Pulling to stand - The City University of New York

Developmental Psychobiology
Osnat Atun-Einy1
Sarah E. Berger2
Anat Scher1
1
Department of Counseling
and Human Development
Faculty of Education
University of Haifa
Haifa, Israel
2
Department of Psychology
College of Staten Island and the
Graduate Center of the City University
of New York, 2800 Victory Blvd., 4A-108
Staten Island, New York 10314
E-mail: [email protected]
Pulling to Stand: Common
Trajectories and Individual
Differences in Development
ABSTRACT: This longitudinal study of 27 infants examined the development of
pulling-to-stand (PTS). In general, infants began PTS using a two-leg strategy
and transitioned to a half-kneel strategy. As a group, infants showed no preference for either strategy at the onset of PTS, switching between strategies until
half-kneeling became the dominant pattern about 1 month after the onset of PTS.
Examination of individual developmental trajectories revealed variability in age
at PTS onset, time between PTS onset and half-kneel strategy onset, duration of
the two-leg strategy as the dominant pattern, time until the half-kneel strategy
became the dominant pattern, shape of the transition between strategies (gradual
vs. abrupt), and timing of PTS relative to onset of other motor milestones. We
discuss variation in developmental trajectory in terms of adaptive behavior during the acquisition of new skills and as a process shaped by infants’ unique
experiences prior to and during the acquisition period. ß 2011 Wiley Periodicals,
Inc. Dev Psychobiol
Keywords: motor development; individual differences; infancy; pulling-to-stand;
pattern preference index; motor coordination
INTRODUCTION
Pulling-to-stand (PTS) is one of the most memorable
motor achievements in infancy. Parents experience
surprise and delight the first morning they enter their
infants’ rooms to find them upright holding onto the
railing of the crib (Alexander, Boehme, & Cupps,
1993). When asked to remember important motor milestones, 98% of parents reported when their infants first
pulled to a standing position, the highest report of
any motor milestone on a 12-item checklist (Capute,
Shapiro, Palmer, Ross, & Wachtel, 1985). For all of
its salience with parents, however, compared to other
motor milestones, PTS has been relatively overlooked
in the research literature.
Received 14 February 2011; Accepted 6 July 2011
Correspondence to: S. E. Berger
Contract grant sponsor: Israel Science Foundation
Contract grant number: 208/07
Published online in Wiley Online Library
(wileyonlinelibrary.com). DOI 10.1002/dev.20593
ß 2011 Wiley Periodicals, Inc.
The primary focus on PTS has been documenting
age of attainment, but there has been inconsistency
in the literature depending on the definition used
to operationalize PTS, the age at which infants were
observed, and whether documentation was for assessment or descriptive purposes (Bayley, 1993; Capute
et al., 1985; Frankenburg & Dodds, 1975; Largo et al.,
1985; Piper & Darrah, 1994; Wijnhoven et al., 2004).
When PTS has been included in empirical studies it is
usually as the anchoring milestone in the transition to
independent standing and walking, after infants already
know how to PTS, but not a focus of study per se
(e.g., Sundermier & Woollacott, 1998; Woollacott &
Sveistrup, 1992). For example, infants use less force to
hold onto a surface of support after they learn to stand
alone than they did when they could only PTS (Barela,
Jeka, & Clark, 1999) and muscle activation in the
ankle, knee, and hip was less coordinated in infants
who could only PTS than in infants who could stand
independently (e.g., Sveistrup & Woollacott, 1996;
Woollacott and Sveistrup, 1992). In the few studies to
document the process by which infants move through
the sub-sequences of PTS, the focus has been on atypical
development in children with Down syndrome (Lauteslager, 1995; Lauteslager, Vermeer, & Helders, 1998).
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Atun-Einy, Berger and Scher
It takes infants several months to proceed through
the typical sequence of upright postures that precede
independent mobility. This sequence of milestones
starts between 7 and 10 months of age (8 months on
average) with infants using furniture or another object
for support to help them pull up to a standing position
and continues with infants cruising along a surface of
support, keeping balance without holding on, taking
their first independent steps, and, finally, between 11
and 15 months of age, independent walking (Capute
et al., 1985; Frankenburg & Dodds, 1975; Piper &
Darrah, 1994). Summaries of this sequence typically
describe PTS briefly as a transitional milestone that
launches the subsequent events (Adolph & Berger,
2006; Piek, 2006; Trettien, 1900) and the sequence as a
whole reflects a protracted period during which leg
strength and balance control increase (Thelen & Ulrich,
1991).
PTS represents an important achievement in the
development of postural control, including upward
displacement in the body’s center of gravity and
narrowing the base of support (Alexander et al., 1993).
