Postural adjustments due to external perturbations during sitting in 1-month-old infants: evidence for the innate origin of direction specificity

Abstract

The aim of the study was to examine whether infants, at an age when they have no or little experience in sitting, can produce direction specific postural adjustments, i.e. synergies of muscle activity on the ventral side of the body during backward sway and on the dorsal side during forward sway. In addition, we addressed the question whether postural adjustments at this young age are restricted to single muscle responses or consist of a variable repertoire of muscle activation patterns including one during which all direction specific muscles participate ('complete' pattern). Postural adjustments due to external perturbations in a sitting position were studied in eight healthy infants aged 1 month. Multiple surface EMGs of neck, trunk and leg muscles and kinematics were recorded while the infants were exposed to horizontal forward (Fw) and backward (Bw) displacements of the surface of support. Direction specific postural adjustments, defined as adjustments during which agonist activation or antagonist inhibition preceded antagonist activation, were present in 85% of Bw and 72% of Fw translations. The direction specific adjustments showed a large variability with the repertoire of adjustments including the activation of one, two or all of the recorded direction specific muscles. The finding of direction specific adjustments at 1 month of age support the opinion that the basic level of organisation of postural adjustments has an innate origin. The finding of a variable repertoire of muscle response patterns, including the 'complete' pattern, refutes the idea that the development of postural adjustments results from gradual addition of appropriate muscles to the synergies.

Variation and Variability: Key Words in Human Motor Development

“…The basic level of control deals with the directional specificity of the adjustments: when the body sways forward, primarily the dorsal muscles are recruited; when the body sways backward, primarily the ventral muscles are activated. 89 A study by Hedberg et al 90 indicated that 1-month-old infants have direction-specific adjustments, which suggests that the basic level of postural control has an innate origin. Young infants show a variable repertoire of direction-specific adjustments from which, from the age of 4 months onward, they learn to select by means of active trial and error the adjustment that fits the situation best.…”

No stable arm preference during the pre‐reaching period: A comparison of right and left hand kinematics with and without a toy present

“…Moreover, such left-right differences found in object oriented movements could then be attributed to further development of neural control as opposed to organismic constraints such as genetics. Such rapid emergence has been noted between reaching without grasping to reaching with grasping (Wimmers, Savelsbergh, Beek, & Hopkins, 1998) and with early postural control (Hedberg, Forssberg, & Hadders-Algra, 2004).…”

Learning about gravity: segmental assessment of upright control as infants develop independent sitting

“…All of these studies have considered the trunk as a single segment. They have dealt with the lack of trunk control in their subjects by using semireclined and supported seating (Bertenthal et al 1997;van der Fits et al 1999a, 199b;Woollacott et al 1987), propping on arms (Cignetti et al 2011;Harbourne and Stergiou 2003), or allowing the infant's spine to collapse and/or holding the infant up from the chest and releasing the support (Harbourne 1993;Harbourne and Stergiou 2003) just prior to surface perturbation (Hedberg et al 2004(Hedberg et al , 2005. None of these methods has allowed evaluation of, or offered control for, variable contributions of different trunk segments to postural control, and none has addressed the question of how infants acquire a vertical sitting position.…”