J Rehabil Health Sci 2012; 10:1-5Ⓒ 2012 Osaka Prefecture University

Journal of Rehabilitation and Health Science
Online ISSN 2185-8004

Original Article

The Characteristics of Sit-to-stand Movement in Infants aged 1 Year:
A Preliminary Study
Ryo YONETSU,†１ Hiroyuki SAKITA,２ Noriko YAMASHITA,３ Kumiko ITO,１ Nao YAMAMOTO,１ Himeka WAKITA,１ YukikoMIKI,２ Yoshihiro KAWARAI,２ and Junichi SHIMIZU４1Department of Physical therapy, School of Comprehensive Rehabilitation, Osaka Prefecture University, 3-7-30 Habikino, Habikinocity, Osaka 583-8555, Japan; 2Department of Rehabilitation, Osaka Medical Center and Research Institute for Maternal and Child
Health, 840 Murodocho, Izumi-city, Osaka 594-1101, Japan; 3Devision of community health, Osaka Medical Center and ResearchInstitute for Maternal and Child Health, 840 Murodocho, Izum-city, Osaka 594-1101, Japan; 4Graduate Course of Rehabilitation
Science, Division of Health Science, Kanazawa University Graduate School of Medical Science, 5-11-80 Kodatsuno, Kanazawa-city,Ishikawa, 920-0942, Japan
Received 14 March 2012; accepted 10 April 2012

Sit-to-stand (STS) movement requires coordinated movement of multiple body segments. However, thekinematic characteristics of STS movement in infants have not been fully clarified. The purpose of thisstudy was to assess the kinematic characteristics of STS movement in infants. Six infants aged 12 to 14
months and 6 adults aged 21 to 22 years old took part in this study. In order to assess STS movement, amotion analysis system consisted of 2 cameras was used. STS movement data which included the durationfrom initial point to hip off the seat (Phase I) of STS movement and angular movement of each joint (trunk,hip, knee, and ankle) were collected. To compare the sampled data, Mann-Whitney U-test was used.
Statistical significance was set at P<0.05. The duration time of Phase I of the infant group significantlyincreased compared with that of the adult group (P<0.05). The transitional trunk angular movement
significantly decreased in the infant group compared with that of the adult group (P<0.05). On the otherhand, the transitional hip and ankle angular movement significantly increased in the infant group comparedwith that of the adult group (P<0.05). These findings suggest that the pattern of STS movement in theinfants is characterized by not only less trunk inclination, but also less hip flexion motion and more ankle
dorsiflexion. These observed movements exemplify how infants are able to stand from sitting position
independently.
Key words : sit-to-stand movement; motion analysis; infant

1 Introduction
Sit-to-stand (STS) is one of the important movements for infants to develop, and this ability is a necessary prerequisite for walking. STS movement requires
forward and upward displacement of the body's center ofmass from a larger to a relatively smaller base of support.
Moreover, adequate body balance, equilibrium reaction
and coordination of muscle activation are required simultaneously 1. However, infants would have some difficultyin accomplishing this movement, due to their immatureequilibrium function. Thus, understanding how infants
performed STS movement independently is invaluable
infor ma tion in making a rational evaluation of motor
development of disabled children, such as cerebral palsy.
†Corresponding author, Email : yonetsu@rehab.osakafu-u.ac.jp

There have been few studies on motion analysis of
infant's STS movement 2, 3; hence, the kinetic and kinematic characteristics of STS movement in infants have notbeen clarified fully. Cahill's study 2 has reported thatyounger infants (12-18 months) had the smallest trunk
inclination and higher angular trunk flexion velocity.
McMillan's study 3 has also suggested that infants would
have larger trunk inclination with age. In this manner,
one of the features of infant's STS movement would besmaller trunk inclination. However, this hypothesized
fea ture would lead us to one important question. Ifinfants could not incline their trunk fully, their body'scenter of mass would not also be shifted forward.
Therefore, it was hypothesized that infants would shift thebody's center of mass forward by utilizing different
strategies.