Standard behavioral assessment tools (e.g., Bayley,
1993; Frankenburg & Dodds, 1975; Piper & Darrah,
1994) and guidelines for practitioners (e.g., Alexander
Developmental Psychobiology
et al., 1993; Bly, 1994) provide good qualitative
descriptions of the endpoints of the development of
PTS and the ages at which the intra-task sub-sequences
are achieved. For example, the Alberta Infant Motor
Scale (AIMS; Piper & Darrah, 1994) includes a 16item subscale for standing, that has three items regarding the transition to stand: PTS with support, PTS as a
way to assume independent standing, and a half-kneel
posture as a position to play. In the beginning, infants
pull-to-stand supporting most of their weight with their
arms, and with their legs extended symmetrically
behind them (Fig. 1A). Gradually, infants use the lower
extremities more actively when pulling up against
gravity to supported standing (Vander Linden &
Wilhelm, 1991). As greater control of the hips develops, infants progress to PTS from a half-kneeling
position. Half-kneeling is a qualitatively different
movement pattern than PTS from two feet because it
requires a dissociation of the lower extremities while in
a weight-bearing position (Alexander et al., 1993). To
pull to a stand from a half-kneel, infants start with both
legs flexed towards the trunk, with one or both rotated
to one side. Then, one leg is flexed forward and pushes
into extension as the other leg extends from kneeling
and moves forward (Fig. 1B). In the AIMS, the only
FIGURE 1 Schematic drawing of (A) two-leg and (B) half-kneel strategies for pulling to
stand.
Developmental Psychobiology
assessment tool that has half-kneeling as an independent item, infants receive credit for half-kneeling as
a transitional posture used for PTS or a permanent
posture in which they play. Fifty percent of infants
demonstrate half-kneeling by 8.5 months of age and
90% by 11 months of age (Piper & Darrah, 1994).
Traditionally, the transitions to sitting, crawling, and
walking have been a well-studied part of motor development research because of the psychological changes
that accompany dramatic change from prone to upright
postures or from immobility to independent locomotion
(e.g., Adolph, Vereijken, & Denney, 1998; Adolph,
Vereijken, & Shrout, 2003; Deffeyes et al., 2009). The
onset of crawling, for example, is related to increased
conflict between parents and infants, infants’ sense of
mastery at their newfound independence, and cognitive
changes as infants explore more of their environment
(Acredolo, Adams, & Goodwyn, 1984; Campos,
Kermoian, & Zumbahlen, 1992; Campos et al., 2000;
Koterba and Iverson, 2005). Like crawling, PTS may
also mark a significant psychological change in infancy.
PTS provides infants with a new upright vantage point
or a position to play (Alexander et al., 1993; Bly, 1994;
Piper & Darrah, 1994), new opportunities for different
exploratory behaviors, the ability to get upright under
their own volition, and when they do, new visual,
vestibular, and proprioceptive information (Adolph,
1997; Chen, Metcalfe, Jeka, & Clark, 2007; Metcalfe
et al., 2005). Learning to pull to a stand is associated
with emotional changes, such as self-awareness, pride,
and joy (Trettien, 1900).
OBJECTIVES
Studying the naturalistic, spontaneous transition as
infants learn to pull to a stand has important theoretical
and clinical implications. First, PTS marks a meaningful postural change from quadruped to biped (Barela
et al., 1999; Sveistrup & Woollacott, 1996; Woollacott
& Sveistrup, 1992). Second, disturbances in motor
development may be indicated by stereotyped and
invariant behaviors during the acquisition of PTS
(Alexander et al., 1993; Bly, 1994; Lauteslager et al.,
1998; Winders, 1997), but to know what is truly invariant or atypical, we must first examine and understand
the normative range of development. Thus, the first
aim of this study was to describe in detail the typical
acquisition and developmental trajectory of PTS.
Our second aim was to examine individual differences in the acquisition of PTS. We examined both
within- and between-posture variability, specifically
individual differences in the timing of the subcomponents comprising the pull-to-stand posture, and
Common Trajectories and Differences in Development
3
variability in timing of PTS relative to other motor
milestones (Adolph, Berger, & Leo, 2011; Berger,
Theuring, & Adolph, 2007). Because PTS is a closed
task with a clear starting and ending position and minimal variability in the environment over the course of
executing the motor skill, we expected to see fewer patterns of movement than typically observed in open motor tasks, such as crawling, walking, and reaching, in
which actors respond to a constantly changing environment as the task is executed (Adolph et al., 1998, 2003;
Thelen et al., 1993). Akin to the pattern of transitioning
between two strategies over the course of learning to
stand up from a chair independently (a closed task;
McMillan & Scholz, 2000), we predicted that infants
would first pull to a stand using both legs simultaneously before eventually pulling to a stand from a halfkneeling position. We also had the opportunity to
examine individual patterns of timing as infants learned
to pull to a stand, previously done only with open-task
skills. We expected to find variation in the amount of
time that infants spent in each sub-posture and in the
time it took them to move from one to the other.
