Upright posture characterizes most primates, especially hominids. This posture was developed early in human evolution, notably marked by the emergence of Homo erectus approximately two million years ago (Niemitz, 2010). Bipedalism, a consequence of this upright posture, is a milestone for human locomotion. While for other primates upright posture is not the most common daily position, for human beings upright posture is largely associated with the activities of daily life. Despite this, postural control is a not easy task, it includes balance in standing, walking and steps transitions but also reaction to perturbations (Wittenberg et al., 2017). Deficits in postural control can lead to dangerous situations, with poor postural control increasing the risk of falls and injuries. This is particularly true for groups of individuals with special needs. In an attempt to reduce the impact or risk of such dangerous situations, a body of research has been dedicated to examining how perturbations affect human postural control.

Individuals with special needs represents a group with high risk of falling and consequent injury (Masud and Morris, 2001; Finlayson et al., 2010). Individuals with intellectual disabilities and poor motor coordination show an elevated risk of falls and poor standing stability (Cherng et al., 2007; Du et al., 2015; Speedtsberg et al., 2017). Individuals with intellectual disabilities have demonstrated deficits in coordination of body movements, difficulties in mastering simple motor activities, including fine and/or gross motor skills, balance and posture (Finlayson et al., 2010; Westendorp et al., 2011; Houwen et al., 2016). Motor performance in such group is usually slower, less accurate, and more variable than that of their peers (Westendorp et al., 2011; Zwicker et al., 2012; Caçola, 2016; Houwen et al., 2016). These deficits can persist into adulthood, and can cause social and emotional difficulties (Mandich and Polatajko, 2003).

The partial absence of nervous system structures, in particular Astrocytes and Oligodendrocytes, which are involved on myelination process, (Kirby et al., 2014; Simons and Nave, 2016) play a role in the motor control of individuals with Down syndrome (Ábrahám et al., 2012). There is evidence that individuals with intellectual disabilities have reduced myelination in the brain, which implicates alterations in structural brain networks (Debrabant et al., 2016). As such, it is possible that, as in Down syndrome, myelination may influence motor control in intellectual disabilities. However, it is not known whether the amount of myelin sheathing in the spinal cord, specific in motor and inter-neurons, of intellectual disabilities individuals is impaired to the extent that it could explain the deficits in the postural control.

It is difficult to measure postural control and spinal cord activation/processing without invasive experimentation. However, we have developed a non-invasive apparatus that produces lateral perturbations, while simultaneously being connected to an Electromyograph that measures the muscular latency of a postural reflex. Recent studies using this apparatus have shown how posture is controlled when uni and bilateral perturbations are applied (Mühlbeier et al., 2017). In addition, chronic low back pain is associated with delayed muscle reflex responses of trunk muscles, which mainly is processed in spinal cord level (Liebetrau et al., 2013). Other study of our group has showed that neural circuits in the spinal cord could substantially contribute to muscular reflex bursting pattern (Wulf et al., 2012). Liebetrau et al. (2013) also suggests that, a delayed muscular reflex latency could have relevant influence on spinal stability, if subjects do not adapt their reflex amplitudes.

Therefore, the aim of the current study was to use the method mentioned above to investigate the latency responses of postural muscles after unexpected upper side perturbation limb loading in typically developed and intellectual disabilities participants. We expected that compared to typical participants, individuals with intellectual disabilities will show delayed (slower) postural control latency after external perturbation. Based on the possibility of poor myelination and atypical processing at the spinal cord level, we also hypothesized that participants with intellectual disabilities will have different latencies and patterns of muscular activity when compared to typically developed individuals, independently of the side of perturbation.

Means and standard deviations of the latency of the right and left M. obliquus externus abdominis for the groups ID and CG are presented in Figure 2. Two-way ANOVA showed that this muscle latency was affected by the perturbation side (contralateral and ipsilateral) [F_{(1, 71)} = 0.8, 0.9, p = 0.004] and not affected by group [F_{(1, 71)} = 2.0, p = 0.15], nor the interaction perturbation side versus group [F_{(1, 71)} = 1.6, p = 0.20]. The Post hoc test showed that the latency of M. obliquus externus abdominis was shorter for the contralateral side than for the ipsilateral side (p < 0.05).

For ID group, the confidence intervals of 95% was 72.4 and 98.9 ms on contralateral side, for ipsilateral side was 83.3 ms and 109.8 ms. For CG group, the confidence intervals of 95% was between 56.7 ms and 80.5 ms on contralateral side, for ipsilateral side was between 83.6 ms and 107.4 ms.

In this study, we measured the muscular onset after unexpected external lateral perturbations applied to participants with intellectual disabilities and typically developing persons. Our results do not support the hypothesis that the intellectual disabilities group would have delayed muscle activation compared with typically developing persons. Moreover, both groups presented shorter latency for the contralateral side.

