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Very interesting examples to me are Trapezius or Latissimus - they both have places with more tendons than muscles. Taking into account that now they contract weaker and slower with all these tendons, what are the profits?

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If you observe the general structure of the locomotor system, you will be able to roughly separate muscles in two main groups:

  • the muscles used for static work and position maintenance.

  • those exclusively used for manipulation of the external world.

You will also notice that most muscles with extensive tendinous attachments reside on the axial skeleton (i.e the trunk). This is because they are used for maintenance of the upright position, and examples of those are: Trapezius, Latissmus dorsi, Rectus abdominis, and many more... Given their static nature, they contain a lot of slow muscle fibers, allowing them to keep working over extended periods of time without rest. However, this does not make them weak. If the postural muscles were to have a structure similar to more peripheral muscles, for example biceps brachii, they would require an enormous number of different circular tendons extending in every direction to account for all the different body positions they act to maintain. To build a progressive example:

  • the biceps brachii is a fusiform muscle including two main parts. It is designed to pull hard in a limited range of movement types (i.e arm flexion). For that, it has evolved a small number of strong tendon attachments.

  • the serratus anterior was designed for a more extensive panel of moves. It therefore possesses several main bodies of muscles extending in different directions, with their own tendon attachment on the ribs.

  • the trapezius muscle has been designed to allow the head to keep its upright position (among other functions), and therefore is used as a positional link to guarantee the positional relation between head and shoulders. Its a static muscle and must be able to pull efficiently in a wide range of different directions, so its tendinous apparatus is extensive.

There are also special cases. If you take the digastric muscle of the neck for example, the tendons are used as attachments, but also as a kind of hinge to redirect the directional strength of the muscle.

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  • $\begingroup$ I think they get weaker because they have less muscle in it. If the muscle is longer, then it has more sarcomeres and thus there is more stuff that contracts. Could you please also add info on what benefits does this structure (large amount of tendons) give to the posture maintaining muscles? $\endgroup$ – Stanislav Bashkyrtsev Aug 18 '14 at 11:11
  • $\begingroup$ In fact, muscle strength is in relation to its tranverse section. The more sarcomeres are pulling in parallel, the stronger. $\endgroup$ – Raoul Aug 18 '14 at 11:24
  • $\begingroup$ Raoul is correct - afterall, if you exercise your biceps to increase its strength, it gets thicker, not longer. I think it would also help to add explicitly that tendons provide the mechanical attachment of muscle to bone, meaning that tendons determine at what place and in what angle the muscle's force is applied to the bone. I would expect that to be important in regards to force distribution, bone stability and energy efficiency (though correct me if I'm wrong). $\endgroup$ – Armatus Aug 18 '14 at 11:28

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