Correlation between muscle strength and aerobic capacity from age 34 to 63, both cross- sectionally and in terms of longitudinal changes within a cohort of Swedish men and women.

Abstract: Background: The proportion of elderly in the population is increasing globally. Aerobic capacity (AC) and muscle strength (MS) are important factors for health and quality of life. AC as well as MS decrease with higher age. Increased knowledge about how longitudinal changes in AC and MS covaries could lead to more accurate assessments and exercise recommendations depending on age and fitness level.  Objective: The overall aim was to examine changes in MS and AC from 34 to 63 years of age and if there was an association between the two variables. Specific research questions: (Q1) Is the Two-hand lift (THL) a valid measuring method of knee extensor strength compared to isokinetic knee extension in Biodex®? (Q2) Is there a correlation between VO2max and knee extensor strength on a cross-sectional level at the ages of 63, 52 and 34? (Q3) Is there a correlation between longitudinal changes in knee extensor strength and VO2max from 34 to 63 years of age? (Q4) Is the longitudinal change in physical capacity between 34 and 63 years of age the same in terms of MS and AC?  Methods: Data from a Swedish longitudinal, prospective cohort-study (SPAF) with objective measures of physical capacity from the same sample of women and men since 1974 was used for statistical analysis. A linear regression model was conducted to investigate construct validity of the THL. Knee extension strength (Nm) in Biodex® was set as the dependent variable and kilogram force (kp) in the THL, Jamar® Hand Grip Strength (kg), bodyweight (kg), height (cm) were set as independent variables with sex and exerciser yes/no as factors. A multiple linear regression model was used to explore correlation between MS and AC on a cross-sectional level. VO2max (L/min) estimated according to Ekblom-Bak’s ergometer cycle test was defined as the dependent variable at age 63 and Åstrand’s test at age 34 and 52. MS (kp) in the THL-test, height (cm) and bodyweight (kg) were defined as independent variables. To explore correlation between change in AC and change in MS from 34 to 63 years of age, data from Åstrand’s ergometer cycle test and the THL was used. Differences between repeated measures of VO2max (L/min), bodyweight (kg) and MS (kp) were calculated as quotients that could be analysed with multiple linear regression. A dependent t-test was executed to find out if there was any statistical difference between change in VO2max (L/min) and change in MS (kp) expressed as quotients between 63 and 34 years of age. Results: (Q1)THL showed to be a significant predictor for knee-extensor strength in the Biodex® (R = 0.78, p <.001). (Q2) For the women (n = 67) there was a positive correlation between MS and VO2max (p <.05) at 63 years of age. For the men (n = 74) the correlation was also significant (p <.05) but inversed. At the age of 52 there was a positive correlation (p <.05) for women but not significant for men, and at the age of 34 there was no significant correlation for neither women nor men. (Q3) Change in MS and change in AC did not correlate from 34 to 63 years of age, but from 34 to 52 years of age there was a significant correlation (p <.05). This applied for both women (n = 42) and men (n = 48). (Q4) For the women there was no significant difference between change in MS and change in AC. The average decrease in MS was 7,2% and 15,7% in AC. For the men there was a significant difference between decrease in MS and decrease in AC (p <0.001). The average decrease in MS was 4.5% and 26% in AC. The results were partly dependent on an unexplainable increase in performance on the THL from 52 to 63 years of age that could be due to a systematic measuring error.  Conclusion: The overall results indicate that there is an association between knee extensor muscle strength and VO2max that especially applies to individuals with low physical fitness. Further research with more reliable measuring methods of knee extensor strength is needed to better understand the relationship between muscle strength and aerobic capacity in the ageing population.