Physical fitness levels and trends of kindergarteners in Hong Kong during the COVID-19 pandemic.

Background/objectives: The study aimed to examine the physical fitness and activity levels of kindergarteners in Hong Kong during the coronavirus disease 2019 pandemic. Methods: A total of 2052 kindergarteners (48% girls; 32.9% Grade 1, 34% Grade 2, and 33.1% Grade 3) were recruited from July 2020 to November 2021. Participants completed the physical fitness tests, including body composition, flexibility, lower-limb muscle strength, upper-limb muscle strength, lower-limb muscle endurance, and agility. Children's physical activity and overall well-being were examined using parental proxy reports. Parents also reported their physical activity and parental support to children's physical activity engagement, as well as their perception of children's kindergarten physical activity environment. Fitness differences by age and gender were analyzed using one-way ANOVA and ANCOVA measuring effect size with partial eta-squared. Additionally, correlations assessed the relationship between children's fitness and parents' proxy reports. Results: The results of the physical fitness tests were higher than those in previous studies conducted by the Physical Fitness Association of Hong Kong in 2015-2018. Gender-based differences were observed in most tests for children aged 4 years and older, with boys showing higher scores in the standing long jump, shot put, and balance tests, while girls had higher scores in the sit-and-reach test. Parents' proxy questionnaire answers indicated that children's continuous jump test performances were significantly related to their frequency of physical activity per week (r = 0.19, p < 0.001), and that children's health was significantly and positively correlated with their fitness level (r = 0.179, p < 0.009). Inadequate school physical activity was associated with poor upper-limb strength (r = 0.078, p < 0.005). Moreover, a high level of parental support for their children's participation in physical activity was correlated with a high level of parental participation in vigorous-intensity physical activities (r = 0.167, p < 0.005). Conclusion: The physical fitness of children in Hong Kong was less affected by the epidemic. Parents' healthy behaviors and support were related to children's participation in PA. Efforts to improve children's physical fitness and motor development should include parent education and physical activity involvement.

Development and validation of a test instrument for the assessment of perceived basic motor competencies in first and second graders: the SEMOK-1-2 instrument.

Both actual motor competencies (AMC) and perceived motor competencies (PMC) play an important role in motor development research and children's physical and psychological development. PMC refer to children's perceptions of their motor competencies. To assess the PMC of first and second grade children (aged 6-9 years), the SEMOK-1-2 instrument was developed. The instrument is aligned to the validated MOBAK-1-2 instrument which assesses AMC in the competence areas "object movement" and "self-movement" Accounting for possible reading difficulties in younger children, the motor tasks and answer options were illustrated and explained verbally. The purpose of this study was to test and validate the SEMOK-1-2 instrument and investigate the associations between the constructs AMC, PMC and physical activity (PA), whereby PA was measured by the participation in team and individual sports. Data from N = 404 pupils in the German-speaking part of Switzerland from first and second grades (M = 7.8 years, SD = 0.69, 49% boys) were analyzed. Confirmatory factor analyses were conducted to test the factorial validity of the SEMOK-1-2 instrument. Structural equation models were used to investigate the association between the constructs. The analyses confirmed a two-factor structure with the factors PMC "object movement" and PMC "self-movement", corresponding to the factors existing in the MOBAK-1-2 instrument. Latent correlations between AMC factors and the corresponding PMC factors were r = 0.79 for "object movement" and r = 0.76 for "self-movement". Associations with external criteria and covariates, such as sex, were associated with both AMC and PMC. Analyses also revealed that children who participated more often in individual and team sports showed higher levels in both AMC and PMC. The confirmation of the two-factorial structure of the SEMOK-1-2 instrument and the associations between AMC and PMC as well as external criteria indicate construct and criterion validity. The SEMOK-1-2 instrument can be economically utilized for assessing PMC and is also suitable for the monitoring of PMC in the context of Physical Education.

The relationships between language, functional hearing, social, and motor development skills in children with early cochlear implants.

