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Research Reports |
SF Jeng, PT, ScD, is Associate Professor, School and Graduate Institute of Physical Therapy, College of Medicine, National Taiwan University, and Adjunct Physical Therapist, Department of Rehabilitation and Physical Medicine, National Taiwan University Hospital, No. 7 Chun-Shan South Rd, Taipei, Taiwan (jeng{at}ntu.edu.tw)
LC Chen, PT, MS, is a doctoral student, Department of Kinesiology; University of Maryland, College Park, Md
KI Tsou, MD, is Professor, School of Medicine, College of Medicine, Fu-Jen Catholic University, and Deputy Director of Education and Neonatologist, Cardinal Tien Hospital, Taipei, Taiwan
WJ Chen, MD, ScD, is Professor, Institute of Epidemiology, College of Public Health, National Taiwan University, and Adjunct Research Fellow, Department of Psychiatry, National Taiwan University Hospital
HJ Luo, PT, MS, is Physical Therapist, Department of Rehabilitation and Physical Medicine, Kaohsiung Chang-Gung Memorial Hospital, Kaohsiung, Taiwan
Dr Jeng and Ms Li-Chiou Chen provided concept/idea/research design and data collection. Dr Jeng provided writing and project management. Ms Li-Chiou Chen, Dr Wei Chen and Mr Luo provided data analysis. Dr Jeng and Dr Tsou provided fund procurement. Dr Tsou provided subjects, and Mr Luo provided clerical support. Ms Li-Chiou Chen and Dr Wei Chen provided consultation (including review of manuscript before submission). The authors thank the infants and their parents for their participation in this study; Ms Hsian-Feng Chen and Ms Shiu-Ying Yu for their assistance in data collection; Mr Hong-Nan Chen for his assistance in program development; and Dr Linda Fetters, Dr Tung-Wu Lu, and Ms Hua-Fang Liao for their guidance during the study
Address all correspondence to Dr Jeng
Submitted June 3, 2003;
Accepted August 20, 2003
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Key Words: Kicking movement • Kinematic analysis • Prematurity • Walking
| Introduction |
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 TopAbstract Introduction MethodResultsDiscussionConclusionReferences |
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Spontaneous movements in early infancy have been proposed as related to later motor control.8,9 Increasing evidence indicates that examination of spontaneous motility may provide a sensitive and reliable indication of an infant's present condition and later neurological condition.10–12 Among these spontaneous movements, rhythmical kicking has frequently been chosen for research because of its potential role in walking development. This can be seen in the studies of Thelen and colleagues,13–15 in which kinematic analysis was used to examine longitudinally the leg movements of full-term infants from birth until attainment of walking. Kinematic variables examined included frequency, spatiotemporal organization, interjoint coordination, and interlimb coordination. They observed that spontaneous kicking, which is locomotion-like leg movements of the newborn infant in the supine position, has a spatial and temporal organization similar to that of mature walking. Rhythmical kicking, therefore, has been postulated as the precursory motor pattern that is later incorporated into upright locomotion.13
Several researchers16–24 have used kinematic analysis to examine the kicking development of preterm infants. Some authors17–19,22,23 aimed to find early kicking differences between full-term infants without known impairments or pathology and preterm infants who sustained severe perinatal brain damage (ie, extensive hemorrhage or ischemia as detected by cranial ultrasound) and later developed major neurological disorders such as cerebral palsy. Certain kicking features have been found among preterm infants with overt cranial sonographic abnormalities. For example, Droit et al19 documented a longer intra-kick pause and less alternate movements (simultaneous flexion of one leg and extension of the other leg) together with more semi-both-leg movements (simultaneous flexion and nonsimultaneous extension of both legs) in preterm infants with overt neurosonographic abnormalities when they reached full-term age. Heriza,17 Yokochi et al,18 and Vaal et al23 reported a higher correlation of interjoint coordination and a lower variability of spatiotemporal organization in preterm infants with overt cranial sonographic abnormalities from 3 months corrected age onward. Van der Heide et al,22 however, found no differences between groups in interjoint coordination at 1 and 3 months corrected age. Although not entirely consistent, these studies on kicking have helped detect early neuromotor predictors for major neurodevelopmental disorders in infants born prematurely.