METHOD
Participants
Twenty-seven infants (17 male; 10 female) participated from
the time they were 7 months old until they were 12 months.
We chose 7 months to begin observation because the average
age at which infants first pull to stand is around 8 months and
we wanted to capture the onset (Capute et al., 1985; Piper &
Darrah, 1994). One boy began contributing data at 6 months
of age because he had earlier motor milestone onsets than the
rest of the sample. Three infants continued to be studied
after they were 12 months until we obtained a minimum of
3 sessions where we observed them PTS. No infants could
pull-to-stand when the study began. By the time the study
ended, 7 infants had begun to walk.
Families were recruited to participate in the study by posting fliers about the research around the university where the
research was being conducted and by leaving fliers at health
care centers. Participants were also recruited via a ‘‘snowball’’ technique where participants were asked to mention the
research to friends or contacts via word of mouth. All infants
were born at full term and were in good health. All families
but one were urban, Caucasian, and of middle to upper-middle socio-economic status. Mothers’ average age at the start
of the study was 33 years; fathers’ average age was 35 years.
Average years of education for both mothers and fathers was
17 years. Families received disks with the movies from each
observation session and a children’s book as thank you gifts.
Procedures
Naturalistic Observation. Infants were videotaped for 20–
30 min in their own homes every 3 weeks. Infants were
4
Atun-Einy, Berger and Scher
observed at least seven times. Due to individual differences in
timing of PTS onset, some infants had 3–4 additional observation sessions. The number of total home observations for
each infant ranged from 7 to 11 (M ¼ 8.5; SD ¼ .89). The
number of times infants pulled to a stand at each session
ranged from 3 to 8 (M ¼ 6). From the videotapes of each
session, a primary coder rated infants’ PTS ability:
no attempt to get upright, attempt to get upright, success at
getting upright with two legs, success at PTS from a halfkneel posture. After infants achieved success at PTS using
either strategy, a primary coder rated subsequent sessions to
determine when infants were skilled at PTS, which was defined as PTS with minimum effort and concentration, with the
ability to change positions quickly.
Motor Milestone Checklist. Parents documented the onsets
of getting to sit, pulling to stand, hands-and-knees crawling,
and cruising. Parents were given an illustrated checklist of
motor milestones based on Scher and Cohen’s (2005) motor
diary. They were instructed to use the checklist to monitor
changes in their infants’ motor skills between observation
sessions. The onset of getting to sit was when the infant could
get into a sitting position independently and hold the position.
The onset of PTS was defined as the day when infants first
successfully used furniture or another object as support to
pull themselves up and maintain an upright position without
falling. When infants met the attempt criteria, parents were
instructed to pay special attention to infants’ behavior and
to contact the researcher as soon as infants first met either
of the success criteria. The onset of crawling was when
infants could execute two full cycles of forward movement on
hands and knees with their belly off the ground. The onset of
cruising was when infants could support an upright posture
by holding onto a surface of support with their hands and
execute two full cycles of movement using hands and feet.
Infants’ acquisition of motor milestones according to checklist criteria was confirmed via video coding.
Developmental Psychobiology
for PTS with a half-kneel as between .5 and 1.0, and no
pattern preference around 0. For each infant, a preference
index was calculated for each observation and for the overall
study.
Motor Development Screening. To ensure that participants
fell in the normal range of motor development, infants were
screened in two ways. Trained pediatric physical therapists
rated videotapes using the Alberta Infant Motor Scale (AIMS;
Piper & Darrah, 1994) for each home observation of each
infant. The AIMS is an assessment tool used to identify motor
delay and evaluate motor performance from birth to independent walking. The scale was normed on a cross-sectional,
random sample of over 2000 infants, has high inter-rater and
test–retest reliability and validity, and has been cross-culturally validated (Darrah, Piper, & Watt, 1998; Darrah, Redfern,
Maguire, Beaulne, & Watt, 1998; Jeng, Yau, Chen, & Hsiao,
2000; Manacero & Nunes, 2008; Piper & Darrah, 1994;
Syrengelas et al., 2010). In addition, a trained pediatric physical therapist manually evaluated infants’ muscle tone and
range of motion in the hip, knee, and ankle joints in the lower
limbs and shoulder, elbow, and wrist joints in the upper limbs,
during one session via passive limb movement. If infants
were evaluated to have relatively excessive range of motion
in more than five joints, then they were classified as having
hypermobility.
