Sit happens: Does sitting development perturb reaching development, or vice versa?

Highlights

• We followed infants longitudinally while sitting and reaching skills developed.

• We used observational coding, motion analysis and EMG to describe changes over time.

• Reaching development was not perturbed by unstable sitting postural control.

• Variable postural strategies characterized early sitting, often causing falls.

• Infants appear to prioritize reaching for objects in spite of postural errors.

Abstract

The development of reaching and of sitting during the first year of life is typically studied as separate yet related behaviors. Interestingly, very soon after learning to reach, 4–7-month-old infants start coordinating their arms with their trunk and legs for sitting. In this longitudinal study, we focused, for the first time, on how infants learn to use their arms for the dual tasks of reaching for objects while providing arm support as they learn to sit. We hypothesized that the use of arms for support during sitting development would be a temporary perturbation to reaching and result in a nonlinear progression of reaching skill.

Eleven infants were studied monthly from the time they began to prop sit to the time of sitting independence (5–8 months of age). Behavioral coding, kinematics, and electromyography (EMG) characterized reaching and posture while infants sat as independently as possible. Results revealed significant changes across time in trunk movement and hand use as infants transitioned through three stages of sitting: with arm support, sitting briefly without arm support, and sitting independently. Infants used their hands more for contacting objects and less for posture support linearly across time. In contrast, changes in posture control as indicated by pelvis and trunk movement demonstrated a U-shaped curve with more movement of these two body segments during the middle stage of sitting than in the first or last stage. During the middle stage of sitting infants reached persistently even though posture control, measured by pelvis and trunk movement, appeared to be significantly challenged. Muscle activation consisted of tonic and variable combinations of muscle pairings in early sitting. As infants progressed to sitting without hand support, variable but successful strategies utilizing lower extremity muscles in a tight linkage with reach onset emerged to provide prospective control for reaching. Our findings support the contention that reaching both drives the development of sitting in infancy as well as perturbs sitting posture, factoring into the assembly of the complex dual sit–reach behavior that supports and expands flexible interaction with the environment.

Introduction

This study seeks to better understand how the experiences encountered by infants during the initial development of sitting impact the continued development of reaching. Reaching, which emerges around 4–5 months of age (Thelen et al., 1993, von Hofsten, 1991), provides the first independent exploration of objects in the local environment. Not surprisingly, reaching creates an impact on perceptual, cognitive and social development (Corbetta, Thelen & Johnson, 2000) including knowledge about objects (Needham, 2000, Ruff et al., 1992) and plans for action with objects (Claxton, Keen & McCarty, 2003). Sitting also begins emerging around 4–5 months of age as infants gain head and upper trunk control in supported sitting. Corresponding changes in perceptual, cognitive and social development appear to be related to the achievement of sitting (Out et al., 1998, Rochat, 1992, Soska et al., 2010). Unlike reaching, which is a behavior that allows infants to be successful interacting with objects shortly after the behavior first emerges, sitting takes an additional 3–4 months to transition from infants using their arms for support (aka ‘prop sitting’) to independent sitting (Folio and Fewell, 2000, McGraw, 1945, Piper et al., 1992). Thus, sitting and reaching overlap both in developmental time, and in relation to other areas of development.

We propose that learning to combine sitting and reaching requires a period of problem solving to plan and execute movement and exploration of objects. Specifically, newly sitting infants who are offered objects are faced with a dilemma that must be solved in real time: risk falling by using one or both arms to reach for the object, or remain safely prop sitting but inhibit the drive to explore a reachable object. The general purpose of this study was to test hypotheses regarding the process by which infants learn to combine these two behaviors.

In spite of the obvious interaction of these two behaviors, studies of reaching and sitting development typically focus on each behavior separately. For reaching, infants are generally studied when they are in a supported supine or reclined sitting position with reduced postural control requirements. For example, Thelen et al. (1993) stabilized infants’ trunk in a specialized reclined seat during reaching throughout the first year of development for quantification of the reach. Savelsbergh and van der Kamp (1994) also utilized a specialized seat to reduce the influence of limits in postural control when examining infant reaching in different orientations to gravity. Together, these studies found individual differences between children in the way reach developed, as well as differences within each child depending on their position in space (vertical sitting or reclined sitting). Infant reaching generally becomes smoother and more accurate from the emergence of reaching at 4 months to skillful reaching at 8 months of age (von Hofsten, 2004, Thelen et al., 1993). However, the constrained posture of the infants in these studies limits the translation of these findings to everyday, naturally occurring behavior, which involves the ongoing coordination of reaching and sitting.

