Our findings show that handgrip strength is longitudinally associated with depression, MCI, and QoL in Mexico's nationally representative sample of older adults. Overall, higher levels of HGS associated with lower odds of depression and MCI, and better QoL. We also found that a higher proportion of this association could be attributed to between-person differences rather than within-person.
Our results pointing to the association between HGS and depression are similar to those reported in several previous longitudinal studies. Kandola et al. (30) identified that a 5kg increase in HGS was associated with reduced odds of depression in OLDER ADULTS age 65 or more (OR = 0.727; 95% CI: 0.623, 0.850), assessing depression with the Patient Health Questionnaire-9 in a large sample from the UK Biobank. A recent systematic review and meta-analysis reported that muscle strength (MS) was inversely associated with depression/depressive symptoms (OR = 0.85 CI95% 0.80, 0.89) in older adults, with most of the studies assessing MS using HGS (12).
The findings of the longitudinal association between HGS and MCI have also been documented previously in Kim et al.’s (31) analysis with participants from the Korean Longitudinal Study of Aging. Those with lower HGS quintiles –as compared to the 5th quintile (higher HGS) as reference– had an augmented probability of experiencing cognitive decline and lower scores in the Korean Mini-Mental State Examination (OR per quintile for cognitive decline: Q1 = 4.99 Q2 = 2.57, Q3 = 1.93, Q4 = 1.41, p < 0.01). Similarly, a study with a sample of more than 14,000 participants > 50y found that a 5kg increase in HGS was associated with a reduced probability of cognitive impairment (OR = 0.97 CI95% 0.93, 0.99). The trends were in the same direction even after categorizing by weak (reference) and normal HGS (OR = 0.54 CI95% 0.43, 0.69) (14).
QoL has often been studied in relation to specific health events such as epilepsy (32) and cancer (33). However, evidence about the maintenance or increase of QoL in older adults has been growing over the last several years. The evidence for the association between HGS and QoL has been mixed. One study found that a 1kg increase in HGS was associated with a higher score of QoL, using the Control, Autonomy, Self-realization, and Pleasure-16 test (β = 0.24 CI95% 0.11, 0.37 in women, and β = 0.18 CI95% 0.06, 0.30 in men) (15). Another study also found that higher values of HGS were associated with an increased probability of better physical and mental component scores (above the median) using the Medical Outcomes Study Short Form-12 (SF-12) (OR = 1.05 CI95% 1.01, 1.10, and OR = 1.05 CI95% 1.01, 1.10, respectively) (16). On the contrary, Gopinath et al., 2007, used the extended version of the SF-12 did, not find significant associations (17). The evaluation of QoL is complex; in fact, several tools have been developed to capture some or all the domains of this concept (health status, psycho-social status, and other aspects of life) (34). Differences in the assessment of QoL and study design (cross-sectional or longitudinal) could partially explain the mixed results among these studies.
Muscle strength and HGS decline with age (35). Nonetheless, we know that this process is dynamic rather than static over time (36). Despite the above, most longitudinal studies have incorporated HGS as a time-constant exposure in their analysis. In this study, we considered HGS and WGS as time-varying exposures to further explore their relationship with depression, MCI, and QoL. Our results are consistent with two previous studies that analyzed HGS as a variable that changes over time. One study revealed that negative changes in HGS (a 1SD decrease) increase the odds of depressive symptoms after one year of follow-up in a sample of American older adults (OR = 1.13; CI95%: 1.01–1.27) (37). Another study, using a similar analytical approach to ours, found significant associations of HGS and several indicators of cognitive function (fluid reasoning, working memory, episodic memory, semantic memory, and crystallized ability). Specifically, the authors reported that higher levels of HGS attenuate the decline of fluid reasoning (7.5%) and semantic memory (19%) for within-person effects. For the rest of the indicators, the between-person effects were predominant (38).
Our results add to the available evidence in several ways. First, we have estimated the association of HGS with three of the most critical outcomes for older adults: depression, MCI, and QoL. Second, we have considered the dynamic nature of HGS in our estimates. Third, to the best of our knowledge, there are no studies that have looked at the association between HGS and QoL considering HGS as a time-varying exposure.
The results of the disaggregation analysis (between- and within-person) showed that between-person effects were statistically significant across all associations, for both HGS and WGS, except for in the case of MCI. Meanwhile, significant associations were observed for the within-person effects for MCI and partially with QoL, but not for depression. The predominance of between-person effects has already been reported in longitudinal studies with older adults (39). We hypothesize that this focus on between-person effects is likely because within-person effects may require more extended periods to observe significant changes in the outcomes analyzed. It has also previously been thought that between-person differences with respect to modifiable risk factors may have more substantial influence on outcomes like MCI and QoL than intra-individual differences. However, more longitudinal studies are required that disaggregate between- and within-person effects to confirm or dismiss these hypotheses.
The hand has been recognized as one of the most evolved structures of the human body (40). Consequently, HGS is a complex mechanism that requires strict control of the central nervous system for neuromuscular activation through motor unit organization and coordination to achieve the highly demanded task (41). The integrity of brain processes essential for optimal and efficient muscular coordination is reflected in HGS (42). Thus, changes in neurological processes may be responsible for the variations in grip strength and its associations with cognitive function.
The mechanisms driving the association between HGS, depression, and QoL are likely related to lifestyle factors, such as a decreased in physical function and activity (43, 44), and specific responses like oxidative stress and inflammation (45). For example, cytokines released in inflammation response trigger changes in the metabolism of neurotransmitters like serotonin, dopamine and glutamate that may lead to depression and other psychological disorders (46). Also, chronic inflammation appears to have a minor but constant effect on people's overall happiness as well as their contentment in intimate relationships, all of which are conditions associated with QoL in OLDER ADULTS (47). More evidence is needed to clarify the multiple processes linking HGS with these outcomes.
The results of our study should be interpreted within the context of its strengths and limitations. A major strength in this study was the large community-dwelling sample of OLDER ADULTS and the prospective design of the study. Also, all outcomes were measured through previously validated tests. There were also several potential limitations of our study. Although there was an acceptable response rate, included participants overall had healthier baseline characteristics than excluded OLDER ADULTS, leading to a likely sub-estimation of the observed associations. Even so, significant associations between HGS and the outcomes of interest were found. Additionally, even though we included a variety of potential confounders, there could have been residual confounding from variables such as drug use, inflammatory biomarkers, and food intake.