The relationship of testosterone and AR CAG repeat genotype with knee extensor muscle function of young and older men

The inter-relationship between muscle strength and serum testosterone is not fully understood, and may be confounded or influenced by age. The polymorphism of androgen receptor gene CAG number (AR CAGn) could also influence these variables. The study examined the relationship between total testosterone (TT), free testosterone (FT) and AR CAGn with the muscle strength of young (YM, 18-30 yrs, n=82) and older (OM, 60-70 yrs, n=101) Caucasian men. Knee extensor strength was measured isometrically and isokinetically, and thigh and whole-body lean mass of the OM was determined by DXA. TT and serum hormone binding globulin (SHBG) were assayed by ELISA and used to calculate FT. AR CAGn was determined using polymerase chain reaction and microchip electrophoresis. OM were weaker than YM (-20 to -29%, all P<0.001), and serum androgens were lower (TT, -13%; FT, -13%; both P<0.001). TT was unrelated to any strength measurement in YM or OM. In the OM only, FT had a weak positive association with all three strength measures (r(2)=4.1-9.3%, P<0.036) and both whole body and thigh lean mass (r(2)=6.1-8.6%; P<0.013). Muscle strength was unrelated to AR CAGn for either the YM or OM, or when data were collapsed across both age groups (age normalised strength). Lean mass in the older cohort was also independent of AR CAGn. In conclusion, FT, but not TT or AR CAGn, was positively associated with muscle strength, but only as values declined with age.