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Strength training reveals strange sex bias
A new US study of the effects of strength training on inactive men and women has produced a fascinating and unexpected new finding: while training produced significant increases in resting metabolic rate in young and older men, it had no effect on the resting metabolism of women.
The study involved 46 physically inactive subjects divided into the following four groups: young men (aged 20-30), young women, older men (65-75) and older women. After testing of various parameters, including aerobic capacity, body composition and resting metabolic rate (RMR), all the subjects took part in a whole body strength training (ST) programme for three days per week for an average of 24 weeks, using Keiser K-300 air-powered exercise equipment. They were then re-tested, with final results available for seven young men and women, 10 older men and eight older women.
a. Each of the four groups increase fat-free mass (FFM) significantly in response to training, with young subjects showing significantly greater increases than older ones, but no apparent gender differences.
b. By contrast, changes in fat mass were affected by gender but not by age, with men showing a significant reduction and women no change.
c. All groups showed significant increases in 1-M strength for all exercises, with changes for chest press and leg press analysed separately for any effects of age and gender. Combined young subjects increased 1-RM strength in the leg press more than older subjects (31% v 23%). Young subjects increase chest press.
1-RM strength significantly more than older subjects (28% v 16%) and men increased significantly more than women.
d. Significant increases in absolute RMR (9%) were observed for both young and older men but not for young and older women, despite similar increases in fat-free mass.
e. There was no change in energy expenditure of physical activity (EEPA – the other major component of total energy expenditure apart from RMR) outside the ST sessions for any of the groups.
‘The results of this study show for the first time that changes in RMR in response to ST is affected by gender and not by age,’ the researchers report. ‘Furthermore, when RMR was corrected for [fat-free mass] there was still a significant gender effect, with men having a ST-induced significant increase in RMR, whereas women still showed no change.’
‘In addition, contrary to what has been suggested previously, ST does not cause an increase in EEPA outside of the training sessions in healthy, sedentary young and older men and women.’
What can explain the clear gender difference in the impact of strength training on resting metabolic rate? One possible explanation advanced by the researchers is ‘differences in sympathetic nervous system activity responses to ST’, which was not, unfortunately, measured in the study.