Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC326
Temperature effects on muscle force in pre- and post- menopausal women.
J. Bieles1, S. A. Bruce1, R. C. Woledge1
1. Centre of Human and Aerospace Physiological Sciences, King's College, London, United Kingdom.
Postmenopausal women (Post-M) are weaker than premenopausal (Pre-M) women because their muscles are smaller and because the remaining muscle exerts less force per cross sectional area (CSA) (Phillips et al, 1993). Cooling muscles below their optimum temperature also reduces force per CSA (Ranatunga et al, 1987). These factors may interact, and we have investigated whether the temperature sensitivity of muscle force alters at the menopause. We measured the maximum voluntary force (MVF) exerted by intrinsic hand muscles during thumb adduction in 11 Post-M and 11 Pre-M women. Hand temperature was varied between 18 and 38○C by immersion in a water bath. Skin temperature was monitored; it has previously been shown to closely follow intramuscular temperature (Ranatunga et al, 1987). Study approval was obtained from King’s College Research Ethics Committee, ref BDM/08/09-29, and subjects provided written informed consent. Of 426 MVF measurements five were discarded as outliers. For temperatures above 35.2○C the average MVF was 52±3 N for Pre-M and 54±4 for Post-M (mean ± SEM). To compare temperature sensitivity, results are expressed with respect to the mean MVF above 35.2○C (MVFS) for each subject: MVFREL = MVF/MVFS. Fig 1 shows the MVFREL values grouped by menopausal status and by temperature. There are clear differences between Pre-M and Post-M at the three lower temperatures (p<0.002, t-test). A two-way ANOVA with menopausal status and temperature as factors gave significant main effects and also a significant interaction between temperature and menopausal status (p<0.01). The null hypothesis that temperature dependence of force is independent of menopausal status can therefore be rejected. So we consider temperature effects separately for each group. We found peak force at about 30○C in the Pre-M group with a fall at both higher and lower temperatures and a steep fall below 20○C. This agrees with the findings of previous studies with young subjects (Ranatunga et al, 1987). When muscle force in other species has been investigated over a wide range of temperatures a plateau has been found near physiological temperature (Rall and Woledge, 1990), with a fall at higher and lower temperatures. The Post-M group showed a linear decline of MVFREL with cooling below 35○C. We propose that hormonal changes at the menopause produce a rightward shift in the force temperature curve by about 5○C. It seems likely the effect we have demonstrated is hormone related rather than simply an effect of age, because specific muscle force in women changes abruptly in the peri-menopausal stage, and is otherwise independent of age below 65 (Phillips et al., 1993). It seems probable that the change in temperature sensitivity accompanies the change in specific muscle force.
Where applicable, experiments conform with Society ethical requirements