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Testosterone-Induced Muscle Growth Linked To Muscle Fibre Hypertrophy

Increases in muscle volume when young men in good gonadal health are treated with graded doses of testosterone are due to the growth of muscle fibre.

The increases are associated with concentration-dependent rises in cross-sectional areas of both type I and type II muscle fibres and myonuclear number, say specialists at the Charles R. Drew University of Medicine and Science, Los Angeles, California and Harbor-University of California at Los Angeles Medical Center, Torrance, California.

The specialists noted that muscle size increased when replacement doses of testosterone were given to healthy hypogonadal men and supraphysiological doses were given to eugonadal men.

To see if these increases were due to muscle fibre hypertrophy, the specialists gave monthly injections of a long-acting gonadotropin-releasing hormone agonist to suppress endogenous testosterone secretion to 61 healthy men aged 18-35 years. The men also received weekly injections of 25, 50, 125, 300 or 600 mg testosterone enanthate (TE) for 20 weeks.

Thigh muscle volume was measured by magnetic resonance imaging. Muscle biopsies were taken from vastus lateralis muscle in 39 men before and after the 20 weeks.

The combined treatment resulted in a mean nadir testosterone concentration of 234 ng/dl with the 25 mg dose. This rose to 289 ng/dl with 50 mg, to 695 ng/dl with 125 mg and to 1,344 ng/dl with 300 mg. The highest concentration, 2,435 ng/dl, was reached with 600 mg doses.

Graded doses of testosterone administration were associated with testosterone dose and concentration-dependent increase in thigh muscle volume. Changes in cross-sectional areas of both type I and II fibres were dependent on testosterone dose and correlated significantly with total and free testosterone concentrations during treatment.

The men who received 300 mg and 600 mg of testosterone enanthate weekly experienced significant increases from baseline in areas of type I muscle fibres. Those in the 600 mg group had significant rises in cross-sectional area of type II fibres also.

The relative proportions of type I and type II fibres did not change significantly after treatment in any group.

The myonuclear number per fibre increased significantly in men on the two highest doses of testosterone enanthate and correlated significantly with testosterone concentration and muscle fibre cross-sectional area.
Endocrinology and Metabolism 2002;283(1):E154-E164. "Testosterone-induced increase in muscle size in healthy young men is associated with muscle fiber hypertrophy"

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