Muscular strength was significantly and inversely associated with risk of death from all causes and cancer after controlling for potential confounders, including cardiorespiratory fitness. The inverse association was consistent in strata of age (<60 and ≥60 years) and body mass index (18.5-24.9 and ≥25 kg/m2). Muscular strength was significantly and inversely associated with risk of death from cardiovascular disease after controlling for age, yet the association was attenuated once other potential confounders were entered in the model and was not significant after further adjustment for cardiorespiratory fitness. The analysis on the combined effects of muscular strength and cardiorespiratory fitness with all cause mortality showed that the age adjusted death rate in men with high levels of both muscular strength and cardiorespiratory fitness was 60% lower (P<0.001) than the death rate in the group of unfit men with the lowest levels of muscular strength. These results highlight the importance of having at least moderate levels of both muscular strength and cardiorespiratory fitness to reduce risk of death from all causes and cancer in this population of men.
We investigated the association between standardised measures of upper and lower body muscular strength and disease specific risk of mortality in a large cohort of men with extensive follow-up. Muscular strength and cardiorespiratory fitness were moderately correlated (age adjusted partial r=0.33), suggesting that the association between muscular strength and risk of death from cancer works at least partially through different mechanisms than those associated with the protective effects of cardiorespiratory fitness. That the association between muscular strength and risk of death from cardiovascular disease was not significantly independent of cardiorespiratory fitness highlights the key role of cardiorespiratory fitness in the development of cardiovascular disease in men; however, their combined effects cannot be easily disentangled in an observational study. In this cohort the number of deaths from cardiovascular disease was lower than that from cancer (145 and 199, respectively). This may have reduced the statistical power to detect a significant independent association between muscular strength and risk of death from cardiovascular disease.
Apart from our preliminary analyses in the aerobics centre longitudinal study,28
only one study has assessed the association between muscular strength and all cause mortality after adjusting for cardiorespiratory fitness and age, smoking status, and body mass index and found that handgrip strength and upper body strength (push-ups) were not significantly associated with risk of death from all causes.18
A significant inverse association between muscular strength (measured by handgrip strength) and risk of mortality has been reported in several other studies.15 16 17 19 20 21 22 23 24 25 27
The main limitation of this test is that the measurement is highly influenced by the grip span of the dynamometer and hand size.44 45 46
None of these previous studies standardised the grip span or assessed a second muscle group. Furthermore, hand grip uses a relatively small muscle group and is not well correlated with measures of overall muscular strength as determined by measurements of strength using large muscle groups.47
Assessing additional muscle groups may provide a better overall index of muscular strength, especially when measured in large muscle groups. Moreover, cardiorespiratory fitness was not measured in these studies, and we know that cardiorespiratory fitness is strongly associated with morbidity and mortality.7 8 9 10 11 12 13 37 38 39
The apparent protective effect of muscular strength against risk of death might be due to muscular strength in itself, to muscle fibre type or configuration, or as a consequence of regular physical exercise, specifically resistance exercise. Muscle fibre type and configuration has a genetic component and influences strength, yet it is clear that resistance type physical activities are major determinants of muscular strength.5 48
We have previously reported a strong and positive association between the frequency of self reported resistance exercise and maximal muscular strength in men enrolled in the aerobics centre longitudinal study—that is, the higher the participation in resistance exercise the higher the muscular strength.32
This observation suggests that the measurements of muscular strength obtained in the present study provide an adequate representation of the resistance exercise habits in our cohort. Results from intervention studies indicate that resistance training enhances muscular strength and endurance, muscle mass, functional capacity, daily physical activity, risk profile for cardiovascular disease, and quality of life.5
These factors are well known predictors of higher risk of mortality. The benefits of resistance training are evident in men and women, young adults, and older people, in overweight and obese adults as well as in people of normal weight, and in people with or without disability, or with cardiovascular disease.5
We observed an inverse association between muscular strength and risk of death from all causes and cancer in older men (≥60 years) and younger men (<60 years). This suggests that older men may also benefit from having higher levels of muscular strength. In older men, functional performance becomes more directly dependent on strength, as these men also experience age associated declines in muscle mass and consequently in muscular strength.
The observed association between muscular strength and risk of death from all causes or cancer was also independent of body weight. We showed an inverse association between muscular strength and risk of death from all causes and cancer in overweight and obese men, as well as between muscular strength and risk of all cause mortality in those of normal body weight. Body mass index may have a different meaning in those who have greater muscular strength. For example, leg press strength for a man weighing 60 kg would be expected to be lower than that for a man weighing 90 kg. That is why we included body mass index in the multivariate analyses. Thus we not only controlled for the effect of weight but also height, which might have influenced the torque or force production. A high body mass index can result from a greater amount of fat or muscle. Yet in epidemiological studies most of the people with a higher body mass index also had higher fat levels.
