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Pediatric Physical Therapy:
doi: 10.1097/01.pep.0000234963.72945.b1
Research Report

The Effects of Prone Positioning on the Quality and Acquisition of Developmental Milestones in Four-Month-Old Infants

Dudek-Shriber, Linda EdD, OTR/L; Zelazny, Susan MS, OTR/L

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Author Information

State University of New York at Buffalo (L.D.S.) and Medina Memorial Hospital, Medina, New York (S.Z.)

Address correspondence to: Linda Dudek-Shriber, EdD, OTR/L, State University of New York at Buffalo, 3425 Main Street, Buffalo, New York 14214. E-mail: lshriber@buffalo.edu

Funded by the Neurodevelopmental Treatment Association.

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Abstract

Purpose: This study investigated whether positioning infants in the prone position while awake affected the quality and achievement of developmental motor milestones.

Methods: One hundred four-month-old infants were administered the Alberta Infant Motor Scale to assess motor development. A Parent Questionnaire was used to gather information on time spent in various positions throughout a typical 24-hour day.

Results: Significant differences (p < 0.001) in the achievement of seven prone, three supine, and three sitting milestones were evidenced in infants who spent time awake in the prone position compared to those whose time in the prone position was limited.

Conclusions: Prone positioning while awake appears to be associated with certain motor milestones achieved by four-month old infants.

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INTRODUCTION

Occupational and physical therapists working in early intervention are responsible for evaluating various aspects of infant motor development with particular emphasis on the acquisition of developmental motor milestones. Progressing through an established milestone continuum in a timely manner has been considered a crucial indicator of an infant’s neuromaturation and development. Although it is understood that a specific motor behavior cannot be explained exclusively by one factor,1 placement in the prone position has been viewed by therapists as being important for the development of antigravity movements. These movements are proposed to facilitate the infant’s achievement of head, neck, and postural control, as well as the development of shoulder girdle stability and upper-extremity weight bearing, shifting, and reaching skills.2–4

Today, because of the success of the American Academy of Pediatrics (AAP) Back to Sleep Campaign, the majority of parents in the United States no longer place their infants in the prone position for sleep. In 1992, when the AAP first associated the prone sleeping position with Sudden Infant Death Syndrome (SIDS),5 more than 70% of infants in the United States were being placed in the prone position for sleep,6 and the SIDS death rate was 1.2 infants per 1000 births.7 By 2001, 87% of infants were being placed in the supine or side lying positions for sleep, and the death rate from SIDS decreased to 0.56 infants per 1000 births.8 These data have supported and reinforced the positive impact of the Back to Sleep campaign.

Although the benefits in SIDS reduction have been substantial, some questions and concerns have been generated as a result of infants spending increased time in the supine position, and less time in the prone position. One of those concerns has been the possible effect on motor milestone development.

According to the neuromaturational theory first put forward by Gesell9 and McGraw,10 the infant’s acquisition of developmental motor milestones reflects a process of maturation that corresponds to changes in functional behavior. Gesell’s9 years of observation of thousands of children led to the identification of age-referenced motor milestones that are still considered valid for clinical assessment today. McGraw’s10 longitudinal observations of twins led her to conclude that a certain level of readiness is necessary for acquiring motor skills. Traditionally, experts who have studied child development have agreed that the acquisition of skills that occurs during the first year of life is characteristically predictable.1,2,9,10

While neuromaturational theory has provided essential knowledge regarding development, other theories also have helped to expand and refine our understanding of how infants acquire skills. Dynamical systems theory (DST) is one that offers a comprehensive perspective on the various influences that can affect an infant’s development. According to this theory, internal subsystems such as the musculoskeletal and central nervous systems are recognized for their importance; however, the influence of the environment and the demands of the task also are emphasized for their impact on outcomes and variations in motor behavior. Dynamical systems theorists propose that cognition, emotions, intrinsic drive, and the nervous, musculoskeletal, sensory, and perceptual systems are some of the internal components needed for purposeful action.1,11 Simultaneously, however, the environment, which includes factors such as the position in which the infant is placed, the toys available, and the quality of care, also can impact the infant’s actions. DST is used to explain that subtle contextual and temporal influences can affect real life behavior, and that a change in any component can directly influence the infant’s motor response. The extent of this influence and the effect of these changes however, cannot always be predicted.1,11 When the AAP recommended the supine versus the prone position for sleep in 1992, a change in the environmental influence on the developing infant occurred. This change has been reflected in the fact that the majority of parents are now placing their infants in the supine position for sleep.6 As a result, potential effects on motor development based on this change in infant sleep position began to be studied.