Other studies that contrasted the early kicking of full-term infants without known impairments or pathology and preterm infants who had no overt cranial sonographic abnormalities also showed differences.16,20,21,24 Heriza16 documented a lower kick frequency and a longer inter-kick pause in preterm infants who had no overt neurosonographic abnormalities when they approached full-term age. Jeng et al24 found distinct kicking features at post-term ages in preterm infants who had no overt neurosonographic abnormalities, particularly those born at young gestational ages. The kicking features included a higher kick frequency together with a shorter flexion phase at 4 months corrected age and a higher correlation of interjoint coordination together with a lower variability of interlimb coordination at 2 and 4 months corrected age. Nevertheless, Geerdink et al20 and Piek and Gasson21 observed a paradoxical direction of differences in interjoint coordination at post-term ages. Despite the reported early kicking differences for preterm infants without overt cranial sonographic abnormalities, their functional importance remains unclear because no outcomes have been measured.
As the survival rate of preterm infants with VLBW continues to improve, there has been a concomitant reduction of overt cranial sonographic abnormalities and rate of cerebral palsy.25,26 In the absence of cerebral palsy, however, considerable proportions (20%-30%) of preterm infants with VLBW have been found to exhibit neurobehavioral problems in follow-ups during preschool and school ages.27–29 Thus, scrutiny of the early neurodevelopmental process of preterm infants without overt cranial sonographic abnormalities may help identify those who will exhibit neurodevelopmental problems in the future. To this end, age of walking attainment is a useful measure of early neurodevelopment because it reflects various degrees of motor delay and has been increasingly used for studies of infants with prematurity.30–32 Several studies31,33,34 have shown that preterm infants attain the ability to walk later than full-term infants, after correction for prematurity. Furthermore, failure to walk by 18 months corrected age has been found to be associated with neurodevelopmental disorders.31,35,36 Thus, identification of early kicking predictors for the age of walking attainment in preterm infants may provide valuable clues for the genesis of neurodevelopmental disorders.
This study extended the findings of Jeng et al24 by examining the age of walking attainment in preterm infants who had VLBW but no overt neurosonographic abnormalities and full-term infants without known impairments or pathology and by examining the relationship between spontaneous kicking and age of walking attainment in these infants. Infants were prospectively examined for their kicking movement at 2 and 4 months corrected age, and they were monitored for their age of walking attainment until 18 months corrected age.
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Twenty-two infants with VLBW and 22 full-term infants were included in this study between June 1997 and December 1998. The 22 infants with VLBW (15 boys [68%] and 7 girls [32%]) had a mean gestational age of 30.1 weeks (SD=2.5, range=26–35) and a mean birth weight of 1,180 g (SD=243, range=636–1,492). Among the infants with VLBW, 3 (14%) had minor intraventricular hemorrhage (grade I-II) and 9 (41%) had chronic lung disease (respiratory disease that requires oxygen therapy for 28 days or longer39). Fifteen of the mothers (68%) had more than 12 years of education, 5 mothers (23%) had 9 to 12 years of education, and 2 mothers (9%) had less than 9 years of education. The 22 full-term infants (8 boys [36%] and 14 girls [64%]) had a mean gestational age of 39.1 weeks (SD=3.1, range=38–41) and a mean birth weight of 3,298 g (SD=219, range=2,910–3,632). Among the full-term infants, 15 of the mothers (68%) had more than 12 years of education and 7 mothers (32%) had 9 to 12 years of education. Comparison of perinatal and demographic characteristics between groups revealed that the infants with VLBW included more boys (
2 =4.5, df=1, P=.03) and had lower gestational age (F=253; df=1,43; P=.0001) and birth weight (F=923.6; df=1,43; P=.0001) than the full-term infants. The groups were comparable in maternal education (2=2.9, df=2, P=.2).
Instrumentation and Testing Procedure
The infants were prospectively examined for kicking movements at 2 and 4 months corrected age and were monitored for their age of walking attainment until 18 months corrected age. During the follow-up period, all infants had regularly scheduled clinic visits for vaccinations at 1, 2, 4, 6, 9, 12, 15, and 18 months corrected age at the pediatric department of the hospital.
