Physiology
Whilst males and females are structurally similar, there are many physiological differences that affect the sexes’ ability to gain muscle mass.
Hormones
The primary reason that females cannot gain muscle mass as fast or to the extent as males is the difference in hormone status.
Testosterone is one of the androgenic hormones responsible for anabolism in the body (Kraemer & Ratamess). It is testosterone that is responsible for masculine traits (i.e. excess hair (especially facial), deepening of voice, increase in muscle mass). Both males and females produce testosterone, as it is necessary for hormonal balance and body function (Marieb, 2004). However, males have much HIGHER levels of testosterone than females, with the ‘normal’ range of total testosterone (in the bloodstream) being 0.95-4.3 pg/dl, compared to the 0.7-3.6 pg/dl of females. However, it is not so much the total amount of testosterone that an individual has that determines their potential/ability for muscle growth, since most of the testosterone in the body is bound to either sex hormone binding globulin (SHBG) or other non-specific proteins such as albumin (Wheeler, 1995), but their levels of FREE testosterone (i.e. the amount of testosterone that is NOT bound in the body). In males 0.3-5% (with an average of 2%) of their total testosterone if free, with their free testosterone normal values being 270-1100 ng/dl, compared to only 6-86 ng/dl of free testosterone available to females.
The female ‘equivalent’ of testosterone is estrogen (Marieb, 2004). Whilst estrogen may increase Growth Hormone (GH), it also increases a) SHBG, which decreases the amount of free testosterone in the body; and b) cortisol, which reduces muscle mass (Hakkinen, 1989).
Muscle Fibres and Types
There is a similar distribution of the percentage of Type I, Type IIa, and Type IIb muscle fibres in both males and females. However, females have ~60-80% of the muscle cross-sectional area (CSA) and whole muscle anatomical cross-sectional area (ACSA) than that of males. Therefore, despite the potential for muscle hypertrophy in a relatively short period of time (Gregory, et al., 2006), similar percentage increases in either muscle mass or volume as a result of resistance training, results in smaller total overall gains in CSA and ACSA in females than in males (Folland & Williams, 2007).