Our results enhance the evidence in the relationship between muscular function and cognition. We found that motor function exhibited the largest association with the orientation domain (Final model r2 = 0.289 ; GS r2 = 0.16 ; HS r2 = 0.175) followed by the language domain (Final model r2 = 0,272 GS r2 = 0.15 ; HS r2 = 0.164). Our findings complement preliminary reports that have showed associations between early motor function and dysexecutive symptoms and semantic memory impairments (32).
The underlying neural mechanisms of the motor-cognition relationship is set on the place and grid cells located in the entorhinal cortex, with an important role in spatial orientation (33). Thus, a correlation between small volume of the left entorhinal cortex and muscular dysfunction was reported in those without cognitive impairment during the dual-task gait analysis (34). Previously publication determine that executive functions are essential for gait control, since gait requires the integration of sensory and perceptual information, a continuous updating of the input, and quick adaptations of the gait pattern (35–37). Similarly, a longitudinal study (38), in dual-task GS 27.4% of the variance was attributed to executive attention, while orientation and attention are cognitive domains linked to each other(38).
In the same line, speech and language are proven to be among the most reliable markers to distinguish between the types of dementia in which motor dysfunction can be observed (39). Also, speech and language are cognitive domains strongly associated with the supplementary motor area (SMA), a very important structure in motor execution, thus, alterations in the SMA and its correlated circuits (subcortical circuits, basal ganglia) may be clinically seen as alterations in language and motor performance (40). Muscle strength, specifically HS can be an overall indication of the integrity of the central nervous system and the total force of the upper limb muscles. Stronger HS is related to better performance on functional tasks as it can be an indicator of the ability to walk, rise from a chair, hold small items, toothbrush or comb (16).
Our results showed that the coefficient of variability in the cognitive domains is similar when analizing GS or HS, suggesting that both variables have similar performance when evaluating motor function correlates the cognitive status of individuals. Even though GS has emerged as one of the motor domains strongly correlated with incident dementia, our results showed that GS and HS may be an alternative parameter that can be used in the clinical practice to assess individuals at risk to develop dementia. GS and HS has been reported as a useful single marker of frailty (41). Likewise, handgrip dynamometers are inexpensive, easily portable, non-invasive, reliable, do not require extensive training, and are not biased by learning effects that can be seen in neuropsychological tests (16).
In line with our findings, previous reports have shown that poor physical performance is associated with cognitive decline. Besides, GS and HS represent a core determinant of physical frailty and sarcopenia, which are both associated with cognitive impairment (42). More recently, there has been proposed that physical and cognitive decline can occur simultaneously and they can share common etiologies (43). Hormonal levels and inflammatory biomarkers are thought to be implicated in cognitive dysfunction; indeed, hormones like Irisin, are expressed not only in the muscle but also in the brain and it reduces neuroinflammation and post-ischemic oxidative stress, suggesting that this molecule may play an important role in neuroprotection and synaptic plasticity (44). Similarly, higher levels of pro-inflammatory cytokines, such as IL-6, were associated with greater cognitive decline as well as lower HS. Associations between impaired cognitive performance and poor physical performance in GS and balance suggest that abnormalities in the nervous system processing speed may be linked to changes in cognitive function (16).
In our analyses, the accuracy of GS and HS as methods to identify cognitive impairment was 65% and 63%, respectively. The cut-off point set for GS was 0,59 m/s, and for HS was 17.5 kg. This is one of the main contributions of our study based on the fact that we propose a non-cognitive way to classify older adults with cognitive impairment in a national representative sample in a middle-income country, filling knowledge gaps in motor biomarkers of cognitive decline research. Currently, there is no sufficient evidence using GS or HS to identify cognitive impairment, for this reason, many recent publications suggest that combining motor and cognitive measures improves the classification of older adults at risk of dementia (45, 46). Around the world, there is an imperious need to find low-cost, accurate, and accessible biomarkers to identify preclinical stages of dementia (47). Therefore, the results of the present study enhance the opportunity of diagnosis in countries without access to expensive biomarkers such as Positron Emision Tomography (PET) imagining. However, further research in this area is needed to extend the generalizability of our findings.
Our study has some limitations. First, the cross-sectional did not make us able to establish causality. This points to the importance of conducting longitudinal studies evaluating the predictive validity of HS and GS and standardizing the optimal cut-off for detecting individuals with impaired cognition. Second, we calculated the ROC curve taking as a comparator a score greater than 24 for the MMSE, which is not validated in our population. GS in the 3 meters test is not a routingly used and this results may underestimated speed(48). However, we do not consider that it biases our results as has been used in previous works(12, 18). Nevertheless, the SABE Colombia study has the largest sample of Latin American older adults, providing to our analyses a good statistical power.