592 VITALITY AND EFFICIENCY WITH RESTRICTED DIET. 



(8) LATENCY, AMPLITUDE, AND REFRACTORY PERIOD OF PATELLAR REFLEX.i 



Of all the human reflexes that may be elicited by appropriate stimuli, 

 that of the patellar lends itself most conveniently to measurement. 

 No doubt the method of recording the latency, amphtude, and 

 refractory period of this reflex from the thickening of the quadriceps 

 muscle is, all things considered, the most satisfactory technique.^ 

 The term ''reflex arc" is associated with such ideas as simpHcity and 

 invariabiUty, in short, constancy of response. Physiologists, however, 

 who have worked with human reflexes, and specifically with the 

 patellar reflex, have soon discovered that this is not a constant in its 

 time relations or in its amphtude. Our data for the 63 young men of 

 the normal series of 1917, with whom the patellar reflex was measured 

 with identical apparatus and procedure as in the low-diet research, 

 may be examined in connection with this matter of the variability of 

 this reflex. In the first place, it is noteworthy that of the 63 men there 

 were 15 from whom the reflex could not be obtained in measurable 

 amplitude. Every effort was made to adjust the apparatus to exactly 

 the proper height on the tendon, to see that the leg of the subject was 

 in a comfortable position, and that the subject was relaxed, as indi- 

 cated by the condition of his leg muscles. Our failure was, therefore, 

 not because of lack of time or care in trying to secure the reflex. It 

 was because under our conditions for stimulation and on that date 

 the reflex could not be produced. In several of these cases, when trial 

 was made with the legs crossed and by the usual clinical method, there 

 was some reflex, but of course this was not measurable as to latency 

 or amphtude under such conditions. 



The 48 men from whom measurable series of reflexes were obtained 

 gave an average latency (average of all individual averages) of 32 <t.^ 

 The standard deviation for this series of 48 patellar reflex latencies 

 was 6.6 (T, the coefficient of variabihty, therefore, being 21 per cent. 

 We may anticipate our results slightly here and point out that this 

 coefficient of variability is larger than a similar coefficient for neuro- 

 muscular processes, such as reactions and muscle coordinations, which 

 are usually considered much more complex than the patellar reflex. 

 The average amplitude of the primary reflex for the 48 indiv' duals 

 was 15 mm. This is a magnification of 6 times the extent of actual 

 muscle-thickening. With different subjects the average amplitude 



^'Measurement No. 7 (changes in pulse-rate occasioned by short periods of exertion) is discussed 

 with the other pulse data; see p. 415. 



^Dodge and Benedict, Carnegie Inst. Wash. Pub. No. 232, 1915, p. 36. 



^This average is for the reflexes of the first stimulus. On page 155 it is explained that two 

 pendulum hammers were used to stimulate for the reflex. The second hammer was to fall against 

 the knee at a variable interval from the stimulus of the first. When this interval was short, 

 there was no response, that is, the reflex was refractory. When it was fairly long there was a 

 response, in some cases almost as large as the primarj^ reflex. These secondary reflexes, however, 

 which followed shortly upon the primary, are not counted in the averages, a is used as an abbre- 

 viation for 0.001 second. 



