Title, Date of Publication & Journal

Muscular strength in male adolescents and premature death: cohort study of one million participants

BMJ, 11.30pm 20/11/12

Claim supported by evidence?

The paper supports the claim that there is a small increased risk of premature death in the weakest adolescents, but not that the strongest have a reduced risk.

Summary

This study focuses on premature death, which is a very rare event for all groups.

This is a good quality cohort study which uses a large sample size.

Study Conclusions

The study concludes that the physically weakest adolescents are at the highest risk of premature death.

This study does not conclude that muscle-building exercises will make you live longer.  There is no suggestion that you improve your chances of survival by making yourself stronger. 

The study claims that “People at increased risk of long term mortality, because of lower muscular strength, should be encouraged to engage in exercise programmes and other forms of physical activity.”  This may well be true but cannot be concluded from the data in this study.

The data in Figure 1 appears to show that the group at by far the most risk is the weakest 10%.  For the next 10%, the risk appears slightly raised although to a much lower extent.  There is no discernible difference in the remaining 80%.

Strengths/Limitations

It is impossible to establish causal links from this kind of study.

The data cannot say whether physical weakness results from ill health or causes ill health.  Either is possible.

The weakest 10% of people in the study were probably just individuals of physically poor quality.  That means a finding of excess premature deaths in that group is not surprising.

The most likely explanation of the results is that the lowest muscular strength is a marker for the poorest physical health. 

The relative risks appear large but the absolute risks are actually very small.  Say we take 1000 young people, we would expect 12 of them to die over the next ten years if they are all in the 10% weakest category, compared with 9 of them to die if they are all in the 10% strongest category.

The most plausible explanation is that physical strength is a marker of general
physical resilience and cardiovascular fitness, which you would expect would be associated with mortality.

We also know that people with poor mental health are less likely than others to be physically active, which could explain a higher suicide rate in the weakest category.  It does not follow that strengthening your muscles directly reduces your chance of suicide. 

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