Statement of principal findings
The objective of this study was to review the current state of knowledge on the the presence of functional Kinematic parameters that may be impaired in mild cognitive impairment during the performance of functional tasks. Moreover, this systematic review hoped to investigate if these functional objective parameters could be improved by different physical interventions. To our knowledge, this is the first systematic review that provides a comprehensive overview of longitudinal studies (RCTs and cohort studies) using objective instrumented kinematic assessment of functional task as outcome measures or as parameters which could be impaired in mild cognitive impairment patients or may predict an incident MCI. Furthermore, most of the studies included in this review were published after 2015, which indicates the novelty of the topic [57-62, 64, 67, 69-71]. Therefore, we are still in an exploratory stage regarding the use of kinematic parameters in the detection of functional impairment related to mild cognitive impairment or as outcome measures in RCTs. The present review included fifteen studies. On the one hand, Cohort studies showed that the coefficient of variation in the median walking speed, day-to-day pattern, gait speed, gait variability as well as gait symmetry, specially in Dual Task conditions, were parameters which could be impaired in MCI Patients [62, 63, 66-68]. Furthermore, these prospective longitudinal studies suggested that gait variability, the coefficient of variation of the walking speed and trajectories of weekly walking speed may be significant predictors of MCI, whereas gait speed was not associated with incident MCI risk [69-71]. On the other hand, RCTs suggested gait speed, stride length, stride time, balance and the time to perform the TUG may be improved after some physical interventios [56-58, 61]. Nevertheless, the quality and the strength of evidence per outcome was low and the risk of bias was substantial to draw firm conclusions.
Comparision with other studies
The present systematic review establish that gait speed, especially on the dual task conditions [62, 67, 68], was significantly reduced in subjects with mild cognitive impairment. These results were in line with some studies that have identified reduced gait speed as a predictor of preclinical stages of dementia or mild cognitive impairmetnt [23-25, 29, 30, 33-36]. Within the kinematic analysis of the walking speed, the coefficient of variation in the median walking speed, gait speed, gait variability as well as gait symmetry, specially in Dual Task conditions, were parameters which may be impaired in MCI Patients [62, 63, 66-68] according to included studies of the present systematic review. Moreover, the coefficient of variation of the walking speed, trajectories of weekly walking speed and gait variability might be a significant predictor of MCI [69-71]. This statement is in accordance with a previous 5 years of follow-up prospective study which determined that gait pace and variability may predict a future risk of cognitive decline and dementia in initially non-demented older adults [72]. Other studies also showed that stride time variability in dual task may be a sensitive indicator of cognitive change [26, 73]. Furthermore, Bahureksa et al. [40], in a systematic review and meta-analysis, also revealed that gait parameters such as velocity (p<0.01), stride length (p<0.01), stride time (p=0.02) and coefficient of variation (p<0.01), could discriminate best between MCI and healthy controls under single task conditions. Another systematic review [74] also demonstrated that physical activity in home or everyday life activities monitored by sensor technologies in home, were parameters indicative of MCI. Finally, it is important to underline our studies included different walking distances and different kinematic instruments. In the literature, it has been demonstrated that participant walking strategy changes with walking distance, resulting in a significant effect on gait variability [75], so walking distance could be highly relevant in order to measure gait variability as a marker for MCI. Randomized Controlled Trials included in this systematic review showed that gait speed, stride length, stride time, balance, specially the center of mass sway in anterior-posterior and medial-lateral directions, and the time to perform the TUG may be improved after some physical interventios [56-58, 61]. Nevertheless, there is still room for improvement in current interventions. For instance, there is limited evidence on intervention effects on stride time variability [76] although this parameter seems to be a important predictor of MCI [58, 61, 62]. It has been demonstrated in other RCTs that the combination of aerobic exercise, balance training and cognitive training could help reduce the risk of further cognitive impaiment and may improve cognition, mobility, balance and Quality of life [41, 43]. Furthermore, some Systematic Reviews and Meta-analysis formed by Randomized Controlled Trial showed that exercise, specifically aerobic and resistance (strength) exercises, join cognitive training could imporve cognitive function, activities in daily living and modo [77-80]. However, It has not been identified others Systematic Reviews that assess changes in functional objective parameters after physical interventions and It has not been found either RCTs which use Kinematic parameters as outcome measures, so It would be necessary more clinical trials using these parameters as outcome measures.
Strengths and weaknesses of the study
The strengths of this systematic review included the use of a pre-specified protocol registered on PROSPERO, the PRISMA checklist, the NOS and The Cochrane Collaboration's tool to determine the risk of bias of each study and the GRADE system to evaluate the overall quality and the strength of the evidence per outcome. Furthermore, another strength of this review is that we performed a systematic review using studies only which provided a validated diagnostic criteria of MCI. There are several limitations that should be mentioned. First, despite this review was designed to be comprehensive with a robust search strategy, using a long variety of MeSH terms, and searching in other sources (grey literature), it is possible that some studies were not identified. Second, the lack of uniformity among the study design (e.g. walking distance, variables measured, differents instruments used in kinematic analysis) should be taken into account when interpreting the results. Furthermore, studies did not report the reliability or validity data of the instruments used in kinematic analysis. Third, reported bias were found in several included studies, especially in clinical trials where the risk of bias in most domains was “unclear”. Moreover, the quality and the strength of evidence per outcome was low. This could also limit the findings of the present systematic review.
Implications for clinical practice
Our results showed that, overall, kinematic gait parameters could be impaired and may predict an incident MCI. Moreover, these functional objective parameters could be improved by physical interventions in MCI patients. This is an important step forward in developing a clinically validated approach for measuring MCI related functional deficits which could predict a future risk of MCI and could even help its early diagnosis, although further studies are required in order to validate the findings of this review. Findings of this systematic review could be useful for promoting specific interventions aiming reverse early functional changes associated with MCI, since RCTs included in this systematic reviews have demonstrated that physical interventions could improve gait speed, stride length, stride time, balance, specially the center of mass sway in AP and ML directions [56-58].
Implications for further research
Despite the promising results of the present study, some flaws observed in most of the included in this review should be resolved. Hence, there are some recommendations to guide future research: (i) studies should use the same instrument to perform the kinematic analysis which would allow a better comparison of data between studies; (ii) these instruments should be valid and reliable as established in the Cosmin taxonomy; (iii) it should be conducted RCTs and Cohort studies with high quality of evidence since studies included in this systematic review often showed an unclear risk of bias and a low quality of evidence; (iv) it would be necessary conducting more Clinical trials which use functional objective parameters as outcome measures of physical interventions in MCI