Abstract
Primate locomotor evolution, particularly the evolution of bipedalism, is often examined through morphological studies. Many of these studies have examined the uniqueness of the primate forelimb, and others have examined the primate hip and thigh. Few data exist, however, regarding the myology and function of the leg muscles, even though the ankle plantar flexors are highly important during human bipedalism. In this paper, we draw together data on the fiber type and muscle mass variation in the ankle plantar flexors of primates and make comparisons to other mammals. The data suggest that great apes, atelines, and lorisines exhibit similarity in the mass distribution of the triceps surae. We conclude that variation in triceps surae may be related to the shared locomotor mode exhibited by these groups and that triceps surae morphology, which approaches that of humans, may be related to frequent use of semiplantigrade locomotion and vertical climbing.
Highlights
From Aristotle’s thoughts in De Motu Animalium [1], to Borelli’s [2] comprehensive review of biomechanics in the 1600s, to Muybridge’s [3] original documentation of gaits in horses around the turn of the last century, animal movement has been a vibrant and productive area of research providing insights into critical aspects of form-function relationships and selection pressures on limb and body design in many vertebrates, including primates
More data on the dynamics of vertical climbing need to be collected before definite conclusions can be drawn about the relationship between triceps surae (TS) morphology and vertical climbing
Numerous authors have suggested that there is an evolutionary relationship between vertical climbing and bipedalism (e.g., [45, 46, 65, 66, 196,197,198,199,200,201,202]), and at the very least, kinematic data on joint angles and EMG recordings support similarities in movements and muscle activity between these two locomotor modes (e.g., [44, 45, 175, 176, 183])
Summary
From Aristotle’s thoughts in De Motu Animalium [1], to Borelli’s [2] comprehensive review of biomechanics in the 1600s, to Muybridge’s [3] original documentation of gaits in horses around the turn of the last century, animal movement has been a vibrant and productive area of research providing insights into critical aspects of form-function relationships and selection pressures on limb and body design in many vertebrates, including primates. Since researchers have been compiling a long list of features that distinguish the walking gaits of most primates from those of most other mammals These features include the use of a diagonal sequence footfall pattern (e.g., [8,9,10,11,12,13,14,15,16]), the lack of a running trot with the use of an amble instead (e.g., [17,18,19,20,21,22]), relatively high hindlimb peak vertical forces (e.g., [16, 23,24,25,26]), highly protracted arms at touchdown (e.g., [27, 28]), and a deeply yielding elbow [29, 30]. These locomotor characteristics are hypothesized to have been important for the evolution of a diverse array of locomotor modes (e.g., [5, 25, 31,32,33,34]), but most of these locomotor modes appear to have been facilitated by a basal differentiation of the functional role of the forelimb and hindlimb
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