In 1997, Jantz, Blosser, and Fruechting12 conducted a retrospective study of 257 infants at four and six months of age to determine whether the recommended change in sleep position was having an impact on their motor development. Their results indicated that the four-month old infants who slept in supine or sidelying were less likely to roll over than prone sleepers. Other factors considered such as ethnicity and maternal age were not found to contribute to this difference. At six months of age, no significant differences were found in the infants’ motor development. The authors concluded that sleep position had an impact only on the rolling milestone at four months of age. They also suggested however, that the change in sleep position for infants might necessitate a modification of when some developmental milestones should be expected to occur.

Before this study,12 which was conducted in the United States, Mildred et al13 conducted a study in Australia on the positioning of 100 infants between the ages of one and six months. They examined both the sleep and play positions of infants and sought to determine whether the caregivers’ knowledge regarding prone sleeping and SIDS influenced their positioning choices during play. The results indicated a significant relationship between the caregivers’ knowledge of SIDS and the avoidance of the prone position for play (p = 0.002). The authors suggested that the campaign in Australia that promoted the supine position for sleep also seemed to be having an effect on how infants were being positioned while awake.

In a study of 351 infants in the United States, Davis et al14 examined differences between prone and supine sleepers’ acquisition of early milestones. Parents were asked to record their infant’s sleep position and time awake in the prone position until six months of age. Eight developmental milestones were assessed. In general, it was determined that prone sleepers acquired motor milestones at an earlier age than supine sleepers. This study also determined that increased playtime in the prone position was significant to the earlier attainment of supported sitting, sitting alone, crawling, and pulling to stand (p < 0.05). When controlling for maternal education, race, infant gender, birth weight, and the number of older siblings, however, a difference was found only for pull to stand. The authors concluded that sleep position significantly affects early motor development, but that the differences in milestone achievement were not characterized by delays.

In one of the most recent studies done on infant sleep and play positioning and its relationship to milestone achievement, Salls et al15 assessed 66 infants at two, four, and six months of age, using select milestones from the 1988 Denver Developmental Screening Test. In addition, caregivers were asked to identify their infant’s primary sleep position and estimate the amount of time their infant spent in the prone position while awake. The results indicated that 59 of the 66 parents placed their infant in the supine or side lying positions for sleep, with the majority also indicating that their infant spent very little time in the prone position while awake. In regards to milestone achievement and prone awake time, significant differences were found for two-month-old infants in their ability to hold the head up to 45 degrees, and sit with the head steady if they spent 15 minutes or less of time awake in the prone position. No significant differences in milestone achievement related to time spent in the prone position while awake were noted at the fourth or sixth months. They recommended that future research with a more sensitive infant assessment be conducted to better evaluate both the qualitative and quantitative aspects of infant motor development.

Although investigators in each of these studies examined sleeping patterns and their relationship to motor milestone achievement, infants’ time spent in the prone position while awake continues to require examination for its possible effects on motor development and its quality. In 2000, the AAP specifically suggested that “A certain amount of tummy time while the infant is awake and observed is recommended for developmental reasons….”6(p. 654) Information has remained limited, however, regarding how many parents are following through with this recommendation, and if the time spent in the prone position while awake has been sufficient to influence the quality and development of motor milestones.

Having an understanding of how much time infants spend in the prone position while awake is important for occupational and physical therapists since positioning has been consistently emphasized as being essential for functional performance. If infants are spending less time in the prone position while awake, the acquisition of milestones thought to be important for postural control and the achievement of motor tasks also may be influenced by this change. If infants are not being placed in the prone position while awake, therapists need to be aware of this when examining the infants’ development and when making decisions about their intervention needs.

