Familial risk ratios for high and low physical fitness levels in the Canadian population

Purpose: To estimate the familial risk of being physically fit or unfit in Canada. Methods: The sample consisted of 11,680 participants from 4144 nuclear families of the 1981 Canada Fitness Survey. Indicators of physical fitness included estimated physical working capacity at a heart rate of 150 beats·min^-1 derived from a step test (PWC₁₅₀), hand grip strength, sit- ups, and trunk flexibility. Probands were defined as reference individuals who were physically fit (95th, 85th, and 75th percentiles) or physically unfit (25th, 15th, and 5th percentiles) for each fitness indicator, respectively. Results: Standardized risk ratios, adjusted for age and sex (SRR), for spouses and first-degree relatives of probands exceeding the 95th percentile are 1.63 and 1.81 for PWC₁₅₀, 2.38 and 3.16 for grip strength, 2.63 and 3.98 for sit-ups, and 2.59 and 3.56 for trunk flexibility, respectively, whereas the SRR for spouses and first-degree relatives of probands below the 5th percentile are 1.54 and 1.34 for PWC₁₅₀, 1.83 and 1.85 for grip strength, 1.13 and 1.53 for sit-ups, and 1.42 and 1.84 for trunk flexibility, respectively. The familial risks tend to be greatest at the extremes (95th and 5th percentiles) and the risks for first-degree relatives of physically fit probands are generally greater than those for spouses for grip strength, sit-ups and trunk flexibility, whereas those for PWC₁₅₀ show no clear pattern. Conclusion: There is significant familial risk for being physically fit or unfit in the Canadian population. The pattern of SRR suggests that the familial risk for indicators of strength and flexibility may be due, in part, to genetic factors, whereas the shared family environment is largely contributing to the familial risk for PWC₁₅₀.