January 28, 2007 | David F. Coppedge

Muscles Use Gears, Automatic Transmission

Analogies may not be perfect representations of reality, but it must pique the interest of all of us the way Elisabeth Pennisi in Science1 compared muscle to cars and bicycles:

One look at a ballerina as she pirouettes and poses drives home the remarkable ability of our muscles to adapt to diverse biomechanical demands.  Manny Azizi and Thomas Roberts, biomechanists at Brown University, have now found that as certain muscles contract, they vary their shape to balance the need for speed and force.  It’s as if these muscles have a builtin automatic transmission, says Azizi….
[Azizi’s] simulations showed that certain muscle shapes caused contracting pinnate fibers to shift to a less steep angle.  When that happens, the muscle’s overall height decreases more than it would have had the fibers maintained their angle.  In other words, the virtual muscle shifted into the equivalent of a high gear ratio, increasing the speed of contraction…. Azizi then looked at whether real muscles acted this way.  He had expected that each pinnate muscle would have just one gear ratio, that is, undergo a characteristic shape change, and therefore be strong or contract fast but not have both features…. [they found] the muscle operated at a lower gear and took full advantage of the dense packing of pinnate fibers….
    Just as one changes gears on a bicycle to crawl up an ever-steeper hill, “the direction of change in the muscle gears matches the mechanical demands of contraction,” Azizi said.  Moreover, the muscle’s shifting of gears required no nervous system input, occurring automatically depending on the load applied.

Imagine–your muscles are like a bicycle with automatic transmission.  The gearbox of muscle surprised the researchers.  “A single muscle undergoes not one shape change but a range of different shape changes under different circumstances,” Azizi found.  While pinnate muscles can rotate under light loads, they are prevented from rotation under heavy loads by the pull on the fibers.  “Thus, although pinnate muscles are supposedly specialized for force, under light demand, they can also work fast,” Pennisi explained.  A colleague admired this study “assessing muscle architecture with relation to function.”


1Elisabeth Pennisi, “News Focus: SOCIETY FOR INTEGRATIVE AND COMPARATIVE BIOLOGY MEETING: Muscle Fibers Shift Into High Gear,” Science, 26 January 2007: Vol. 315. no. 5811, p. 456, DOI: 10.1126/science.315.5811.456b.

Need we say?  There was no mention of evolution in this article.  Picture some examples of human muscle in action: the ballerina on tiptoes, a skater doing a triple lutz, the contestant in the World’s Strongest Man Competition hoisting a car, the concert pianist pounding a fortissimo section of a Rachmaninoff concerto, a gymnast doing an iron cross on the rings, a sprinter doing the high hurdles, a Chinese contortionist balancing water-filled glasses all over her body while lifting herself by a mouth grip – or just you, reaching on tiptoe for an item in the top cupboard.  Did anyone score you a 10 for that?  You’re amazing.  You knew that, of course.  But the right response should be, “Shucks, I’m just enjoying the gifts I got for my birthday.”

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