PURPOSE: The purpose of this study was to investigate the relations between functional hearing, language, social, bilateral coordination and manual dexterity skills in children with early cochlear implants (CIs). METHODS: Thirty children with CIs were included in this study. The manual dexterity and bilateral coordination development of the participants were evaluated with Manual Dexterity and Bilateral Coordination subtests of Bruininks-Oseretsky Motor Proficiency-2 (BOT-2). Their language skills were assessed by the Test of Early language Development-3. To assess the functional hearing of participants the Functioning After Pediatric Cochlear Implantation scale (FAPCI) was administered their caregivers. Also, the Social Skills Evaluation Scale was administered to participants' teachers to asses their social skills. RESULTS: There were significant correlations between participants' receptive and expressive language skills, Manual Dexterity, and FAPCI scores (p < 0.05). There were also significant relationships between the SSES and FAPCI scores of the participants (p < 0.05). However, the Bilateral Coordination subtest did not show any significant correlation with any of the measurements (p > 0.05). CONCLUSION: The results suggest that the language, manual dexterity and functional hearing abilities of children with CIs are closely related. Although, there were no significant correlations between all of the measurement, it is important to look beyond hearing and speech evaluations to assess the whole child.

Reduction in limb-movement complexity at term-equivalent age is associated with motor developmental delay in very-preterm or very-low-birth-weight infants.

Reduced complexity during the writhing period can be crucial in the spontaneous movements of high-risk infants for neurologic impairment. This study aimed to verify the association between quantified complexity of upper and lower-limb movements at term-equivalent age and motor development in very-preterm or very-low-birth-weight infants. Video images of spontaneous movements at term-equivalent age were collected from very-preterm or very-low-birth-weight infants. A pretrained pose-estimation model and sample entropy (SE) quantified the complexity of the upper- and lower-limb movements. Motor development was evaluated at 9 months of corrected age using Bayley Scales of Infant and Toddler Development, Third Edition. The SE measures were compared between infants with and without motor developmental delay (MDD). Among 90 infants, 11 exhibited MDD. SE measures at most of the upper and lower limbs were significantly reduced in infants with MDD compared to those without MDD (p < 0.05). Composite scores in the motor domain showed significant positive correlations with SE measures at most upper and lower limbs (p < 0.05). The results show that limb-movement complexity at term-equivalent age is reduced in infants with MDD at 9 months of corrected age. SE of limb movements can be a potentially useful kinematic parameter to detect high-risk infants for MDD.

The effect of 10 weeks of karate training on the development of motor skills in children who are new to karate.

This study investigated the effect of a 10-week karate training program on the motor skill development of 5-7-year-old children new to karate with two different test batteries. A total of 28 participants were included in the study: 18 in the Karate group and 10 in the control group. The karate group was subjected to a fundamental karate training (kihon) program consisting of 90-minute sessions four days a week for ten weeks in addition to physical education classes at their schools. In contrast, the control group did not participate in any sportive activities except physical education classes in their schools. Data were collected using the Eurofit test battery and the TGMD-2 test. In the pre-post test comparison of the anthropometric measurements of the karate group, no significant difference was found in the control group. In contrast, a significant difference was obtained in height, body mass index, and body fat percentage. In the post-test analysis of the two independent groups, there were statistically significant differences in favor of the karate group regarding height and body fat percentage (p < 0.005). In the pre-post analysis of the Eurofit test and the TGMD-2 for the karate group, all parameters showed statistically significant improvements (p < 0.001), while the control group showed no statistical difference. After comparing the karate and control groups, the Eurofit Test and TGMD-2 post-test results showed significantly higher scores (statistically significant differences) in all parameters for the karate group. In conclusion, the study shows that the 10-week karate training program positively affected the motor development of the participating children.

Sensor-Based Quantitative Assessment of Children's Fine Motor Competence: An Instrumented Version of the Placing Bricks Test.

