The testicles, which manufacture both sperm cells and the hormone testosterone, are located outside of the abdomen as a result of their requirement to be at a lower temperature than body temperature. As a result they are eminently vulnerable to physical damage, especially during strenuous activity. Pain in your right or left testicle after cycling can result from various sources, some directly related to cycling and relatively benign, others more generalized and potentially serious.
Trauma
Given the fact that each of your legs pushes down on the pedals about 80 or 90 times a minute during a ride that may last well over an hour, it's likely that at some point you may accidentally catch one of your testicles against the edge of the seat, especially if it's improperly positioned. If this happens, you may not notice immediately, thanks to a combination of adrenalin, endorphins and concentration as well as the fact that the nerves supplying the genitals can become compressed and cause temporary numbness. In such cases, post-ride discomfort is usually transient, but watch for discoloration or swelling just in case.
Poorly Positioned Seat
If you're riding on the wrong kind of seat or have it set too high or too low, you may experience a condition called pudendal neuralgia. While this sometimes leads to numbness, it can often be painful. If your left testicle hurts after a ride, you can try looking for a seat without a nose or find one of the specially designed seats made to prevent this problem from occurring. Riding more upright or raising the handlebars to keep you from leaning too far forward can also help.
Testicular Torsion
Interruption of the blood supply to the testicles is common in bicycle riders and can rapidly lead to serious pain. A condition called testicular torsion results when the testicle twists around on its spermatic cord, cutting off the blood supply. Testicular torsion is often a surgical emergency. If this happens, seek treatment immediately, as you are at risk for losing the organ owing to the onset of gangrene.
Fluid Accumulation
A number of conditions with etiologies unrelated to cycling can be aggravated by riding your bike because of the effect on the local vasculature. These include a varicocele, which is caused by enlarged veins in the scrotum that drain the blood from the testicles; a spermatocele, which is fluid in the epididymis behind the testicle that results in the formation of a cyst; and a hydrocele, which is fluid buildup inside the scrotum itself. Pain is typically mild, but infection may result, so watch for swelling that does not quickly resolve.
