Given the fundamental role movement plays in all human behavior, delays and impairments in the acquisition of motor skills during the first year of life have significant implications for the subsequent development of the brain and behavior and for an individual's ultimate quality of life. Researchers have established that the acquisition of motor skills during the first year of life has cascading effects on the later development of skills in the motor and psychological domains in at-term and preterm infants (6–13). Several studies have shown that the quality of early movements in preterm infants predict the quality of motor skills at later ages (12, 14, 15). Hua and colleagues recently showed that even a mild delay in crawling and walking onsets increases the risk for subsequent motor impairments in childhood (16). Research on the psychological revolutions that follow the acquisition of independent crawling and walking provides some of the strongest evidence for the fundamental contribution motor development makes to broader developmental outcomes (6, 7, 17–19). Researchers have linked the emergence and practice of independent crawling around 8–9 months of age to changes in perceptual-action coupling, spatial cognition, memory, social and emotional functioning and brain functioning (6, 17, 20–25) and the acquisition of walking to a range of developmental changes (26), particularly in the language domain (8, 27, 28).

The pervasive effect of locomotor experience on a child's development represents one of the primary reasons clinicians target locomotor skills for therapeutic intervention for children at risk for developmental delay. However, most of the interventions designed to promote locomotion begin well after the age of 12 months, largely because clinicians cannot diagnose locomotor problems and neurological disorders like CP until infants have already demonstrated delayed acquisition of motor skills. A recent systematic review by Dumuids-Vernet and collaborators reported that since the year 2000, only ten motor/locomotor interventions commenced before the age of one year in infants at risk for motor delays and only three targeted infants born preterm, the other studies targeted infants with an established disability at birth (Myelomeningocele, Down syndrome or high brain damage) (29).

A clear exception to the prevailing tendency to initiate interventions after the infant's first birthday is the Neurohabilitation method developed by Ferenc Katona over 30 years ago in Hungary (30, 31). The method focuses on frequent stimulation of “elementary neuromotor patterns” that contribute towards upright postures and locomotion, from the first months of life and continuing for several months thereafter. Unfortunately, much of the world knows little about Katona's work and methods because Katona published most of his findings in Hungarian (32, 33) and he believed it was ethically wrong to conduct a randomized controlled trial to compare a group of infants who had received his treatment to a group who had not (personal communication, April, 2013). However, Thalia Harmony and her colleagues have written comprehensively about the implementation of Katona's method in Mexico and its effectiveness for preterm and term infants with and without perinatal brain damage (34–36). For example, in one study she and her colleagues compared the neurodevelopmental outcomes at six to eight years of age of 20 children who had received Katona's Neurohabilitation treatment to the outcomes of 13 infants whose families had voluntarily discontinued treatment after one month. The treated infants received treatment for several months after discharge from the hospital and received consistent follow up evaluations until the evaluations between six and eight years of age. All infants had brain damage, as indicated by their first clinical and MRI examinations, and were matched in terms of age, gender, birth weight, Apgar, risk factors and diagnosis. The findings revealed that the treated group had a higher percentage (90%) of children with normal neurodevelopmental outcomes between six and eight years of age than the non-treated group (38%), with particularly clear differences for children born at or before 34 weeks gestational age.

Given the heightened risk of motor/locomotor delays and impairments in very preterm infants, it is essential to have early intervention programs that clinicians could start even before the diagnosis of a behavioral or neurological disorder to improve long-term motor outcomes for these infants (37, 38). As indicated by a growing body of literature (37, 39) these programs should be implemented as early as possible to take advantage of the heightened plasticity of body and supra spinal structures (i.e., brain and corticospinal tract) and spinal circuitries (40–42). It is also essential (37) that these programs: (1) promote active, self-generated movements, (2) are high in frequency (37, 38, 42–44), and (3) target specific functions that are likely to generalize more broadly to motor and muscular development and be feasible for infants limited in their postural control and mobility (29, 45). Aside from Katona's Neurohabilitation method, what type of very early intervention could be implemented to stimulate active locomotion in very premature infants while adhering to these recommendations? Could an intervention designed to stimulate locomotor development be started as early as term equivalent age and would such an intervention facilitate the development of mature locomotion and other gross motor skills?

