Sex differences in mechanomyographic responses to voluntary isometric contractions.
OBJECTIVE
This study was designed to compare mechanomyography (MMG) and the force relationship during isometric ramp contractions of biceps brachii muscles in females and males to identify sex differences in the MMG responses.


METHODS
Subjects (10 females and 9 males; age range, 20-26 yr) were asked to exert an isometric elbow flexion torque from 5 to 80% maximal voluntary contraction (MVC) at a constant rate of 10% MVC per second. The MMG signal was normalized to muscle cross-sectional area (CSA) as measured by ultrasound imaging.


RESULTS
MVC and CSA were significantly different between the two sex groups (males>females); however, there was no sex difference in the MVC relative to muscle CSA (MVC/CSA). The root mean squared amplitude of the MMG (RMSMMG) was significantly greater in the male group than the female group. The RMSMMG relative to muscle CSA was also different between the two sex groups (males>females). The sex difference in the RMSMMG/CSA was more pronounced with increasing torque. The torque levels at which the inflection points in the MMG amplitude were located were different between the two sex groups. The mean power frequency (MPF) of the MMG in the female group increased monotonously, which was different from that in the male group.


CONCLUSIONS
Our results suggest that the sex differences in MMG responses and motor unit (MU) activation strategy result from the predominant activity of the MU with slow-twitch fibers and an effective fused tetanus in females. In addition, the sex-related differences in muscle morphology, subcutaneous adipose tissue, and muscle stiffness appear to be insufficiently reflected in the present MMG responses, particularly relative to muscle CSA.
Nonaka
Hisako
H
School of Humanities and Social Sciences, Nagoya City University, Nagoya, Japan.
Mita
Katsumi
K
Akataki
Kumi
K
Watakabe
Makoto
M
Itoh
Yasushi
Y
D000328:Adult; D019412:Anatomy, Cross-Sectional; D001132:Arm / Q000502:physiology; D001696:Biomechanical Phenomena; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D007564:Japan; D008297:Male; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging; D009213:Myography; D012737:Sex Factors; D019415:Torque; D014463:Ultrasonography