Posts Tagged "Exercise Mechanics"

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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Exam Study Strategy – Honesty! Honestly?

Posted by on Mar 7, 2014 in Anatomy, Study Strategy | 6 comments

Today after answering a few reader questions, I was thinking about what attributes make a person a successful test taker.  Some of these attributes may strike you as pretty obvious.  A test taker needs to exhibit or spend the following things to be successful.

  • Time – to study
  • Effort – Not just reading and letting the words slip past your brain while you study
  • Intelligence – The ability to grasp complicated concepts

Throughout school I met plenty of people who had all three of these things and still did poorly on test day.

Why?

Honesty.

I’m talking about being honest with yourself.  This is a different kind of honesty than just always telling the truth to other people, because being honest with yourself can be quite hard.  Let me give you an example:

I spent a good deal of time covering levers. But levers are simple right?  FLE123, boom. Done. How can that be hard?

Well for many it might be that simple, but when I sat down with the practice exams and started getting lever questions wrong…I had to stop and think. I had just learned a lot of anatomy, and so I started to overthink things.  Instead of a biceps curl being a first class lever, I started thinking everything was a third class lever because I was arguing (with myself) that the applied effort was where the muscle inserted and thus, applied its force to the bone. So for a biceps curl I was arguing effort was on the forearm, load at the hand, and fulcrum at the elbow – making it a third class lever (E in the center, third class).

But it’s not where the effort is applied, it’s NOT where the insertion is.  The ‘E’ in effort stands for where the force is generated, in this case the belly of the biceps muscle. My first run in with this topic had me thinking it was easy for me, but I was glossing over some details that tripped me up later. I had to have the conversation with myself and hash out where I was confused, but in the end I understood it more completely than ever.

So be honest with yourself, don’t gloss over things and say “I got that” when you really don’t.

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Exercise Science Review 1-6, Intro to Plyometrics

Posted by on Jan 23, 2013 in Exercise Science, Review Topics | 0 comments

Plyometrics is defined as a quick, powerful movement that begins with a counter movement, or “pre-stretch”.

Imagine you are about to take a jump, instinctively you bend at the hips and knees before exploding with violent hip and knee extension propelling yourself into the air.  The initial bend at the hips and knees is the “pre-stretch” and it stores energy in your muscle fibers and connective tissue and releases it upon contraction.  This is known as the Stretch Shortening Cycle, and as you can tell from the wikipedia article some of the science around exactly why this works is difficult to nail down.

Nevertheless, the bottom line is dynamically pre-stretching a muscle increases output power.  Whether this is by increased innervation from the CNS or stored energy in the elastic tissue, or some of both, the goal here is to know some of the theory and practical applications.

Muscle Mechanical Diagram

Muscle Mechanical Diagram

Above is a mechanical diagram that is often used to represent the components involved in the stretch shortening cycle.

Parallel Elastic Component (PEC)

The PEC or Parallel Elastic Component, is comprised of the epimysium, perimysium, endomysium, and sarcolemme.  It exerts a passive force when the muscles are being stretched but not activated.

Series Elastic Component (SEC)

The Series Elastic Component stores the majority of the energy in the plyometric exercise.  The SEC is comprised mostly of tendons, and they act like a spring.  As energy is stored during the stretch phase, the tendons lengthen and store elastic energy.  Following the eccentric phase, the energy is released if immediately followed by a concentric phase.  Otherwise the stored energy is lost as heat.

Contractile Component (CC)

The contractile component is exactly what it sounds like, the component of the muscle doing what we always expect muscles to do: contract.  Composed of actin, myosin, and cross-bridges as covered in chapter 4 of the book.  If you don’t know what those last words meant, start here on wikipedia: Actin, Myosin…though the wikipedia articles probably go into more detail than you need for the CSCS Exam.

There is more on plyometrics you will need to know for the CSCS Exam, specifically the three phases of the stretch shortening cycle as well as program design for plyometrics.  I will get into these topics later, but won’t be covering them in my first review series.

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Exercise Science Review 1-5, Levers and Mechanical Advantage

Posted by on Jan 19, 2013 in Exercise Science, Review Topics | 0 comments

This is perhaps one of the most challenging topics of the Exercise Science section.  In physics class, I always forgot whether a lever was class I, class II, or class III.  On top of this, you need to know points of origin of muscles and where they insert.  Basically, you have to know Anatomy AND Physics, two topics renowned for their difficulty…though I am exaggerating.  These are really two subtopics of both fields, and we should be able to get through them just fine with some careful thinking and good images (lol @ my photoshop skills).

To fully understand this post, you may need to review what Torque is, what a Lever is, and what Mechanical Advantage (or leverage) is.

Levers

  • Class I – A lever in which the load (dumbbell for example), and the applied force (by muscle in this case) act on the same side of the fulcrum.
  • Class II – A lever in which the load and the applied force act on the same side of the fulcrum, with the applied force having greater mechanical advantage (a longer moment-arm) than the load.
  • Class III – A lever in which the load and the applied force act on the same side of the fulcrum, with the applied force having lower mechanical advantage than the load (and thus a shorter moment arm).
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