During the kicking test, infants were placed in the supine position on an examination table and wore black shorts to reveal the following anatomical landmarks on both sides of the body: mid trunk, greater trochanter, lateral femoral condyle, lateral malleolus, and fifth metatarsal head. Reflective ball-shaped markers with 1- to 2-cm diameter were placed at these landmarks to define the hip, knee, and ankle angles. The infants were tested 1 hour before feeding and were kept in an alert state (state 5, according to the Brazelton Neonatal Behavioral Assessment Scale40). Kicking movements were recorded for 5 minutes using 4 synchronized video cameras (2 Peak High Speed Video cameras* and 2 WV CL-350 video cameras
). Each camera was operated at 60 Hz and was connected to a videocassette recorder (SVHS AG-1960 and 1970) and a time code generator (Horita SDR-50
). Two cameras were placed on each side of the infants at a distance of 2 m. The cameras were angled at 70 degrees to construct a 3-dimensional analysis of the movements of the same limb with calibration errors of <3 mm. The testing procedure was described in more detail in Jeng et al.24
During the first visit to the clinic, when the infants were 2 months corrected age, the parents were asked to prospectively monitor and record their child's age of walking attainment, which was defined as the time the infant began walking 5 successive steps without support.33 The data were collected within a week of this event and were recorded by corrected age. To reduce observer bias in the determination of age of walking due to parental report alone or other problems such as faulty memory, a research assistant made biweekly telephone calls when the infants were 9 to 18 months corrected age and asked a specific set of questions regarding the child's motor status and age of walking. Parental report of locomotor status has been found to have a high degree of agreement (>95%) with telephone call response.31 For those infants whose telephone call response did not correspond to the parental report (<5%), the former information was used for analysis.
All infants also were examined for their neurological condition at 18 months corrected age by a physiatrist in the hospital. The physiatrist was masked to the study's purpose and infant groups. The examination included aspects of neurological signs, primitive and pathological reflexes, and motor function. In addition, the information of health state and developmental intervention received during the follow-up period was recorded. The physiatrist made the clinical diagnosis of the infants based on the neurological examination.
Data Acquisition and Reduction
Previous studies by Droit et al19 and Piek and Carman41 indicated that bouts of spontaneous kicking usually last only for a few seconds. Therefore, a 20-second segment of recorded movement that best represented continuous kicking movements was selected for the individual infant at each month. The criteria for selection of the segment were: (1) when the behavioral state of the infant was awake but not crying, (2) when continuous kicking was present, and (3) when all joint markers were visible in the camera view. The selected video record was analyzed using the Peak Performance Motion Analysis System (Motus version 3.01*) with a fourth-order Butterworth filter and a filtering rate at 6 Hz. The 2-dimensional video data set generated from the cameras that recorded the same limb movements was converted into 3-dimensional spatial coordinates. Joint angles were defined as the relative angles between the anatomical landmarks, with 180 degrees regarded as full extension at each joint and 0 degree regarded as full flexion. Angular displacement and velocity of the hip, knee, and ankle joints were calculated from the coordinate data. The obtained angular displacement and velocity data were used to calculate the kinematic variables of kick frequency, spatiotemporal organization, interjoint coordination, and interlimb coordination using the Visual Basic Software (version 5.0) program.
Kick frequency was measured as the number of kicks by each leg during the 20-second period and was converted to cycles per minute. Spatiotemporal organization included kick amplitude and movement phases. Kick amplitude was measured by the range of hip flexion during a kick cycle, which consisted of 4 movement phases: flexion phase, intra-kick pause, extension phase, and inter-kick pause. The flexion phase was the time from the initiation of hip flexion until the movement ceased; the intra-kick pause was the time from the end of hip flexion until the initiation of hip extension; the extension phase was the time from the initiation of hip extension until the movement ceased; and the inter-kick pause was the time from the end of hip extension until the initiation of the next flexion phase.15 The initiation of hip flexion was the frame at which continuous hip flexion (angular velocity of <0°/s for more than 10 frames) was first noticed (angular velocity=–14°/s). The termination of hip flexion was the frame at which hip flexion stopped, as indicated by the occurrence of minimum absolute value of angular velocity. The initiation of hip extension was the frame at which continuous hip extension (angular velocity of >0°/s for more than 10 frames) was first noticed (angular velocity=14°/s). The termination of hip extension was the frame at which hip extension stopped, as indicated by the occurrence of minimum absolute value of angular velocity.
