Your stamina is fading. Lifting heavy items is more difficult. You might be working out less than usual, but there is nothing you can think of that explains this loss of strength. Studies have found that, when men between the ages of 25 and 60 are relatively inactive, they will lose half a percent of muscle strength each year. After 60, the rate of muscle loss doubles every 10 years. While decreased strength can be attributed to age, some can be tied to a decline in testosterone levels. In other words: as testosterone levels decrease, so does muscle mass. Testosterone is a necessary component in gaining and retaining lean muscle mass. In a recent study, men who used testosterone replacement for six months to bring their levels into the “normal” range experienced an increase in muscle mass of 2.5 pounds.