The development of goal-directed reaching in infants
II. Learning to produce task-adequate patterns of joint torque |
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Authors: | J Konczak Maike Borutta Johannes Dichgans |
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Institution: | (1) Department of Neurology, University of Tübingen, Hoppe-Seyler-Strasse 3, 72076 Tübingen, Germany; Fax: +49–7071–296504, e-mail: konczak@uni-tuebingen.de, DE |
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Abstract: | Nine young infants were followed longitudinally from 4 to 15 months of age. They performed multijoint reaching movements
to a stationary target presented at shoulder height. Time-position data of the hand, shoulder, and elbow were collected using
an optoelectronic measurement system. In addition, we recorded electromyographic activity (EMG) from arm extensors and flexors.
This paper documents how control problems of proximal torque generation may account for the segmented hand paths seen during
early reaching. Our analysis revealed the following results: first, muscular impulse (integral of torque) increased significantly
between the ages of 20 (reaching onset) and 64 weeks. That is, as infants got older they produced higher levels of mean muscular
flexor torque during reaching. Data were normalized by body weight and movement time, so differences are not explained by
anthropometric changes or systematic variations in movement time. Second, while adults produced solely flexor muscle torque
to accomplish the task, infants generated flexor and extensor muscle torque at shoulder and elbow throughout a reach. At reaching
onset more than half of the trials revealed this latter kinetic profile. Its frequency declined systematically as infants
got older. Third, we examined the pattern of muscle coordination in those trials that exhibited elbow extensor muscle torque.
We found that during elbow extension coactivation of flexor and extensor muscles was the predominant pattern in 67% of the trials. This pattern was notably absent in comparable
adult reaching movements. Fourth, fluctuations in force generation, as measured by the rate of change of total torque (NET)
and muscular torque (MUS), were more frequent in early reaching (20–28 weeks) than in the older cohort (52–64 weeks), indicating
that muscular torque production became increasingly smoother and task-efficient. Our data demonstrate that young infants have
problems in generating smooth profiles of proximal joint torques. One possible reason for this imprecision in infant force
control is their inexperience in predicting the magnitude and direction of external forces. That infants learned to consider
external forces is documented by their increasing reliance on these forces when performing voluntary elbow extensions. The
patterns of muscle coordination underlying active elbow extensions were basically the same as during the prereaching phase,
indicating that the formation of functional synergies is based on a basal repertoire of innervation patterns already observable
in very early, spontaneous movements.
Received: 5 January 1996 / Accepted: 19 August 1996 |
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Keywords: | Arm movement EMG Motor learning Torque Human |
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