Human newborns can perform alternating locomotor movements of their legs at birth when supported upright in a stepping position on a table (46, 47) and can propel themselves forward when breast crawling on their mother's abdomen (48, 49) and via swimming movements when supported in the prone position in water (50). Researchers and clinicians have observed these patterns in preterm infants in neonatal care units when they crawl occasionally in their incubators (51) and when they perform stepping movements while supported upright under their armpits (30, 31). Though traditionally considered simple spinal reflexes, destined to rapidly disappear under the increasing influence of cortical maturation and playing no role in the development of independent locomotion, a growing body of literature has revealed that these primitive locomotor behaviors display much greater complexity than simple reflexes and serve as important precursors to mature crawling and walking. First, primitive locomotion does not disappear during development; biomechanical constraints mask its expression. For example, a rapid increase in fat mass during the first two months of life make the legs difficult to lift in the upright position if the infant has not received stepping practice, leading to an apparent disappearance of the stepping pattern. However, minimizing the gravitational force on the legs by submerging the infant in water (52, 53) or driving leg extension with a moving treadmill belt (54) can reactivate the stepping pattern between 2 and 6 months of age.

The second reason researchers now consider early motor patterns as complex behaviors is that several studies have shown that a range of higher order stimuli processed supra spinally can initiate and modulate newborn stepping and crawling, including visual, olfactory, and auditory stimuli (55–59). Finally, researchers have used a variety of experimental approaches to document a link between primitive and mature walking. One approach has used EMG recordings to show common patterns of muscle activation in newborn stepping and mature walking (60), even though such patterns remain plastic during development and can be fractioned into more controlling units (61). In another approach, researchers have shown that two months of daily stepping on a solid surface from birth leads to an earlier emergence and/or higher quality of mature walking in typically-developing infants (62, 63) and training stepping on a treadmill leads to an earlier emergence and higher quality of walking in infants with Down syndrome (64–68). Though less studied, researchers have shown that daily training of supported crawling in typically developing infants from four months of age onward leads not only to an earlier emergence of independent crawling and walking but boosts cognitive development (19). In addition, researchers have shown that Katona's method, which includes crawling as part of more comprehensive intervention for very young infants, also boosts motor and cognitive development (31, 35).

In summary, a large body of literature provides evidence for three important linkages in early development, each with connections to the other linkages. First, infants born very preterm have a much greater risk for delayed and/or impaired motor development, including locomotor development. Second, early primitive locomotor patterns appear to serve as precursors for later emerging independent crawling and walking. Third, the acquisition of crawling and walking make fundamental contributions to the development of a diversity of skills in the motor and psychological domains. Taken together, these linkages suggest that training locomotion in infants born very preterm could have positive and pervasive effects on their developmental outcomes. This suggestion raises some interesting questions. For example, what type of active locomotor movements should clinicians stimulate? Should the infant's movements be active or passive? When should the stimulation begin? Is there a preferred method or paradigm for providing the stimulation? Researchers have used a solid surface or treadmill to promote stepping in young infants, however stepping on these surfaces could create problems for very preterm infants, who are more fragile and often hypo or hypertonic. Furthermore, this type of stepping practice requires an experimenter to support the infant's body and head and constrains independent movement and forward propulsion. Moreover, supporting the infant's head and body under the armpits blocks potential locomotor movements of the arms and associated trunk and head movements, important in mature crawling and walking (69, 70).

The stimulation of quadrupedal locomotion has several advantages over the stimulation of other motor activities, such as isolated head movements, stepping on a treadmill, or repetitive limb movements. Crawling represents a whole-body activity that involves the coordination and sequencing of multiple muscles and joints, leading to observable propulsion and reorientation of the infant to her environment, even in the newborn (30, 31, 71). Notably, the neuromuscular substrate underlying the quadrupedal organization of crawling also seems to underlie bipedal stepping and walking (69, 70). Consequently, the active practice of crawling potentially facilitates the development of not only quadrupedal locomotion but also bipedal locomotion, holding of the head, unsupported sitting (by strengthening the muscles of the neck and trunk) and standing. However, due to the weight of their head, even typically developing newborns cannot lift their trunk and head to move forward by themselves in a prone position at birth; an experimenter must support very young infants (30, 31, 72), preventing them from moving actively in any direction in their environment.

Given the aforementioned limitations in newborns' ability to propel themselves via crawling movements, we designed and constructed a mini-skateboard, which raises the head and trunk off the ground and allows the arms to move freely, to facilitate and stimulate quadrupedal locomotion during the first months of life (71). This mini-skateboard, known as the Crawliskate, promotes a lengthening of the spine and a neutral position of the head to facilitate the arm and leg movements necessary to generate independent propulsion (see a detailed description in the methods). The results of a previous study on 60 typically-developing newborns highlighted the advantages of using the Crawliskate to promote newborn crawling (71). The newborns propelled themselves significantly further on the Crawliskate and they demonstrated more mature crawling patterns, in terms of limb kinematics and interlimb coordination, than when observed crawling without the Crawliskate.

