Running can trigger spasms in the legs, although experts do not fully understand the true physiological cause of cramps and spasms caused by running, according to Runner's World. Several theories attempt to describe this phenomenon.
One theory describes muscle cramping during running and other physical exercise as the result of a failure in the pathways of neural communication, reports Competitor.com. When the brain sends a signal for the muscle to contract, but the muscle fatigues instead, it creates a miscommunication and the muscle remains contracted when it should not. This miscommunication can lead to cramps or spasms in the legs.