Behavioral flexibility in learning to sit

Abstract

What do infants learn when they learn to sit upright? We tested behavioral flexibility in learning to sit-the ability to adapt posture to changes in the environment-in 6- to 9-month-old infants sitting on forward and backward slopes. Infants began with slant at 0°; then slant increased in 2° increments until infants lost balance. Infants kept balance on impressively steep slopes, especially in the forward direction, despite the unexpected movements of the apparatus. Between slant adjustments while the slope was stationary, infants adapted posture to the direction and degree of slant by leaning backward on forward slopes and forward on backward slopes. Postural adaptations were nearly optimal for backward slopes. Sitting experience predicted greater postural adaptations and increased ability to keep balance on steeper changes of slant, but only for forward slopes. We suggest that behavioral flexibility is integral to learning to sit and increases with sitting experience.

Keywords: flexibility; infant; postural control; sitting; sloping surface.

Figure 1

Sitting on various surfaces. (A) Sitting with legs outstretched on a rigid, horizontal surface. Stick figure shows a perfectly erect posture with a trunk-thigh angle of 90°. Figure shows that with the trunk perfectly upright, the body is at the edge of the base of support. (B) Infants, like older children and adults, sit with their torso leaning slightly forward, keeping the trunk well within the base of support. Trunk-thigh angle of 81.2° represents the average across infants in the current study. (C) Sitting on a forward slope. To keep the trunk perfectly upright as in A infants would need to increase their trunk-thigh angle by leaning backward (black stick figure shown with trunk-thigh angle of 120°). Green stick figure shows under-compensation (with a trunk-thigh angle of 90°, the infant would be pulled off balance and fall forward) and orange stick figure shows over-compensation (e.g., with a trunk-thigh angle of 140°, infants would need to recruit abdominal muscles to hold themselves stationary and risk losing balance). (D) Sitting on a backward slope. To keep the trunk perfectly upright, infants would need to decrease their trunk-thigh angle by leaning forward (black stick figure shown with trunk-thigh angle of 70°). Green stick figure shows under-compensation (with a trunk-thigh angle of 90°, the infant would be pulled off balance and topple backward) and orange stick figure shows over-compensation (e.g., with a trunk-thigh angle of 40° bringing the trunk close to the knees, infants would also need to recruit abdominal muscles). Note that C-D also give a visual representation of the steepest “sitable” slopes that infants achieved, on average, in the current study in the forward (M = 30°) and backward directions (M = 19°).

Figure 2

Line drawing of a typical infant in the (A) forward and (B) backward test directions at the average steepest “sitable” slopes that infants achieved. Because the slope only slanted in one direction, we created the two conditions by facing infants down or up the slope. Orange lines represent trunk-thigh angles derived from manual digitization of videos. (A) Forward: The trunk-thigh angle is 98°, so the infant is under-compensating (not leaning sufficiently backward). (B) Backward: The trunk-thigh angle is 63°, so the infant is slightly over-compensating (leaning slightly more forward than necessary).

Figure 3

Summary of major findings. Trunk-thigh angles and slants are measured in degrees. Blue circles represent the average trunk-thigh angles for the forward direction, and the red circles show the average trunk-thigh angles for the backward direction at each increment of slant. Error bars denote ±1 SE. Light green bar shows the range of baseline trunk-thigh angles with slant set to 0°. The dark green circle and dashed line denote the average baseline trunk-thigh angle. The increasing distance between circles and dashed line emphasizes the faster rate of change in trunk-thigh angle in the backward direction compared with the forward direction. Ns represent the number of infants contributing data at each slant in each direction. All 22 infants contributed data to 2°-18° slants in the forward direction and to 2°–6° in the backward direction. Two infants achieved steepest sitable slopes of 42° in the forward direction and 1 achieved 36° in the backward direction. Bar graph shows the group average for steepest sitable slope for each direction. Error bars denote ±1 SE.

Figure 4

Individual differences in infants’ ability to cope with forward (top panel) and backward (bottom panel) slopes. Trunk-thigh angles and slants are measured in degrees. Each infant is represented by the same participant number in each direction; infants who fussed out in each direction are noted. Data are ordered in columns by the steepest sitable slope in each direction (steepest sitable slope is also represented by the width of each graph). Large graphs on left side of each panel show the most successful infant in each direction and the extent of the axes. Stick figures above each graph illustrate (A) the increase in infant #08’s trunk-thigh angle from baseline to steeper slants and (B) the decrease in infant #01’s trunk-thigh angle from baseline to steeper slants. The legs-down posture in A and legs-up posture in B reflect the thigh angle relative to horizontal; the trunk position shows infant’s trunk relative to vertical. Circles represent trunk-thigh angle at each observed degree of slant. Delta values (Δ), the change in trunk-thigh angle across slants, are denoted for each infant in each direction based on the fitted regression line through each infant’s data (solid lines). Dashed lines represent the optimal regression line if infants adjusted their trunk-thigh angle from baseline by 2° for each 2° increase in slant. Size of the yellow colored region between the solid and dashed lines reflects infants’ deviation from optimal responding. Scatterplots in gray boxes on left side of each panel show delta values across months of sitting experience.