The purpose of the current study, therefore, was to determine (1) the number of and amount of time that four-month-old infants spend in various developmental positions during a 24-hour period; (2) the number of infants who were able to achieve specific milestones as measured by the Alberta Infant Motor Scale (AIMS); (3) if time spent in the prone position awake is predictive of the achievement of particular motor milestones, and if so, to what extent; (4) how much time is spent in the prone position while awake in a 24-hour period by infants who are able to achieve milestones as opposed to those who are not; and (5) what proportion of infants in the study obtained the AIMS normative percentile scores?

The four-month age range was chosen for this study since current developmental literature suggests that infants older than this age may begin to change positions on their own, rather than maintain the position in which they are placed.4

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METHODS

Participants

Participants were recruited from the well-child patient population of two private pediatric practices, three-day care centers, published birth announcements in two regional newspapers, and through referrals from parents who knew other four-month-old infants and their families. The criteria for infants to be included in this study were that they (1) were born full term after an uncomplicated pregnancy; (2) had a birth weight of at least 2.268 kg (five pounds); (3) had no chronic or acute medical conditions; and (4) were between four-months-zero days and four-months-29 days old.

To determine the sample size for this study, the research by Salls et al15 that included four-month-old infants as part of the sample was consulted. A medium effect size of omega = 0.32 (the square root of chi square over the sample size) was detected. The omega value was transferred to a d-index to determine the sample size for two group comparisons.17 Specifically, the statistic was used to compare time spent in the prone position while awake in infants who achieved and didn’t achieve milestones. To achieve a power of 0.80 at α1 = 0.05, with d = 0.50 (a medium effect size), a total sample size of 100 was needed.17

A convenience sample of 125 parents of four-month-old infants was contacted to request their participation. Of the 125 parents contacted, 14 declined to participate. Two parents initially agreed to have their infant be a part of the study but elected to withdraw prior to data collection. Information obtained from the parents on nine of the infants was either incomplete or unusable. This resulted in a sample of 100 four-month-old infants for the study. A nearly equal distribution of gender was represented, with 48 males and 52 females participating. The overwhelming majority was Caucasian (94%), with two African Americans, and four infants of mixed heritage represented.

The age ranges, current weight, length, and mean Ponderal Indexes of the 100 participating four-month-old infants are reported in Table 1. The mean ponderal index, which is a numeric value that represents the ratio of the infants’ weight in grams to length in centimeters, was calculated because it is a measure of the relative mass of the body and allows for a comparison between subjects having different body compositions.17

Table 1
Table 1
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Instruments

The AIMS was used in this study. It is a norm-referenced performance based instrument designed for the naturalistic observation of the quality of motor development in infants from birth to 18 months of age. The AIMS normative data allows for the determination of the percentile ranking of an infant’s motor development with a peer group matched for his or her age.

Specifically, the AIMS is designed to evaluate postural control relative to four developmental positions, which make up the subscales. These include the prone (21 items), supine (nine items), sitting (12 items), and standing (16 items) positions. A separate score for each of the prone, supine, sitting, and standing subscales is obtained.1 The sum of the four subscale scores provides the infant’s total AIMS score. From the total AIMS score, the percentile ranking of the infant’s motor performance is compared with the normative age-matched sample. For the purposes of this study, all developmental positions that the infants were able to achieve were observed so that an overall assessment of their milestone achievement and its quality could be obtained. In addition, all positions were assessed to determine if prone positioning had an impact on the quality and acquisition of milestones in the other developmental positions.

