The nervous system controls the movements of the body. Signals travel from the brain, down the nerves in the spinal cord and then out to the appendages, such as the arms and the legs. However, occasionally the nerves can become trapped or compressed in the joints. When this occurs in the knee, it is called peroneal nerve entrapment and has a number of symptoms that can also cause the leg to swell.
Swelling of any organ or appendage is caused by a build-up of fluid, causing the skin to appear stretched and often feel spongy to the touch. A swollen leg is not a symptom of nerve entrapment, which can, in fact, cause the leg to atrophy (or lose mass). However, the symptoms that a trapped nerve does cause can, in turn, cause damage to the leg that results in swelling.
A trapped nerve not only makes it difficult for messages from the brain to reach the affected leg, but can also cause a number of sensations in the leg. The nerve itself can feel quite painful, while the leg may begin to tingle on the outer side of the upper or lower leg. The leg may begin to feel numb on a regular basis.
Because the nerve cannot get messages from the brain to the leg properly, the leg may begin to feel inexplicably weak. This is not because the muscles of the leg are becoming weaker necessarily, but because the messages telling the muscles to move are weak, increasing the effort required to move them. As this happens, it may become more appealing to walk less. However, sitting for long periods of time can cause the fluids in the leg to begin to build up, causing swelling.
Because the leg muscles are made to feel weak as a result of a trapped nerve, you may begin to walk with an abnormal gait. This can include walking with an inexplicable limp, or with the leg making movements that it would not normally make as it becomes harder and harder to control. Another cause of an abnormal walk may be that the foot is affected by the same weakness as the leg. These walking problems may cause you to put weight on the affected leg in ways and positions that it is not used to, the shock of which can cause the leg to begin to swell. In addition, a changed walking gait may make it more likely that you will bump the affected leg against things, again causing swelling.
As leg swelling is not a symptom of a trapped nerve, it may actually be symptomatic of a different condition. Among other things, blood clots, lymph node blockages and infections can cause the leg to swell. If your leg begins to swell, consult your doctor as soon as possible.