INTRODUCTION
Autism spectrum disorder (ASD) is a heterogeneous disability affecting 1 in every 68 children in the United States.1 2 3 , 4 3 , 4 5
One aspect that has been explored minimally in children with ASD is the neuromuscular components of potential motor deficits. One group of researchers has explored the neuromuscular differences in gross motor behaviors between children with ASD and children who are developing typically (TD).6 6
In addition to walking, jumping and landing are also common motor skills of children. Focusing on a fundamental motor skill, such as landing , is important, given the established relationship between fundamental motor skill and cognition7 8 landing task in children with ASD and compare their responses to children TD. This study is an attempt to expand the literature and provide guidance for future research.
METHODS
Participants
Six children, 3 with ASD and 3 TD between the ages of 3 and 4.5 years, participated. Parents provided information regarding ASD diagnosis. These children were part of a larger study conducted at California State University, Northridge, exploring the development of landing strategies in children TD. Children were recruited from schools in the San Fernando Valley and service learning programs at the university. Parents provided informed consent to participate in the study. The Institutional Review Board at California State University, Northridge, approved the study.
Procedures
The participants and their caregivers came to the motor development laboratory where a questionnaire regarding diagnosis of developmental disabilities was completed.
Participants were asked to change into compression shorts and a tank top and to remove shoes. Anthropometrics, including leg length, and maximal vertical reach (MVR) and maximal vertical jump (MVJ) were assessed and recorded.
A trained laboratory member placed 10 Trigno Delsys surface electromyography (sEMG) devices and 15 reflective markers on participants. sEMG was placed and recorded on both sides of the body for gastrocnemius (G), tibialis anterior (T), rectus femoris (Q), semitendinosus (H), and erector spinae (E). Reflective markers were placed on the following anatomical landmarks: center of the forehead, base of the skull, cervical vertebrae 7 (C7), and on both sides for the acromion, lateral epicondyle of the elbow, greater trochanter of the hip, lateral side of the knee, lateral malleolus of the ankle, and the fifth metatarsal.
Participants were asked to land from a height-adjusted bar. Height of the bar was calculated according to the following formula: Bar height = (40% leg length + 40% MVJ + MVR). For safety reasons, the individual bar height was tested for each child using few assisted drops before data collection.
After the bar height was tested and deemed safe, the participants completed 15 drop trials, where they were asked to land onto 2 force plates (Kistler, 9286BA; one foot on each force plate) and react to a randomized light cue. The light cue instructed the child to stay in place (stay trial), or to run to the left or right (left or right trials). Five trials for each condition were completed per condition.
Surface Electromyography
Delsys EMG Works software was used to rectify and smooth the sEMG data using a root mean square technique (window length 0.1 seconds, window overlap 0.005 seconds). For each sEMG channel, 2 independent coders manually identified the onset/offset of muscle bursts (≥80 ms) during preimpact (750 ms before), impact, and postimpact (750 ms after). The onset and offset times, the duration of the burst, and percentages of trials that contained a muscle burst were calculated.
Kinematics
A Qualisys three-dimensional motion capture system was used to collect kinematic data. Hip and knee angles at impact and maximum flexion were calculated. The duration from impact to maximum hip and knee flexion was calculated.
Statistical Analysis
Descriptive statistics were calculated for anthropometric, sEMG, and kinematic variables using SPSS v.22. Independent t tests were used to guide our exploration of the data but not to determine statistical significance. Results with P values lower than .05 are reported. Mean values across all trials for the sEMG and kinematic data are presented and used to compare groups. This aggregated average was conducted since results were similar regardless of drop condition (ie, left, right, and stay). Lastly, interrater reliability for the sEMG data was calculated, achieving 96.2% agreement for muscle bursts and 91.8% agreement for the timing of onset/offset.
RESULTS
Participants
Three children with ASD (4.18 ± 0.25 years; 66.6% male) and 3 children with TD (3.63 ± 0.73 years; 66.6% male) had differences between groups for average height (ASD 110.633 ± 2.72 cm, TD 99.13 ± 4.94 cm), weight (ASD 19.23 ± 0.93 kg, TD 14.8 ± 0.85 kg), and reach height (ASD 135.17 ± 2.75 cm, TD 120.27 ± 2.75 cm). There were no group differences in average BMI (ASD 15.75 ± 1.43 kg/m2 , TD 15.08 ± 0.71 kg/m2 ), leg length (ASD 45.97 ± 2.50 cm, TD 41.30 ± 4.48 cm), bar height used (ASD 157.52 ± 11.39 cm, TD 139.20 ± 11.39 cm), or MVJ (ASD 145.07 ± 7.28 cm, TD 126.30 ± 11.48 cm) between groups.