2The purpose of this study was to assess the kinetic
and kinematic characteristics of STS movements ininfants.
2 Methods
2.1 Subjects
Six infants (2 males and 4 females), aged 12 to 14
months, and were able to walk independently, participatedin this study. In addition, all infants attained independent
walking within 3 months. The infant subjects included apreterm and/or low birth weight infants. Jeng has reported that preterm infants attained independent walking at
significantly older age than term infants, after corrected
age for prematurity 4. Infants in this study showed nodifference in Apgar scores [1-and-5 min] and delay of
motor development (Table 1); as such, all infants havealmost the same motor function.
Moreover, 6 adults (3 males and 3 females), aged 21
to 22 years old, who had no history of neurological and/ororthopedic diseases, participated. This research study wasconducted after having obtained the approval of Osaka
Prefecture University Research Ethics Committee (2011P05). The purpose of this study was explained to theinfant's parents and adult subjects orally, and writtenconsent were obtained.
2.2 Materials
A chair with no arms or backrest and as high thesubject's knee joint in sitting position was prepared. The
size of the support surface of this chair was 40 cm
wide × 30 cm long.
An originally developed pressure-sensitive trigger
device (30 cm wide × 20 cm long × 3 mm thick), whichhelps to detect the point of hip off the seat, was placed on
this chair. This device can record loss of contact with theseat by detecting when subjects' weight is reduced by lessthan 3 kg.
In order to assess subject's STS movement, a motion
analysis system using a motion analyzer with 2 digital
cameras (Kinema tracer: made by Kissei Comtec, Japan)(30 fps), which was synchronized to this trigger device,was used. Each camera was placed on the dom i nantTable 1

oblique and lateral side.
2.3 Movement procedures
Markers were placed unilaterally on the following
body landmarks: lateral malleolus, lateral femoral
condyle, greater trochanter, and acromion. Since theinfants could not understand fully our verbal commands
and/or movement procedures, each parent gave verbal
encouragements and various gestures in each STS trial.
Thus, each parent sat in front of the child, within 1-meterdistance. This procedure was similar as in Cahill's study2.
Moreover, the infants were permitted to perform STS
movements as they preferred. STS movements were performed more than 5 times in bare feet, with a brief pause
between each trial. Equally important, infant subjects
have a difficulty in standing still 2. Therefore, the infants
were allowed to take two or three steps forward.
2.4 Analysis procedures
For the purpose of data analysis, one trial was selected. The inclusion criteria were as follows: (i) trunk andbilateral lower thighs were erect as straight as possible;(ii) sole of the feet to the floor; and (iii) hands resting
between the thighs. The subjects were instructed to standup from the chair without using their arms to push up tostanding position. Then, STS movement data which
included the duration of STS movement, angular movement and angular velocity were collected.
The end point of STS was not defined in this study
since the infants have difficulty in standing still 2 .
Therefore, only the duration time from the initial point ofSTS (T1) to hip off the seat (T2) (Phase I) was calculated.
The initial point of STS (T1) was defined as the point
when the marker of acromion started to move, and the hipoff the seat point of STS (T2) was measured from the dataon our developed trigger device and was defined as thepoint when the waveform is located in first lowest electrical voltage (Fig. 1).
As for the angular movement, the angles which
included trunk, hip, knee, and ankle segments were collect ed. Angular movement of trunk was defined as
between the lines from the acromion to greater trochanterand the vertical line (Z-axis) through the greater trochan-

The profile of infant subjects

3Fig. 1

The definition of hip off the seat (T2)
Vertical and horizontal axis demonstrate electricalvolt age, time, respectively. The electrical voltage
records approximately 2 voltages when the subject's
buttocks are on the chair. This device can record lossof contact with the seat by detecting when subjects'
weight is reduced by less than 3 kg.

ter. In a similar way, angular movement of hip wasdefined as between the line from the acromion to greater
trochanter and the line from the greater trochanter to thelateral femoral condyle. Angular movement of knee wasdefined as between the line from the greater trochanter tothe lateral femoral condyle and the line from the lateralfemoral condyle to the lateral malleolus. Angular movement of ankle was defined as between the line from thelateral femoral condyle to the lateral malleolus and thevertical line (Z-axis) through the lateral malleolus. Thesesettings were similar as in our previous study 5. Then, theangle of the 4 segments at T1 and T2 was calculated.
Trunk inclination and ankle dorsifexion in STS
movement shift the body's center of mass forward 1, 6.
Consequently, these segments assessed not only the angular movement, but also the maximum positive peak angular velocity in Phase I.
In the analysis, SPSS version 16 was used. Due tosmall sample size, nonparametric analysis (MannWhitney U-test) was selected for comparing the infant
and the adult group. P-value less than 0.05 was considered statistically significant.
3 Results
3.1 The duration timeThe duration time of Phase I of the infant group significantly increased compared with that of the adult group(P = 0.030)(Fig. 2). The Median [min-max] time of Phase
I of the infant group was 1.13 [0.76 - 1.97] seconds; thatof the adult group was 0.86 [0.75 - 0.95] seconds.
3.2 Angular movement
There was no significant difference on the 4 segments
of initial angular movement in the two groups (Table 2).
As for the transitional angular movement, there wasFig. 2 The duration time of Phase I
Vertical axis indicates second. *: P<0.05

