Increasing age is accompanied by a gradual decline in muscle strength (1) which may be explained by reduced muscle mass (e.g., loss of muscle fibers and reduced size of muscle fibers, especially type II fibers), inactivity, and neural factors (e.g., loss of motor neurons) (2, 3). This age-related loss of muscle strength may impair older adults' physical function (e.g., ability to walk, rise from a chair) (3, 4). The reduction of muscle strength and physical function are important components in both sarcopenia and frailty (5, 6) and increases the risk of dependency, institutionalization, and mortality among older adults (5–8). Thus, assessing these aspects is important to develop effective preventive- and treatment strategies especially among the oldest old (>80 years).

Previous cross-sectional studies show that higher muscle strength in the lower body is related to better physical function, such as walking and rising from a chair (9–20). Most previous studies examining the association between muscle strength and physical function have focused on healthy older adults in their 60-s and 70-s (9, 12, 14–18, 20, 21). However, as life expectancy increases worldwide, many live into their 80-s and 90-s and the age-related physiological changes affecting muscle strength and physical function become more prominent after the age of 80 years (1). Consequently, a higher proportion of the aging population will depend on mobility devices (e.g., rollator, walker, canes) and home care services (22, 23). Despite this, only a handful of studies have examined the oldest old (>80 years) (10, 11, 13, 19) and these studies are limited to healthy older adults (10, 19) and/or institutionalized participants able to walk independently (11, 13). Use of mobility devices could influence both muscle strength and physical function as such devices may compensate for lower extremity weakness and loss of mobility (24). This leaves a gap in the literature, and it is important to examine the association between muscle strength and physical function among very old (>80 years) frail individuals who receive home care services, where the need for mobility devices might be high (22, 23).

Most studies examining the association between muscle strength and physical function have measured muscle strength dynamically, especially explosive strength [i.e., power (force × velocity)] (11–13, 15, 16, 18–21, 25). However, evaluating dynamic strength can be challenging for older adults, as it may require high technical skills, sufficient balance and coordination, proper equipment and familiarization, and multiple attempts (26, 27). These challenges become even more apparent for the oldest old (>80 years) and/or those who depend on mobility devices. A possible alternative to overcome the abovementioned challenges is to measure muscle strength isometrically. This enables measurement of maximal strength as maximal voluntary contraction (MVC) and explosive strength as isometric rate of force development (RFD) with high level of control, making it safe, easy, and practical to perform for older adults (26, 27). Despite this, few studies on the oldest old (>80 years) have used isometric measures for muscle strength (13). Furthermore, RFD which is obtained from the slope of the force-time curve (Δforce/Δtime), has been proposed as an important determinant for daily life activities, maintaining postural balance, and avoiding falls among older adults (27–29). Although a few cross-sectional studies have shown that higher RFD is associated with better physical function in 60- and 70-year-olds (9, 14, 15, 17), more research is needed to understand the relationship between RFD and physical function, especially among the oldest old (>80 years). However, to our knowledge, this has not been reported in the existing literature. Finally, although studies indicate that explosive strength is more important for physical function than maximal strength (25), there is a lack of studies including the oldest old and examining explosive strength (i.e., isometric RFD). Thus, the aim of this cross-sectional study was to investigate the association between maximal- and explosive strength with physical function among very old community-dwelling individuals receiving home care.

Baseline characteristics of the participants are presented in Table 1. The sample consisted of 64 females (60%, body mass 65.5 kg, height 157 cm) and 43 males (40%, body mass 77.4 kg, height 169 cm). The males were slightly younger (85 vs. 87 years), stronger, and had a higher percentage of mobility device usage (68 vs. 55%) than the women. Data for physical function were available in 97–98% of participants, while data on MVC and RFD was available in 94% of participants. The number of participants included in the analyses ranged from 99 to 100.

This cross-sectional study showed that higher maximal- and explosive strength were associated with better physical function in the oldest old who receive home care. These findings suggest that maintaining and/or improving muscle strength is important for perseverance of physical function into old age.

