I came across the following paragraph on the working of muscles in a book:

 

Muscles bring about movement by contracting (shortening) & relaxing (lengthening). One end of a muscle is attached to a movable bone where as the other end is attached to a fixed bone. When the muscle contracts, it pulls the movable bone. Since muscles are only capable of pulling, they work in pairs.

 

Now what does the line in bold mean? Aren't muscles capable of pushing?

 

Thanks,

 

Ron

No, they cannot. The molecular mechanisms of muscle movement exert force exclusively by contraction (if you are interested in details you should look up actin-myosin filaments).

And even if the muscles could push, it would be pretty useless to push on tendons.

The muscle fibers are composed of proteins. Energy molecules added to these muscle proteins, cause these proteins to change their configurational shape and compact. Without the energy molecules they will relax back to the previous configurational shape that is more expanded.

 

When we have two opposing muscles, a simple solution is shift the energy chemical back and forth so one contracts and one expands. Since the muscles are controlled by the nervous system, this can be done more efficiently via ionic signals from nerves, which alter the local water potential and thereby alter the equilibrium protein shape, even when the energy molecules are present in both sets of muscles. This allows quicker turn-around without energy molecule flush.

 

Relative to a mechanism, if we use ATP this withdraws electron density from the proteins. The proteins sees this loss of electron density and contract. When we recycle the attached phosphate, the electron density is released back to the protein and they fluff out. Cationic currents from nerves are positive and also withdraw electron density. This adds extra equilibrium contraction to one muscle group, so even if the other muscle has ATP, the first can pull harder. The result is muscle tone, with both wanting to pull but nerves making one pull harder.

Edited October 4, 201015 yr by pioneer

Also, if you are interested in understanding how muscles contract, ignore pioneer's post.

Edited October 4, 201015 yr by CharonY

But what are they capable of pulling? What do they pull?

 

Ron

Muscles are attached to bone (via tendons) and by contracting they pull at them.