The M. obliquus externus abdominis is highly related to postural stability. Lateral perturbations induce a specific reaction pattern of the postural regulation, because the contralateral trunk muscle response is faster and has a higher amplitude compared to the ipsilateral muscular response (Wulf et al., 2012; Liebetrau et al., 2013; Mühlbeier et al., 2017); however, participants with intellectual disabilities have not required longer to process changes to the current postural position. This may play a role in critical situations, such as falls or injuries (Gueze et al., 2001; Finlayson et al., 2010). Similar muscle onset suggests that the intellectual impairment may not affect the short latency postural responses. On the other hand, this deviant muscle response pattern has been suggested as factor that predisposes individuals to lower back injuries and may function as a compensatory mechanism to stabilize the spine (Radebold et al., 2000). Muscle recruitment and timing patterns plays an important role in maintaining postural control and lumbar spine stability (Liebetrau et al., 2013). The recovery of postural stability happens fast, usually within a range of 20–200 ms,(Granata et al., 2004) and does not involve the brain. Most computational process required to perceive alterations in one's pose, as well alterations required to regain a stable posture, happens at the spinal cord level via the integration of information from the skin and muscles, through neurons and inter-neurons (Bizzi et al., 1991; Wolpert and Ghahramani, 2000).

As well as being involved in postural control, the spinal cord is heavily involved in other complex motor computations and commands, such as the acquisition and maintenance of motor skills (Bizzi et al., 1991; Kiehn, 2006; Lemon, 2008; Shmuelof and Krakauer, 2012). The spinal cord is a special part of central nervous system, exhibiting functional and structural plasticity, which includes changes in motor neuron firing thresholds, axonal conduction velocity and in synaptic terminals on motor neurons (Wolpaw and Tennissen, 2001; Kiehn, 2006; Lemon, 2008). Consequently, the atypical functioning of the spinal cord is related to an increased risk of falls (Wolpaw, 2007). Our results suggest that intellectual disabilities are not associated with abnormal asymmetrical M. obliquus externus abdominis onset (Liebetrau et al., 2013). The mechanisms that might explain why persons with intellectual disabilities have more falls (Cherng et al., 2007; Du et al., 2015; Speedtsberg et al., 2017) is still unknown.

Further research is required to ascertain whether the possible atypical spinal processing in individuals with intellectual disabilities is related to the enhancement of falls and injuries. Although their muscle onset is similar, medular and cortical regulations within the postural control could be different in such populations. If this were the case, it would allow the development of interventions to develop and improve postural control for this population. We suggest that this is particularly important, as people with special needs are known as high risk group of falls, especially as they age (Masud and Morris, 2001; Finlayson et al., 2010). In addition to the research presented here, there are already some indications that individuals with intellectual disabilities have a unique brain signature (Zwicker et al., 2011; Biotteau et al., 2016). This may be related to neuroanatomical abnormalities, specifically a reduction in the amount of myelination and white matter and spine cells function (Ba et al., 2013; Verpelli et al., 2013; Debrabant et al., 2016). We suggest that it is possible that the spinal cord also presents similar abnormalities.

It should be noted that we were able to measured the outcome of the integrative processing between the skin, muscles, motor neurons, inter-neurons and others structures at the surface of the skin, using the EMG Apparatus. As such, we were not able to measure whether whole or part of the spinal cord in individuals with intellectual disabilities is structurally and functionally different from typically developing participants. Even though our study does not provides evidence that such differences may exist; we suggest that more studies are required to confirm or reject this hypothesis.

The current study furthers the knowledge about how the muscles involved in postural control of individuals with intellectual disabilities works. Our results show no difference of OE muscle latencies in individuals with intellectual disabilities and in typically developed persons. The knowledge about muscle and spinal functioning of persons with ID is useful for fall prevention as well for health care of injuries, and may provide insights for new physical therapies. Interestingly, similar muscle activation are employed for ID individuals compared to CG, independent of the perturbation side. Further research are required to ascertain how posture is controlled in participants with intellectual disabilities.

In summary, we showed that when participants with intellectual disabilities are offered an unexpected lateral external perturbation they take similar time “as typically developing participants” to react to this perturbation. To our knowledge, this is one of the first reports that suggest spinal cord processing of participants with ID is similar to that of typically developing participants. This provides an important starting point for future research into postural control in individuals with ID. Although, there have been prior reports on the atypical postural control of adults (Finlayson et al., 2010; Du et al., 2015) and children with intellectual disabilities (Johnston et al., 2002; Finlayson et al., 2010; Kane and Barden, 2012) we show that unexpected mechanical perturbations did not differ OE muscular latency, compared to CG, which is directly involved in postural control and processed at spinal cord.

JWT-C and AM made substantial contributions to the conception of the research and interpretation of the data, being also heavily involved in preparing the drafts of the manuscript while HW and LM provided comments on the final revisions. All four authors reviewed and approved of the final manuscript to be submitted. All four authors agree to be accountable for all aspects of the work to ensure that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.