For the purposes of the current study, interrater reliability for the AIMS was established for the two investigators using intraclass correlation coefficient (ICC 2,K). Ten videotapes of infants representing each of the five AIMS percentile ranks were observed by the co-investigator who did not directly assess the infant. As a result, two videotapes of infants in each of the five percentile ranks were observed and scored by the co-investigator for the purpose of determining reliability. The videotapes were observed and scored at various points of time during the data collection process. The calculations of inter-rater reliability were completed at the end of the study. Inter-rater reliability obtained for the subscales was 0.88 for prone, 0.87 for supine, 0.86 for sitting, and 0.50 for standing. For the total AIMS score, 0.98 agreement was obtained. The average inter-rater reliability achieved was 0.80.

The Parent Questionnaire, which also was used for this study, was developed by the researchers to establish and verify the participants’ inclusion criteria and demographics. The demographics obtained included the infants’ date of birth, birth weight, race, gender, current weight, length, the number of weeks of the pregnancy, and general information about the infant’s health. In addition, the parents needed to delineate the amount of time within a typical 24- hour period that the infant spent on his or her back, belly, seated in a seat, being held, and in other possible positions. The parents could check this information on the questionnaire according to fixed increments of time equal to none, less than one hour, and in one-hour increments. The time frames indicated by the parents were based on a recollection of their infant’s typical day. The parents completed the Parent Questionnaire after the AIMS was administered and while the investigator was still in the room.

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Procedures

Approval was obtained from the Social and Behavioral Sciences Institutional Review Board (IRB) of the State University of New York at Buffalo and the D’Youville College IRB before initiating the study. Initial contact with the parents was made by telephone by one of the investigators. At this time, the purpose of the study was explained, and it was determined whether or not the parents were interested in participating. If the parents chose to participate, arrangements were made to assess the infant at a convenient time for the parents, but also when the infant was typically awake and alert. On the day of data collection, the purpose of the study was again reviewed, and consent forms for testing and videotaping were signed. The investigator then evaluated the infant’s motor performance with the AIMS, with the parent present. Parents were allowed to interact with and be on the floor with their infants. However they were requested not to assist their infant in achieving a milestone. Simultaneous videotaping was completed by the two investigators on 50 of the 100 infants so that the investigator that did not directly observe the infant could potentially observe the tape for reliability purposes. Videotapes were also used to enable the accuracy of the scoring to be checked later by the original investigator if needed. Both investigators had experience in administering the AIMS and in observing infant development. To ensure adequate recording of responses, the investigator would score the infant’s responses in each developmental position before proceeding with observations in a different position. Following collection of all of the infant’s performance data, the parents were asked to complete the Parent Questionnaire. They were asked to complete the questionnaire after the infant was evaluated to avoid any potential researcher bias during the assessment. Before leaving the test site, the infant’s AIMS test booklet, the Parent Questionnaire, and the videotape were all coded for purposes of maintaining confidentiality. To reinforce the accuracy of the observations, the investigator reviewed the data collected and calculated both the AIMS subscale and total percentage scores on the same day of data collection. During the study, ten videotapes were viewed for the purpose of determining inter-rater reliability. The co-investigator did a separate scoring of the AIMS at the time the videotapes were observed.

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Data Analysis

To answer the first research question, the number of four-month-old infants who spent time in the various developmental positions as indicated by the fixed time intervals on the Parent Questionnaire was calculated. Means, standard deviations, and medians were determined to describe the time spent in each position. To find out what portion of the sample achieved specific milestones as measured by the AIMS, the number of infants who were able to achieve the milestone was determined. To find out if demographics and time spent in the prone position while awake were important predictors of the achievement of a particular milestone, logistic regression using a hierarchical stepwise method was applied. Demographics were put in to the analysis first, followed by time spent in the prone position while awake. The significance of the overall variance accounted for by the predictor was calculated and reported. Independent t tests were used to compare how much time infants who achieved milestones spent in the prone position while awake, as opposed to infants who did not achieve them. The number of the four-month-old infants in the study in each percentile was then compared with AIMS norms using a chi-square analysis and standardized residuals.

