Functional fitness

A small set of objective tests is used across ageing research. Grip strength is measured by squeezing a handheld dynamometer and reflects overall muscle strength (see grip strength). Gait speed is the usual walking pace timed over a short course. The chair-rise test times five stands from a chair without using the arms. Standing balance tests ask the person to hold progressively harder stances: feet side by side, one foot half in front of the other, then heel to toe.

The Short Physical Performance Battery (SPPB) combines three of these, standing balance, gait speed and chair rises, into a score from 0 to 12, with higher scores indicating better lower-limb function [5]. Newer tests with survival data include the 10-second one-leg stance and the sitting-rising test, in which a person sits down on the floor and stands up again.

The tests are quick, cheap and hard to fake. Because they measure the whole person, they also capture the effects of any illness already present.

A 2010 meta-analysis collected published and unpublished results from community-dwelling populations. Comparing the weakest with the strongest quarter of grip strength (14 studies, 53,476 participants), the summary hazard ratio (the risk in one group relative to another) for all-cause mortality was 1.67 (95% CI 1.45 to 1.93) after adjusting for age, sex and body size, although results varied considerably between studies. For walking speed, the slowest quarter compared with the fastest (5 studies, 14,692 participants) had a hazard ratio of 2.87 (95% CI 2.22 to 3.72), with more consistent results [1].

In five studies of chair-rise time with 28,036 participants, the slowest quarter had higher mortality than the fastest (hazard ratio 1.96, 95% CI 1.56 to 2.45), again with wide variation between studies. People unable to complete the chair rises had the highest mortality (hazard ratio 4.09, 95% CI 2.24 to 4.42). All five studies of standing balance found some link between poorer balance and higher mortality, though not every result was statistically significant and the tests were too varied to combine. Studies of walking speed, chair rises and balance were all in older populations, most averaging over 70; grip strength was the only measure also studied in younger groups [1].

In the Prospective Urban Rural Epidemiology (PURE) study of 139,691 adults in 17 countries, followed for a median of four years, every 5 kg lower grip strength was linked to 16% higher all-cause mortality (hazard ratio 1.16, 95% CI 1.13 to 1.20). Grip strength predicted death more strongly than systolic blood pressure, but was not significantly linked to injury from a fall or to fracture [4].

Walking speed combines many systems in one measure: leg strength, balance, joint health, heart and lung capacity, and the nervous system that coordinates them. A 2011 pooled analysis of individual data from nine studies followed 34,485 community-dwelling adults aged 65 and over (mean age 73.5, mean gait speed 0.92 metres per second) for 6 to 21 years, during which 17,528 died [2].

Survival increased across the full range of speeds, with no threshold: each 0.1 metre per second faster was linked to a hazard ratio of 0.88 (95% CI 0.87 to 0.90). At age 75, predicted 10-year survival ranged from 19% to 87% in men and from 35% to 91% in women across the range of gait speeds. A prediction based on age, sex and gait speed alone was as accurate as one based on age, sex, chronic conditions, smoking history, blood pressure, body mass index and hospital stays [2].

Gait speed therefore works as a summary of overall health. Walking as exercise, including brisk pace, is covered in walking and steps.

A 2016 meta-analysis gathered standardised data from 17 studies with 16,534 participants (mean age 76) and compared SPPB score groups with the best group, scores of 10 to 12, after adjusting for age, sex and body mass index. Scores of 0 to 3 were linked to an odds ratio (the odds of death in one group relative to another) of 3.25 (95% CI 2.86 to 3.79), scores of 4 to 6 to 2.14 (95% CI 1.92 to 2.39), and scores of 7 to 9 to 1.50 (95% CI 1.32 to 1.71) [5].

The link was consistent across follow-up lengths, participant groups, regions and ages. The authors concluded that a score below 10 predicts all-cause mortality [5].

The sitting-rising test scores a person from 0 to 10 on sitting down to the floor and standing up again, losing a point for each hand, knee or other support used. In a Brazilian study of 2,002 adults aged 51 to 80 (68% men), followed for a median of 6.3 years with 159 deaths, scores of 8 to 10 were the reference. After adjusting for age, sex and body mass index, scores of 6 to 7.5 were linked to a hazard ratio of 1.84 (95% CI 1.1 to 3.0), 3.5 to 5.5 to 3.44 (95% CI 2.0 to 5.9), and 0 to 3 to 5.44 (95% CI 3.1 to 9.5). Each one-point increase in score was linked to 21% better survival [6].

In a separate study from the same research group, 1,702 adults aged 51 to 75 who could not stand on one leg for 10 seconds had 84% higher all-cause mortality (hazard ratio 1.84, 95% CI 1.23 to 2.78) after adjustment for age, sex, body mass index and illnesses [7]. This test is discussed further in balance training.

Both floor-based tests come from single studies, adjusted for relatively few factors. They need no equipment, but their evidence base is weaker than the pooled data for grip strength and walking speed.

No trial has randomly assigned people to practise a capability test and then measured mortality. Trials instead train the underlying capacities. In the 2019 Cochrane review of 108 randomised trials in adults aged 60 and over, programmes combining several types of exercise, most often balance and functional exercises plus resistance training, probably reduced the rate of falls by 34% (rate ratio 0.66, 95% CI 0.50 to 0.88; a rate ratio of 0.66 means 34% fewer falls; moderate-certainty evidence) [3].

The strength side of these programmes is described in resistance training and muscle mass; age-related loss of muscle strength and function is described in sarcopenia.

All the mortality figures above come from observational studies. Illness often slows people and weakens them before it causes death, so part of the link between a poor score and early death reflects disease already present (reverse causation). The hazard ratios describe how well a test predicts death, not how many years would be gained by improving the score [1, 2].

Heterogeneity between studies was high for grip strength and chair rises [1], and the floor-based tests each rest on one study [6, 7]. A rapid decline in test performance can itself signal an underlying medical problem.