Autism spectrum disorder (ASD) is one of the most prevalent neurological disorders (1,2). Children with ASD are less involved in social and/or organized activities (3) and are less likely to engage in new activities (4). In addition, families face challenges finding appropriate care to address the child’s individual needs (5). Typically, ASD is characterized by social communication deficits, movement stereotypy, restricted interests, and both hyper- and hyposensitivity to sensory stimuli (6). However, motor skill impairment is present in at least 80%–90% of children with ASD (7,8). Moreover, motor impairment is thought to precede both impaired social and communication skills (9) and contribute to diminished interests to participate in social activities (1), resulting in a much greater risk for chronic disease development (e.g., obesity, diabetes, cancer, and cardiovascular disease) compared with individuals without ASD (10). Because motor skills are quantifiable and behavioral measures are subjective to the evaluator (11), some researchers have accepted motor impairment as a core symptom of ASD (12–14). What appears lacking, however, is the limited understanding we currently have regarding methods to improve motor function or mitigate motor impairment in ASD.
Walking is the most fundamental motor ability, as it is the primary form of human locomotion. Generally, children with ASD present with a number of gait differences compared with their peers with typical development (TD). Recently observed differences in groups of children with ASD compared with children with TD include decreased cadence, velocity, and step length (15) in addition to decreased peak joint torques at the hip and ankle (16). In addition, decreased coordination (17), decreased stride length (18), greater stride length variability (19), and decreased range of motion at the ankle joint (20) have been observed in children with ASD during walking. More recent studies have used matched-pair and single-subject analyses to reveal heterogeneous gait mechanics exhibited among children with ASD (21,22). From these studies, it was determined that the angular positioning of the hip, knee, and ankle joints were distinctly different throughout the gait cycle in children with ASD compared with age- and sex-matched children with TD (21). Further, varying amounts of kinematic asymmetry were observed between the left and the right limbs among children with ASD (22). As mentioned previously, it may be of interest to clinicians and researchers to identify interventions that can address motor impairments in children with ASD, and the evidence reviewed highlights the need for such interventions to mitigate motor impairment during fundamental human movements such as walking.
A common intervention tool used by therapists to target sensory integration issues in this population is the weighted vest (WV), which applies a compressive external load to users (23). A survey of occupational therapists indicated that 349 (82%) of 514 have used or continue to use added mass interventions in children with ASD (24). The compression produced by the added load can calm users by providing sensory input to the thalamus and sensory areas in the parietal lobe of the cerebral cortex, resulting in greater functional attention to produce more purposeful activities (25). Contemporary literature suggests that the application of external loads leads to negligible improvements in the traditional characteristics of ASD, such as stereotypy, attentiveness, and hyperactivity (23,26,27). However, in consideration of the growing acceptance of motor impairment as a core feature of ASD, it is reasonable to presume that an external load might modulate sensory input responses during walking to improve overall movement control. A recent study revealed that WV use does not negatively affect gait mechanics, as defined by bilateral asymmetry, and it might actually decrease the number of asymmetries between the hip joints (13). However, it remains unclear whether the WV use can be a useful tool for the improvement of more direct motor function parameters during walking. If WV use can positively influence motor function during walking, it may be more feasible for clinicians and researchers to adopt best practices in the ASD community for children presenting with poor walking abilities. Therefore, the purpose of this investigation was to examine the effects of an external load (delivered via WV) on the linear smoothness of the center of mass (COM); the angular smoothness of the hip, knee, and ankle joint rotations; and the stride length and stride time during overground walking in children with ASD. On the basis of the assumption that the WV will modulate sensory responses such that the selection of an appropriate motor strategy is simplified, it was hypothesized that the WV would alter gait function (decrease stride length and stride time) and improve control (increase gait smoothness) during walking.
