Is a wide stride for higher top speed misleading? 

In Part 4 of the Physics of Hockey Skating, we solidified our understanding of how we can leverage knee bend to maximize the amount of our stride push that lies in a horizontal direction.

The next two parts will consider what can be done with the direction of our pushes while only considering horizontal push directions across the ice surface. The goal within this picture is to maximize force in the forward direction (and thus maximize our potential for speed).

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 width 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 phenomena 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". This is to say that, when two things occur together on a consistent basis, there are a few possibilities about what is happening;

1.       One thing could be causing the other

2.       The reverse (“the other” could be causing the first thing considered)

3.       A third phenomenon is 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 on the website of Dr. Michael Bracko who has executed many studies on hockey skating (http://www.hockeyinstitute.org/power-skating-nhl.htm). 

If you surf around on his site to the areas that focus on his research and look at some of the published studies, you will find 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).

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 counterproductive. 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 a certain 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 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.  Anytime 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 describes 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 impressive speed.

However, 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.

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 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 one can produce when 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 using 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 mostly 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 down the ice during the time that 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. 

Thus, if we turn our toes forward so we can glide down the ice as we push, we can reduce the muscle contraction velocity for that push and increase the amount of force we can generate from that push.  This is the reason why we don’t push straight back in the forward stride. 

However, the story is not quite complete just yet as we don’t have any way to determine how much we should turn our toes to the outside or what angle to push between rearward and lateral in the forward stride.

In Part 6 we’ll put the pieces together to create the full understanding of how stride width is advantageous to any ice skater looking to maximize speed and acceleration.