This current study investigates the feasibility and the effect of early crawling training using the Crawliskate on the locomotor and motor development of very preterm infants. The intervention complies with the essential program elements for early interventions described earlier in the introduction, particularly for infants with limited postural control or who are hypotonic, and it preserves the natural alignment of the head, neck and spine when the infant is in the prone position, which is particularly important for infants born premature. The “Crawliskate” can be used at even younger ages than a robotic device recently designed to train crawling in infants with or at risk for cerebral palsy (73, 74) and it has the added benefits of being portable and easy to use in the home. As noted earlier, studies of more than four hundred typically-developing newborns revealed that neonatal crawling can be regulated by visual, olfactory and auditory stimuli processed supra spinally, and it appears to engage and stimulate the neural circuitry underlying the control of mature bipedal and quadrupedal locomotion (58, 59, 71). Consequently, newborn crawling represents an ideal target for early intervention. Early crawling training has the potential to influence the plasticity and development of the supra spinal tracts, ultimately contributing to the development of not only locomotion but a range of gross motor skills and the cognitive and psychological skills to which these motor skills have been linked (6, 17, 19).

Given the aforementioned observations of typically-developing newborns, we hypothesized that stimulating very premature infants' crawling on the Crawliskate would enhance their mature crawling (and possibly their walking), gross motor, and general development during the first year of life. We based our expectations on the findings from studies referred to earlier showing that five minutes of daily upright stepping practice over the first eight weeks of life accelerates walking onset of typically developing infants (62, 63), presumably because of the well-established neurological links between newborn stepping and later walking (60), and 15 min of daily crawling training in 5–9 month-old typically developing infants accelerates crawling onset, motor development and intellectual development relative to controls (19). We anticipated these latter training effects would be more pronounced in very young infants given their heightened body and brain plasticity (41, 42). The long-term aim of the study was to compare the motor and general development of infants up to 60 months corrected age (CA), however, in the current paper we present the motor and general outcomes up to 12 months CA.

The study was registered at www.clinicaltrials.gov; registration number: NCT05278286. Because of the limited number of subjects in our study due to the home-based nature of our intervention (see training protocol below), we chose not to include very preterm infants with major brain lesions because this would have required us to match lesion levels and locations across the three groups and we could not test a large enough sample to accomplish such matching. We randomly allocated forty-four very preterm infants without major brain lesions to one of three groups: Crawling training on the Crawliskate (Crawli), prone positioning on a Mattress training without the Crawliskate (Mattress), or standard care (Control), in which infants received pediatric checkups at 4, 9, 12, 18 and 24 months CA but no other treatment unless recommended by the pediatrician. It is important to note that we designed the Mattress condition to act as an active Hawthorne-effect control for the effect of prone positioning on the Crawliskate [prone positioning has positive effects on motor development, e.g., (75)] and the effect of visits from the osteopaths who implemented the training in the infants' homes. Because infants born between 24 + 0 and 27 + 6 (GA + Days) have higher risk for developmental delays than those born between 28 + 0 and 32 + 0 GA (76), the randomization process stratified infants on gestational age (24–27 + 6 GA vs. 28–32 GA) to ensure equal rates of GA at term equivalent age between groups.

We recruited eligible infants between March 2017 and October 2019 from four Neonatal Intensive Care Units (NICU) in Paris, France. Infants were included if they were born between 24 and 32 weeks of gestation (WG), lived within 10 kms of our laboratory (because of daily home visits by our team for the training groups), and could leave the NICU and begin training between 37 and 42 WG. Exclusion criteria included major brain damage defined by transcranial ultrasound, such as uni or bilateral IVH grade 3 and 4, periventricular leukomalacia, punctate lesions of the white matter and non-isolated lesions of posterior fossa (MRI was not used to screen for brain lesions as MRI was not available for all participants). Exclusion criteria also included Hypoxic-Ischemic Encephalopathy defined as infants presenting Apgar scores <5 at 5- and 10 min and a pH < 7 of the umbilical artery. Finally, exclusion criteria further included congenital anomalies, bronchopulmonary dysplasia with oxygen dependence after 36 WG, any digestive or other problems preventing prone positioning, limb deformities, and presence of retinopathy or sensory pathology that may delay motor development. We excluded infants if only one of the criteria was present. The study was approved by the ethical committee Ile de France 3 (CPP n°3465; ANSM n°2016-A01320-51). All parents provided written informed consent for their infants to participate in the study.