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RESULTS

Numbers of Infants and Time Spent in Developmental Positions Over the Course of 24 Hours

Table 2 illustrates the number of infants and the time they spent in various developmental positions throughout a 24-hour period. The results indicated that the majority of infants (n = 74) slept more than five hours a day in supine. Time spent in the prone position while awake was limited to 30 minutes or less for 60 of the 100 infants in this study. Seventy-nine infants sat supported while awake for more than two hours to five hours of their days, and 73 infants spent this same amount of time being held.

Table 2
Table 2
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Table 3 reports the amount of time the infants spent in various developmental positions during a 24-hour period according to means, medians, ranges, and standard deviations. The results indicate that time spent in supine while awake (x = 2.31 hours; SD = 1.25) was greater than the time spent in the prone position while awake (x = 1.16 hours; SD = 1.07). Median scores however showed a more substantial difference, with 2.5 hours being spent in supine while awake and only one-half hour being spent in the prone position.

Table 3
Table 3
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Prediction of Infant Motor Milestone Achievement by Prone Awake Time

Logistic regression using a hierarchical stepwise method indicated that none of the demographic factors, which were entered first, were found to be predictive of the infants’ ability to achieve developmental milestones. Time spent in the prone position while awake however was found to be significant to the four-month-old infants’ achievement of seven of the 21 prone, three of the nine supine, and three of the 12 sitting milestones. Table 4 shows the number of infants achieving a milestone for which time spent in the prone position while awake was a predictor. It also shows the odds ratio which indicates the likelihood of achieving a particular milestone over not achieving a milestone, when an infant spends one or more hours in the prone position while awake.

Table 4
Table 4
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Time Spent in the Prone Position While Awake for Milestone Achievers Versus Nonachievers

Figure 1 shows the differences in the mean amount of time spent in the prone position while awake for the four-month-old infants who were able to achieve a particular prone, supine, or sitting milestone in comparison to those who were not. All differences between achieved and not achieved were significant at p < 0.001 using a t test. The figure also illustrates that as the milestones in each position got progressively more difficult, there was a decrease in the number of four-month-old infants who were able to achieve them, but a steady increase in the mean amount of time spent in the prone position while awake by those who were successful.

Fig. 1
Fig. 1
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Proportion of Infants Obtaining AIMS Percentile Norms

The proportion of the sample in this study that obtained the various percentile ranks was compared with the normative data of the AIMS provided by Piper and Darrah.1(p. 205) First, z-scores were calculated for each ranking. The frequency of each ranking in the normative sample was then compared with the frequency of the study sample using the goodness of fit test. The chi-square for a normal distribution indicated that the distribution of the AIMS percentile scores for the four-month-old infants in this study were significantly different from the percentile distribution of the infants in the AIMS normative sample (χ2 = 20.71, p < 0.01; Table 5). Standardized residuals were calculated to identify categories that contributed to significant chi square values. Large discrepancies were found for the fifth, 25th, and 50th percentiles. In general, more infants in the study were in the 10th and 25th percentiles than in the normative sample, and fewer infants were in the 50th, 75th, and 90th percentiles or greater.

Table 5
Table 5
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DISCUSSION

In examining how infants spent their time while awake, it was clear that they spent more time in supine awake than in the prone position while awake. These findings provided support for other studies which have suggested that some parents are placing their infants more in supine while awake, rather than in the prone position.13,14 During the course of this study, most of the parents (n = 60) who placed their infants in the prone position for less that 30 minutes per day indicated to the researchers that their infants were intolerant of the prone position. This reported intolerance was consistent with the Salls et al15 study, as well as the Davis et al14 study, where the authors indicated that many of the infants who slept in the supine position did not want to be placed in the prone position while awake. Despite reports that the infants’ pediatricians had informed the parents in the current study about the importance of “tummy time for play,” the infants’ intolerance led them to discontinue this position when their baby became upset.

This reported lack of tolerance for the prone position might correspond with another of the study’s results, which indicated that being held, and sitting supported actually comprised the greatest amount of the infants’ time while awake. While it can only be speculated that some of the infants were held, or placed in supported sitting during the day due to their intolerance for being in the prone position, the effects of the time spent in both of these positions also merits attention for their possible influences on motor milestones and the quality of their development.

