Accordingly, the body segments act as pair oscillators, in which the symmetric relationships (in-phase and out-phase) are more easily maintained at higher speeds than other complex phase conditions. For example, feet are more likely to experience non-coupling and apply different functional strategies at lower speeds, while motion patterns are carried out with higher coupling and symmetry at higher speeds [
13]. This finding is consistent with Plotnik et al. [
35] finding, that measured GRF for gait asymmetry. Based on the results of this study (Figure 2), although no relationship was observed between three walking speeds and gate asymmetry, a significant and inverse correlation existed between walking speed and Phase Coordination Index (PCI) at low speeds, i.e. increasing PCI was inversely related to decreasing speed.
PCI is, in fact, a factor for examining a combination of precision and coordination of the walking phase, where the lower values represent a more accurate and stable walking phase, and the higher values indicate a disorder in the lateral coordination of the limbs [
20]. Also, these conditions may result from sending neural signals to higher centers of the nervous system and the need for more attention at lower walking speeds [
35]. Regression analysis showed only a significant linear relationship between PCI and walking at slow speed (P=0.002, R2=0.53). Curve fitting showed that the following relationship is established between PCI and walking at slow speed:
PCI=B+walking speed×A
, where A=-4.41±2.31 and B=8.8±1.20 (Dark thick line). No significant relationship was found for other speeds (R2<0.09, P>0.288) (derived from study [
35]).
The effect of age on gait symmetry
In general, aging can affect walking; for example, young people walk faster, with a longer pace, and higher stepping rates than the elderly [
57]. These changes in walking may result from degradation of neurological and physical performance due to aging [
37]. Since gait symmetry depends on the nervous and physical function of the individual [
1], it can be expected that it decreases by aging. However, studies have reported different results. Although several studies have shown that age is not effective on gait symmetry [
4,
6,
24,
48], recent studies have reported otherwise, which seems to differ in the age distribution of subjects [
20,
36,
37].
Contrary to study results of nonsignificant effect over a continuous age range [
4,
24,
48], some studies with significant effects [
36,
37] compared two different age groups. Meanwhile, Himann et al. showed that gait function remains unchanged until the age of 60, which then varies greatly [
58]. This finding justifies the report on the insignificant effect of aging on gait symmetry in subjects over a continuous age range. Because aging, especially at the age of 60 and above, can affect gait symmetry, this factor should be considered in studies along with clinical examinations. However, because it is unclear that changes in the gait symmetry occur exactly at what age, further studies in this field seem essential.
The effect of gender on gait symmetry
Like age, gender also has a significant effect on human gait parameters. Usually, women walk slower and with shorter pace than men [
57,
59]. Also, previous studies have reported a significant difference between the sexes in terms of kinematic gait characteristics, including the range of motion in hip and ankle joints [
60], as well as other parameters such as the mechanical energy of joints [
59]. Accordingly, due to the morphological differences between two sexes, one can expect differences in the gait symmetry characteristics.
However, according to the conducted studies, the slight effect of gender on gait symmetry has not been clearly disclosed yet. Given that the gait symmetry indices used in the studies are different, several studies have reported non-significant gender effects on gait symmetry [
4,
24,
48]. On the other hand, some papers have shown a significant sex-specific differences in terms of gait symmetry [
32,
37]. However, drawing a definite conclusion is impossible because of few studies in this regard. In the meantime, further studies aiming at more complex variables such as joint torque during walking can provide a more comprehensive insight into the characteristics of gait symmetry and the difference between the two sexes.
4. Discussion
Based on the results of various studies, walking is an asymmetric behavior regarding the different performance of the lower limbs. Having a disease and anomaly affecting the function of the lower limbs increases this asymmetry. In this regard, achieving full symmetry after a rehabilitation period cannot be hypothetical, although its reduction is desirable. It is unclear, however, that the degree of asymmetry in the function of the lower limbs in healthy people is as high as to be considered a criterion that necessitates further research.
Also, considering the probable effect of walking speed and age on gait symmetry, it is necessary to control these factors in future studies. Finally, such information can be useful for improving the performance of athletes, as well as for gait assessment, clinical prescriptions for patients with abnormalities, and design of orthosis and prosthesis.
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Conflict of Interest
The authors declared no conflicts of interest.
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