Acute Increases in Testosterone after Exercise are Essential! 
The magnitude of the increase in testosterone can be affected by the amount of muscle mass used during exercise18, 19, intensity and volume2, training experience20, and nutrition intake21.   There are a lot of variables influencing testosterone's secretion after exercise, but it seems that acute elevations in testosterone are necessary component for strength gains. Hansen et al23. measured muscle strength changes in muscle flexion in the arms following 9 weeks of resistance exercise.  One group performed muscle flexion exercises only, whereas the second group performed a lower body exercise prior to doing the elbow flexion exercises.  Performing elbow flexion exercises only failed to elevate testosterone after exercise, however testosterone was significantly elevated when lower body exercise were performed before elbow flexion exercises.  Muscle strength increased to a greater extent in the arms when the lower and upper body exercises that were combined that produced increases in testosterone compared to training arms only which yielded no changes in testosterone.  Another study reported that the acute increase in both testosterone and free testosterone correlated with the individual changes in strength that occurred during a 6-month training period12.  Although some studies have reported no long term changes in resting testosterone after resistance exercise, acute increases in testosterone which may only last for 15 minutes after exercise is essential for muscle strength and hypertrophy gains.   It appears that the acute response to resistance exercise is more important to muscle growth and remodeling than chronic changes in resting hormonal changes, as many researchers have not shown a significant change in resting hormonal levels despite increases in muscle strength and hypertrophy.

Testosterone Dose-Dependently Increases Muscle Strength
During adult life, the average male produces about 7 mg of testosterone daily.  The normal range of plasma testosterone in males is 300-1000ng/dl, but the average value declines by age 80 to approximately 50% of that at age 20 years of age26. The low level of anabolic hormone testosterone may be a limiting factor as why older adults tend to have lower gains in strength compared to younger men. The impressive gains in strength from testosterone can be demonstrated in which one study in hypogonadal men whom received testosterone increased muscle strength on the bench press by 22% without exercise27.  Testosterone is a wonderful hormone for men, but it can’t perform miracles if you lay on your ass!  NASA investigated the usefulness of using testosterone to prevent muscle atrophy and strength loss during space flight.  Men were assigned to bed-rest while using low supraphysiological range testosterone. Interestingly a significant anabolic response was achieved, as both whole body nitrogen balance and leucine kinetics were improved by testosterone treatment, but there was still a decline in muscle strength.  These results suggest that in the absence of daily physical activity, testosterone administration will not increase or, in the case of this bed rest model, preserve muscle strength28.  The natural changes in testosterone throughout the day make measuring testosterone complex as there are several studies that have shown no change in testosterone over several weeks of training, but when pharmacological testosterone is administered in combination with resistance exercise there are clear increases in muscle strength.  When supraphysiological dosages of testosterone are administered to healthy young men, there was a significant increase in muscle size and strength without exercise24.   When exercise is added to supraphysiological dosages of testosterone an even greater effect on strength is achieved.  The same research group conducted an interesting study on different dosages of testosterone and how it affected muscle strength.  They first gave all the healthy young men in the study a gonadotropin-releasing hormone agonist (GnRH antagonist) to suppress natural testosterone similar to the earlier study mentioned.  The men then received 25, 50, 125, 300, or 600mg of testosterone for 20 weeks.  The researchers discovered that testosterone was associated with a dose dependent increase in leg press strength and power.  Meaning that the changes in leg press strength and power was highest for the 600 mg group and lowest for the 25 mg group.  Another interesting finding was that the strength and muscle power were not correlated with serum IGF-1 levels.