*4.2. GRF*

Ground reaction force (GRF), which can measure braking and propulsive forces during gait, is a summation of forces produced by all body segments [48]. Increases in magnitude and variability of the peaks of GRF during the weight acceptance and push-off phases are to be found in people with unstable locomotion [48]. Our result found that different heel-toe drops have no significant effect on the first and second peaks of vertical GRF. Therefore, it is reasonable to speculate that wearing NHS, NS, and PHS has little effect on the walking stability of pregnant women in the third trimester.

The results showed no significant change in the ML-GRF during the stance phase, except in the early stance phase, the ML-GRF of NHS was significantly smaller than PHS and NS. Previous studies have shown that in the early stages of the stance phase (0–6% stance phase), the maximum ground reaction of the supporting foot is directed laterally and increases significantly with increasing walking speed [49]. This is similar to the results of our research. Our research results show that with the increase in heel-toe drop, the velocity also increases, which may be the reason for the difference in ML-GRF. Less energy is expended when the body is stable on the inside. Therefore, NHS has a smaller ML-GRF, which may be evidence of reduced energy consumption in pregnant women wearing NHS.

The AP-GRF included braking and propulsion peaks [50]. Our study found that the AP-GRF of the NHS propulsion peak was significantly smaller than NS and PHS. At present, there is controversy about the change in AP-GRF during pregnancy. Some researchers believe that there is no significant difference in AP-GRF during pregnancy, and other studies have shown that the AP-GRF decreases during pregnancy [51]. This may be due to edema of the pregnant foot during pregnancy, which interferes with flexion by increasing the width of the foot, resulting in reduced thrust. Our study found that it may be due to the thickness of the front palm of the NHS, which leads to disturbance of the flexion of the metatarsophalangeal joints, which may be the reason for the small AP-GRF during the propulsion phase.

#### *4.3. COP Trajectory*

COP is used to describe the complex dynamic functions of the foot and foot-ground interface during gait [52]. The COP is not only used as a dynamic stability index and measured risk or consequence of various lower limb musculoskeletal disorders [52–55]. The lack of lateral stability is known to be a risk factor for falls [52,55]. The results showed no significant difference in the range of ML-COP in NHS, NS, and PHS, which is consistent with the previous study [56]. No significant differences in the range and velocity of ML-COP were found in the flat shoes, medium heel lift shoes (16 mm), heel lift shoes (25 mm), and heel lift shoes (34 mm) [56]. NHS and PHS may not pose a more significant biomechanical challenge to the medial–lateral control.

The AP-COP displacement measures the fluency of the stance phase during regular gait, with higher AP-COP displacement and gait line length indicating a more physiological gait pattern [57,58]. The results showed that the AP-COP of NHS is significantly smaller than NS and PHS. Previous studies have shown that AP-COP moves forward and decreases during the stance phase in pregnant women [13,14]. Reduced COP displacement in the AP

direction could be linked to the waddling type of gait adopted by pregnant women [13]. Raymaks et al. found that the AP-COP increases with the increase in heel height, the AP-COP of NS is significantly smaller than that of PHS, and leg muscle activation increases when walking in high heels [59]. The results of our study may indicate that women in the third trimester of pregnancy have the lowest degree of muscle activation when walking with NHS.