Inter-Rater Reliability. To ensure reliability, a second coder
recoded 30% of observations making sure to choose samples
so that all ages were represented. Interrater reliability for
both the naturalistic observations of PTS and the AIMS was
greater than 90%. Disagreements were resolved through
discussion.
RESULTS
Pattern Preference Index. A primary coder calculated
infants’ PTS pattern preference at each observation point by
identifying whether infants preferred to pull to a stand using
two legs or from a half-kneeling position. Coders used the
index ½ðBUÞ=ðB þ UÞ which is frequently used for calculating right or left hand/leg preferences or uni- or bi-manual
reaching preferences (e.g., Armitage & Larkin, 1993;
Corbetta & Thelen, 1999; Corbetta, Williams, & SnappChilds, 2006; Cornwell, Harris, & Fitzgerald, 1991; Fagard,
1998; McCormick & Maurer, 1988; Michel, Sheu, &
Brumley, 2002). For this study, B stands for the number
of times that infants used two legs to PTS (‘‘bipedal’’)
and U stands for the number of times infants PTS from a
half-kneel (‘‘unipedal’’). This index generates a continuous
measure of preference enabling researchers to describe how
strong a preference is, rather than create a possibly artificial,
dichotomous forced choice (Bryden & Sprott, 1981; Michel
et al., 2002). The limited range from 1 to 1 makes it possible to compare preferences between infants. We defined dominance for PTS with two legs as an index between .5 and 1.0,
Preliminary Data Analysis
The distribution of infants’ scores on the AIMS was
close to a normal distribution (skewness ¼ .11,
SD ¼ .44; kurtosis ¼ .18, SD ¼ .87), with all scores
falling within the normal range. The physical exam
identified hypermobility in 6 of the 27 participants. For
an additional two infants, the physical exam revealed
hypermobility and muscle tone at the low end of the
normal range. Nonparametric Mann–Whitney analyses
tested whether the eight participants with excessive
range of motion were different from infants with normal muscle tone and normal range of motion. Analyses
revealed no differences based on physical examination
for age of attainment for standing, crawling, half-kneeling, cruising, or AIMS score. Therefore, subsequent
analyses make no distinction between participants
based on the results of the physical exam.
Developmental Psychobiology
Onset of Pulling to Stand
Figure 2 shows individual differences in the age and
timing of PTS onset and strategies. All infants learned
to pull to a stand over the course of the study. As indicated by the end of the dark gray sections of Figure 2,
the age of onset of PTS ranged from 5.92 to 11.89
months (M ¼ 8.66 months; SD ¼ 1.26 months), which
fell within the expected normal developmental range
(Bayley, 1993; Piper & Darrah, 1994). At onset, only 2
infants (7%) used half-kneeling exclusively to PTS, 10
infants (37%) used 2 legs exclusively to PTS, and 15
(55%) infants PTS using both strategies in the same
session.
The start of the light gray sections of Figure 2 indicate the age of onset of PTS using the half-kneel pattern (range ¼ 6.56–11.89 months; M ¼ 9.23 months;
SD ¼ 1.26 months). All infants except one (who used
two legs to PTS throughout the entire study) used the
half-kneel pattern during the study. The half-kneeling
strategy was observed within a week of the onset of
PTS in 34.6% of participants, between 1 and 2 weeks
after the onset of PTS in 15.4% of participants, and
greater than 2 weeks after the onset of PTS in 50% of
participants (range ¼ 0–1.51 months).
The endpoint of each individual graph in Figure 2
marks the age at which skilled half-kneel PTS was
attained. The time from PTS onset to skilled half-kneel
PTS ranged from 1.11 to 2.89 months (M ¼ 1.91
months, SD ¼ .41 months).
We examined when infants achieved the PTS milestone relative to other locomotor milestones (getting
into a sitting position independently, crawling on hands
and knees, cruising). On average, infants pulled to a
stand after the onset of hands and knees crawling and
getting into a sitting position and before cruising. There
Common Trajectories and Differences in Development
5
were no gender differences for onset ages for any of
the motor milestones. An examination of individual
patterns of motor milestone achievement revealed a variety of motor milestone orders that were not apparent
when ages of onset were averaged across the whole
sample. Only seven infants (26%) matched the average
group pattern. The rest of the sample achieved their
motor milestones in three different additional orders
(Fig. 3).
In addition to variability in the order in which milestones were achieved, we also observed variability in
the timing in which milestones were achieved relative
to each other. Close to half of the sample achieved at
least two milestones within a week of each other. Seven
infants (26%) began PTS and hands-and-knees crawling
in the same week, four (15%) began getting to sit and
PTS in the same week, and two (7.5%) began handsand-knees crawling, getting to sit, and PTS all in the
same week.
