Appreciating the Design of Human Muscles
Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, The Miracle of the Cell, will be published in September.
The muscles of all organisms have the same basic design, consisting of densely packed arrays of the basic contractile elements known as molecular motors. Movement comes about as a result of a sequence of three conformational changes.
As I described in Nature’s Destiny, Chapter Eleven:
[E]ach basic working component in the muscle cell is an individual protein molecule consisting of a long tail and short head rather like an elongated tadpole, known as a myosin motor. Movement comes about as a result of a sequence of three conformational changes. First, the myosin head attaches itself to another long fibrillar molecule known as actin… Second… the head bends suddenly — the power stroke — and this bending causes the myosin molecule and the actin to move in opposite directions. Third… the head unbends and attaches itself to the actin… The sequence is repeated again, and gradually, via a series of small steps, the two molecules slide past each other.1
Recent work has also shown that each myosin head moves about 8 nanometers with each power stroke and that the heads are stacked in the muscle fibrils in a helical conformation about 14 nanometers apart.2 Each of these tiny units has the same strength, so they exert the same “pulling” force per cross sectional area.
Tightly Packed Indeed
From consideration of the geometrical constraints on the size and movement of the myosin heads, it is likely that no further improvement in muscle power can be achieved by increasing the density of packing of the myosin motors. They are packed as tightly as possible!3 As Schmidt-Nielsen points out, it is unreasonable to expect that this mechanism could be improved to provide a greater force per cross sectional area, for the maximal force should be related to the number of filaments that can be packed within that area, and this again depends on the size of the protein molecules that make up the filaments:
All muscle contraction we know about is based on sliding filaments of actin and myosin, and if we could pack more filaments into a given cross-sectional area, the force would be increased. This is most unlikely because the diameter of the filaments is determined by the size of protein molecules that make up the filaments, and their size is probably determined by the requirement of the molecular mechanism.4
Increasing the power of muscles by increasing the force of the individual power strokes that each myosin head makes as it bends and pushes on the actin fiber is also difficult to envisage. Recent measurements of the force of an individual power stroke show that this is about three piconewtons, and this is already several times greater than the strength of individual weak bonds.5 Because it is the weak bonds that hold all the cells constituents together, including the components of the myosin motor and the actin fiber on which it pushes, it is impossible to increase the force of the power stroke to any significant degree or each stoke would cause damage not only to the myosin motor itself but also to other delicate adjacent structures in the cell, including the actin fiber.
Tomorrow, “What if Our Muscles Were Less Powerful?”
- Michael Denton, Nature’s Destiny (New York: The Free Press, 1998), 245.
- Steven M. Block, “Nanometres and Piconewtons: the Macromolecular Mechanisms of Kinesin,” Trends in Cell Biology 5 (1996): 169-175. R. Anthony Crowther, Raúl Padron, Roger Craig, “Arrangement of the Heads of Myosin in Relaxed thick Filaments from Tarantular Muscle,” Journal of Molecular Biology 184 (1985): 429-439.
- Knut Schmidt-Nielsen, Animal Physiology: Adaptation and Environment. 5th ed. (Cambridge: Cambridge University Press, 1997), Chapter Ten.
- Robert Simmons, “Molecular motors: Single-molecule mechanics,” Current Biology 6, (1996): 392-394. See also Block, op. cit. For strength of weak bonds see Bruce Alberts, Alexander Johnson, Julian Lewis, Keith Roberts, Martin Raff, Peter Walter, Molecular Biology of the Cell, 3rd ed. (New York: Garland Publishing, 1994), 90-92. For energy levels in kJ of myosin cross bridges see William F. Harrington, “On the Origin of the contractile force in skeletal muscle,” Proceedings of the National Academy of Sciences USA 76 (1979): 5066-5070. For energy levels of affinity bonds composed of multiple weak bonds, see J.M. Batz, and R.A. Cone, “The Strength of Non-Covalent Biological Bonds and Adhesions by Multiple Independent Bonds,” Journal of Theoretical Biology 142 (1990): 163-178; François Amblard, Charles Auffray, Rafick Sekaly, Alain Fischer, “Molecular analysis of antigen-independent adhesion forces between T and B lymphocytes,” Proceedings of the National Academy of Science USA 91 (1994): 3628-3632.