Eight male children (11 ± 4 yr, 1.45 ± 0.25 m, 55.28 ± 26.20 kg) volunteered to participate in this study. This sample was determined via an a priori power analyses using G*Power 3.1 software (28) and the velocity/height data of Nobile and colleagues (18). This power analysis indicated a total sample of seven participants was necessary to achieve a proposed effect size of 1.08, a power (1 − β) of 0.8, and an alpha level of 0.05 (18). As such, the sample of eight participants provided sufficient statistical power. Participants were recruited through fliers posted in local ASD clinics and word of mouth. All participants were required to have a previous clinical diagnosis of ASD from a medical professional, verbally confirmed by a parent, be able to walk while carrying an external load of up to 15% body mass, be aged between 8 and 17 yr, and be present without a toe-walking style gait pattern. Before completing any laboratory tasks, the testing protocol was explained to the participants in age-appropriate language. Written informed parental consent and child assent was provided to the investigators as approved by the local institutional review board at the site of data collection (University of Nevada, Las Vegas, NV) in accordance with the Declaration of Helsinki.
Participants completed all laboratory activities during a single session. Demographic and anthropometric measures (age, height, and mass) were recorded. Three-dimensional kinematic data were obtained using a 10-camera motion capture system (200 Hz; Vicon Motion Systems, Ltd., Oxford, UK). Spherical reflective markers were adhered bilaterally to the following locations using hypoallergenic adhesive tape and elastic wraps: acromion process, anterior superior iliac spine, posterior superior iliac spine, iliac crest, medial and lateral femoral condyles, medial and lateral malleoli, and base of the second toe. A single reflective marker was adhered to the sacrum. Finally, thermoplastic shells with four noncollinear markers were placed bilaterally on the thigh and leg using elastic wraps and adhesive tape, and three noncollinear markers were placed bilaterally over the calcaneus/heel counter of the shoe.
Participants were provided up to 5 min to familiarize themselves with each experimental condition as well as the laboratory environment. During familiarization, participants were monitored by the researchers for overt verbal and nonverbal responses to the task and conditions related to discomfort or a desire not to complete the walking task in a given condition. If concerning responses were observed, the children were asked whether they felt they were okay to perform the current walking task. The participants were asked to walk in a straight-line distance of approximately 9 m at a pace that best represented the pace they would use if they were “going for a walk outside.” The first and the last 2 m of walking were discarded to account for initial acceleration and terminal deceleration. We discarded uncharacteristically fast or slow trials, which were visually monitored by the research team as speeds approaching the run–walk transition or slower than a pace used to go for a walk outside. Fifteen trials were recorded during two conditions, for a total of 30 experimental trials. Parents were asked to not involve themselves during testing by minimizing interactions with their child and not reacting to or providing instructions for their child’s performance during the study. The first condition consisted of overground walking with no added mass (bodyweight [BW]). The second condition consisted of walking overground while wearing a modified WV (MiR Vest, Inc., San Jose, CA) loaded with 15% body mass distributed evenly within the vest over the anterior and posterior aspects of the trunk (WV). The conditions were presented such that the BW condition was performed before WV to ensure that the data obtained during the BW condition were not influenced by a previously worn WV condition (13) and best reflected the typical BW walking strategy.
Raw data were exported to the Visual 3D biomechanical software suite (C-Motion, Inc., Germantown, MD). A five-segment model was built from the marker trajectories to represent the trunk, pelvis, thigh, leg, and foot segments bilaterally. Data were smoothed using a low-pass Butterworth digital filter with a cutoff frequency of 6 Hz. The 15 trials recorded for each condition were then reduced to left and right strides, defined as the time between consecutive ipsilateral heel strikes, which were identified using a velocity-based algorithm for overground walking (29). From the smoothed data, the model COM was calculated from estimated segmental properties. Then, a Cardan rotational sequence (x-y-z) was used for 3D joint angular computations, where x represents the medial–lateral axis, y represents the anterior–posterior axis, and z represents the longitudinal axis. Hip, knee, and ankle joint angular positions were expressed in degrees, such that positive polarities indicated a flexed/dorsiflexed position. For each stride, the linear jerk of the COM (third derivative of the COM position with respect to time; m·s−3) was calculated along the anterior–posterior axis in the direction of motion. In addition, the angular jerk of the hip, knee, and ankle joints (third derivative of joint angular position with respect to time; °·s−3) was calculated bilaterally in the sagittal plane. Stride length was calculated as the linear distance in meters between consecutive ipsilateral heel strikes, and stride time was calculated as the elapsed time of the stride in seconds.