Changes in Pattern Preference
A repeated measures ANOVA on Pattern Preference Index score at the first four observations after onset of
PTS revealed a main effect for session, F(3,
72) ¼ 17.50, p < .00, h2 ¼ .41 (Fig. 4). As expected,
the preference for PTS using half kneel increased over
the course of the study. A series of post hoc, least significant difference, pairwise comparisons revealed that
the preference for half kneeling was significantly lower
in session 1 than in sessions 2, 3, and 4 (p’s < .04);
and lower in session 2 than in session 3 (p < .01).
Relationship Between Skill and Pattern
Dominance
To test the relationship between skilled PTS and pattern
preference, we identified for each infant the session at
which skilled half kneel PTS was first achieved. A repeated measures ANOVA comparing infants’ Pattern
Preference Index score at the ‘‘skilled’’ session with the
sessions immediately before and after revealed a main
effect for session, F(2, 50) ¼ 11.56, p < .00, h2 ¼ .32.
Infants showed a significant increase in preference for
the half-kneel strategy from the session prior to skilled
half-kneeling to the session where they were skilled at
using it. Infants’ preference for the half-kneel strategy
remained unchanged between the session where skill
was first observed and the subsequent session.
Pulling to Stand Trajectory Profiles
FIGURE 2 Total length of each bar represents infants’ time
course of pulling to stand until skilled half-kneeling was
attained.
To address our aim of examining the developmental
trajectory of PTS and individual differences, trajectory
profiles for individual infants were determined by
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Atun-Einy, Berger and Scher
Developmental Psychobiology
FIGURE 3
Individual patterns of motor milestone onsets.
examining the shape of change of infants’ Pattern Preference Index over the course of the study. By examining the shape of the trajectory, rather than only a static
index, profiles contained additional information about
the time from the first day of PTS onset until the first
day the half-kneel pattern was attained, and time until
the half-kneel pattern was the dominant pattern.
We first sorted participants into groups based on
whether their individual trajectory fit the overall group
pattern preference. Sorting continued with profiles of
individuals who did not fit the overall pattern based on
how long infants spent in each PTS pattern. Three distinct profiles emerged: gradual transition to half-kneel
preference (group pattern), prolonged use of two legs,
and rapid transition to half-kneeling.
FIGURE 4 Mean Pattern Preference Index score for the
first 4 observation sessions that infants pulled to a stand.
Scores from .50 to 1.00 indicate a dominant preference for
PTS with two legs. Scores from 1.00 to .50 indicate dominant preference for PTS with half-kneel. Scores around 0 indicate no preference.
Profile 1: Gradual Transition to Half-Kneel Preference. Forty-four percent of infants (n ¼ 12) fit the profile
of having a gradual change in PTS strategies before
exclusively using half-kneeling to PTS (Fig. 5A).
Infants in this profile began PTS either with a two-leg
Developmental Psychobiology
Common Trajectories and Differences in Development
7
FIGURE 5 Representative individual trajectories for PTS fitting (A) Profile 1, (B) Profile 2,
and (C) Profile 3. Dashed vertical lines in graphs (D)–(F) indicate the onset of key motor
milestones for the three case studies. For (D), the last three observation sessions fell during
the follow-up period after the formal period of study had ended. Scores from .50 to 1.00
indicate a dominant preference for PTS with two legs. Scores from 1.00 to .50 indicate
dominant preference for PTS with half-kneel. Scores around 0 indicate no preference.
preference or without a preference and then gradually
acquired a preference for the half-kneel strategy. By
the last observation session, all infants were using the
half-kneel strategy exclusively. The physical examination revealed hypermobility in only two infants fitting
Profile 1.
Profile 2: Prolonged Use of Two Legs to PTS. Twentysix percent of the infants in the sample (n ¼ 7) fit the
profile of prolonged use of two legs to PTS as the dominant pattern (Fig. 5B). Infants in this profile typically
did not begin to use half-kneel to PTS as their dominant strategy until the 5th or 6th session. Of the seven
infants fitting Profile 2, the physical examination
revealed one with low tone and hypermobility, three
with hypermobility, and two with excessive range of
motion in the legs but not the arms.