The assessment of fine motor competence plays a pivotal role in neuropsychological examinations for the identification of developmental deficits. Several tests have been proposed for the characterization of fine motor competence, with evaluation metrics primarily based on qualitative observation, limiting quantitative assessment to measures such as test durations. The Placing Bricks (PB) test evaluates fine motor competence across the lifespan, relying on the measurement of time to completion. The present study aims at instrumenting the PB test using wearable inertial sensors to complement PB standard assessment with reliable and objective process-oriented measures of performance. Fifty-four primary school children (27 6-year-olds and 27 7-year-olds) performed the PB according to standard protocol with their dominant and non-dominant hands, while wearing two tri-axial inertial sensors, one per wrist. An ad hoc algorithm based on the analysis of forearm angular velocity data was developed to automatically identify task events, and to quantify phases and their variability. The algorithm performance was tested against video recordings in data from five children. Cycle and Placing durations showed a strong agreement between IMU- and Video-derived measurements, with a mean difference <0.1 s, 95% confidence intervals <50% median phase duration, and very high positive correlation (ρ > 0.9). Analyzing the whole population, significant differences were found for age, as follows: six-year-olds exhibited longer cycle durations and higher variability, indicating a stage of development and potential differences in hand dominance; seven-year-olds demonstrated quicker and less variable performance, aligning with the expected maturation and the refined motor control associated with dominant hand training during the first year of school. The proposed sensor-based approach allowed the quantitative assessment of fine motor competence in children, providing a portable and rapid tool for monitoring developmental progress.

Assessment of the measurement properties of the Peabody Developmental Motor Scales-2 by applying the COSMIN methodology.

The Peabody Developmental Motor Scales-2 (PDMS-2) has been used to assess the gross and fine motor skills of children (0-6 years); however, the measurement properties of the PDMS-2 are inconclusive. Here, we aimed to systematically review the measurement properties of PDMS-2, and synthesize the quality of evidence using the Consensus-based Standards for the Selection of Health Measurements Instruments (COSMIN) methodology. Electronic databases, including PubMed, EMBASE, Web of Science, CINAHL and MEDLINE, were searched for relevant studies through January 2023; these studies used PDMS-2. The methodological quality of each study was assessed by the COSMIN risk-of-bias checklist, and the measurement properties of PDMS-2 were evaluated by the COSMIN quality criteria. Modified GRADE was used to evaluate the quality of the evidence. We included a total of 22 articles in the assessment. Among the assessed measurement properties, the content validity of PDMS-2 was found to be sufficient with moderate-quality evidence. The structural validity, internal consistency, test-retest reliability and interrater reliability of the PDMS-2 were sufficient for high-quality evidence, while the intrarater reliability was sufficient for moderate-quality evidence. Sufficient high-quality evidence was also found for the measurement error of PDMS-2. The overall construct validity of the PDMS-2 was sufficient but showed inconsistent quality of evidence. The responsiveness of PDMS-2 appears to be sufficient with low-quality evidence. Our findings demonstrate that the PDMS-2 has sufficient content validity, structural validity, internal consistency, reliability and measurement error with moderate to high-quality evidence. Therefore, PDMS-2 is graded as 'A' and can be used in motor development research and clinical settings.

Agreement of synchronous remote and in-person application of the Alberta Infant Motor Scale: Cohort study.

INTRODUCTION: Using standardized scales to assess motor development via telemedicine can increase access for low-income populations. Our aim was to verify the agreement and feasibility between remotely and synchronously applying the Alberta Infant Motor Scale (AIMS) and the in-person format. METHODS: This was a concordance study, with 77 typical infants aged 4-18 months (mean = 13 months). The AIMS was applied remote via video calls and face-to-face. We applied a questionnaire to caregivers to verify feasibility. RESULTS: There was a high level of agreement between the remote and in-person assessments, with intraclass correlation coefficients above 0.98 and low standard error measure values (<1 item for each posture, <2 items for the total raw score, and =5% for the normative score). The smallest detectable change was between 1.67 and 2.45 for each posture, 3 for the total raw score, and 6% for the normative score. The Bland-Altman analysis showed low bias with the mean difference close to zero (<0.80) and low error with little dispersion of the difference points around the mean. Caregivers' perspectives on the synchronous remote assessment were positive, with good quality, clear information during the assessment, and comfort with the method. DISCUSSION: The synchronous remote application of the AIMS may be an alternative for families without access to in-person services that assess motor development.

