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I know that when ADP binds to the myosin head, it moves along and as it does so, it releases the ADP. The ATP attaches to the myosin head and releases the myosin head from the actin filament. Then the enzyme ATPase hydrolises the ATP to ADP, causing the myosin head to move to the original position.

My question is:

  1. How does the actin filament (troponin) move to its original position? Does it use a mechanism similar to a spring?
  2. Is there a limit to how much the myosin head can pull the actin filament?
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ad 1. : In skeletal muscle, contraction of the antagonist stretches the agonist to its original position (Fig. 1).

ad 2. : Yes, ATP generates a fixed amount of energy that limits the force generated.

muscles
Fig. 1. Agonist/antagonist pairs in the skeletal musculature have opposing actions. source: wikipedia

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  • $\begingroup$ Does the (maximum attainable) force saturate when available ATP is above a certain threshold? If so, what would be the new limiting factor? $\endgroup$
    – TLDR
    May 17, 2016 at 19:03
  • $\begingroup$ @fs137 - At the protein scale, each power stroke uses 1 ATP per cycle, so ATP concentration is not the limiting factor $\endgroup$
    – AliceD ♦
    May 17, 2016 at 19:05
  • $\begingroup$ Thanks. So the time scale for each power stroke (and number of 'units') limits the amount of force that can be generated? $\endgroup$
    – TLDR
    May 17, 2016 at 19:07
  • $\begingroup$ @fs137 - afaik, yes $\endgroup$
    – AliceD ♦
    May 17, 2016 at 19:16
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Christiaan has answered your first question, it is external constraints that restore the muscle `rest' shape — this is also the case for nonskeletal muscle as the bladder.

On question 2, I will give a little more details, broadly based on Huxley 1957: imagine the muscle is trying to contract against something infinitely resistive. Full contraction cannot be achieved. Myosin heads still bind to actin, but actin cannot slide wrt myosin filament, and only a local deformation is possible under this tension increment. But myosin heads don't stay bound for ever, and when one detaches, this tension increment is lost locally (there'll be heat produced, dissipating this energy), thus the global muscle tension doesn't increase further. This is a built-in limitation rather than a question of how much energy is available.

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