From 5 to 10 years old, the coordination gait variability decreased. The coordination variability, as CV, was calculated for each
coordination mode duration. In our results, coordination variability decreased from the youngest group to the oldest group. Ankle joint angle
variability decreased from 4-6 to 7-9 years old, and from 7-9 to 15 years old
27
; while 4-years old typically developing children had higher
gait stride time variability than 7-years old children, 6-7-years old children were more variable than 11-years old, and 11-14-years old
children and young adults had similar gait stride time variability
28
. The coordination mode duration indicates how long a coordination mode
occurs during gait. Lower variability in the coordination modes suggests there are less changes in the types of coordination mode along
children's development.
Double and single support phases have different coordination variability. Typically developing children showed higher
coordination variability at the early stance than for the late stance. Comfortable and fast walking speeds induce different propulsion
strategies in younger (6-9 years old) and older (9-13 years old) children
29
. Only fast walking induced changes in propulsion strategy in
older children. Our results must be carefully evaluated. Comparing stance subphases, the coordination variability reduced during double
support phases compared with the mid-stance; and it was the lowest for the late stance, during propulsion and toe-off. We could expect
coordination variability during the double support phases to be similar because right and left lower limbs have similar motion patterns in
typically developing children, suggesting a coordination symmetry. Children showed increased symmetry for loading foot parameters during
gait as they become older
30
, and for spatial-temporal gait measures (stance time, single and double support)
31
; although, gait may not be
mature by age 13. Typically developing children have similar lateral stability for both dominant and non-dominant limbs, while children with
cerebral palsy, only for the non-dominant limb, were more stable in body weight support compared with typically developing children
32
. As
maturity develops, older children walk faster, increasing the single support stance phase and step length4. Less coordination variability
during push-off suggests an optimal coordination mode pattern is achieved earlier than for foot strike in childhood.
Coordination modes can be grouped into parallel and serial coordination. The serial coordination is a less complex coordination
because just one joint is moving, while for parallel coordination, two joints are moving at the same time. Two joints moving at the same time
is an expected movement pattern in gait
19,20
. For all groups, the foot strike and midstance had a serial coordination predominance. For
older children, a predominant serial coordination strategy shifts to a mixed coordination at the push off. Therefore, an optimal coordination
quest during push off is moving towards the parallel coordination. In serial coordination (proximal or distal coordination modes), just one
joint is moving while the other is still; on the other hand, in parallel coordination (in phase and antiphase coordination modes), both joints
are moving at the same time. Thus, one-joint motion is less complex than two-joint motion, suggesting parallel coordination might be more
complex than serial coordination. The shift from serial coordination to parallel coordination over the years can be explained by motor control
maturation, allowing more joints to be controlled at the same time. To overcome an incoordination condition, people with cerebellar ataxia
18
decreases the swing phase and increases the double support phase because during the single support the balance control is unstable
33
.
One limitation of this study is not measuring any other motor development milestone to describe the participants. This is an
observational study, with a small sample based on a snowball sampling strategy. In our study, variability measures reflect inter-joint
coordination changes, not the standard joint motion patterns in time. Another limitation is sample size; and future research should develop
larger-scale cross-sectional and longitudinal studies based on these specifications to confirm our proposed coordination milestones.
CONCLUSION
Mastering balance control is crucial to develop the bipedal locomotion in childhood
4,5,14
. During the single support, balance control
manages the combination of a static equilibrium condition (by the support leg) and dynamical equilibrium condition (by the swing leg); while
during the double support, the balance control has a closed kinetic chain to stabilize the whole body. These completely different balance
situations that children need to master to walk independently over any surface and condition. Since development arrow might overcome
the incoordination states to coordinate states, we would expect that the coordination variability should be different between single and
double support phases across groups. This was true for our results, but the coordination variability decreased during the single support
phase, and not for the single support as we were expecting, rejecting our hypothesis.
Changes in inter-joint coordination were mostly related to the mid-stance phase. Its variability decreased as children were older.
Results showed a shift from serial coordination to parallel during the mid-stance phase for older children. Our results suggest gait changes
could be oriented towards a less coordination mode variability with development. Based on our findings, the clinical gait analysis could hold
attention to the coordination modes in different populations to describe how a health condition changes motor coordination. Studies based
on larger samples could describe how the coordination variability changes during motor development to evaluate whether our suggestion
of coordination milestone is correct. Such findings will significantly improve how educators, healthcare professionals, and parents could
track motor development, offering insights into effective strategies for enhancing children’s motor skills. Furthermore, further investigations
could explore the factors shaping the gait developmental trajectory, including the influence of physical activity, environmental conditions,
and neurological factors. Such inquiries can refine our understanding and inform targeted interventions for promoting optimal motor
development in children.