Processed data were exported from Visual 3D to Matlab (R2015b; Mathworks, Inc., Natick, MA). Gait smoothness was calculated using a modified jerk cost (JC) (18) along the anterior–posterior axis for the system COM (JCCOM) and for the sagittal rotations of the hip, knee, and ankle joints (JCHIP, JCKNEE, and JCANKLE respectively). We chose to evaluate gait smoothness in this fashion because it is well suited to identify motor control strategies dominated by feedback, as opposed to feed-forward responses, in children with ASD in which they react to environmental stimuli rather than predicting necessary actions (18). Specifically, JCCOM was calculated by dividing the time integral of squared COM jerk by stride length squared divided by stride time to the fifth power (30). JCHIP, JCKNEE, and JCANKLE were then calculated at each joint using the JC equation described previously while substituting the angular jerk of the respective joint for the linear jerk of the COM, and stride length was substituted by the angular excursion of the joint.
Mean and SD values were calculated across strides for each participant for the following parameters of interest: stride length, stride time, JCCOM, JCHIP, JCKNEE, and JCANKLE. From the participant mean values, a group mean was calculated for each parameter. A 2 (limb) × 2 (load condition) ANOVA analyses were used to examine differences between limbs and conditions at the group level (α = 0.05). If an interaction was detected, paired-samples t-tests (α = 0.05) with Bonferroni corrections were used for between-limb comparisons within conditions and within-limb comparisons between conditions. If no interaction was detected, the Sidak adjustment was used for main effect multiple comparisons. Despite adequate a priori statistical power, we acknowledged the possibility of statistical error associated with the relatively small sample size that could mask true differences. As such, a single-subject analysis was also performed to reveal such differences and identify any unique individual responses (21,22) to the WV that would otherwise be unidentifiable using a group statistical design (31). For the single-subject analysis, the model statistic (α = 0.05) technique was used (32). This technique is similar to a t-test, although it accounts for both the SD for each comparative mean value and the number of observations (strides) used to calculate the critical difference for each individual based on their own generated movement variability (33). Four model statistic tests were performed for each participant to evaluate differences within-limbs between-conditions and between-limbs within-conditions. A MATLAB® function to conduct model statistic tests is provided as Supplemental Digital Content 1, http://links.lww.com/TJACSM/A43.
Stride length and stride time data are documented in Table 1. Stride length was not influenced by the interaction of condition and limb (P = 0.717), and no limb main effect was detected (P = 0.969). However, a significant condition main effect (P = 0.018) revealed a reduction in stride length during WV use compared with BW. For stride time, no significant interaction (P = 0.743), limb main effect (P = 0.915), or condition main effect (P = 0.703) was detected. JCCOM, JCHIP, JCKNEE, and JCANKLE magnitudes are presented in Table 1. No significant interactions, limb main effects, or condition main effects were detected for JCCOM (P = 0.636, P = 0.728, P = 0.621), JCHIP (P = 0.488, P = 0.349, P = 0.437), JCKNEE (P = 0.652, P = 0.211, P = 0.629), or JCANKLE (P = 0.466, P = 0.079, P = 0.901).
Stride length data at the single-subject level are presented in Table 2. No significant differences (P > 0.05) were detected between left and right limbs for stride length during the BW condition. However, 25% of the participants exhibited stride length differences between the left and the right limbs during the WV condition. In addition, 75% of the participants exhibited stride length differences between both BW and WV for the left limb and the right limb. Stride time data at the single-subject level are presented in Table 3. No significant differences (P > 0.05) in stride time were detected between left and right limbs for any participant during the BW condition. For the WV condition, 13% of the participants exhibited significantly different (P < 0.05) stride times between the left and the right limbs. For the comparison between BW and WV for left limb stride time, 75% of the participants exhibited significant differences (P < 0.05) between conditions, whereas 50% of the participants exhibited significantly different (P < 0.05) right limb stride times between BW and WV.