The Crawliskate, or Crawli, is a mini skateboard, adapted to newborns and young infants, that permits comfortable positioning on the belly. The slight upward slope (15 degrees) of the Crawli allows the upper part of the trunk and the head to be elevated, with the head resting on a platform in front (Figure 1D). The slope also allows maintenance of the natural flexion of the trunk and head, avoiding the possible hyper-extension of the head that can occur when the infant is placed directly in a prone position on a flat surface. The other advantage of the Crawli is that it frees the movements of the arms because the upper trunk is raised. Under these conditions, the arms can actively participate in the child's propulsion along with the legs (71). With the minimum slope of the Crawli at the infant's pelvis, the infant's legs rest on the surface of support, which allows the infant to learn to elevate the pelvis to make effective propulsive thrusts. To ensure the safety of the infant, he/she is secured to the Crawli by a system of fabric fins and straps that wrap the child on the skate (Figures 1A,B). Two stabilizers placed on each side and in front of the headrest prevent any lateral imbalance during the infant's propulsive pushes. The wheels placed under the Crawli allow the infant to move 360 degrees. The Crawli thus allows the infant to move actively in any direction by pushing with all four limbs (Figures 1B,C for a newborn on the Crawliskate).

Figure 3 summarizes the outcome assessments. The assessors of all outcomes were blind to the infants’ group and the osteopaths who performed the training had no contact with the assessors. Note, we do not present the crawling efficiency data (assessed via three-dimensional kinematics) at term equivalent age, 2, 6, 9, and 12 months CA in this manuscript because we continue to reduce and analyze those data.

The planned 1,600 home training sessions supervised by the 30 osteopaths constrained the sample size to 44. The stratified randomization process ensured we had a similar number of 24–27 + 6 and 28–32 GA infants in each group.

We used Statistica software (TIBCO Software Inc.) to run the analyses. Differences are given as mean differences with 95% confidence intervals and effect sizes are reported with confidence intervals when the effects are significant.

Among 107 participants assessed for eligibility, 49 infants were included and randomly assigned to the Crawli (n = 17), Mattress (n = 15) or Control (n = 17) group. Five infants were excluded immediately after inclusion. Forty-four subjects were included in the final analysis with 15 in the Crawli group, 14 in the Mattress group and 15 in the Control group (See full description in Figure 4). The participants were predominantly Caucasian 11/15, 10/14 and 11/15 respectively in the Crawli, Mattress and Control groups. The others were of North or Central African descent.

There was no difference in perinatal characteristics and parental education level between groups (See full description in Table 1). There was no difference in either the ATNAT scores [F(2,41) = 0.21, p = 0.81, partial η²= 0.01; Figure 5] or the Motor Optimality scores [F(2,33) = 1.71, p = 0.20, partial η²= 0.93; Table 2] between groups at inclusion, confirming the homogeneity of the groups at inclusion.

Adherence to the training was high, likely because the osteopaths were the ones who conducted the training in the infants' homes each day and this motivated the parents' compliance with the study. Of the 56 sessions, the Crawli group and Mattress groups completed an average of 52 (SD = 2) and 51 (SD = 2) sessions, respectively. The difference was not significant. The maximum number of sessions missed by a single infant was nine.

All infants trained in the Crawli group were able to move forward on the Crawliskate with a mean traveled distance per session of 235.4 cm (SD = 58.5) and a range of the mean from 106.8 to 393.3 cm (see Figure 6A). As expected, in contrast to the Crawli group, infants positioned prone on the mattress were only able to move between 0.2 and 15.3 cm (mean = 6.9 cm, SD = 7.6 cm) (U = 210, p = 0.000005, rrb = 1); Figure 6A). We observed a high variability in the mean distances traveled by the infants on the Crawliskate during each of the eight weeks (Figure 6B).