Another finding of this study the investigators considered important was that some skills in the supine and sitting positions also were associated with time spent awake in the prone position. Supine skills associated with time spent awake in the prone position included hands to knees (90% by five months), active extension (90% by five and one-half months), and rolling supine to prone without rotation (90% by approximately eight and one-half months). Sitting skills associated with time spent in the prone position awake included sitting with propped arms (90% by four and one-half months), unsustained sitting (90% at six months), and sitting with arm support (90% at six months).1 All of the sitting skills require either some trunk extension or upper extremity weight bearing which is hypothesized to be facilitated through positioning in the prone position.

The results of this study therefore seem to suggest that infants who are provided with the opportunity to be in the prone position appear to be able to develop movement skills and weight-bearing patterns against gravity that not only support the attainment of prone milestones, but milestones in other positions as well. While this result is inconsistent with the findings of Salls et al,15 who found no significant differences in four-month-old infants’ milestones in relationship to awake time in the prone position, the authors conceded that an assessment that is sensitive to both the qualitative and developmental aspects of motor behavior would be preferred over the one they had used. The AIMS that was used in the current study was specifically designed for this purpose and therefore is an assessment that is more sensitive and specific than the Denver II to ascertain differences in developmental milestones based on qualitative indicators. This may have accounted in part for the differences in the studies’ results.

Specifically, the results of this study also appear to suggest that those infants who spent slightly more than an hour or more in the prone position while awake per day achieved greater success in acquiring certain prone, supine, and sitting milestones that begin developing incrementally at approximately four months of age. It was beginning at one hour and 21 minutes of time spent in the prone position while awake (for the supine hands to knees milestone) that significant differences began to be seen in infants who achieved specific milestones as opposed to those who did not. In addition, many of the motor milestones achieved by the infants in this study who were placed in the prone position for longer time periods were above the four-month old developmental level. These results although preliminary, seem to offer some support for the literature that has traditionally suggested that prone positioning provides a basis from which more advanced skills are built.2,4,18

Finally, when comparing the total percentile scores of the 100, four-month-old infants in the study with the AIMS normative sample, it was clear that as a whole, the infants in the study had scores lower than those in the AIMS sample. Perhaps associated with this was the finding that 60 of the 100 infants in this study spent 30 minutes or less in the prone position while awake. Since it is not known how much time the infants in the Canadian normative sample spent in the prone position while awake when the AIMS was published in 1994, it is most likely that the knowledge of the relationship between SIDS and positioning was not as widespread then, as it is today. As Jantz et al12 have implied, the change that has occurred in positioning might necessitate a modification of when we can expect early milestones to appear. Further research into the relationship between positioning and milestones should help to determine whether this modification is warranted.

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Limitations and Recommendations for Future Research

Generalization of this study’s findings is restricted by the use of a non-random convenience sample of limited ethnic diversity from one geographic location. Future research should include a randomly selected sample of infants from different ethnic backgrounds and geographical locations, which would help determine the consistency of results reported here and ascertain whether ethnic and perhaps regional differences exist in the positioning and development of infants. The birth order of the infants and maternal education level may have an impact on how the parents positioned their infants and complied with the AAP’s recommendations. These factors should be included in future research as well. Another way to determine differences would be to design an experimental study to determine if significant differences in milestones were obtained when infants were placed in the prone position while awake for a specified time period during the day. A control group could follow their typical schedules.

Parents were asked via a Parent Questionnaire, to estimate the time their infants typically spent in various positions within a 24-hour period rather than to maintain a log. Relying on this retrospective estimate may have inadvertently encouraged a response bias and decreased the accuracy of the time estimates the parents provided. To encourage greater precision in providing this information, a 24-hour time log should be completed that reflects the actual time the infant spent in various positions throughout the day.

Another limitation of this study was the 0.50 interrater reliability coefficient obtained by the raters for the standing subscale, indicating moderate reliability19 for a clinical measurement. This most likely affected the standing milestone outcomes for this research. An additional limitation was that interrater reliability calculations were not completed until the end of this study. In future studies investigators should establish interrater reliability early in the research process to ensure greater confidence in the results.

