player needs to focus on the tactical and strategic options he, his team, and the other team will bring to the game.   Likewise, during the learning process, players must focus on a concept they can understand in order to master that concept in the form of a movement pattern.  Once that becomes a solid habit we can build on it with a more complex concept.   

 

This small adjustment to the recovery phase of the stride and the debate it incited among hockey skating experts demonstrated a lack of consistent understanding among the most knowledgeable hockey skating minds in the world.  The thought that more understanding was needed led me to analyze the physics behind many parts of the hockey stride.  I look forward to bringing more to you in future segments.

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