Figure 5D shows an extreme case of prolonged exclusive use of two legs to PTS. Infant GK’s trajectory
was so unusual that we describe it in detail here as a
case study, but his data were not included in the group
statistical analyses. In 8 observations in which GK
pulled to stand 53 times, he did not use the half-kneel
strategy once. PTS using the half-kneel strategy was
observed only in follow-up observation sessions when
he was 14 months and 21 days—after cruising and
close to the onset of walking. Because of the unusual
timing, we observed GK closely and noted that he used
the ‘‘w-sit’’ pattern exclusively for the first month after
he began to sit independently. W-sitting may indicate
insufficient muscle control around the hip compensated
for with a symmetrical sitting posture. GK also demonstrated excessive range of motion during the physical
exam but only in the lower limbs. Despite the unusual
developmental trajectory for PTS, GK was within the
normal range on the AIMS, and his locomotor milestone onsets were in the normal range. Onset of sitting
may have been delayed—his parents sat him passively
at around 9 months of age and he got into a sitting
position independently at 11 months.
Profile 3: Rapid Transition to Half-Kneeling. Thirty
percent of the infants in the sample (n ¼ 8) fit the profile of a rapid transition to PTS using a half-kneeling
strategy (Fig. 5C). Infants in this profile typically began
using half-kneeling as a strategy for PTS immediately
after the onset of PTS and began using it almost exclusively. One infant demonstrated hypermobility during
the physical examination and another infant demonstrated hypermobility and low tone.
Figure 5E shows an extreme case of rapid and exclusive use of PTS using a half-kneeling strategy. Infant
8
Atun-Einy, Berger and Scher
AB was observed 11 times, but only pulled to stand in
5 observations. During the first observation session
where PTS was observed, it occurred only once. AB
was the last infant in the study to attain PTS and he
demonstrated the half-kneel pattern exclusively. Despite
the unusual developmental trajectory for PTS, AB was
within the normal range on the AIMS.
Kruskal–Wallis tests comparing the three profiles
confirmed significant differences between the groups
for Overall Pattern Preference Index score and time
until the half-kneel pattern became a dominant pattern
(x2 ¼ 16.79, p < .01 and x2 ¼ 11.27, p < .01, respectively), as well as the Pattern Preference Index scores
at each of the first three observations sessions, all
p’s < .02. Mann–Whitney tests revealed that each profile was significantly different from the other two on all
measures (all p’s < .02), except for Profiles 1 and 2 on
time until half-kneel pattern dominance. There were no
differences between the groups for age of onset for
PTS, crawling on hands and knees, cruising, or AIMS
scores. A chi-square test revealed no significant relationship between PTS trajectory profile and physical
examination results.
Microgenetic Case Study
We had the opportunity to observe 1 infant, GE, at least
once per week, but sometimes more frequently, from
the age of 8.5 months until the age of 13 months, for a
total of 32 observation sessions. The most striking characteristic of the development of PTS was an almost exclusive use of the half-kneel strategy from the onset
(Fig. 5F). On only three observations did GE pull to a
stand with two legs, and on those occasions PTS with
two legs was sporadic and GE still showed a half-kneel
preference at those sessions. GE’s motor development
was typical on all measures, with the only noteworthy
event being a modified crawl on one knee and one foot
as her typical crawling pattern immediately prior to the
onset of PTS.
DISCUSSION
This study longitudinally examined the development of
PTS in infancy. On average, infants learned to pull to a
stand at 8.75 months of age using two legs before eventually starting to use the half-kneel strategy about
2 weeks later. The Pattern Preference Index showed
that, in general, infants showed no pattern preference at
the onset of PTS, but typically switched between strategies within and between sessions until half-kneeling
became the dominant pattern about 1 month after the
onset of PTS. Our findings about the development of
Developmental Psychobiology
PTS at the group level showed that the traditional ways
of describing this transition were accurate in the aggregate (Alexander et al., 1993; Bly, 1994). However,
observing the development of PTS at the individual
level revealed that traditional methods of observation
were unable to capture the nuances of variability
within and between participants. As with other welldocumented motor milestones, the development of PTS
is characterized by individual variation in trajectory,
timing relative to other motor milestones, and strategy
preference.
To address our first aim of describing the acquisition
and developmental trajectory of PTS, we documented
the change over time of the strategies that infants used
as they were learning to pull to stand. As expected,
infants transitioned from the two-leg strategy to the
half-kneel strategy for PTS. PTS with two legs requires
the legs to move simultaneously as one unit in two
dimensions. The half-kneeling pattern requires a lateral
weight shift to the supporting leg, greater coordination
between the flexors and extensors to keep balance
around the relevant joints (i.e., hip, knees), dissociation
between the legs, and movement in three dimensions
(Bly, 1994; Meade, 1998). When infants first learn
to pull to a stand, with minimal strength and balance
control for getting upright, stability takes the form of
limiting the movement of some joints and muscles
(‘‘freezing’’ the degrees of freedom) and shifting
the center of mass upward until the initial sub-tasks of
the skill are mastered (Bernstein, 1967; Vereijken, van
Emmerik, Whiting, & Newell, 1992). Later, as strength
and coordination increase, infants are free to introduce
more complexity into the solution, such as shifting the
center of mass laterally and upward (e.g., Harbourne &
Stergiou, 2003; McMillan & Scholz, 2000; Vereijken &
Waardenburg, 1996). Variability in pattern dominance
over the course of skill acquisition resulted from fluctuating motor abilities as infants acquired expertise about
keeping balance in their new upright posture (Piek,
2002).
