Is There a Sex Difference With Aging In Stimulus-Response Characteristics of The Lower Limb?
Format of Original
American College of Sports Medicine
Medicine & Science in Sports & Exercise
PURPOSE: The aim of this study was to compare the input-output characteristics of the motor cortex of young and old men and women.
METHODS: MEPs of the vastus lateralis (VL) were recorded from 11 young (19-30 yr, 6 women) and 26 old adults (61-79 yr, 13 women) with bipolar EMG electrodes by delivering single-pulse TMS over the motor cortex. The active motor threshold (AMT) was determined and MEPs elicited at increasing intensities (5% increments of simulator output, SO) during intermittent isometric contractions at 10% maximal voluntary contraction. MEP amplitudes were expressed relative to the resting maximal compound muscle action potential of the VL (Mmax) and plotted against the SO (%AMT) to generate an input-output curve and determine the following parameters: the estimated maximal MEP amplitude (MEPmax), the stimulus intensity required to elicit a response equal to half MEPmax (S50), and the peak slope of the sigmoidal curve.
RESULTS: AMT was similar between the young and old adults (44.9 ± 9 vs 47.4 ± 4.4 %SO, respectively) and between men and women (54.4 ± 9.6 vs 54.8 ± 12.2 %SO respectively). Mmax was less in young women (10.41 ± 1.72 mV) compared with young men (17.28 ± 2.8 mV, Pmax, young women and men were similar for the MEPmax (35.3 ± 15.1 vs 34.8 ± 23.7 %Mmax, P>0.05), peak slope (0.08 ± 0.04 vs 0.13 ± 0.06, P>0.05). However, S50 was lower in young women compared with young men (110.9 ± 5.9 vs 124.3 ± 5.5 %AMT). Mmax was less in old women compared with old men (6.5 ± 2.4 vs 10.9 ± 3.7 mV, Pmax, old women and men were similar for the MEPmax (30.7 ± 17.5 vs 31.7 ± 11.7 %Mmax, P>0.05), peak slope (0.091 ± 0.02 vs 0.084 ± 0.03, P>0.05) and the S50 (113.9 ± 13.2 vs 123.3 ± 17.5 %AMT, P>0.005).
CONCLUSION: When normalized to the compound muscle action potential, there were minimal sex differences for the young and old adults in most input-output characteristics and AMT of the knee extensor muscles.