Table 2Angular movement of each segment on the initialpoint (T1)no significant difference on knee segment in the twogroup (P=0.234); whereas there was significant differenceat the other 3 segments (Table 3, Fig. 3).
The transitional trunk angular movement of theinfant group significantly decreased compared with thatof the adult group (P=0.009). The Median [min-max]transitional trunk angular of the infant group was 37.6
[11.4 - 51.5] degrees, and that of the adult group was 55.5
[45.3 - 60.1] degrees (Fig. 3A).
On the other hand, the transitional hip angular movement of the infant group significantly increased comparedwith that of the adult group (P=0.009). The Median [minmax] transitional hip angular of the infant group was 84.2
[63.4 - 101.1] degrees, and that of the adult group was
58.0 [56.2 - 71.0] degrees (Fig. 3B).
The transitional ankle angular movement of theinfant group also significantly increased compared withthat of the adult group (P=0.041). The Median [min-max]transitional ankle angular of the infant group was 20.0
[9.8 - 25.3] degrees, and that of the adult group was 13.4
[6.9 - 18.0] degrees (Fig. 3D).
3.3 Angular velocity
The maximum positive peak of trunk angular velocity of the infant group significantly decreased comparedwith that of the adult group (P=0.004). The Median [minmax] maximum positive peak of trunk angular velocity of

4

Table 3

Angular movement of each segment on hip off theseat (T2)

Fig. 3

The angular movement of each joint on T2
Vertical axis indicates degree. A), B), C) and D) show the angular movement oftrunk, hip, knee, and ankle, respectively. *: P<0.05 **: P<0.01 n.s.: not significant

Fig. 4The maximum peak angular velocity in Phase I
Vertical axis indicates degree/second. A) and B) show the maximum peak angularvelocity of trunk and ankle, respectively. **: P<0.01

5

the infant group was 75.3 [40.1 - 87.5] degrees/second,
and that of the adult group was 105.2 [82.1 - 111.2]
degrees/second (Fig. 4A).
On the other hand, the maximum positive peak ofankle angular velocity of the infant group significantly
increased compared with that of the adult group(P=0.002). The Median [min-max] maximum positive
peak of ankle angular velocity of the infant group was
53.4 [38.9 - 133.8] degrees/second, and that of the adultgroup was 28.3 [21.1 - 36.6] degrees/second (Fig. 4B).
4 Discussion
The present study focused on the kinetic and kinematic characteristics of STS movement in infants. The
findings indicate that the pattern of STS movement ininfants is characterized by shortened duration in Phase I,
less hip flexion motion and more ankle dorsiflexion, andhigher ankle angular velocity, compared with that of theadults'. In the following paragraphs, the features of STS
movement in infants will be discussed.
Trunk inclination in STS movement shifts the body's
center of mass forward from the buttocks to the feet,
which propels seat off the hip 1, 6. If for some reason thismovement is restricted significantly, STS movement would
be performed by various compensatory patterns 5, 7, 8, 9. In
this study, trunk angular and angular velocity of infant'sSTS movement at T2 showed significantly lower angularmovement and velocity than that of adults'. These findings have been reported by Cahill's study 2. Thus, theinfant subjects might have also difficulty in shifting thebody's center of mass forward. This presumption explains
for the longer duration of Phase I.
In order to shift the body's center of mass forward,
the infants tilted their lower thighs forward larger andsooner instead of utilizing trunk inclination. In general,tilting lower thighs forward has been caused by trunk
inclination 6. This motion helps to produce hip flexion
moment to shift the body's center of mass forward andupward 1, 6. Although knee angular movement at T2
between the 2 groups had almost similar angle, the infants
could not flex their hip segments fully, due to immaturity.
This phenomenon indicated that STS movement in infants
could not generate hip flexion moment. Therefore, STS
movement in infants might be more heavily dependent onthese lower thighs motions than those in adults. In otherwords, it could be assumed that STS movement in infants
was characterized by less selective muscle control. This
presumption was explained by McMillan's study 3, which
has reported that infants performed their STS movement
by not only little trunk inclination but also by sliding theirbuttocks forward at earlier stage.

These findings lead us to conclude that the pattern of
STS movement in the infants is characterized by not only
less trunk inclination, but also less hip flexion motion andmore ankle dorsiflexion. These observed movements
exemplify how infants are able to stand from sitting position independently.
The limitation of the present study is high variabilityof STS movement in infants. Several researchers 3, 10 havereported that STS movement within children has high variability. Therefore, the kinetic and kinematic characteristics of STS movement in infants have not been fully clarified. To have a better understanding of STS movement ininfants, more infant subjects are needed in future study.
Acknowledgements
This study was supported by a Research Promotionof practical hospital of Osaka Prefecture University(2011). We greatly appreciate John Surya for looking
over our research paper.
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