Some previous cross-sectional studies have investigated the relationship between muscle strength and physical function among the oldest old (>80 years) (10, 11, 13, 19). Barbat-Artigas et al. (10) showed that ambulatory women (mean age 80.4 years) in the lowest maximal leg-strength quartile was 12–25-fold more likely to have impairments in chair rise, preferred-, and maximal gait speed compared to those in the highest strength quartile. Likewise, Bassey et al. (11) found that explosive strength, measured as leg extension power, was related to chair rise, stair climb, and gait speed (r = 0.65–0.81) in residents of a rehabilitation center where 65% used mobility devices. These previous findings are in line with ours, however, direct comparisons between studies are difficult due to the focus on slightly different populations and aspects of physical function. Moreover, explosive strength has in previous studies been assessed by dynamic measures (i.e., power), especially among the oldest old (11, 19). Although Altubasi (9) showed that higher isometric rate of torque development (RTD) was moderately correlated with stair climb time (r = −0.59), correlations were weak for TUG, ramp up, and preferred gait speed (r = −0.12 to −0.29) in healthy older adults in their 60s and 70s. Similarly, Osawa et al. (17) found that RTD was important for some, but not all, measures of physical function among healthy older adults in their 60s. However, these results might not be entirely comparable to ours as the relationship between muscle strength and physical function is believed to be curvilinear, creating a threshold where muscle strength is less important for physical function, especially in younger, stronger older adults (12, 41). Thus, our results support those of previous studies showing that higher muscle strength is associated with better physical function in the oldest old and expand on the existing literature by including individuals who receive home care and with high mobility device dependency (60%), which is an important and increasing group of older adults.

Explosive strength has been found to be more important for physical function than maximal strength among older adults in their 60-s and 70-s (12, 18, 42). Although evidence suggest that explosive strength decreases more rapidly than maximal strength with increasing age (2) few of the previous studies have examined the oldest old (>80 years) have included measures of both maximal- and explosive strength (13). To indicate the strength of the associations, we calculated standardized regression coefficients which indicated a slightly stronger association for explosive strength (RFD) than maximal strength (MVC) with all measures of physical function. Rising from a chair as fast as possible involves repetitive acceleration of one's body mass and may demand less time to develop force and a higher level of explosive- than maximal strength (43). Similarly, the acceleration of body mass is also relevant for TUG-8ft performance and walking as fast as possible. It should be mentioned that the 95% CIs for the standardized regression coeffects overlap substantially, making it difficult to draw inferences regarding the importance of maximal- vs. explosive strength from our results. Furthermore, we used a 200-ms window to assess RFD, and RFD measured during the later phase of rising muscle force has been found to be closely related to MVC (44). Thus, there is most likely a relation between the two measures. However, a stronger association for explosive strength can be supported by the age-related degeneration in the muscle (e.g., atrophy of type II fibers, cross-sectional area, fewer motor units, and reduced motor unit firing rate) (45). We cannot exclude the possibility that some extreme values affected our findings. Therefore, we performed a sensitivity analysis to assess the influence of extreme values showing no major changes in the standardized regression coefficients. However, it should be noted that the standardized regression coefficient for MVC with 5TSTS increased from −0.26 to −0.40, possibly indicating that maximal strength is even more important for the ability to rise from a chair than initially found.

The progressive atrophy of muscle fibers reported with increasing age is greater for type II muscle fibers than for type I muscle fibers (46). Type II muscle fibers are especially important during fast movements and, consequently, explosive strength might be more impaired than maximal strength (27, 38). Resistance training using maximal intentional acceleration of load (i.e., explosive type) has shown superior effects on explosive strength and physical function when compared to traditional heavy load resistance training (47). However, heavy loads resistance training has shown to increase the size of type II muscle fibers and myosin heavy chain II A proportion in 85–97-year-olds (48), which might be effective for eliciting gains in explosive strength (27). Thus, designing heavy loads resistance training programs with maximal intentional acceleration of the load (“explosive heavy load type”) (38, 48) could be the optimal combination for improving older adults' explosive strength, and consequently maintaining or improving physical function in old age. Additionally, such a training program would be beneficial for increasing maximal strength as well.

Isometric testing of older adults' muscle strength holds several advantages, as it requires less technical skills, balance, and coordination than dynamic strength testing (26, 27). Furthermore, isometric testing enables a high level of control, making it safe, easy, and practical to perform (26, 27). Although dynamic power has been measured previously in the oldest old during chair rise (11) and a facilitated jump test (19), these tests require higher technical skills and can be difficult to perform for older adults, especially for those who depend on mobility devices. Furthermore, many daily life movements (e.g., rapid walking, postural balance, preventing a fall) require rapid force production over a short time frame (e.g., 50–300 ms) (27–29). As RFD can be obtained from the force-time curve (27) it is a relevant measure of older adults' explosive strength. Thus, the present study show that isometric testing is a viable, practical, and safe alternative for assessment of muscle strength in older adults, also when the proportion of mobility device use is high.

