A study of the effects on mortality of muscle strength in young people

CLINICAL CONTEXT

Although multiple studies have focused on the relationship between aerobic fitness and the risk for mortality, less research has been conducted on the possible association between muscle strength and mortality outcomes. Ruiz and colleagues addressed this issue in a study published in the July 1, 2008, issue of the BMJ. They found that higher muscle strength in the legs and chest among men between 20 and 80 years old was associated with reduced rates of overall mortality and cancer mortality. After adjustment for the degree of cardiorespiratory fitness, muscle strength did not significantly affect the risk for cardiovascular mortality.

Young adults have a very different epidemiology of mortality vs middle-aged and older adults. The current study by Ortega and colleagues evaluates the effect of muscular strength on the risk for early mortality among young people.

STUDY SYNOPSIS AND PERSPECTIVE

It is well known that hypertension and high body mass index during adolescence are associated with premature mortality. Now, a long-term prospective study of more than a million Swedish adolescents has turned up yet another risk factor: low levels of muscular strength.

The study was published online November 20 in the BMJ.

The authors of the study, led by Francisco Ortega,  point out that muscle strength in adulthood has been linked to all-cause and cardiovascular mortality in adults. Until now, however, similar data have been lacking in adolescents.

Similarly, they say, better cardiorespiratory fitness is associated with better mental health in adults, but there has been scant evidence whether muscle strength at any age is associated with future mental health and suicide mortality.

"This study provides strong evidence that a low level of muscular strength in late adolescence, as measured by knee extension and handgrip strength tests, is associated with all cause premature mortality to a similar extent as classic risk factors such as body mass index or blood pressure," the researchers write.

Ortega and colleagues followed up 1,142,599 young men aged between 16 and 19 years who were evaluated for service in the Swedish military. During a median follow-up of 24 years, the researchers assessed the extent to which muscle strength in adolescence was associated with all-cause premature mortality, which they defined as death before age 55 years. 

Participants who scored high on the knee extension and handgrip muscle strength tests had a 20% to 35% lower risk for death from any cause and from cardiovascular disease compared with those who scored low on the strength tests. Further, the researchers note, those differences were independent of body mass index or blood pressure status.

Ortega and his colleagues conclude, "Low muscular strength should be considered an emerging risk factor for major causes of death in young adulthood."

BMJ. Published online November 20, 2012. Full text

STUDY HIGHLIGHTS

• Study data were drawn from Swedish military conscripts registered between 1951 and 1976. Researchers focused on individuals who were 16 to 19 years old at the time of baseline testing.
• Participants also completed strength testing at baseline, including handgrip, elbow flexion, and knee extension power.
• Participants were observed until the time of death or immigration away from Sweden. Death registers were used to follow the main study outcome of early mortality, which was defined as death before age 50 years.
• Researchers focused on the relationship between muscle strength at baseline and mortality outcomes, with particular attention to death from suicide, cardiovascular disease, and cancer.
• The study outcomes were adjusted for year of birth and conscription office.
• 1,142,599 men provided data for study analysis. The mean follow-up period was 24.2 years.
• 26,145 men (2.3% of the study sample) died during follow-up. Suicide and unintentional injuries comprised nearly half of the deaths recorded. Cardiovascular disease was responsible for 7.8% of deaths, and cancer accounted for 14.9% of deaths.
• Overall, higher levels of muscle strength reduced the risks for early overall mortality and cardiovascular mortality by 20% to 35%, but strength did not affect the risk for cancer mortality.
• A mortality benefit of improved muscle strength was evident regardless of body mass index at baseline.