Trunk orientation, stability, and quadrupedalism

Abstract

Interesting cases of human quadrupedalism described by Tan and Colleagues (2005-2012) have attracted the attention of geneticists, neurologists, and anthropologists. Since his first publications in 2005, the main attention has focused on the genetic aspects of disorders that lead to quadrupedalism within an evolutionary framework. In recent years this area has undergone a convincing critique (Downey, 2010) and ended with a call "… to move in a different direction … away from thinking solely in terms of genetic abnormality and evolutionary atavism." We consider quadrupedalism as a "natural experiment" that may contribute to our knowledge of the physiological mechanisms underlying our balance system and our tendency toward normal (upright) posture. Bipedalism necessitates a number of characteristics that distinguish us from our ancestors and present-day mammals, including: size and shape of the bones of the foot, structure of the axial and proximal musculature, and the orientation of the human body and head. In this review we address the results of experimental studies on the mechanisms that stabilize the body in healthy people, as well as how these mechanisms may be disturbed in various forms of clinical pathology. These disturbances are related primarily to automatic rather than voluntary control of posture and suggest that human quadrupedalism is a behavior that can result from adaptive processes triggered by disorders in postural tone and environmental cues. These results will serve as a starting point for comparing and contrasting bi- and quadrupedalism.

Keywords: bipedalism and quadrupedalism; gait pathology; humans; locomotion; muscle tone; posture control.

Figure 1

(A) Postural fixation of the head is impaired at all times (left), but as soon as the patient closes her eyes her head falls forward until her chin rests on her chest (right). (B) Patient standing as erect as she can (left), and 2 or 3 min later (right). (C) Patient’s usual position when on all fours (left), and 1 min after blindfolding (right). (D) “Flexed posture” in a monkey with severe bilateral degeneration of the Globus pallidus [adapted from Martin (1967), with permission].

Figure 2

A case of Camptocormia. (A) A kinematic study of the time-course of the involuntary forward bending immediately following arising from a chair reveals a profound loss of postural alignment, however, balance control is intact. The segments of the body move in a coordinated way that ensures that the center of pressure remains steady over the base of support. (B) Trunk flexion when walking forward, but NOT when walking backward or sideways [the data were adapted from St George et al. (2010)].