Neuromuscular development of the human infant as exemplified in the achievement of erect locomotion

Summary The purpose of this study has been to demonstrate successive changes in neuromuscular activities which culminate in the development of a mature, integrated, human gait. A total of 3,387 observations were made upon a group of 82 infants ranging in age from birth to 5 years. On the basis of these observations it was possible to select criteria which differentiated seven fairly distinct phases in the development of erect locomotion. Once the criteria were defined, it was possible to rate the accumulated observations in accordance with the phase representative of the mode of behavior described. These ratings reduced the data to a symbolic system so that curves showing the trend and duration of each phase could be obtained. Similar curves were presented showing the development of erect progression as manifested by five individual children, two of whom were identical twin boys. In addition, calculations were made which indicate the rate of development in this activity as represented by the group and the individuals. In determining the criteria of each phase, it was recognized that certain changes in overt behavior reflect reorganization of the neural centers involved in the activity. On the basis of cytologic studies cited, the analysis embarked with the basic assumption that behavior characteristic of the newborn infant is controlled at a nuclear or subcortical level. Further analysis concerned those qualities of overt behavior which reflected (a) an inhibitory influence from the cortex upon the functioning of infracortical centers, (b) the onset of cortical participation in activating or conducting a particular aspect of the behavior pattern, and (c) the expansion and integration of various cortically controlled movements which are essential to an integrated bipedal gait. Similarly analyses were made of the changing configurations in neuromuscular movements which lead to the assumption of an erect posture. Criteria were proposed for the differentiation of seven significant phases. Because of the relative sparseness of the observations on this activity the data were not given the critical analysis as were those concerning the development of erect locomotion. However, the general trend of development is indicated, and the qualities of overt behavior which reflect neural reorganization are described. It was pointed out that changes in anatomical dimensions, as well as neural reorganization, may be factors affecting the overt expression of neuromuscular performances. In various motor performances the onset of cortical functioning appears to progress in a cephalocaudal direction. The criteria proposed in the selection of the several phases of each activity have embodied those features which indicate the fundamental course of development without regard for individual peculiarities. On the other hand, individual variations may signify temporary differences as to the order in which various aspects of the function advance. For example, some infants may, at a given time, show greater advancement in the aspect of progression while others manifest control over the equilibratory in advance of progressive mechanisms. Such variations are relatively unimportant since all aspects are finally interconnected for the performance of an integrated act. However, there is reason to believe that variations as to the manner or order in which different aspects of neuromuscular development gain ascendency may provide a means for the study of predominating or identifying qualities in individuals.