Given that the prevalence of overweight and obesity exceeds 66% in the United States49
and that overweight and obese people are at a substantially higher risk of disability50
and death,51 52
these results have important implications for public health. Exercise recommendations to prevent or treat obesity have focused mainly on aerobic activities, yet resistance exercise is an important complement for weight control, mainly as a result of the increases in metabolically active muscle mass.2
Under most circumstances, and especially during physical inactivity, resting energy expenditure is the largest component of total energy expenditure. The energy expenditure related to muscle metabolism is the only component of resting energy expenditure that might vary considerably.2
The resting metabolic requirements of splanchnic tissues, brain, and skin vary little under normal conditions, mainly because of their relatively constant mass and protein turnover rates. In contrast, large variations in muscle mass are possible, and the rate of muscle protein turnover (synthesis and breakdown) may vary as well. The synthesis and breakdown of muscle protein are principally responsible for the energy expenditure of resting muscle. In theory, every 10 kg difference in lean mass translates to a difference in energy expenditure of about 100 kcal daily, assuming a constant rate of protein turnover.2
A difference in energy expenditure of about 100 kcal daily translates to about 4.7 kg of fat mass yearly. Over a long period the maintenance of a large muscle mass and consequent muscle protein turnover can contribute to the prevention of obesity. Therefore it is reasonable to presume that when sustained over time, resistance exercise training should help to prevent or revert increases in body fat.5
Strengths and limitations
The results of the present study should be interpreted with caution. Generalisation of the findings may only apply to well educated white men of middle to upper socioeconomic status. Values for blood pressure and cholesterol levels, body weight, and cardiorespiratory fitness from participants in the aerobics centre longitudinal study were similar to those reported in two population based studies in North America.8
Moreover, there is no reason to believe that the benefits of muscular strength would be different in other ethnic or socioeconomic groups. Because of the limited sample of women, who contributed relatively few deaths to the main study, we were unable to perform a meaningful parallel analysis on women. Therefore women were not included in this substudy. No detailed information about drug use or diet was available, which may have biased the results through residual confounding. It seems unlikely, however, that these factors would account for all of the observed association between muscular strength and mortality. That none of the participants reported a family history of cancer might be a limitation of the main study owing to self selection bias. In fact, only 1.16% of men in the entire cohort of the aerobics centre longitudinal study reported a family history of cancer. Future studies should include such information whenever possible.
A major strength of this study was the inclusion of objective and standardised maximal tests for muscular strength (upper and lower body) and cardiorespiratory fitness using highly reliable measurement protocols in a large cohort of men with extensive follow-up. Undetected subclinical disease is always a concern in any observational study, but it is less likely to have occurred in our cohort because of the comprehensive physical examination and the clinical assessment completed by each participant. Moreover, participants were healthy enough to achieve at least 85% of aged predicted maximal heart rate during the treadmill test.
Muscular strength was independently associated with risk of death from all causes and cancer in men. These findings are valid for men of normal weight, those who are overweight, and younger or older men, and are valid even after adjusting for several potential confounders, including cardiorespiratory fitness. Muscular strength seems to add to the protective effect of cardiorespiratory fitness against the risk of death in men. Whether the association between muscular strength and risk of death from cardiovascular disease is independent of typical confounders as well as of cardiorespiratory fitness warrants further investigation.
Prospective studies among diverse populations and among women are needed to examine the independent and combined associations of muscular strength and cardiorespiratory fitness with disease specific mortality. It might be possible to reduce all cause mortality among men by promoting regular resistance training involving the major muscle groups of the upper and lower body two or three days a week.5
Resistance training should be a complement to rather than a replacement for aerobic exercise. The recommendation for moderate to vigorous physical activity and resistance training are supported by the current research owing to the reduction in risk of death from all causes and cancer associated with increased cardiorespiratory fitness or muscular strength.3 4 5 6
What is already known on this topic
- Cardiorespiratory fitness provides strong and independent prognostic information about the overall risk of illness and death
- Most prospective studies examining the association between muscular strength and death have had limitations
What this study adds
- Muscular strength in major muscle groups is independently associated with death from all causes and cancer in men aged 20-82
- These findings are valid for those who are of normal weight or overweight, younger or older, and even after adjusting for several potential confounders, including cardiorespiratory fitness
- Muscular strength seems to add to the protective effect of cardiorespiratory fitness against the risk of death in men