Evaluating the association between duration of breastfeeding and fine motor development among children aged 20 to 24 months in Butajira, Ethiopia: a case-control study.

BACKGROUND: A Suitable environment and proper child nutrition are paramount to a child's physical and mental development. Different environmental factors contribute to proper child development. Breast milk is an important source of nutrition during the early years of life and contains essential nutrients that are the building blocks for growth and development. OBJECTIVE: To assess the association between the duration of breastfeeding and fine motor development among children aged 20 to 24 months living in Butajira, southern Ethiopia. METHOD: Community-based case-control study design was employed among mother-child dyads of children aged 20 to 24 months in Butajira Southern Ethiopia. Children were screened for fine motor delay using the Denver II developmental screening and identified as cases and controls. A repeated visit was done to gather the rest of the information and 332 samples, 83 cases, and 249 controls were available and assessed. Epi-data version 4.4.2.1 software was used to prepare a data entry template, which was later exported to and analyzed using STATA version 14 statistical software. Finally, a Multivariable logistic regression model was used to adjust for confounders and estimate the independent effect of breastfeeding duration on fine motor development. RESULT: We didn't find a significant association between the duration of breastfeeding from 21 to 24 months and fine motor delay compared to children who were breastfed less than 18 months[AOR: 0.86, 95% CI: (0.36, 2.05)]. Children who have mothers > 35 years of age were 78% less likely than children who had mothers younger than 25 years, Children who had mothers in secondary school and above were 77% less likely than mothers who didn't have formal education, Females were 1.86 times more likely than males, and Children who scored 20-29 on the Home score were 51% less likely than Children who scored < 20 to have fine motor delay. CONCLUSION: Duration of breastfeeding was not significantly associated with fine motor delay for children aged 20 to 24 months old. The age of the mother, the educational status of the mother, being female, and Home score were identified to have a significant association with fine motor delay. Improving the educational status and empowerment of women is essential. Further work should be done on avoiding gender differences starting from a young age and creating a conducive environment for child development is crucial.

Infant vocal productions coincide with body movements.

Producing recognizable words is a difficult motor task; a one-syllable word can require the coordination of over 80 muscles. Thus, it is not surprising that the development of word productions in infancy lags considerably behind receptive language and is a known limiting factor in language development. A large literature has focused on the vocal apparatus, its articulators, and language development. There has been limited study of the relations between non-speech motor skills and the quality of early speech productions. Here we present evidence that the spontaneous vocalizations of 9- to 24-month-old infants recruit extraneous, synergistic co-activations of hand and head movements and that the temporal precision of the co-activation of vocal and extraneous muscle groups tightens with age and improved recognizability of speech. These results implicate an interaction between the muscle groups that produce speech and other body movements and provide new empirical pathways for understanding the role of motor development in language acquisition. RESEARCH HIGHLIGHTS: The spontaneous vocalizations of 9- to 24-month-old infants recruit extraneous, synergistic co-activations of hand and head movements. The temporal precision of these hand and head movements during vocal production tighten with age and improved speech recognition. These results implicate an interaction between the muscle groups producing speech with other body movements. These results provide new empirical pathways for understanding the role of motor development in language acquisition.

The relationship between home environment affordances and motor development and sensory processing skills in premature infants.