The osteopaths reported no harms to the trained infants and the ATNAT follow-up assessment at each age up to 12 months CA showed no deleterious effects of training in the Crawli and Mattress groups: similar ATNAT scores were observed between Crawli, Mattress and Control groups except for an unexpected lower score for the Mattress group than the Crawli group at 2 months CA (F(2,40) = 3.62, p = 0.04, partial η² = 0.15 [CI 95%(0.08–0.55)]; HSD Tukey Crawli > Mattress, p = 0.03; Mattress = control, p = 0.28 and Crawli = control, p = 0.51) but not thereafter (ATNAT 6: F(2,40) = .13, p = 0.88, partial η² = 0.01; ATNAT 9: F(2,39) = 1.00, p = 0.37, partial η² = 0.05; ATNAT 12: F(2,38) = 0.98, p = 0.39, partial η² = 0.05; Figure 5). The temporary lower score for the Mattress group at 2 months CA was due to a greater spinal axis hyper extension on the corresponding specific ATNAT item on spinal axis (data not shown). The groups demonstrated similar MOS at 2 months CA [F(2,38) = 1.32, p = 0.28, partial η² = 0.06; Table 3].

All Gross Motor (GM) scaled scores are summarized in Figure 7 and Table 4.

The 3 groups were similar in their Bayley Fine Motor scaled scores (results are summarized in Table 5).

ASQ-3 results are summarized in Tables 6, 7.

First, we observed excellent adherence to the training, likely because the osteopaths we recruited conducted the training in the infants' homes each day. The results show the feasibility of such early home-based crawling training performed 5 min/day for 8 consecutive weeks following discharge from the NICU. Specifically, all infants in the Crawli groups were able to propel themselves using quadrupedal movements with the help of the Crawliskate without deleterious effects, as shown by similar scores on the ATNAT at 2, 6, 9 and 12 months CA and GMA at 2 months CA. As predicted, infants positioned directly on the mattress without the Crawliskate were unable to move very far and unexpectedly developed a temporary head and trunk hyper extension at 2 months CA.

It is remarkable that even at this very early age, premature infants could travel long distances with the help of the Crawliskate, up to a maximum of 7 m in only 5 min for some of the infants. However, we observed a large variability within and between infants in traveled distance. We can likely attribute the variability in distance traveled to variation in the infant's state of alertness at each of the training sessions. Sometimes infants appeared too tired or sleepy to move much during the session. We also saw variability within sessions, with infants often starting to crawl, then stopping, then resuming again at the end of the session. Consequently, we found it difficult to construct a learning profile for crawling during the 8 consecutive weeks of training for each infant. We are presently working to code the number of leg and arm steps during each session to determine the distance traveled per step, which should provide a better representation of the progression of crawling skill during the training.

One of the remarkable effects we saw from stimulating the very premature infants crawling at an early age was the impact it had on the development of mature crawling at 9 months CA, with infants in the Crawli group showing significantly more mature crawling than infants in the Mattress and Control groups. This result is not simply the consequence of prone positioning because the Mattress group infants demonstrated a similar level of crawling competence as infants in the Control group. The effect of early crawling training on mature crawling confirms, with a new paradigm, the existence of a link between neonatal and mature locomotion. Researchers have reported previously a link between stepping and walking by showing that daily stepping on a solid surface from birth enhances the emergence and quality of mature walking in typically-developing infants (62, 63) and training stepping on a treadmill leads to an earlier emergence and higher quality of walking in infants with Down syndrome (64, 65, 67, 68, 82). Our results on crawling training also confirm, but with a much earlier intervention, results reported 40 years ago showing that daily training of supported crawling in typically developing infants from four months of age onward leads to an earlier emergence of independent crawling and walking (19). However, we did not find an effect of our intervention on walking, as scored by Bayley items at 12 months CA, only a strong trend for the Crawli infants to have higher scores relative to the Control group on the items related to walking at 9 months CA (p = 0.07). This result for walking could result from the fact that, in contrast to typically-developing infants, very premature infants may need more than 5 min per day and/or more than 8 weeks of crawling training to accelerate the development of their walking proficiency. Alternately, the Bayley items related to walking at 12 months CA may lack the sensitivity to detect subtle differences in the quality of walking between groups of very preterm infants without brain damage. As we also collected the spatio-temporal parameters of these infants walking patterns on a GAITRite pressure sensitive walkway at 24 months CA, we may still find an effect of our intervention on walking.