Differences in sEMG
Differences were found in the duration and percentage of sEMG burst. Participants with TD had longer burst during impact for G (left and right), T (left), and H (left and right). The children with ASD had longer burst during preimpact for E (left and right) and G (right) (Figure ). The Figure graphs the average onset/offset times and the average length of muscle bursts during preimpact (A) and impact for both groups (B). Postimpact data had fewer differences between the groups for onset/offset time and durations and therefore were not presented.
Fig.: sEMG onsets and offsets for all conditions during preimpact (A) and impact (B). Length of the vertical bars indicates duration of the muscle burst. Horizontal black continuous line denotes impact. Horizontal black dashed line denotes max hip flexion for ASD. Horizontal gray dashed line denotes max hip flexion for TD. The circled muscles indicate group differences. ASD indicates autism spectrum disorder; E, erector spinae; G, gastrocnemius; H, semitendinosus; L, left; Q, rectus femoris; R, right; T, tibialis anterior; TD, typical development.
Children TD, on average, had a greater percentage of trials that included an EMG burst in E (left [ASD 45.0% ± 21.0%, TD 69.8 ± 27.5%] and right [ASD 50.2% ± 19.9%, TD 69.3% ± 17.5%]), G (left [ASD 60.6% ± 19.6%, TD 79.4% ± 14.2%]), and Q (left [ASD 43.4% ± 28.6%, TD 71.3% ± 26.2%]) at postimpact; and H (right [ASD 94.3% ± 8.6%, TD 100% ± 0%]) and E (right [ASD 94.3% ± 8.4%, TD 100% ± 0%]) at impact. Taken together, these results indicate that children with ASD had longer muscle bursts prior to impact, whereas children TD had longer and more muscle activation at and after impact.
Kinematics
No differences were found between the groups for the angle of the knee and hip at impact or maximum flexion. However, there were differences in the duration from impact to maximum knee (ASD 1.09 ± 0.25 seconds, TD 0.99 ± 0.15 seconds) and hip flexion (ASD 1.14 ± 0.2 seconds, TD 1.07 ± 0.11 seconds). Children TD had shorter duration for both maximum knee and hip flexion.
DISCUSSION
The results from this pilot study support that children TD had more and longer sEMG bursts during impact compared with children with ASD. Prior literature in children TD suggests that longer bursts of muscle activation, during what we defined as impact, represent a more mature landing strategy.9 , 10 landing strategy in children with ASD, as defined by the timing of muscle activation. Compared with previous studies,3 , 4
The various landing conditions (stay, left, and right) had similar results for both kinematics and sEMG data. This could be due to the fact that the light cue was triggered as the landing was initiated, and therefore, children may have performed this task as 2 separate actions (land and stay, or land and run). A more detailed analysis of the postimpact data including reaction time, and stepping and attention strategies, where potential condition differences may exist, might provide insight into group differences in the decision-making process for our task.
Kinematic differences were found in the duration from impact to maximum knee and hip flexion. Children TD showed shorter duration to achieve maximum knee and hip flexion. Paired with the results from the sEMG data, where most extensor muscles (E, Q, and G) had greater percentage of activation postimpact, children TD were exhibiting a more efficient landing strategy from impact to maximum flexion and postimpact. However, further study is needed to investigate this claim.
There were no significant differences in maximum hip and knee flexion. Lack of group differences may be explained by a small sample size (low power) and/or large variability in the kinematics. However, no angular amplitude differences in the lower extremity may exist between these 2 populations during landing . The only differences might be in the timing variables and duration of muscle activation. If this is accurate, observation of movement amplitude patterns in young children with ASD may not elucidate underlying neuromuscular deficiency. Caution should be used in interpreting our results. Further investigation is needed using a larger sample size and more robust kinematics.
Due to the limitations of this pilot study, we can only suggest that differences in sEMG may exist between children with and without ASD. If correct, our findings might have therapeutic relevance, suggesting that interventions for children with ASD should target muscle activation timing to prevent potential deficits in landing , a skill widely used for all children during recreational and structured physical activities.
ACKNOWLEDGMENTS
Thank you to all of the participants and their families for being part of the study. Also, a special thank you to the laboratory interns in the laboratory that helped with the project.
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