The availability of stimulating materials in the home environment is of great importance to optimizing an infant's development. This study, which has a cross-sectional study design, was conducted to examine the relationship between home environment conditions and equipment support and the motor development and sensory processing skills of premature infants. Children born premature, aged 10-16 months, were included in the study. Motor development was evaluated with the Peabody Motor Development Scale-2, and sensory processing skills were evaluated with the Test of Sensory Function in Infants. The Affordances in the Home Environment for Motor Development-Infant Scale was used to evaluate the home environment. A total of 51 premature infants were included in the study. It was determined that there was a significant relationship between physical space, stimulus variety and fine motor toys in the home environment and Peabody Motor Development Scale-2 gross motor and fine motor development scores. It was also shown that there was a relationship between the tactile and total scores of the Test of Sensory Function in Infants Scale and the variety of stimuli in the home environment and gross motor toy scores. These findings show that the opportunities provided in the home environment of premature infants may be related to their motor development and sensory processing skills. Consequently, the home environment may be associated with experiencing movements and sensory experiences.

Exploring Facilitators and Barriers to Physical Activity for Families of Rural Preschoolers Participating in a Motor Skill Program.

While schools provide one opportunity to encourage physical activity, caregivers play an exceedingly important role in creating an environment conducive to preschool children's physical activity. Yet, little is known regarding the perceptions of caregivers, important choice agents for young children's physical activity behavior after participating in a motor skill program. The purpose of this study was to examine caregivers' perceptions of facilitators and barriers to children's physical activity at home among rural, low-income families who participated in a school-based early childhood physical activity program, SKIPping with PALS, designed to increase physical activity and improve motor development. Eleven caregivers consented to participate in a semi-structured interview regarding their perceptions of physical activity and their experience after six months of participation in the program. An inductive, naturalistic evaluation approach was utilized for qualitative data analysis, following the six recursive phases of thematic analysis. A review of the interview transcripts revealed that all caregivers valued physical activity and encouraged their children to be active. Four major facilitators, four major barriers, and an overarching theme of parental support for childhood physical activity were identified. These factors are largely circumstantial and attitudinal and, thus, are difficult to modify but are important to be cognizant of when designing interventions.

Developmental associations between motor and communication outcomes in Fragile X syndrome: Variation in the context of co-occurring autism.

LAY ABSTRACT: Fragile X syndrome (FXS), the leading heritable cause of intellectual disability, has a co-occurrence rate of autism spectrum disorder (ASD) estimated at ~60%. Children with FXS experience delayed achievement and slower development of key motor abilities, which happens to an even greater extent for children with both FXS and ASD. A multitude of studies have demonstrated that motor abilities are foundational skills related to later communication outcomes in neurotypical development, as well as in the context of ASD. However, these associations remain unexamined in FXS, or FXS + ASD. In this study, we aimed to determine the associations between early motor skills and their rate of development on communication outcomes in FXS. Furthermore, we investigated whether these associations varied in the context of co-occurring FXS + ASD. Results revealed within-FXS variation in the context of co-occurring ASD between some aspects of motor development and communication outcomes, yet within-FXS consistency between others. Findings provide evidence for variability in developmental processes and outcomes in FXS in the context of co-occurring ASD and offer implications for intervention.

Mechanical environment influences muscle activity during infant rolling.

An infant's musculoskeletal and motor development is largely affected by their environment. Understanding how different mechanical environments affect an infant's movements and muscle use is necessary to inform the juvenile products industry and reduce incidents involving inclined nursery products each year. The purpose of this study was to determine how the coordinated movements and corresponding muscle activation patterns are affected by different mechanical environments, specifically the back incline angle. Thirty-eight healthy infants (age: 6.5 ± 0.7 months; 23 M/15 F) were enrolled in this IRB-approved in-vivo biomechanics study. Surface electromyography sensors recorded muscle activity of the erector spinae, abdominal muscles, quadriceps, and hamstrings while infants rolled in five different mechanical environments: a flat surface and four device configurations representing a range of inclines infants are commonly exposed to. Coordinated movements were determined using video. In all configurations featuring an inclined seatback angle, infants experienced significantly higher erector spinae muscle activation and significantly lower abdominal muscle activation compared to the flat surface. Infants also exhibited a different coordinated movement featuring spinal extension and a pelvic thrust in the inclined device configurations that was not previously observed on the flat surface alone. Understanding how infants coordinate their movements and use their muscles during rolling in different inclined environments provides more insight into motor development and may inform the juvenile products industry. Many factors impact an infant's movements, therefore future work should explore how other environmental interactions influence an infant's movements and muscle activation, particularly for rolling.