JC data at the single-subject level are documented in Table 4. The left versus right limb comparison during BW revealed significant differences (P < 0.05) between limbs for JCCOM, JCHIP, JCKNEE, and JCANKLE in 50%, 88%, 50%, and 75% of the participants, respectively. The left versus right limb comparison during WV revealed significant differences (P < 0.05) between limbs for JCCOM, JCHIP, JCKNEE, and JCANKLE in 50%, 50%, 63%, and 75% of the participants, respectively. The comparison between BW and WV for the left limb strides revealed significant JCCOM, JCHIP, JCKNEE, and JCANKLE differences (P < 0.05) in 75%, 63%, 38%, and 63% of the participants, respectively. The comparison between BW and WV for the right limb strides revealed significant JCCOM, JCHIP, JCKNEE, and JCANKLE differences in 63%, 50%, 63%, and 63% of the participants, respectively. To present the variation among participants, exemplar plots from two participants showing the unique responses to the WV are documented in Figs. 1 and 2.
The purpose of this investigation was to examine the effects of an external load (delivered via WV) on the linear smoothness of the COM; the angular smoothness of the hip, knee, and ankle joint rotations; and the stride length and stride time during overground walking in children with ASD. The main findings of the current study indicate that WV can alter spatiotemporal gait parameters and improve the smoothness of the COM during walking in children with ASD. At the group level, the WV perturbation significantly reduced stride length and increased COM smoothness. Children with ASD are known to exhibit decreased stride length in comparison with children with TD to ensure the COM remains inside the base of support (34), and the decrease in stride length observed in the current sample (group level, regardless of limb) is consistent with findings from a previous study of children with TD carrying 15% BW (35). This could be considered a favorable result because it suggests bilateral gait mechanics in children with ASD undergo a similar inherent neurological adaptation to that of children with TD when carrying external loads despite having movement planning deficits (36). Because the observed decrease in stride length during the weighted walking occurred when the load was arranged symmetrically over the upper body, the decreased stride length was not solely due to a potential shift in the location of the total body COM. Instead, the decrease in stride length reflects an accommodation the new inertial characteristics of the trunk (13) by reducing the degrees of freedom available during ambulation.
At the individual level, the significant stride length reductions observed during WV use versus BW occurred for both the right and the left limbs in all but two participants. Although this evidence may seem favorable due its similarity with the group-level result, the individual responses for stride length varied substantially among the participants with respect to asymmetry. For example, significant stride length asymmetries were not observed in any participant during BW, although three participants exhibited significant stride length asymmetries during the WV condition with two of those participants exhibiting a reduction in stride length. Movement asymmetry is important for this population, as asymmetry during walking indicates the presence of uncoordinated interlimb control (13) and pathological gait (37). As such, the three participants who exhibited significant stride length asymmetries may have lesser abilities to execute coordinated bilateral movements in response to the WV perturbation regardless of whether they can decrease stride length, unilaterally or bilaterally, to accommodate the trunk’s new inertial characteristics. Nonetheless, these findings suggest children with ASD possess the neurological capacity to adapt spatiotemporal gait parameters to maintain the ability to functionally ambulate at self-selected speeds under greater mechanical demands (added WV mass). Interestingly, these three participants also exhibited a decrease in stride time, indicating an increase in walking speed as a coinciding response to the WV, which may reflect improved gait performance during WV use.
Previous work (18) similarly observed that normalized JC was greater at the ankle versus the knee, and the JC at the knee was greater than at the hip. These similar responses suggest that the organization of movement control among the hip, knee, and ankle joints is consistent among individuals with ASD. However, the JC values we observed at the hip, knee, and ankle were approximately 3, 1.5, and 5 times greater, respectively, than the those presented by Nobile and colleagues (18). Because we did not assess or control for the severity of ASD symptoms, it is possible that the current sample had more severe ASD symptoms and corresponding motor deficits than the sample assessed by Nobile and colleagues (18).
It is likely that proprioceptive input is a primary mechanism for improved smoothness and gait characteristics with load carriage in children with ASD because contemporary evidence suggests that both proprioceptive (38) and multisensory integration (39) is impaired in this population. Wearing a WV has been shown to increase proprioceptive input (40), thereby creating an environment that can enhance implicit learning and adaptation during a motor task. From the current data at the single-subject level, increased proprioceptive input associated with WV use likely contributed to improved motor skill proficiency through increased gait smoothness that may be realized through augmented body awareness and/or stronger sensory inputs to the cerebellum to facilitate improved motor error correction. Because greater smoothness occurred only in some participants during WV use and the sample as a whole was associated with heterogeneous responses, some children might not respond well to a WV intervention with respect to lower extremity coordination during walking. As shown in Figs. 1 and 2, the linear jerk of the COM and the angular jerk of lower extremity joints were markedly different between participant 1 (Fig. 1) and participant 8 (Fig. 2). Moreover, in participant 1, marked reductions in the variability of linear and angular jerk during WV use versus BW were especially evident throughout the entire stride, whereas portions of the stride with high variability, such as preswing, were similar in participant 8 during both BW and WV use. These unique observations may reflect different motor abilities between these participants and potentially among all participants in the sample. Accordingly, identifying characteristics of gait smoothness may be useful in both determining the effectiveness of a WV intervention for ambulatory improvements and stratifying children with ASD into more specific motor skill related subpopulations.
