Grip strength

Grip strength is measured with a hand dynamometer, a hydraulic, spring or electronic gauge that the person squeezes as hard as possible, usually while seated with the elbow bent at the side. Depending on the protocol, the highest or the average of several attempts with one or both hands is recorded in kilograms. The PURE study used the Jamar hydraulic dynamometer, a standard clinical device [1].

Grip strength varies with sex, age and body size. Men are on average stronger than women, and grip declines with age after early adulthood, so studies compare people within the same sex and age group or adjust for these factors. Because the test is quick and cheap, it is used both in research and in clinical screening, where it is the recommended first measure of muscle strength for diagnosing sarcopenia [3].

The revised European consensus on sarcopenia (EWGSOP2) defines low grip strength as under 27 kg in men and under 16 kg in women; cut-off points in the consensus are generally set about two standard deviations below the average of healthy young adults [3]. A chair-rise test (more than 15 seconds to stand up five times) can be used instead. In UK Biobank, researchers defined muscle weakness as grip under 26 kg in men and under 16 kg in women, and weakness was associated with higher risk for almost all outcomes studied [4].

In the large mortality studies, risk rose steadily across the whole range of grip strength rather than only below a threshold, which is why results are usually reported per 5 kg difference [1, 4].

The PURE study enrolled households in 17 countries of varying income between 2003 and 2009 and followed 139,691 adults aged 35 to 70 for a median of 4.0 years, during which 3,379 died [1]. After adjustment, each 5 kg lower grip strength was associated with a hazard ratio (the risk in one group relative to another) of 1.16 (95% CI 1.13 to 1.20) for all-cause mortality and 1.17 (95% CI 1.11 to 1.24) for cardiovascular mortality. Grip strength was a stronger predictor of all-cause and cardiovascular death than systolic blood pressure.

UK Biobank measured grip in 502,293 adults aged 40 to 69 (54% women), of whom 13,322 died over a mean of 7.1 years [4]. Each 5 kg lower grip was associated with a hazard ratio for all-cause mortality of 1.20 (95% CI 1.17 to 1.23) in women and 1.16 (95% CI 1.15 to 1.17) in men. Adding grip strength to an office-based risk score (age, sex, diabetes, body mass index, systolic blood pressure and smoking) slightly improved prediction of death. UK Biobank participants are healthier on average than the UK population.

A meta-analysis of 38 studies in apparently healthy adults (1,907,580 participants, 63,087 deaths) found higher grip strength associated with a hazard ratio of 0.69 (95% CI 0.64 to 0.74) for all-cause mortality compared with lower grip strength, slightly stronger in women (0.60) than in men (0.69) [2]. Higher knee-extension strength was associated with 14% lower mortality (0.86, 95% CI 0.80 to 0.93). Each study used its own definition of high and low strength, so the pooled estimate does not correspond to a single threshold in kilograms.

In PURE, each 5 kg lower grip strength was also associated with higher risk of non-cardiovascular death (hazard ratio 1.17, 95% CI 1.12 to 1.21), heart attack (1.07, 95% CI 1.02 to 1.11) and stroke (1.09, 95% CI 1.05 to 1.15) [1]. Grip strength was not associated with new diabetes, hospital admission for pneumonia or chronic obstructive pulmonary disease, fall injuries or fractures. In high-income countries, stronger grip was associated with slightly higher recorded cancer incidence, a pattern not seen in middle-income or low-income countries.

In UK Biobank, lower grip was associated with higher mortality from cardiovascular disease, respiratory disease, chronic obstructive pulmonary disease, all cancers, colorectal cancer, lung cancer and breast cancer, but not prostate cancer [4]. Per 5 kg lower grip, the hazard ratio for respiratory disease mortality was 1.31 in women and 1.24 in men. Several associations were stronger in younger participants.

Strength declines with age, and faster than muscle mass. Longitudinal studies at age 75 report strength losses of about 3% to 4% a year in men and 2.5% to 3% a year in women, and studies measuring both found strength falling 2 to 5 times faster than mass [5]. The age-related decline in muscle is described in muscle mass.

Resistance training increases strength in adults of all ages, including those over 50 [6]. General strength exercises that load the hands, such as deadlifts, rows, carrying weights and hanging from a bar, also train grip.

Grip strength is generally interpreted as a marker of whole-body strength and general health rather than a cause of health outcomes in itself. It reflects muscle mass and quality, nerve function, physical activity, nutrition and the burden of chronic disease. The meta-analysis of 38 studies found that upper-body (grip) and lower-body (knee-extension) strength both predicted mortality, regardless of age and length of follow-up [2].

The PURE investigators described grip measurement as a simple, inexpensive way to stratify risk of death and cardiovascular disease [1], and in UK Biobank it improved a standard office risk score [4]. Low grip is the entry point to the diagnosis of sarcopenia [3].

All the mortality evidence is observational. Undiagnosed illness such as heart failure, cancer or depression can reduce strength before it causes death, so part of the association may reflect disease causing weakness rather than weakness causing disease. The median follow-up in PURE was four years [1], a period short enough for this kind of reverse causation to contribute.

No trial has tested whether increasing grip strength on its own, for example with hand grippers, reduces mortality. The PURE authors listed testing whether improving strength reduces mortality and cardiovascular disease as a research priority [1]. Whole-body resistance training has separate observational evidence linking it to lower mortality [7].