Previous studies have suggested that the relationship between muscle strength and physical function is curvilinear, creating a threshold above which an increase in strength does not translate into improved physical function (12, 41, 49, 50). Identification of a specific threshold would be useful to target those with an increased risk of functional limitations who would most likely benefit from resistance training. We did not aim to statistically investigate non-linearity. Furthermore, our participants were very old with poor muscle strength and physical function (e.g., 60% used mobility devices), and identification of a clear threshold may not be possible in such a population (41, 49, 50). Nevertheless, visual inspection of the strength-function curves indicated that if a threshold (i.e., point of change in slope) exist, it is at the far range of our data, around 5.6–6.2 N/kg and 14.1–16.7 N/s/kg for MVC and RFD, respectively (Supplementary Figures 1–4). Importantly, there are very few data points above this, thus, the observed threshold may be due to random variation and should be interpreted with caution.

Reference estimates of older adults' physical function are often derived from apparently healthy populations (32, 51, 52), which excludes more frail individuals. However, as life expectancy increases, many older adults will live into their 80-s and 90-s, and many will be dependent on home care and mobility devices to function in their own home. Thus, healthy, younger older adults are not representative for the entire older population. In the present study, the participants were classified as the oldest old, all received home care, and 60% used mobility devices. Accordingly, their physical function was in line with or slightly lower than those reported by Lusardi et al. (22) for older adults (80–101 years) with- and without mobility devices. Furthermore, the maximal strength was low and comparable to those shown by Aas et al. (53) in a comparable sample, although direct comparison is difficult due to different methods used to assess maximal strength. Thus, our findings highlight the importance of obtaining knowledge about the level of, and association between, muscle strength and physical function in this rapidly growing group of older adults, and not only in younger, healthier, and more well-functioning individuals.

The strengths of our study include the choice of participants (i.e., oldest old, receiving home care, mobility devices) which allows for knowledge about an understudied, yet important group of older adults. Furthermore, we examined both maximal- and explosive muscle strength, and used isometric measures to assess muscle strength. Some study limitations should be addressed. First, this was an exploratory study and the cross-sectional design precludes determination of the temporal relationship between muscle strength and physical function, as well as causality. Second, the study may not have been powered to investigate the associations included in the current paper. Third, our data material showed large SDs and some extreme values. This was not surprising given the variation in age, strength, and functional status seen among older adults receiving home care. It may be that the differences in muscle strength between the genders influenced the distribution of the data, and hence, the results. However, we did use relative muscle strength which may take some of the gender differences into account. Fourth, we did not investigate whether the association between muscle strength and physical function differed according to use of mobility devices, as introducing mobility devices as a covariate in this regression analysis would introduce a collider bias (54). Future studies should examine the impact of mobility devices on the association between muscle strength and physical function. Lastly, although we included measures of both maximal- and explosive strength our analyses did not investigate their independent contributions, which should be examined in future studies. Based on the abovementioned limitations we advise reflective interpretation of the results.

In conclusion, the present study shows that higher maximal- and explosive muscle strength is associated with better physical function in the oldest old who receive home care. Our findings add knowledge about a rapidly growing yet understudied group of older adults and highlight the importance of prioritizing strategies aiming to maintain and/or improve muscle strength for perseverance of physical function into old age.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the Regional Committee for Medical and Health Research Ethics (2016/51), Armauer Hansens Hus, nordre fløyel, 2. etasje, Haukelandsveien 28, Bergen; Norwegian Centre for Research Data (49361/s/AGH), Harald Hårfagres gate 29 N-5007, Bergen, Norway. The patients/participants provided their written informed consent to participate in this study.

HB oversaw the main writing of the manuscript and data analyses. AS, VA, MF, and TR contributed to planning the study, while AS and VA were in charge of running the study and collected data. AS, VA, MF, and TR reviewed the manuscript and gave valued input on revisions. All authors read and approved the final manuscript.

This study was funded by the Norwegian Regional Research Council of Western Norway (RFV, Project Number: 257071).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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