Measurements of lower extremity muscle activation on menopausal women

Aging is associated with a variety of changes in the neuromuscular system, which could theoretically reduce maximal voluntary force production. In women, menopause further enhances the loss of muscle mass and strength. These are mainly due to declining estrogen hormone concentrations. Estrogen is capable of affecting muscle function both through the central nervous system or directly through the muscle tissue. The interpolated twitch technique is a common method in the investigation of the ability to activate muscle. Transcranial magnetic stimulation (TMS) on the other hand can be used to study the inhibition-excitation phenomenon in the human brain through a non-invasive procedure. From a research perspective the quantitative assessment of muscle activation is essential in the evaluation of human muscle function. The aim of this thesis was to study the effects of the stage of menopause on the ability to activate the muscles of the lower extremity during isometric plantarand dorsiflexion tasks. Electrical nerve stimulation was performed for a maximal voluntary isometric platarflexion task, while the TMS task consisted of a dorsiflexion task at three different force production levels. The subjects in the study consisted of women in the early and late perimenopause phase as well as subjects in the postmenopause phase. Our results seem to indicate that none of the group-wise differences in the measurement results between subjects in early and late perimenopause can be attributed to serum E2, as the group-wise difference in serum estradiol (E2) levels was not significant (p = 0,597). In addition no significant differences in the measure of voluntary activation (VA) was found between the early and late perimenopause groups (p = 0,307). The results also indicate that only about half of participants, five subjects in the early perimenopause group and four subjects in the late perimenopause group, demonstrate a clear postactivation potentiation (PAP) phenomenon. The late perimenopausal group had generally longer peripheral silent period (PSP) values than the early perimenopausal group (116 ± 22 ms for late vs 92 ± 27 ms for early), which could indicate changes in central drive with advancing menopause. Supporting this is the finding of decreased peak and average voluntary contraction rate of force development (RFD) between the early and late perimenopausal groups (517 ± 207 N/s avg early vs 315 ± 186 N/s avg late and 3418 ± 1174 N/s peak early vs 2954 ± 909 N/s peak late), which also could be indicative of differences in central drive between the groups. This could also signal a decrease in the subjects capacity to control posture. Our results indicate correlation between the voluntary contraction S-gradient and pre and post twitch maximum force (r = 0,74 for early group pre twitch, r = 0,44 for late group pre twitch, r = 0,70 for early group post twitch and r = 0,42 for late group post twitch), which could reflect submaximal firing frequencies and/or incomplete Ca saturation during the early phase of voluntary contraction. The results also indicate significant negative correlation between the serum E2-level and the motor evoked potential (MEP) latency (r = -0,23 early and r = -0,37 late perimenopause), which on the other hand could indicate that serum E2 may act as a mediator in synaptic transmission. The periand postmenopause measurement results are somewhat inconclusive and it is therefore our suggestion that in order to identify possible trends in the results it is vital to perform additional studies with a larger cohort.