Focused attention as a new sitter: How do infants balance it all?

This study investigated the impact of postural control on infants' Focused Attention (FA). Study 1 examined whether and how sitting independently versus with support impacted 6- to 8-month-old infants' ability to focus attention during object exploration. FA measures did not depend on support condition. However, sitting experience was significantly negatively correlated with FA measures in the supported condition, suggesting that infants with more sitting experience performed fewer exploratory movements, possibly due to faster information processing ability compared to infants with less sitting experience. These unexpected findings prompted an exploration of more subtle looking behaviors during FA in Study 2-a case study of three infants who wore a head-mounted eye-tracker during an FA task. The ability to rapidly shift visual attention was key to gathering environmental information useful for problem solving-an interpretation that is supported by prior findings of the relationship between fast looks and faster information processing.

A Matched-Pair Analysis of Gross Motor Skills of 3- to 5-Year-Old Children With and Without a Chronic Physical Illness.

PURPOSE: The purpose of this study was to compare the gross motor skills of children with a chronic physical illness with those of their healthy peers. METHODS: Data for children with a chronic physical illness come from the Multimorbidity in Children and Youth Across the Life Course study, and data from children without a physical illness come from the Health Outcomes and Physical Activity in Preschoolers study. Multimorbidity in Children and Youth Across the Life Course and Health Outcomes and Physical Activity in Preschoolers included children ages 3-5 years and administered the Peabody Development Motor Scales-second edition. Participants were sex and age matched (20 male and 15 female pairs; Mage = 54.03 [9.5] mo). RESULTS: Gross motor skills scores were "below average" for 47% of children with a physical illness compared with 9% of children without a physical illness (P = .003). Matched-paired t tests detected significant differences in total gross motor scores (dz = -0.35), locomotor (dz = -0.31), and object control (dz = -0.39) scores, with healthy children exhibiting better motor skills, and no significant difference in stationary scores (dz = -0.19). CONCLUSIONS: This skill gap may increase burden on children with physical illness and future research should assess gross motor skills longitudinally to establish whether the gap widens with age.

Proficiency Barrier in Track and Field: Adaptation and Generalization Processes.

The literature on motor development and training assumes a hierarchy for learning skills-learning the "fundamentals"-that has yet to be empirically demonstrated. The present study addressed this issue by verifying (1) whether this strong hierarchy (i.e., the proficiency barrier) holds between three fundamental skills and three sport skills and (2) considering different transfer processes (generalization/adaptation) that would occur as a result of the existence of this strong hierarchy. Twenty-seven children/adolescents participated in performing the countermovement jump, standing long jump, leap, high jump, long jump, and hurdle transposition. We identified the proficiency barrier in two pairs of tasks (between the countermovement jump and high jump and between the standing long jump and long jump). Nonetheless, the transfer processes were not related to the proficiency barrier. We conclude that the proposed learning hierarchy holds for some tasks. The underlying reason for this is still unknown.

Is it possible to identify physical-motor profiles of preschool children on their association with selected biosocial factors?

