MadSci Network: Anatomy

Re: What determins the speed of a muscle reaction and execution?

Date: Wed Mar 21 12:09:28 2001
Posted By: David Burton, Post-doc/Fellow, Physiology, University of Oxford
Area of science: Anatomy
ID: 984712419.An

Dear Pradu,

Thank you very much for your interesting question.  I hope you are able to 
understand my anwer.

You seem to be familiar with the sliding filament model of muscle 
contraction.  This is now accepted as the definite answer to the question 
of how muscles shorten, with the exception of one or two individuals who 
are considered to be madder than the rest of us.  This is summarised well 
in a previous Mad Sci network  
answer, which 
includes links that I have included here for speed of access, first to an 
overview of the mechanisms of muscle contraction and 
to a movie 
 of the sliding filament model. 

I shall now get into answering your question, which appears to be in 
several parts so it is probably best if I deal with it in parts.

Muscle shortens because the head of myosin (thick filaments) binds to actin 
(thin filaments) undergoes a change in shape (isomerization), which pulls 
on the actin past the myosin.  The thing that determines how fast a muscle 
can shorten is how fast the myosin can undergo this process and then return 
to its starting position to get hold of actin and pull it along again.  I 
strongly recommend looking at the 
movie of the sliding filament model to help you understand this if you 
haven't done so already. 

For a more detailed look at what a single molecule of myosin is doing to a 
part of an actin filament look here. 

The human body contains different skeletal muscle fibre types, which are 
termed type I and type II.  Type I are much slower compared with type 
II.  The reason for this is they contain different isoforms of myosin.  The 
isoforms of myosin are termed alpha and beta myosin.  alpha myosin is the 
faster isoform of myosin relative to beta myosin and is therefore the 
predominant isoform found in the fast type II fibres.  alpha myosin 
attaches to actin and very quickly undergoes the isomerization pulling on 
actin and then rapidly detaches from actin returning to its original shape 
so that it can reattach to actin and pull on it again.  Therefore it is 
capable of making the muscle shorten very quickly.  beta myosin, on the 
other hand, attaches to actin and then the time taken to undergo 
isomerization and detachment from actin is much longer.  Therefore the 
muscle containing this myosin type will shorten much more slowly.  However 
the myosin remains attached to the actin for a longer period of time.  
Whilst myosin is attached to actin after it has gone through its 
isomerization step then it is generating force, so beta myosin generates 
more force than alpha myosin.  So you can see that muscle that we want to 
shorten quickly, like the ones we use for running would be fast muscles and 
contain mostly alpha myosin, whilst muscles that we use, for example to 
keep us standing upright that need to generate a lot of force but not to 
shorten very quickly, will contain mostly beta myosin.

I will now provide brief answers to your questions in light of this 
What determines the speed of reaction to the enzymes? 
The enzyme in this reaction is myosin and as you can now probably work out 
it is the myosin itself that is responsible for the rate limiting step 
under ideal conditions where there is free availability of all reaction 
products (I shall come back to this in relation to your question about 
oxygen consumption).

Is there a limit to a speed that a muscle can contract?
The simple answer is yes.  This is called the unloaded shortening velocity 
of a muscle, ie how quickly the muscle can shorten when there is no load 
applied to it.  When this is investigated in a piece of muscle, when the 
conditions are set so that there are no other reaction products limiting 
the rate of reaction, then it is as fast as the myosin is able to interact 
with the actin. Because of the arrangement of muscle sarcomeres in series 
then the rate of contraction is dependent on its length.  The maximum 
unloaded shortening velocity is around 6 muscle lengths per second for a 
fast muscle fibre, eg a muscle 1 cm long will contract at 6cm per second.  
However, this can only occur for a very short time because as the muscle 
shortens the sarcomeres shorten and before very long they will not be able 
to shorten any more.

How is the rate of Oxygen input determined?

You may have noticed when looking at what a single 
molecule of myosin is doing to a part of an actin 
that whilst the actin attaches and detaches from actin a single molecule of 
ATP (adenosine triphosphate) is broken down to form 
ADP (adenosinediphosphate)and Pi(phosphate).  This is the energy source for 
the reaction.  So for muscle contraction to continue the muscle needs a 
ready supply of ATP.  This is produced from the breakdown of glucose to 
carbon dioxide and water, which produces 38 molecules of ATP. 

At this point I would like to refer you to a 
previous ans
wer that I have given on this website, which should provide 
all the background information that you need about the production of ATP.

The fast type II muscle fibres can be divided into different categories, 
type IIa that do not fatigue quickly because they fully metabolise the ATP 
by oxidative phosphorylation and type IIb fibres do fatigue quickly because 
they primarily utilise glycolysis only.  This is because they are 
specifically adapted to their particular function.  The type IIb 
(fatigable) fast fibres are designed for very short bursts of high activity 
that can be performed using stored ATP and maybe some glycolysis, but it 
would be inefficient to maintain all the requirements within the cell to 
perform oxidative phosphorylation in these types of fibres so they have 
very low oxygen demands.  Type IIa (non fatigable) fast fibres are utilised 
with slightly less intensity for a longer time and therefore it is 
important for them to utilise oxygen to work efficiently.

So the short answer to your question about how the input of oxygen rate is 
determined is by specialisation of the muscle fibres.  Those fibres that 
have a high oxygen demand have all the components necessary to fully 
metabolise glucose and consume oxygen, whilst those that only need to work 
for short periods without oxygen do not contain these specialised 

If a fibre runs short of ATP then it will no longer be able to contract as 
quickly.  If this happens then it is no longer the myosin that determines 
how quickly the muscle is able to contract but the amount of ATP becomes 
the limiting factor in the equation.
I hope I have managed to answer your questions.  I have tried to be as 
comprehensive as possible, which has resulted in a very long answer for 
which I apologise.  If you have further questions on this then my email 
address is  and I will be happy to explain my 
answer to you further if necessary.

Good luck and thank you for your interest

Dave Burton

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