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HANDBOOK OF PHYSIOLOGY 



NEUROPHN'SlOLOCn' II 



seen only under partial curarization (169), in neuro- 

 muscular disease (120, 278) or in fatigue (222). 

 Should multiple innervation of single muscle fibers 

 of the type shown for the frog (126, 137), rat (131) 

 and cat (126) prove widespread, it may become 

 necessary to re-examine the concept of overall con- 

 traction of individual muscle fibers as well as that of 

 motor units. 



Postural Mainlniame in Man 



The importance of tonic reflex contraction in the 

 postural maintenance of man has been questioned 

 by several investigators who, using needle electrodes, 

 found essential electromyographic silence in the gas- 

 trocnemius, soleus and tibialis anterior muscles 

 during 'easy standing' (11, 36, 121, 134, 144, 235, 

 272). Others deny these findings (47, 133, 206, 209); 

 but the difference in opinions is in part semantic, 

 for all observers agree that at the extremes of postural 

 sway corrective contractions take place. The impor- 

 tance of training in obtaining adequate relaxation 

 should also be emphasized (267). 



Direction of interest to the play of muscles about 

 the ankle only partly reveals the role played by active 

 contractions in correcting imbalances at this joint, 

 for cotnpensations can result as well from shifts in 

 the center of gravity produced by hip or trunk move- 

 ments. The lower reflex threshold and greater promi- 

 nence of tonic contractions (43, 21 7J, and greater 

 sensory discrimination at proximal joints (89) as 

 compared to distal ones, point to the importance of 

 proximal joints in posture. Pigeons, to use another 

 biped as an extreme example, roost fully as effec- 

 tively upon a leg in which the foot and ankle are de- 

 nervated as upon the sound leg (34). In man observa- 

 tions upon deeper muscles of the hip during standing 

 are lacking, but the related sacrospinalis muscle may 

 be silent (64, 221) or nearly so (36), as is also the rec- 

 tus abdominis (63, 140). More important, however, 

 considering that the line of gravity falls posterior to 

 the hip joint (62), are indications that activity is con- 

 tinuous in the oblique abdominal muscles (63, 255). 



Reaction against Sherringtonian views of postural 

 tone has led Clemmesen of the Danish group of mus- 

 cle physiologists to declare that "we must reckon 

 with the passive elastic tensions and forces in the 

 muscles, and (the fact) that this concept is much 

 more correct than the now discarded concept of 

 muscle tone" (36). He and others (2, 121, 235, 267) 

 point out that stretching of muscles in normal man, 

 including those about the ankle (272), leads only to 



reflex contractions at rapid rates or considerable 

 lengths of stretch. Muscles in the stub of a kineplastic 

 amputation, for example, may be lengthened 50 

 per cent without eliciting detectable electrical changes 

 (221). Kelton (144) argues that po.stural contraction 

 may be more closely geared to joint than muscle 

 receptors since activity in the soleus muscle of a 

 standing man may be initiated by a rate and degree 

 of angular deflection which was incapable of eliciting 

 a stretch reflex in the resting muscle. In animals with 

 greater extensor tone, including those decerebrate, 

 on the other hand, the sensitivity of stretch reflexes 

 may be exquisite, the threshold for the gastrocnemius 

 being 50 fi (170), and for the supraspinatus 8 /i 

 (48) of lengthening. 



The ability of man to stand with little expenditure 

 of neuromuscular energy derives from the elasticity 

 of ligaments and muscles, the mechanical features of 

 some joints and the fair compensation with which the 

 centers of gravity of body segments are superimposed 

 (2). The role of elastic nuchal ligaments in suspending 

 the head of a herbivore is well known; but similar 

 postural functions in man of the ligamenta nuchae 

 and flava, the twisted iliofemoral ligaments and 

 crossed ligaments of the knee are insufficiently appre- 

 ciated (2). Elasticity of the muscle and its sheath is 

 not an inconsiderable factor (34, 135, 267), and even 

 in Sherrington's decerebrate animals, denervated 

 muscle exhibited as much as 10 per cent of the ten- 

 sion developed by the innervated and highly hyper- 

 tonic muscle when stretched to some lengths (43). 

 In man the elastic tension of the calf muscles, at the 

 usual angulation (88°) of the ankle, has been meas- 

 ured and found to be essentially equal to the gravi- 

 tational forces tending to tip the body forward (134). 

 The plastic qualities of muscle, on the other hand, 

 are probaijly of less importance in postural main- 

 tenance than they are in locomotion where they 

 become a major factor in limiting the speed of move- 

 ment (75). 



Experimental observation has shown that an aim 

 of postural integration in man is to keep the center of 

 gravity of the body directly (within 7 per cent) over 

 the center of the basis of support (10, 1 13). Contrary 

 to statements based upon a misunderstanding of the 

 meaning of 'Normalstellung', as used by the original 

 workers in this field [evaluated by Brunnstroin (33) 

 and Hirt li al. (11 9)] the line of gravity of the average 

 individual does not pass successively through the 

 a.xes of the major joints. Rather, it passes posterior to 

 the interacetabular line, anterior to the knee joints, 

 and several centimeters in front of the ankle joint to 