Interjoint coordination was measured by pair-wise cross-correlations of the hip, knee, and ankle joint angles for the same leg during the kick cycles using Pearson product-moment correlation, and these measurements were transformed to Fisher Z scores.20 Interlimb coordination was measured by the number of kicks and the percentages of alternate, unilateral, and synchronous kick patterns during the 20-second period.42 An alternate kick was defined as simultaneous flexion of one leg and extension of the other leg with the flexion phase of the 2 legs overlapping for less than 50% of the movement. A unilateral kick was defined as isolated flexion and extension of one leg when the other leg was in intra-kick pause or inter-kick pause. A synchronous kick was defined as simultaneous flexion (or extension) of both legs during more than 50% of the flexion (or extension) phase. The predominant interlimb coordination pattern also was determined for each infant, as defined by the most frequent type of kick pattern that occurred. The calculations for each kinematic variable have been described in detail elsewhere.24
Data Analysis
The perinatal and demographic characteristics were compared between the infants with VLBW and the full-term infants using an analysis of variance (ANOVA) for continuous variables and chi-square tests for categorical variables. The distributions of age of walking attainment for the infants with VLBW and the full-term infants were estimated by the Kaplan-Meier method, a nonparametric survival analysis, and were compared using the log-rank test.43 During the follow-up of subjects until they reach a pre-specified endpoint (attainment of walking), some subjects may complete the follow-up period of time (18 months corrected age) before the endpoint is reached. For such cases, the survival times were censored; that is, subjects survived to a certain time beyond which their status was unknown. The noncensored survival times were referred to as event times. The Kaplan-Meier method incorporates information from both censored and noncensored data.
The relationship between kicking movements and age of walking attainment for the preterm infants with VLBW and the full-term infants was examined using an ANOVA for repeated measures with a 2 (group) x 2 (age) factorial design. The outcome of walking attainment was stratified into normal versus delayed using data from the full-term infants as the reference. Normal walking attainment was defined as age of walking attainment earlier than 2 standard deviations from the mean age of walking attainment. Delayed walking attainment was considered mild delay or severe delay. Mild delay was defined as age of walking attainment later than 2 standard deviations but earlier than 4 standard deviations from the mean, and severe delay was defined as age of walking attainment later than 4 standard deviations from the mean.44
The associations of kicking variables with age of walking attainment also were examined using Cox proportional-hazards univariate regression models in which the outcome of age of walking attainment was a continuous variable.43 The models provide the maximum likelihood estimates of rate ratio (RR) and the 95% confidence interval (CI) for individual kicking variables. A rate ratio of greater than 1 indicates that the level under consideration may lead to an increased rate of walking attainment as compared with a reference level. A rate ratio of less than 1 indicates that the level under consideration may result in a decreased rate of walking attainment. Multivariate regression analyses of Cox proportional-hazards models were subsequently used to adjust for potentially confounding variables such as prematurity and gestational age. A P value of less than .05 was considered as statistically significant, and the acceptable level for statistical significance in post hoc tests was adjusted to .013 (.05/4). All statistical analyses were performed using the Statistical Analysis Software (SAS) program (version 8.0).||
| Results |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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2=6.49, P=.01).
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Examination of the perinatal history of the infants with VLBW revealed that of the 9 infants with a low gestational age (<30 weeks), 3 (33%) showed delay in walking attainment and 6 (67%) showed normal walking attainment. Of the 3 infants with minor intraventricular hemorrhage, 2 (67%) exhibited delay in walking attainment and 1 (33%) had normal walking attainment. Of the 9 infants with chronic lung disease, 3 (33%) showed delay in walking attainment and 6 (67%) had normal walking attainment.
Neurologic Outcome and Health State
Examination of the neurologic outcome at 18 months corrected age revealed normal neuromotor development in all of the infants with normal walking attainment, motor delay in the 4 infants with mild delay in walking attainment (infants A, D, E, and F), and cerebral palsy with spastic diplegia in the 2 infants with severe delay in walking attainment (infants B and C). Cerebral palsy was diagnosed as the presence of persistently abnormal neurological signs, primitive and pathologic reflexes, and motor dysfunction, whereas motor delay was diagnosed as the presence of motor dysfunction but without abnormal neurological signs. The neurological diagnosis was made by the physiatrist.
Review of the health and developmental intervention records during the follow-up period showed that of the infants with VLBW, 9 (40%) had additional clinic visits (>3 visits) and 4 (18%) had hospitalization(s) because of upper respiratory infection, bronchiolitis, renal tubular acidosis, or hepatitis. Three infants (14%) received developmental interventions (2 infants had developmental counseling, and 1 infant had physical therapy and occupational therapy for 28 sessions). Of the full-term infants, none had additional clinic visits, hospitalization, or developmental intervention during the follow-up period.