The crawling intervention had positive effects on infants' gross motor development when the Bayley scores were aggregated over the four age periods. When looking by age, the Crawli group had higher scores than the Control group at 6, 9 and 12 months CA, that is 10 months after the end of the crawling intervention. The Crawli group had higher scores than the Mattress group at 2 and 6 months CA. The similar gross motor scores between the Crawli and Mattress groups at 9 and 12 months CA may be the result of the limited duration of the training, although it is important to note (see the previous section) that the Crawli group had significantly higher scores on the specific Bayley items related to mature crawling at 9 months CA relative to the Mattress group. If the training lasted longer than two months and/or the duration of each training session was increased, the group differences at 9 and 12 months may have been larger and more likely to be significant, particularly with a larger sample. For example, in Katona's method the frequency and duration of training is much higher and the intervention continues for a much longer period of time (30, 31, 35, 36). However, we think a more likely explanation for the attenuated differences between Crawli and Mattress groups at older ages is the additional time parents in the Mattress group reported continuing to place their infants in the prone position for more than two hours per day after the training (58% and 60% for the Mattress group between 2 and 6 months CA and 6 to 9 months CA, respectively and 21% and 38% for the Crawli group during the same age periods). Tummy time, practiced when the infant is older, is known to have beneficial effects on infant motor development (75). Although the Bayley Gross Motor and ASQ-3 scores of the Crawli and Mattress groups did not differ significantly between 9 and 12 months CA, it is pertinent to note that the Crawli group's scores were always higher than the Mattress group's scores. Moreover, the Mattress group's scores were never significantly higher than the Control group's scores at any age tested.

The generalized and enduring effects of the crawling training on the infants' motor development, particularly relative to the Control group, are quite surprising in the context of prior work which generally shows that motor practice effects are task specific (83). The generalized effects on gross motor development may reflect the whole-body nature of crawling, which not only stimulates the central and peripheral nervous system but also engages a large percentage of the infant's musculature.

The broader effects of crawling training on the total ASQ-3 score, and specifically on the scores representing the communication domain, is likely a result of the additional experiences associated with moving through space and their perceptual consequences. These experiences occurred during the eight weeks of training and as a function of the faster acquisition of crawling in the Crawli group. As noted in the introduction, considerable research has linked locomotor development to developmental changes in perception-action coupling, spatial cognition, memory, and social and emotional functioning during the first year of life (6, 17, 84) and to language development at 12 months of age (8, 28). Researchers have speculated that independent locomotion allows infants to have new experiences which in turn recruit the processes that drive developmental change in domains far removed from the locomotor domain (6, 17). Taken together, these findings highlight that an intervention focused on training a specific motor skill, crawling, can have broad effects on the motor domain of functioning as well as on other domains of functioning seemingly unconnected to the motor domain.

The fact that Crawli but not Mattress training facilitates the development of mature crawling permits different hypotheses. Although when infants lie prone on a mattress they cannot propel themselves very far, they make numerous leg movements (not arm movements), which are inefficient but present. This observation suggests that one of the central components of the “Crawli effect” is propulsion and/or the ability to perform coordinated quadrupedal leg and arm movements. It is impossible to list here the many possible mechanisms behind the effects of Crawli training but some would be especially interesting to investigate in the future. These include the effect of training on the morphology and composition of the leg and arm muscles and on the quadrupedal inter-limb coordination and the intra-limb coordination. It would also be interesting to investigate the evolution of muscle synergies using electromyography in relation to training. Another mechanism likely playing an important role in the effect of Crawli training is the stimulation of perception-action loops between the propulsion actively generated by the infant and the resulting proprioceptive, visual and auditory consequences associated with movement through the environment (36). We intentionally restricted motivational and visual/auditory stimuli in this study during training to minimize potential bias. However, it would be particularly interesting to add visual, auditory, and/or olfactory stimuli during clinical trainings given prior research showing that these stimuli can facilitate newborn stepping and crawling (55–59), especially if these stimuli were combined with parental encouragements.

The main limitation of this intervention is the restricted number of participants, which was a function of the number of osteopaths needed to supervise the large number of home training sessions. However, this design feature was necessary to ensure the training protocol was administered faithfully and to monitor for any possible adverse effects of the training. The small sample size meant that it was not possible to do subgroup analyses within the Crawli group to see, for example, whether any individual difference characteristics explained why some infants derived more benefit from the intervention than others or to see if infants who traveled greater distances during training also had better developmental outcomes. Another limitation of the intervention was the decision to exclude infants with major brain lesions. However, the success of the current intervention suggests the Crawli intervention should now be tested on infants with brain damage. A final design limitation was the daily training time limited to 5 min and the decision to end training after 8 weeks. A longer duration of training may have further increased the efficacy of the intervention.