Men work out for a variety of reasons, including to lose weight, build larger and stronger muscles, improve cardiovascular fitness and reduce psychological stress. All moderate- to high-intensity exercise, however, presents certain physical inconveniences, such as sweat-soaked clothing and a ruddy face, and curiosities, such as the discomfiting phenomenon men not-so-lovingly call "shrinkage." If your genitals -- in particular your testicles --appear small after a workout, rest assured that the condition is temporary, benign and attributable to perfectly non-threatening factors.
The testicles are the site of spermatogenesis, or sperm production. Because the ideal temperature for this process is somewhat lower than the normal body temperature of 98.6 degrees F, the testicles hang outside the body in the scrotum, the looseness or tightness of which is controlled by the cremaster muscle in response to various internal and environmental stimuli. If you are performing outdoor exercise such as running, skiing or cycling in cold weather, your scrotum contracts to draw the testicles closer to the body to keep them close to their accustomed temperature. As a result, the testicles temporarily appear to shrink.
When you exercise, your circulatory system makes adaptations that result in the shunting of oxygenated blood pumped from the heart toward the working muscles in the arms, legs and trunk. In fact, while 20 percent of cardiac output passes through the muscles at rest, 80 percent is fed through the muscles during all-out exercise. Since there is only so much blood to go around, this shunting occurs at the expense of tissues not in immediately in need of heavy oxygenation, including the genitals. As a result of diminished blood flow, the penis and testicles appear to shrink during and after heavy exercise.
Most men know from experience that situations evoking nervousness typically cause an immediate retraction of the scrotum toward the body, part of the "fight-or-flight" response that in ancient times helped provide protection to the genitals of men about to either flee or engage in physical combat. Clinically, this effect of the sympathetic nervous system is mediated by the dartos muscle within the scrotal sac. So if your exercise involves you getting pumped up to run a race or you attempt a lifetime bench-press max, nerves can exacerbate testicular retraction already present owing to physiological factors.
Often, the appearance of smaller testicles following a workout is the result not of changes to the testicles or scrotum, but to temporary alterations in the rest of the body. If you engage in heavy weightlifting, for example, your arms and legs will remain "pumped up" for many minutes after the termination of exercise. Should you catch a glimpse of your testicles in the mirror under these conditions, chances are that the engorgement of your muscles will render your testicles meek-looking in comparison.