Patterns of Kicking Development
Comparison of kicking movements between limbs in all infants revealed differences (all P<.05) but no preference of limb use. The kicking performance of the 2 limbs, therefore, was treated as independent data. Table 1 illustrates the group means of each kicking variable for the infants with mild to severe delay in walking attainment and the infants with normal walking attainment at 2 and 4 months corrected age.
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| Discussion |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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The first attempt to identify kicking predictors for age of walking attainment was made by comparing the kicking movements between infants with mild to severe delay in walking attainment (clinically defined walking delay) and those with normal walking attainment. The results showed that (1) a high kick frequency together with a short flexion phase at 4 months corrected age and (2) a low variability of interlimb coordination at 2 and 4 months corrected age were closely related to walking delay. Subsequently, Cox proportional-hazards analyses confirmed the effects of these kicking predictors on age of walking attainment and further indicated that the effect of the flexion phase was confounded by low gestational age. In addition, 2 more predictors were found for delayed walking attainment: a high hip-knee correlation at 2 months corrected age and a short intra-kick pause at 2 and 4 months corrected age. Our findings suggest that kicking frequency, intra-kick pause, hip-knee coordination, and interlimb coordination are important factors related to walking development in prematurity. The identified kicking predictors are useful and relevant measures for day-to-day practice because they are easily observable in home, day care, and clinical settings. Clinicians could use the identified predictors as early markers to detect those infants who may develop walking delay, and they could use these variables as treatment goals when designing early intervention programs. The significance and potential causes of the individual kicking predictors for age of walking attainment are delineated as follows.
A high kick frequency at 4 months corrected age was consistently associated with a decreased rate of walking attainment. From 2 to 4 months corrected age, the infants with normal walking attainment showed a decrease in kick frequency, whereas those with mild to severe delay in walking attainment began with a lower kick frequency and subsequently the frequency increased to higher levels than in infants with normal walking attainment. The different trends in the development of kick frequency among those infants may partly be due to different rates in neural maturation and differences in muscle tension. Van Wulfften and Hopkins45 observed a broad spectrum of interrelated changes in infant behaviors (eg, learning abilities, motor behavior, social competence) around the age of 2 to 4 months, and they attributed these changes to neural maturation. The changes in motor behaviors are characterized by reorganization from spontaneous to fine-distal and goal-directed movements. In our study, the infants with normal walking attainment may have undergone age-appropriate maturation in neural functions, so that toward 4 months corrected age, they manifested only a few kicks and were paying more attention to manipulation and vocalization. Furthermore, differences in the muscle tension of lower limbs also may influence the kick frequency because higher muscle tension is related to shorter pauses and therefore higher kick frequency.16 Future studies can examine various aspects of neural functions and muscle tension during this age period to determine their relationship to walking development.
A short intra-kick pause at 2 and 4 months corrected age also may predict a decreased rate of walking attainment. The infants with normal walking attainment tended to kick with a longer duration of intra-kick pause than did those with mild to severe delay in walking attainment throughout the follow-up period. However, the intersubject variations in the intra-kick pause were large (0.24±0.26 seconds versus 0.71±1.60 seconds at 2 months; 1.11±1.62 seconds versus 2.72±4.14 seconds at 4 months), and the differences between groups did not reach statistical significance. The tendency of the infants with a decreased rate of walking attainment to assume a short intra-kick pause for kicking might be linked to insufficient muscle force in the lower limbs and abdomen for holding the lower limbs in the air. In addition, a high level of stiffness in the passive viscoelastic properties of limb muscles also may result in shorter pauses.16 Further investigation of muscle force and passive viscoelastic properties of muscles can illuminate their roles in early kicking movement and subsequent walking attainment.
A high hip-knee correlation at 2 months corrected age is closely related to a decreased rate in walking attainment. The unfavorable association between high interjoint correlation and motor outcome has previously been documented.17,18,23 The development of interjoint coordination of spontaneous limb movements can be characterized by high pair-wise joint correlations at a newborn age, followed by a decrease in the joint correlations.42,46,47 The release of joints, as indicated by declined joint correlation, from the early obligatory synergism has been considered as an important feature allowing for the emergence of new movement patterns.42 It has been postulated that the tight interjoint coupling may be linked to an inability to distinguish reciprocal excitation of the antagonist muscles and to poor reciprocal inhibition.48
Our data did not support the predictive utility of the hip-ankle and knee-ankle coordination as previously reported.23 These incongruous results may be due to differences in assessment age because Vaal et al23 followed up infant kicking movement from 6 to 26 weeks corrected age. They indicated that high knee-ankle and hip-ankle correlations during the age period of 18 to 26 weeks were associated with poor motor outcome. In contrast, we examined infant kicking movement only at 2 and 4 months corrected age, so the follow-up duration in our study might have been too short to detect the predictive value of the hip-ankle and knee-ankle coordination at older ages.
