Niels Stensen's geometrical theory of muscle contraction (1667): a reappraisal.

From reading the Elementorum Myologiae Specimen of 1667 by Niels Stensen (Steno), I assert that the text and illustrations contain an observation-based theory on the mechanics of muscle contraction: (1) Based on the study of the structure and motion of several muscles in different animals and in man, Stensen described the contraction of parallel equally long motor fibers formated as uni- or multipennate structures, each forming a parallelepipedon between parallel tendon plates. The parallelepipedon was used as a model allowing Stensen to apply mathematical methods in the argumentation. When the motor fibers contract, the tendons move in parallel planes, the muscle shortens, but the distance between the tendon planes does not change. There will appear a swelling, even if the volume of the model remains the same. Therefore, the swelling observed during contraction, according to Stensen, is no argument for an increase in muscle bulk and no argument against contraction without any change of muscle volume. (2) In the first century after its proposal, different arguments were published against Stensen's theory: in 1680 by Borelli (De Motu Animalium), 1694 by Bernoulli (De Motu Musculorum), 1743 by Boerhaave (Praelectiones), and 1762 by Haller (Elementa Physiologiae). When read today, these arguments are irrelevant, erroneous, or without scientific documentation. However, by the end of the 18th century, Stensen's theory all but disappeared from the science literature. (3) Anatomical and biomechanical studies published after 1980 show that the foundation and applicability of Stensen's theory are still valid. (4) While earlier considered to be perhaps Stensen's weakest work, arguments are presented to reappraise Elementorum as one of Stensen's significant publications and as a significant work in the biomechanical sciences.