The main finding here is that isometric strength measures of different muscle groups (i.e., knee extensors and handgrip) significantly differentiated fallers from non-fallers during a 13-month follow-up regardless of potential confounding factors such as age, gender, body mass index, and previous history of falls. By contrast, although fallers did also present a worse performance than non-fallers in all TUG test modalities (i.e., standard, count and cup), these differences disappeared when adjusting for confounding factors, which supports isometric strength as a sensitive screening measure.
The TUG test is one of the widely used test for the identification of individuals at risk of falling [10] and its validity has been studied by some. Shumway-Cook et al. [11] observed that the TUG test had a sensitivity and specificity of 87% to detect individuals prone to falls. However, in line with our findings, more recent evidence has called into question the validity of this tool [13–15]. Some authors suggested that the inclusion of dual/cognitive TUG tasks (e.g., counting back) might increase its validity for this purpose [26]. Accordingly, the inclusion of additional TUG tasks increased its accuracy in identifying pre-frail individuals in some studies [27]. However, other authors found similar sensitivity and specificity when comparing the three (standard, cognitive, motor) TUG modalities [11]. Although the cognitive status of fallers was significantly lower than non-fallers at baseline, none of the TUG modality performances here (including those with cognitive tasks) could discern fallers from non-fallers in adjusted analyses. Thus, the evidence supporting the validity of the TUG test with or without dual-tasks for the identification of individuals at risk of falls is at least inconclusive.
On the other hand, the present study supports the validity of isometric strength measures as a screening tool to detect individuals at risk of falling. Previous studies have shown that fallers present with an impaired function of lower limb muscles - as measured by different tests such as the chair stand test, jumping, or leg press strength - compared to non-fallers [21, 28]. Isometric strength measures such as those implemented here have also been previously related to falls risk in older adults. Pijnappels et al. [21] observed that individuals who fell after gait perturbations presented the lowest isometric knee extension strength. In addition, Menant et al. [20] observed that low isometric knee extension strength was related to different health-related outcomes in the elderly (including balance, functional mobility and falls). Isometric knee extension strength has also appeared as a better prognostic factor than other measures such as muscle mass alone or a combined score (includes both muscle mass and strength/functional performance) as proposed by the European Working Group on Sarcopenia in Older People (EWGSOP) [20]. Thus assessment of isometric lower-limb strength might be a simple and valid screening tool [20].
Interestingly, our results also show that - apart from knee extension strength - handgrip strength differentiates between fallers and non-fallers. The assessment of handgrip strength is arguably the most widely used [29] as well as the simplest option for the evaluation of isometric strength in older adults. This test is recommended by the EWGSOP for the assessment of sarcopenia [9], and has proven to be a valid prognostic factor of different health-related outcomes including low intrinsic capacity, hospitalization risk, or overall mortality [30–32]. Evidence from meta analyses support the validity of handgrip strength for the prediction of age-related declines in cognition, mobility, functional status and mortality in community-dwelling older people [33]. Previous studies have also reported differences between fallers and non-fallers in terms of handgrip strength [21–23]. Combined, these results do support the use of handgrip strength assessments as a simple screening tool in clinical practice, although its combination with other measures (e.g., functional mobility tests) could enhance predictive accuracy [34].
We also observed that isometric strength measures across different muscle groups were not only related between themselves, but were overall related to performance on the TUG test at baseline and during the follow-up. Bohannon [35] already suggested that handgrip and lower limb isometric strength measures could represent a common construct, as reflected by the strong correlation found between the two markers. Other authors have also reported an association between lower-limb strength—including isometric knee extension—and handgrip strength [21]. Moreover, Alonso et al. [36] recently reported that handgrip strength was not only related to knee extension strength, but also to TUG performance and dynamic balance in older adults. Similarly, handgrip strength has been reported to be correlated with ambulatory capacity in frail older adults [37]. Thus, these findings suggest an association between isometric strength and functional mobility, which would further support the clinical relevance of assessing and enhancing muscle strength in the elderly [17].
Limitations of this study include the small sample size and the short follow-up time. Moreover, not all participants could perform the scheduled tests at all time points due to various reasons (e.g., death, health issues). Hence, linear mixed modelling was used to account for missing data [38]. The present findings might not necessarily be applicable to other populations with different characteristics, such as older adults with comorbidities or those living in nursing homes. However, the major strength of this study is its prospective nature and the fact of having assessed different modalities of the TUG test and different measures of isometric strength continuously throughout the follow-up period.