A low variability in interlimb coordination patterns at 2 and 4 months corrected age is associated with a decreased rate of walking attainment. In the development of bilateral movement coordination, all infants exhibited a trend toward more synchronous kicks, together with less unilateral kicks by 4 months corrected age. Infants with normal walking attainment manifested a large variation in the distribution of interlimb coordination patterns. Although most infants adopted unilateral kicks as their predominant pattern at 2 months corrected age and synchronous kicks at 4 months corrected age, some had assumed other forms of coordination as the predominant pattern. Infants with mild to severe delay in walking attainment, however, exhibited a considerably skewed distribution of kicking patterns. An extremely high proportion adopted the unilateral kicking pattern at 2 months corrected age and the synchronous kicking pattern at 4 months corrected age. This obtained relationship between variability of kicking pattern and motor outcome concurs with the data of Vaal et al.23 It has been proposed that a biological movement system must have stable movement patterns, but at the same time variability or flexibility is necessary for allowing adaptation to environmental changes.49–51 We speculate that infants with normal walking attainment may have greater flexibility of the lower limbs, so they can accommodate for ongoing changes in the development of leg movements. Further study is warranted to determine what factors—whether intrinsic or extrinsic—constrained the limb actions and thus the capacity for variation in the infants with mild to severe delay in walking attainment.
It is worthwhile finally to highlight the usefulness of survival analysis in the investigation of age of walking attainment and its predictive factors in this study. Survival analysis has been widely used in clinical trials to measure the time to a certain event, such as death, relapse, response, and the development of a given disease when the observation times for some subjects are unknown (ie, censored).43 In general, survival analysis provides full information regarding the rate of attaining walking ability over the whole follow-up period and allows comparison of such data between groups. The advantage of the Kaplan-Meier method used in our study is that it does not require a parametric assumption of the survival rate and can be readily applied for a small sample size. Nevertheless, when other covariates need to be adjusted for in comparing 2 groups' survival function, the Kaplan-Meier estimation is no longer sufficient. In view of this limitation, we used Cox proportional-hazards models. Under the assumption that the ratio of hazard rate between 2 levels of an independent variable (ie, exposed versus nonexposed) remains constant over variable periods of time, the models provide estimates of rate ratio for each independent variable with adjustment for all the remaining variables. Because Cox proportional-hazards models can incorporate censored information in the analyses, they extract more information from the data than traditional discrete analyses, such as ANOVA for repeated measures. This point was clearly illustrated in the identification of kicking predictors for age of walking attainment in the 2 kinds of analyses performed in this study.
The major limitation of this study was that only a 20-second segment of recorded movement was selected for the individual infant for kinematic analysis. The use of a short segment of movement records for analysis may introduce bias, particularly when kick frequency is a variable of interest. Further research is needed to use a longer section window of kicking data to enhance the generalizability of the findings.
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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Our data provide information that can help physical therapists to understand the complex connection between spontaneous kicking movement and the development of initial functions (ie, walking). This connection is of current importance due to the increasing numbers of preterm infants seen clinically, coupled with the overall goal of offering developmentally appropriate intervention as early as possible. Moreover, due to the general lack of data supporting the use of specific interventions in young infants with special needs, future research is necessary to investigate the control mechanisms for these early kicking features, to examine whether intervention of these kicking variables affects walking outcome, and to determine the functional importance of age of walking attainment on long-term outcome.
| Footnotes |
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This work was supported by a grant from the National Health Research Institute (DOH 88-HR-619) of the Department of Health and a grant from the National Science Council (NSC90–2314-B002–307) in Taiwan.
* Peak Performance Technologies Inc, 7388 S Revere Pkwy, #601, Englewood, CO 80112. 
Panasonic Broadcast & Television System Co, 1 Panasonic Way, Secaucus, NJ 07094.
Horita Co, PO Box 3993, Mission Viejo, CA 92680.
Microsoft Corp, One Microsoft Way, Redmond, WA 98052-6399.
|| SAS Institute Inc, PO Box 800, Cary, NC 27511.
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