All the participants were examined using the K-MoCA and classified into a normal cognition group or a lower cognition group, according to the cut-off point. Their walking on level ground was captured using a three-dimensional motion capture system; then, the gait parameters of spatiotemporal movement, kinematics, and kinetics were analyzed.
4.1. Biomechanical Gait Pattern of Elderly People with Lower Cognitive Function
Although statistical significance between the two groups did not appear in the spatiotemporal parameters, the LC group showed lower gait performance compared to the NC group (
Table 2). However, subdivisions of the gait cycle showed significant differences between the LC group and the NC group (
Table 3). Compared with the NC group, the LC group had a shorter SLS and longer DLS, which was attributed to the change in the ratio of the LR and the MSt. The LC group showed a longer LR and shorter MSt than those of the NC group. Considering that only cognitive function showed a statistically significant difference between the two groups (
Table 1), this means that the neuromotor control of the gait is different according to the status of cognitive function.
In the LR, the LC group showed lower knee extensor moment and power than the NC group, whereas the hip joint showed higher extensor power (
Figure 1). During the LR, tasks such as weight shifting and shock absorption are performed and, if these tasks are unsuccessful, a fall may be induced. The work of the concentric contraction of the knee extensor for shock absorption [
23] was small in the early LR (i.e., mKPP-0 in
Table 7). To compensate for this, the positive work of the hip extensor was in a burst (i.e., pHP-1 in
Figure 1g).
In the MS, the hip extensor work of the LC group was rapidly decreasing, unlike the maintained hip positive power of the NC group (
Figure 1g). In particular, the LC group presented lower knee extensor moment and power than the NC group (
Figure 1e,h). Although there is no statistical significance, it can be seen that the mKEM and mKPP-1 of the LC group are lower than in the NC group, as shown in
Table 6 and
Table 7. During the MSt, the upright alignment of the segments of single-leg movement that supports the body must be achieved and maintained. Therefore, the low knee extensor moment and power of the LC groups means that the activation of the knee extensors of the LC group is lower than in the NC group. In other words, this means that the quadriceps muscle function was lower in the LC group than in the NC group. Hortobágyi et al. [
24] reported that the function of the quadriceps muscles decreases with aging. This low activation of the knee extensor can merely be regarded as a result of aging, but there was no statistical difference in age between the two groups. In other words, elderly people with cognitive decline show lower knee extensor function than elderly people without cognitive decline.
In the second half of the stance phase (i.e., the TSt~PSw), the LC group presented more dorsiflexion and lower hip flexor power. pAA-2 and AAD-1 in
Table 4 and
Table 5 show that the ankle joint is flexing more in the LC group than in the NC group from the TSt to PSw. This dorsiflexion leads to a faster initial contact of the contralateral limb and might contribute to the reduction of step length, as shown in
Table 2. In addition, pHP-3 and mHPP-2 were lower in the LC group. Given that mHFM was similar between both groups, it can be presumed that the angular velocity of the femur, which is flexing when entering the swing phase, is lower in the LC group and thus contributes to a decrease in stride length. This can be attributed to the reduced positive power of plantarflexion in the PSw (
Figure 1i).
To summarize, significant differences between both groups were found in the subdivisions of the gait cycle. Furthermore, biomechanical differences were also found. The LC group has a longer DLS and LR and shorter SLS and MSt than the NC group. In the LR, MSt, and push-off, the extensor activity of the three lower-extremity joints was different between the two groups.
4.2. Gait Adaptation According to Cognitive Function Decline
The LC group had a slower walking speed, longer DLS and shorter SLS, compared to the NC group. As mentioned in
Section 4.1, this difference results from the change in the ratio of the LR and the MSt and the difference in the motor control pattern. This gait, as exhibited by the LC group, may be an adaptation to further ensure gait stability.
In DLS, including LR and PSw, the movement speed of the center of mass (COM) is the fastest. At this time, if the push-off function increases while pAA-2 decreases, the COM movement will be further accelerated. If the movement speed of COM is further accelerated, lower limb collapse may be induced during the first half of the stance phase, as inferred from the low support moment of the LC group. However, in the LC group, pAA-2 increased and mAEM decreased during push-off. That is, as dorsiflexion increases, the upward and forward acceleration of COM decreases, and as the plantar flexor moment decreases, forward propulsion through the push-off decreases. There was also no difference between the two groups in pAA-3 and AAD-2. This means that no extension occurred to counter the more flexed ankle joint. If extension occurs, as mentioned earlier, the COM accelerates further forward and upward, resulting in a higher movement speed, which can lead to instability. This may be supported by pAP-4, which shows that the energy generation for forward propulsion through extension is low.
The shorter SLS of the LC group is due to the reduced MSt proportion. In the MSt, while one leg supports the body, the body moves forward and an upright stand alignment of segments of the supporting leg is achieved. At this moment, the supporting leg must support the body while maintaining postural balance. The shorter MSt of the LC group would be a strategy to reduce this phase to induce double limb support quickly, thereby securing gait stability.
This gait adaptation of the LC group is consistent with the gait changes that occur with aging. Several studies [
21,
25,
26] related to gait in the elderly have reported that walking speed slows, DLS lengthens, and SLS shortens with age. However, there was no statistical difference in age among the subjects who participated in this study. Thus, the differences shown by the LC group were due to the level of cognitive function, not age. In other words, this suggests that the lower the cognitive function, the more trying it is to secure the stability of gait.
Perhaps this gait adaptation is due to lower postural balance or lower quadriceps/biceps femoris function. First, there are many studies reporting a positive association between decreased cognitive function and decreased vestibular function [
27]. The vestibular sensory function, as is well known, is the main sensory system that regulates postural balance. An adaptation of the LC group may be the result of a low vestibular sensory function. Therefore, in future studies, it will be necessary to investigate vestibular sensory function in the subjects who participated in this study and to reveal its relationship with gait adaptation.
Second, as shown in
Figure 1, pKM-1 and mKEM were lower in the LC group than in the NC group. In addition, the knee extensor power is low during the LR to MSt. Furthermore, the support moment was significantly lower in the LC group than in the NC group, indicating that this difference is due to the knee extensor. That is, it can be inferred that the ability of the knee extensor to support the body while maintaining balance with the single lower extremities was lower in the LC group. These results can suggest clinical evidence that knee joint muscles should be trained/supported when performing walking exercises, to protect against progression to dementia, and to assist gait in the daily life of elderly people who suffer from cognitive decline. To further confirm this, further studies analyzing EMG are needed.
The present study has some limitations as follow: (1) The number of subjects who participated in this study is small, compared with the previous study and, in particular, the number of samples of the LC group were smaller than those in the NC group; hence, it is necessary to recruit more elderly participants and then investigate the differences between both groups.
(2) Only the parameters in the sagittal plane were investigated in the present study. Thus, it is necessary to examine the variables of the non-sagittal plane (i.e., hip rotation, hip abduction), because those parameters can provide stability to the trunk and postural perturbation during the gait.
(3) In addition, in order to investigate the causes of gait adaptation in the elderly regarding cognitive decline in the central nervous system domain, it will be necessary to investigate various sensory and cognitive functions.