The heterogeneous findings in the current study for both COM smoothness and joint smoothness may describe an interaction of the 15% BW load and the heterogeneous manifestations of ASD that can alter stability during locomotion. For instance, some participants responded to the WV with improved COM and joint smoothness, whereas others responded with more jerky movements, supporting the working hypothesis that each child with ASD exhibits unique manifestations of motor performance (13). Further, children whose unique motor abilities are determined to be subpar because of unsmooth movements could realize improved motor ability/increased smoothness by wearing a WV. As such, previous conclusions suggesting individuals with ASD should not use a WV as a therapeutic modality may be misleading, as these results indicate WV use does have therapeutic usefulness for some children.
A limitation of the current study was the inability to perform independent ASD diagnoses and the reliance on parental confirmations regarding their child’s diagnosis of ASD from a medical professional. However, the children in this study did exhibit hallmark social, behavioral, and motor symptoms of ASD. Another possible limitation was the absence of IQ testing because children with low IQ scores (<70) can present with more impairments than peers with higher (>70) IQ scores (41). However, all participants were able to follow instructions and communicate adequately enough to ask questions as needed, suggesting the current sample likely consisted of children without a low IQ. The relatively small sample size as it relates to the group analysis is another limitation. However, we addressed this by both performing an a priori sample size estimation and conducting a complementary single-subject analysis, which is not sensitive to sample size for detecting a treatment effect (31). Finally, a possible limitation of this study relates to the practical application of the WV as a chronic intervention or an intervention delivered in social or public environments. Although no child in this sample exhibited aberrant behaviors or reactions to the WV, it may be that some children with ASD will be less inclined to participate in social activities when wearing a WV because of potential stigmas associated with their “unusual” appearance. As such, short-term retention effects should be examined to determine whether a once-a-day usage of a WV can produce lasting effects throughout the remainder of the day such that potential social stigmas are mitigated. We recommend that future work investigate the effects of acute WV perturbations on subsequently performed BW walking trial to determine whether the increased smoothness observed in some children with ASD during WV use continues after removal of the WV stimulus. Future work might also consider assessing additional external loads to help identify appropriate loads that can improve gait smoothness in children with ASD. Future work might also consider studying measures that can be used to create subgroups of children such that we can obtain a better understanding of children who could be categorized as “nonresponders” to a 15% WV intervention.
In summary, the current study revealed acute responses to WV use during walking, highlighting the heterogeneous manifestations of motor responses in ASD. At the group level, the WV loaded with 15% BW produced both a decrease in stride length and trivial changes in gait smoothness, which could be considered a null result because the favorable outcome of reduced stride length coincided with an unfavorable retention of unsmooth movements. However, at the single-subject level, numerous spatiotemporal and gait smoothness adaptations to the WV were detected in the sample, suggesting group-level analyses can mask potential adaptations to WV use even when there is adequate a priori statistical power. Therefore, WV use could hold therapeutic value for some children with ASD who display impaired gait function and poor movement control/unsmooth movements. Further, clinicians and researchers may have foundational evidence for the potential benefits of WV use to refine current practices in the ASD community of children in need of improved function during walking. Unfortunately, additional measures must be examined to understand the mechanisms underlying positive and negative responses to WV use. We suggest researchers and clinicians interested in evaluating WV use as an intervention to mitigate motor impairment in children with ASD proceed at the individual level to avoid masking individual responses.
This research did not receive any grant or funding from agencies in the public, commercial, or not-for-profit sectors.
The authors have no conflict of interest to disclose. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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