CSCS Practice Questions – Exercise Science Part 8

Posted by on Apr 28, 2013 in Exercise Science, Practice Questions, Review Topics | 2 comments

I don’t have any particularly ingenius way of remembering the answer to the previous question, so this is one of those you’ll have to just commit to memory. Calcium binds with B. Troponin to cause another protein, tropomyosin (which runs along the length of the actin filament) to shift allowing for cross bridge attachment.  The number of cross bridges activated by calcium sum up to equal the total force the muscle produces.

I struggled for quite some time with this post, because I really wanted to do this topic more justice.  Also, I see a lot of parallels in this chapter with concepts I’ve seen in engineering.  Depending on what type of learner you are, the rest of this post may or may not be useful to you.

The Twitch
One thing I encountered while reading the NSCA book that reminded me a lot of an engineering concept, is the idea of a twitch.  We learn about fast twich, slow twitch, and different muscle fibers…but let’s talk about the twitch signal in itself.

When your brain sends a signal to your muscle to fire, what does that signal look like?
Is it just an on button, or like plugging a battery in, or flipping a light switch?  I think the signal is actually represented pretty well by a pulse.

pulse

A representation of the signal sent from a motor neuron

The pulse is your neuron firing, an electrical signal which results in the release of calcium in your muscle cells.  If you wanted to represent this like you would in engineering school, your muscle could be represented with a block diagram and a transfer function.  The transfer function represents your muscles and would transform the electrical signal of the pulse into a force output.

Muscle represented by H(s), force by Y(s)

 

Or represented graphically in the time domain y(t) (force produced by muscle) is equal to the electrical signal sent from the brain transformed by the muscle x(t) * h(t)

The calcium binds to troponin (as we discussed earlier) for a brief period of time during which the muscle contracts.  If the first pulse isn’t backed up by another pulse a short time later the calcium will unbind and force produced by the muscle will drop.

muscle-xferfnctn

A single “on” pulse results in a small amount of force that falls quickly as calcium unbinds from troponin

If, however the pulse is backed up by another pulse, then the force produced by the two signals will add.

summation

Force from two signals sums if sent in quick enough succession

It’s in this way that you can recruit a large muscle to produce a range of forces.  Remember that when you recruit a large motor unit, all muscle fibers contract due to the all-or-none law.  But because the force produced by the muscles is modulated by the period in which your motor neuron fires, the amount of force you can produce in a given motor unit isn’t all or nothing but somewhat continuous.

And finally, this is what happens when your motor neuron fires at a sufficiently high frequency for a sufficient amount of time – maximal force production.

maxforce

Enough motor neuron impulses fired in quick succession yields maximal force production

This wraps up this set of questions.  If you would like to go back through them or other questions, visit my practice questions page.

2 Comments

  1. I really like how you’ve explained this, it is much better than the NSCA book does.

    • Thank you! This one took awhile, glad it helped someone =)

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