Reader Question: Late Support Phase of Sprinting

Posted by on Mar 17, 2014 in Reader Question | 0 comments

I got a question today, but the question had pretty specific text and references to NSCA copyrighted material. On top of that, it references a diagram in the textbook…so you can see I have a number of challenges in explaining this material. So this is not a direct quote from the reader, but the text has been changed.

Hey! I got a question regarding the late support phase of running. The question asks what muscle action is acting to propel the runners center of gravity forward. (Text Figure 17.6 p 465-67)

Great question, and I know for a fact that this portion of the book was a little bit confusing for me too. Sometimes my intuition about what’s going on during sprinting wasn’t spot on, part of this is that static diagrams don’t help as much as you would think when you are thinking about high-velocity sprinting.

First let’s think about some of the key concepts from the question:
muscle action – concentric, isometric, eccentric
propel – drive, push, or cause to move in a particular direction, typically forward (google definition)

This question is tricky because a lot of things can propel your center of gravity forward. If you are standing straight and then raise your right knee (concentric hip flexion) your center of gravity just shifted forward a little bit.

So let’s take a look at a diagram I made. I sketched this out by hand from the book (for copyright reasons I’m not using the original) but the book also reprinted this with permission from Track and Field: The East German Textbook of Athletics by Schmolinsky (1).

Five Phases of Sprinting

Figure 1

Questions like these aren’t easy to answer. You can look at the diagram and come up with the wrong answer. Why? The diagram kinda sucks. It doesn’t fluidly show the entire late support phase on one foot. You have to follow the right foot at early support  phase (iv) and then switch to the left foot on the other side of the diagram for late support (v) phase. Would be nice to have a good drawing of the transition from the early to late support phase on the same foot. If any of you are drawers and can make a good drawing of this, let me know! I did an internet search, but most of the diagrams out there are wrong and involve heel striking – a blatant error in running technique.

ANYWAYS – back to the question at hand:

Think about the three muscle actions (isometric, eccentric, or concentric), which ones involves propulsion?
Concentric, of course. Isometric is for stabilizing or holding still, eccentric is for absorbing force or decelerating, and concentric is for movement (this might be an oversimplification…but I can’t think of a counter example at the moment).

Now look closely at the late support phase (v) and you’ll notice a few things happening:

  • Right leg moving forward (concentric hip flexion)
  • Right knee angle opening up (concentric knee extension)
  • Left side of hip going into extension (does this mean concentric hip extension perhaps? or eccentric?)
  • Left knee heading from slightly flexed to extended (concentric knee extension)

This is tricky. 

The two key frames are here:

2sprintframes

Figure 2

 

Let’s get rid of half the possibilities with this argument. Your right side isn’t touching the ground, and since you are already at speed moving your right leg forward is more about getting in position to land for the next foot strike than doing anything for your velocity. So we are left with

  • Is the R-hip in going into concentric flexion? 
  • Is the R-knee going into concentric extension? 
  • Left side of hip going into extension (does this mean concentric hip extension perhaps? or eccentric?)
  • Left knee heading from slightly flexed to extended (concentric knee extension)

Working from top to bottom still, let’s try and figure out if the hip is going into extension eccentrically or concentrically. 

For a moment, let’s think of the leg as a pendulum.

Fig 3 – Oscillating Pendulum – Source: Creative Commons

Notice that when the pendulum reaches horizontal, it’s horizontal velocity is maximum and horizontal acceleration zero.

Now think of your leg as this pendulum. Sure the comparison isn’t perfect, because your leg has muscles and can move actively — but there are also a lot of similarities. Both are at rest horizontally (if rest is considered standing), their equilibrium point is the same, the points where they reach maximal and minimal velocity are the same. Acceleration points may be different, but figuring that out becomes a complex bio dynamics problem.

Let’s review what we know about the left hip (from Figure 2) in the late support phase:

  • It’s past it’s point of maximal velocity, just like the pendulum when it has swung to the left side
  • Since it was at maximal velocity, and is headed towards minimum velocity it must be slowing down (decelerating)

In order for that to happen, you must be in eccentric hip flexion. Your illiacus, psoas, and rectus femoris are actively contracting yet lengthening in order to slow that leg down. Since that is the definition of eccentric muscle action, it is not contributing significantly to forward propulsion.

Thus, only one answer remains:

  • Is the R-hip in going into concentric flexion? 
  • Is the R-knee going into concentric extension? 
  • Left side of hip going into extension (does this mean concentric hip extension perhaps? or eccentric?)
  • Left knee heading from slightly flexed to extended (concentric knee extension)

But, there is one more piece I missed from the initial observations:

Figure 4 - Ankle angle opening up in plantar flexion

Figure 4 – Ankle angle opening up in plantar flexion

Concentric knee extension and concentric plantar flexion both contribute to forward propulsion.
Eccentric hip flexion doesn’t propel you, but it gets you ready for the next stride.

These types of questions are tricky. Recreate them by running yourself and thinking about which muscles are contracting, and in what way. Sometimes figuring these things out requires a lot of sitting around and thinking, and if that’s not your thing just memorize the table.

 

(1) Schmolinsky, G., ed. Track and Field: The East German Textbook of Athletics.  Toronto: Sport Books.  1993.*
*neither the author or publisher were contacted for use of this sketch, please contact me if you wish to have it taken down

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