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    In the OCR A biology a level book, it says that as you fatigue the amplitude of an electromyogram increases due to increased electrical activity while frequency decreases.

    Please can someone explain these changes to me?

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    Apologies for late reply:-

    An EMG can be recorded in 2 ways:-

    - with a skin surface electrode (rather like a very thin 5p coin)


    - it can be recorded by inserting an ultra-thin electrode into a muscle.

    The first would record the total electrical activity of those muscle fibres (myocytes in effect) whose activity can reach the electrode WHEREAS the second would measure the activity of a single muscle fibre.

    It is a known feature of neuromuscular junction physiology that when a muscle contracts with greater force, it is because of a greater number of nerve impulses reaching the muscle rather than an increase in the strength of any nerve stimulation (analogous to the "all or none" phenomenon of the nerve action potential), i.e. the "amplitude" of the neuronal activity is not responsible for increased force of contraction.

    Assuming that the EMG being described in your book is the first one above, when the muscle is fatigued, there will be less nerve impulses [per unit time hence frequency] reaching it to result in a weaker contraction [less force produced when tired] - try to relate this to the point explained In bold above.

    As to the amplitude part of your Q, to be honest I am not sure entirely - however,, if the book is correct [and I have pinpointed several errors over the years], one explanation for the increased amplitude could be that a tired muscle will result in the recruitment of a greater number of individual muscle fibres in an attempt to maintain enough force - so that the sum of electrical activities of all these separate muscle fibres would tend to increase.- in reality a "losing battle" for the muscle.

    Hope this makes a little sense!

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