The question remains whether reaching enhances or challenges the gradual emergence of sitting. For example, independently sitting infants reached longer distances and more frequently compared to infants not yet sitting independently (Rochat & Goubet, 2000). Independent sitting is also linked to the attainment of object manipulation skill (Adolph & Berger, 2011), and is thought to provide variable opportunities for the child to visually orient to and regard the object being manipulated (Soska et al., 2010). Moreover, once sitting and reaching begin to co-occur, this linkage affects the development of language and cognition (Iverson, 2010, Smith and Sheya, 2010) as well as the emergence of crawling and walking (Corbetta and Bojczyk, 2002, Goldfield, 1989). Thus, the motor skill of sitting has been implicated as one of the factors important to the progression of overall skill in the motor domain, and thus has been quantified using both behavioral and kinematic techniques.

Quantification of motor coordination during sitting development indicates changes in patterns of muscle activation, kinematics and/or key behaviors related to increasing stability. Muscle activity patterns became more tightly timed to the intended action of the child (reaching or controlling the trunk in sitting), and more specific and accurate muscle responses emerge as sitting became more stable (Harbourne et al., 1993, Hadders-Algra et al., 1996a, Woollacott et al., 1987). In addition, the variability of muscle activation to control a postural perturbation in sitting decreases as sitting improves (Harbourne et al., 1993). Trunk and pelvic kinematics change as sitting improves. For example, trunk velocity in the forward direction decreases over developmental time with the removal of support in prop sitting infants, indicating that infants gain increasing control of their trunk and head mass over time (Harbourne et al., 1993). Infants change from an in-phase to an out of phase relationship of thorax–pelvis motion as they progress from prop sitting to sitting without arm support, indicating greater axial control of posture (Kyvelidou et al., 2009). Observational techniques describe similar increases in straightness and verticality of the trunk as sitting develops (McGraw, 1945, Piper et al., 1992). These details of sitting progression have not previously been studied in relation to reaching, so the interaction and coordination of these two skills is unclear. Thus, although much has been learned from studying sitting and reaching behaviors separately, the present study built on the limited literature describing how infants solve the problem of staying vertical while using the arms for reaching, which we briefly summarize next.

The development of sitting may advance the continued progression of reaching. Spencer, Vereijken, Diedrich, and Thelen (2000) posited that sitting independently was associated with stable reaching. For example, Kamm (1995) coded the reaching and sitting progression in 5 infants from 8 weeks of age until they had one month of sitting experience. Vertical orientation had a facilitative effect on the frequency and skill of reaching. She proposed that postural control and reaching develop as a unit as the infant learns control of the head and body for vertical orientation. Experience reaching for objects was correlated to improved postural stability in infants sitting on a moving platform, with more experienced reachers showing more mature postural reactions as measured by electromyography (EMG) (Hadders-Algra, Brogren, & Forssberg, 1996b). Thus, sitting and reaching are complimentary once infants have experience in both. But what about the key period in which reaching has been established in supported positions, and the infant now wants to reach from the new, unstable position of self-supported sitting?

The initial development of sitting may constrain the continued progression of reaching. Rochat and Goubet (1995) examined infant reaching and sitting behaviors in a cross-sectional comparison of sitters, non-sitters, and adult supported non-sitters. Infants who were not independent sitters did not utilize the trunk to extend their reach. Interestingly, whenever non-sitters were provided pelvic support they showed a precocious ability in real time to extend their reach using their trunk. Thus, infants’ postural instability may constrain the adaptability of their reaching to more distant objects. Moreover, these researchers proposed that during the development of reaching, infants who are new to self-supported sitting understand their level of stability and the perceived “reachability” of a nearby object. Consequently, they only attempt a reach when they can avoid a loss of verticality through use of their balance and trunk control (Rochat, Goubet & Senders, 1999). This implies that infants may prioritize postural stability to avoid a fall, and thus alter or inhibit reaching if posture is challenged.

Taken together, these studies outline a general relationship between sitting independence and reaching: sitting emergence may initially inhibit reaching as the infant must use the arms preferentially for postural support, but then enhance reaching when sitting is independent. Several basic questions remain to be addressed. This longitudinal study addresses the issue of how sitting and reaching interact developmentally, and whether the development of independent sitting perturbs or interrupts the ongoing advancement of reaching. We predicted that infants’ reaching would be perturbed by their early attempts at self-supported sitting for 3 reasons: (1) reaching is an established skill compared to the new skill of sitting; (2) poor sitting results in falling, thus use of the arms to maintain support would take precedence over reaching; and (3) sitting itself demands significant planning and infants would not be able to quickly plan for two relatively new skills at the same time. Thus, we hypothesized:

• An interruption of the progression of hand use to either (a) contact objects, or (b) provide postural support as sitting progresses to independence.

• Progressively decreased movement of the trunk and pelvis as sitting posture control emerges.

• Increased coupling of synergistic muscles of the arm, legs and trunk as sitting becomes independent.