POSTURE AND LOCOMOTION 



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fall finally midway between calcaneal and meta- 

 tarsal points of support (2, 33, 202, 253). Active mus- 

 cle support is largely avoided at these levels, however, 

 by mechanical arrangements (62, 181). At the hip, 

 for example, the iliofemoral ligaments together with 

 elastic forces of the muscles may in some individuals 

 be adequate to prevent overextension in standing 

 {2, 135). The pull of these ligaments in turn holds 

 the femora in medial rotation, thus preventing the 

 unlocking rotation at the knee joint which is neces- 

 sary for flexion (2); that the quadriceps tendon is 

 relaxed in standing is easily verified (2). At the tibio- 

 astragalic articulation, the body is prevented from 

 falling forward when the foot is abducted by the fact 

 that the planes of flexion of this joint in the two feet 

 form an angle of 60° open to the front. Teetering 

 forward then requires that force in part be directed 

 along the axes of a hinge joint rather than entirely 

 in its plane of flexion (181). 



To summarize, it is apparent that the function of 

 the neuromuscular system in standing man is less 

 one of steady antigravity contraction than one of 

 intermittent correction of balance. On the other 

 hand, consideration only of evidence from subjects 

 "standing at ease' perhaps overemphasizes the im- 

 portance of passive balance, for people assume a 

 symmetrical position only 20 per cent of the time 

 spent in casual standing (250). Furthermore, man is 

 unique since most other mammals, including pre- 

 historic man and the great apes (202, 229), have 

 standing postures characterized by partial flexion of 

 limb joints with the corresponding requirement for 

 tonal support. 



CENTR.^L .ASPECTS OF POSTURE 



Central Facilitation of Postural Reflexes 



The function of the central nervous systein in 

 posture is, in essence, the translation of the variegated 

 influx of afferent impulses into a steady and directed 

 flow of efferent discharge. Segmental levels are basi- 

 cally capable of this integration. The dog, for exainple, 

 several months after transection of the thoracic cord, 

 can rise from a sitting to an erect position and stand 

 for a time (193, 244), occasionally even on one hind 

 leg (241). Similarly, spinal man can be made to sup- 

 port his body weight, if reflex-initiating pressure is 

 maintained over the popliteal region (158). Yet pos- 

 tural performance in the spinal animal is incomplete. 

 After minutes or a half hour of standing, the hind 



quarters of the spinal dog collapse, either gradually 

 and without apparent stiinulus or suddenly in re- 

 sponse to some minor incident favoring a flexor re- 

 sponse (244). Moreover; behavior in such chronic 

 animals hardly represents quantitatively the condi- 

 tions in the intact animal because of the acquired 

 sensitization of spinal centers (257). Acute spinal 

 dogs or cats show no ability to stand, even when the 

 level of central activity is raised by amphetamine 

 administration (190). Reflexes in spinal animals are 

 characteristically abrupt and unsustained (43). 



The additional background activity necessary to 

 bring potential postural patterns to successful ex- 

 pression arises from several .sources: a) somatic sen- 

 .sory inflow from other spinal and brain-stem levels; 

 h) special .sensory inflow from vestibular organs; c) 

 the non.specific activity of the reticular formation; 

 and d) those relatively discrete influences descending 

 from the cortex, basal ganglia and other brain struc- 

 tures. A discussion of the latter important contribu- 

 tions to posture must be left to other chapters. 



SPINAL AFFERENT INFLUENCES. Sensory inflow over a 

 dorsal root affects activity not only at that specific 

 segment but contributes also to that of distant levels. 

 Thus, sensory impulses from receptors of cersical 

 intervertebral joints do not have their sole or even 

 prime effect at the immediate segmental level. The 

 labyrinthectomized cat cannot hold his head up, 

 for instance, although the head be placed initially 

 in the position favoring extension (217). Rather, the 

 influence of these receptors is stronger upon the fore- 

 legs and extends as far as the hind legs. Even somatic 

 alTerents in cranial nerves inay influence spinal 

 levels, for example, the associated movements of 

 distant limbs which accompany clenching of the jaw 

 of the hemiplegic patient (269). It is not surpris- 

 ing in view of these facts that the incidence and in- 

 tensity of reflex activity increases with the length of 

 the cord segment in a spinal animal (104, 214, 215), 

 or the number of dorsal roots remaining in a partialK' 

 deafferented preparation (too, 173). 



VESTIBULAR INFLUENCES. A powerful influence upon 

 segmental postural reflexes is the vestibular inflow. 

 Although under some circumstances the labyrinths 

 appear to impart greater intensity to all components 

 of motor patterns (72, 145), the net efi"ect of this 

 quinquifid inflow (that from the two maculae and 

 the three cristae) is one of unfatiguing facilitation of 

 antigravity inuscle activity, both at spinal (84, 85) 

 and cranial (244) levels. Nevertheless, sensory in- 



