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Topsy‐turvy locomotion: biomechanical specializations of the elbow in suspended quadrupeds reflect inverted gravitational constraints
Author(s) -
Fujiwara Shinichi,
Endo Hideki,
Hutchinson John R.
Publication year - 2011
Publication title -
journal of anatomy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.932
H-Index - 118
eISSN - 1469-7580
pISSN - 0021-8782
DOI - 10.1111/j.1469-7580.2011.01379.x
Subject(s) - quadrupedalism , elbow , anatomy , suspension (topology) , elbow flexion , joint (building) , biology , physical medicine and rehabilitation , medicine , structural engineering , mathematics , engineering , homotopy , pure mathematics
Some tetrapods hang upside down from tree branches when moving horizontally. The ability to walk in quadrupedal suspension has been acquired independently in at least 14 mammalian lineages. During the stance (supportive) phase of quadrupedal suspension, the elbow joint flexor muscles (not the extensors as in upright vertebrates moving overground) are expected to contract to maintain the flexed limb posture. Therefore muscular control in inverted, suspended quadrupeds may require changes of muscle control, and even morphologies, to conditions opposite to those in upright animals. However, the relationships between musculoskeletal morphologies and elbow joint postures during the stance phase in suspended quadrupeds have not been investigated. Our analysis comparing postures and skeletal morphologies in Choloepus (Pilosa), Pteropus (Chiroptera), Nycticebus (Primates) and Cynocephalus (Dermoptera) revealed that the elbow joints of these animals were kept at flexed angles of 70–100 ° during the stance phase of quadrupedal suspension. At these joint angles the moment arms of the elbow joint flexors were roughly maximized, optimizing that component of antigravity support. Our additional measurements from various mammalian species show that suspended quadrupeds have relatively small extensor/flexor ratios in both muscle masses and maximum moment arms. Thus, in contrast to the pattern in normal terrestrial quadrupeds, suspended quadrupeds emphasize flexor over extensor muscles for body support. This condition has evolved independently multiple times, attendant with a loss or reduction of the ability to move in normal upright postures.