To address our second aim of examining individual
differences during the acquisition of PTS, we examined
variability in the developmental trajectory of PTS and
documented three distinct profiles: (Profile 1) a gradual
change from the onset of PTS until the half-kneel strategy was dominant, (Profile 2) prolonged used of two
legs to pull to stand, and (Profile 3) a rapid transition
to half-kneeling to pull to stand soon after the onset of
PTS. In addition, we identified infants’ unique experiences that may have contributed to which profile they
fit. Most infants in the sample fit Profile 1 and it
reflects the average of the groups, whereas the other
two profiles describe trajectories that fall more to the
extremes. Six of the seven infants who fit Profile 2 had
Developmental Psychobiology
physical examinations revealing excessive range of
motion in their joints and/or low muscle tone. Hypermobility is a benign, common (8% of the normal population) phenomenon in infants, which can affect motor
development, but not necessarily (Gedalia & Brewer,
1993; Tirosh, Jaffe, Marmur, Taub, & Rosenberg,
1991). Studies of young children with Down syndrome
have shown that hypermobility is associated with a lack
of variability and increased symmetry as a result
of having to stabilize the joints (Lauteslager, 1995).
Similarly, infants’ prolonged use of two legs for PTS
(relative to the other profiles) may be related to the excessive range of motion of the joints (albeit within the
normal range) observed in this group as they acquired
the strength they needed for stability. On average, the
infants who fit Profile 3 had a PTS onset about 1 month
later than the other profiles. Although it was not statistically significant, this lag may have given them
more time to acquire sufficient control before attempting PTS so that once they did, they were ready to use
the half-kneel strategy (Haehl, Vardaxis, & Ulrich,
2000). Although the variables capturing the developmental trajectory of PTS were not continuous themselves, variation within Profile 1 suggests a possible
continuum of the developmental trajectory, with some
trajectories resembling a less dramatic version of
prolonged PTS with two legs (Profile 2) and other
trajectories resembling a somewhat less rapid transition
to half-kneeling than Profile 3.
The trajectories that infants went through during the
acquisition of PTS parallel the various developmental
trajectories of the acquisition of crawling. Like new
crawlers who explored a variety of crawling strategies
before eventually settling on a mature hands-and-knees,
diagonal gait (Adolph et al., 1998; Freedland & Bertenthal, 1994), Profile 1 infants started off exploring the
two different PTS patterns without showing a preference, before eventually settling on the mature halfkneel pattern. Infants in Profile 2, who initially pulled
to a stand with two legs before switching to half-kneeling, resemble new crawlers who initially belly-crawl
before eventually acquiring sufficient strength and balance to crawl on hands and knees (Adolph et al., 1998;
Freedland & Bertenthal, 1994). Profile 3 infants, who
used the half-kneel strategy to pull to a stand from the
beginning, resembled new crawlers who crawled on
hands-and-knees from the onset. This variation in
developmental trajectory reflects infants’ adaptive behavior during the acquisition of new skills. Infants took
into account their own motor ability and found a solution that was best for their unique sets of abilities and
constraints (Adolph et al., 1998; Thelen et al., 1993).
As predicted, in addition to variability in profiles at
the group level, we also found possible evidence for a
Common Trajectories and Differences in Development
9
relationship between infants’ individual differences and
variation in developmental trajectory at the individual
level. For example, experience in prior locomotor postures may have influenced how infants GE and AB
arrived at their PTS strategies (Corbetta et al., 2006).
Unusually, both infants used the half-kneel strategy
almost exclusively. GE’s unique one foot/one knee
crawling style may have transferred to the half-kneel
strategy and AB’s considerable delay in PTS (at 11.18
months) may have resulted in the more mature manifestation of the skill at onset due to facilitative motor
experience during the period prior to onset (Haehl
et al., 2000). Infant GK acquired the half-kneel strategy
for PTS a full 6 months after exclusively PTS using
two legs—the typical order of events, but on an unusually long time scale. Although still inconclusive, this
unique combination of factors suggests that his trajectory may have been shaped by instability around the pelvic girdle: recall the conflicting findings of w-sitting
and excessive range of motion in the lower limbs, but
AIMS scores and locomotor milestone onsets in the
normal ranges.