Biosocial factors play a crucial role in the physical-motor development (PMD) of children during the preschool age. The present study aims to identify physical-motor profiles throughout preschool age (3-6 years) and explore associations between profiles and selected biosocial factors such as age, sex, prematurity, weight, height, BMI, and participation in extracurricular physical activities. Data from 412 typically developing children (46.6% girls and 53.4% boys), aged 35-71 months (M = 51.21, SD = 10.47) was collected using the Psychomotor Activities Checklist and specifically the scale of Psycho-Motor Aspects. Cluster analysis made it possible to define four different childhood PMD profiles. High PMD; High PMD except left laterality; medium-low PMD; and low PMD. High PMD profile includes older children, with anthropometric measurements closer to the WHO recommendations, fewer preterm children, and greater participation in extracurricular physical activities. Low PMD profile includes younger children, with weight slightly above and height slightly below the WHO recommendations and low participation in extracurricular physical activities. This study allows us to identify specific trends that may be decisive for the motor development of children throughout preschool age, highlighting selected biological variables and participation in extracurricular physical activities.

Short- and Long-Term Changes in Balance After Active Video Game Training in Children With and Without Developmental Coordination Disorder: A Randomized Controlled Trial.

Active video games (AVG) have been used as training tools and are known to ameliorate balance performance in children with Developmental Coordination Disorder (DCD). Our aim was to evaluate balance using clinical tests and by measuring body sway using a force plate with a mixed design of vision (eyes open/eyes closed), surface (rigid/soft), and support (stance/semitandem) before, and after, training and 4 months later (follow-up). Thirty-six DCD children and 40 typically developing children participated in the study, of which 50 children (26 DCD; 24 typically developing) were retested after 4 months. Balance improved on the clinical measures after the training, which was independent of type of AVG (Wii-Fit and Xbox Kinect) used, and this effect was still present after 4 months. The AVG training did not influence general sway behavior, but only sway in the eyes-open condition, corresponding with task demands of the training and indicating a training-specific effect. Overall, DCD children and typically developing children responded comparably to the AVG training, thereby maintaining the gap in performance between the two groups. The changes in postural sway are interpreted as a sign of more confidence and less freezing of the joints, enabling greater flexibility of movements and balance strategies as supported by the improved performance on balance tests in the DCD children. This is the first study that showed long-term effects of AVG training on balance performance. However, these follow-up results should be interpreted with caution given that 35% of the children were lost in follow-up.

Motor patterns of patients with spinal muscular atrophy suggestive of sensory and corticospinal contributions to the development of locomotor muscle synergies.

Complex locomotor patterns are generated by combination of muscle synergies. How genetic processes, early sensorimotor experiences, and the developmental dynamics of neuronal circuits contribute to the expression of muscle synergies remains elusive. We shed light on the factors that influence development of muscle synergies by studying subjects with spinal muscular atrophy (SMA, types II/IIIa), a disorder associated with degeneration and deafferentation of motoneurons and possibly motor cortical and cerebellar abnormalities, from which the afflicted would have atypical sensorimotor histories around typical walking onset. Muscle synergies of children with SMA were identified from electromyographic signals recorded during active-assisted leg motions or walking, and compared with those of age-matched controls. We found that the earlier the SMA onset age, the more different the SMA synergies were from the normative. These alterations could not just be explained by the different degrees of uneven motoneuronal losses across muscles. The SMA-specific synergies had activations in muscles from multiple limb compartments, a finding reminiscent of the neonatal synergies of typically developing infants. Overall, while the synergies shared between SMA and control subjects may reflect components of a core modular infrastructure determined early in life, the SMA-specific synergies may be developmentally immature synergies that arise from inadequate activity-dependent interneuronal sculpting due to abnormal sensorimotor experience and other factors. Other mechanisms including SMA-induced intraspinal changes and altered cortical-spinal interactions may also contribute to synergy changes. Our interpretation highlights the roles of the sensory and descending systems to the typical and abnormal development of locomotor modules.NEW & NOTEWORTHY This is likely the first report of locomotor muscle synergies of children with spinal muscular atrophy (SMA), a subject group with atypical developmental sensorimotor experience. We found that the earlier the SMA onset age, the more the subjects' synergies deviated from those of age-matched controls. This result suggests contributions of the sensory/corticospinal activities to the typical expression of locomotor modules, and how their disruptions during a critical period of development may lead to abnormal motor modules.