The most common reason for cramping when you tense your foot is because of muscle tension fasciitis. Muscle tension fasciitis is generally common due to exertion, cold and lack of muscle strength, when the muscle inflammation exudes more, muscle fatigue, and then tensing the foot to pull the muscle will cause the muscle can not get rest thus causing spasms and cramps. The cramp will aggravate the original injury and inflammation, thus causing persistent pain that is not easily relieved. Other common causes include local muscle injury, nerve entrapment, nerve injury, inadequate blood supply due to vascular disease, and other less common nerve compression by herniated discs or spinal stenosis.