Motivation to move or get upright may also be a
predictor of variation in motor milestone trajectory
(Atun-Einy, Berger, & Scher, 2011; Thelen, 2005). For
example, motivation may account for why belly
crawlers are willing to endure the effects of dragging
their bodies along the floor for the sake of independent
locomotion (Adolph et al., 1998). We observed cases of
infants with similar physical characteristics who arrived
at different solutions in different sequences for PTS.
One infant with low motor tone and excessive range of
motion showed prolonged use of two-legs to pull to a
stand followed by cruising and then half-kneeling to
pull to a stand. In contrast, another infant with the
same outcome from the physical screening showed a
rapid transition to half-kneeling after a relatively
delayed PTS onset followed by cruising. Individual differences in the timing of when infants pulled to a stand,
relative to their other motor milestones may also indicate that attainment of an upright vantage point may
be a strong motivator for some children. Thus, we
observed individual differences in patterns of timing as
infants learned to PTS, both within the trajectory of
acquisition of the skill and relative to the onset of other
motor milestones, suggesting that infants’ experiences
prior to and during the acquisition of PTS, such as
physical characteristics, sensorimotor experiences, or
motivation, influenced the order and timing of patterns
(Thelen, 1995; Haehl et al., 2000).
A general principle of human movement is that we
are drawn to the most efficient, most economical motor
solutions across postures (Galna & Sparrow, 2006). In
this study, the half-kneel strategy eventually became
10
Atun-Einy, Berger and Scher
the dominant pattern for PTS for all infants, suggesting
its optimality. What makes half-kneeling the optimal
solution for PTS? Part of the explanation lies with the
greater stability that a half-kneel provides over a twoleg solution. With one leg turned out to the side, a halfkneel stance provides a wider base of support so that
infants are less susceptible to threats to balance. As
with crawling, asymmetrical limb coordination stabilizes the center of mass during motion (Adolph et al.,
1998). Even after they no longer need support to get
upright, older children and adults often continue to use
the half-kneel strategy to rise from the floor (VanSent,
1988). Another reason why the half-kneel strategy is
the optimal PTS solution is the flexibility that the posture provides. By shifting some of the body’s weight to
the legs and reducing the role of the arms during PTS,
the half-kneel strategy allows infants to have their
hands and arms free for other activities such as object
exploration or play. Infants may want their hands free
during the transition to a stand, prior to object play, in
anticipation of manual activity (McCarty, Clifton, Ashmead, Lee, & Goubet, 2001). In addition, the half-kneel
strategy is adaptive because it allows for controlled
responses to events in the environment. In the halfkneel stance, infants are able to pause mid-PTS as a
play position or turn their bodies in response to something that has captured their attention while maintaining stability.
A strength of this study is the different levels of
analysis used to explore the development of PTS in infancy. In addition to describing the typical sequence of
development, we have also been able to describe the
range of developmental trajectories in the normal population. Although this study makes no claims of being
able to use infants’ PTS trajectory to diagnose or detect
developmental delays, these findings do have clinical
implications. In practice, the timing and age of onset of
PTS in infancy is often used as a marker for developmental delay. Clinicians describe extended periods of
use of the two-leg strategy for PTS with asymmetrical
strategies mastered relatively late (Bly, 1994; Lauteslager, 1995). However, since the developmental trajectory of PTS had never before been tracked, there was no
reference for comparison in a typically developing population for how long was ‘‘too long.’’ Understanding
the sequence and timing of development may give
therapists a reference for when to intervene, perhaps
keeping an eye out for periods of instability, rather than
a specific landmark event. Future studies should incorporate quantitative measures of PTS at close intervals
to capture instability, such as changes in the level of
support provided by the upper extremities relative to
the lower extremities. We now understand that variability in timing of the dominant PTS pattern, in timing of
Developmental Psychobiology
the transition between patterns, and in the way that the
transition occurs (gradual vs. abrupt) is the norm. It is
also apparent that, despite the appearance of a stagelike transition during the acquisition of PTS at the
group level, development is not stage-like at the individual level. The timing of the sub-components of the
skill do not appear in lockstep from one infant to
another.
NOTES
This article is based on data collected by Osnat Atun-Einy in
partial fulfillment of the doctoral dissertation at the University
of Haifa. This research was supported by Israel Science Foundation Grant No. 208/07 to Anat Scher and a 2010–2011 Fulbright Research Fellowship to Sarah E. Berger. We gratefully
acknowledge Moran Samuel for assistance with data collection, data coding and figure preparation; Sandra Zuckerman
for data management and analysis; Heather Szatmary for her
beautiful line drawings; and to all of the infants and their
families for their enthusiasm and commitment to participating
in this research.
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