Finally, the current study was restricted to the analysis and description of four-month-old infants. The authors are aware that since this study began, individuals have begun to assess the effects of positioning on the development of skills at older ages. This will be extremely useful in providing additional information to substantiate whether or not there are any long term effects of positioning on development. Especially important will be any effects observed beyond the first year of life.

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CONCLUSION

The findings of this study suggest prone positioning while awake might promote the acquisition and quality not only of prone skills, but also of skills in other positions. Therapists who are assessing the quality and achievement of motor milestones should inquire about and take into consideration the amount of time infants are spending in the prone position as well as other positions.

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REFERENCES

1. Piper MC, Darrah J. Motor Assessment of the Developing Infant. Philadelphia: W.B. Saunders; 1994.

2. Alexander R, Boehme R, Cupps B. Normal development of functional motor skills. Tucson, AZ: Therapy Skill Builders; 1993.

3. Ayres AJ. Sensory Integration and the Child (12th printing). Los Angeles: Western Psychological Services; 1995.

4. Bly L. Motor Skills Acquisition in the First Year. Tucson, AZ: Therapy Skill Builders; 1994.

5. American Academy of Pediatrics Task Force on Infant Positioning and SIDS. Positioning and SIDS: update. Pediatrics 1992;89:1120–1126.

6. American Academy of Pediatrics Task Force on Infant Sleep Position and SIDS. Changing concepts of sudden infant death syndrome: Implications for infant sleeping environment and sleep position. Pediatrics 2000;105:650–656.

7. Matthews TB, Menacher F, MacDorman MF. Infant mortality statistics from the 2001 period linked birth/infant death data set. National Vital Statistics Reports. Vol. 50, No. 12. Hyattsville, Md: National Center for Health; 2003.

8. National Institute of Child Health and Human Development (NICHD). SIDS rate and sleep position, 1985–2001. Washington, DC: U.S. Center for Disease Control. Available at: nichdclearinghouse@mail.nih.gov. Accessed July 18, 2006.

9. Gesell A. The First Years of Life. New York: Harper & Brothers; 1940.

10. McGraw M. Growth: A Study of Johnny and Jimmy. New York: Appleton-Century Crofts; 1935. (reprinted in 1975 by Arno Press).

11. Thelan E, Schoner G, Scheier C, et al. The dynamics of embodiment: a field theory of infant preservative reaching. Behav Brain Sci. 2001;24:1–86.

12. Jantz JW, Blosser CD, Fruechting LA. A motor milestone change noted with a change in sleep position. Arch Pediatr Adolesc Med. 1997;151:565–568.

13. Mildred J, Beard K, Dallwitz A, Unwin J. Play position is influenced by knowledge of SIDS sleep position recommendations. J Paediatr Child Health 1995;31:499–502.

14. Davis BE, Moon RY, Sachs HC, et al. Effects of sleep position on infant motor development. Pediatrics. 1998;102:1135–1140.

15. Salls JS, Silverman LN, Gatty CM. The relationship of infant sleep and play positioning to motor milestone achievement. Am J Occup Ther. 2002;56:577–580.

16. de Leva, P. Ponderal index, normalized body mass, etc. Message posted to Biomechanics and Movement Science electronic mailing list. 1996; April 23. Archived at http://isb.ri.cef.org/biomch-1–1996-04/00112.html. Accessed April 5, 2005.

17. Cohen J. Statistical Power Analysis for the Behavioral Sciences. 2nd ed. Hillsdale, NJ: Erlbaum; 1988.

18. Richter EW, Montgomery PC. The Sensorymotor Performance Analysis. Hugo, MN: PDP Press; 1988.

19. Portney LG, Watkins MP. Foundations of Clinical Research: Applications to Practice. East Norwalk, CT: Appleton & Lange; 1993.

Keywords:

child development; comparative study; infant; infant care/trends; motor skills; prone position; posture; time factors

© 2007 Lippincott Williams & Wilkins, Inc.

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