A back spasm is a sudden and involuntary contraction, or uncontrollable tightening, of a muscle in response to strain, overuse, weakness, or muscle pain related to injury or a disorder.Back spasms often occur near the spinal cord or near the nerve roots that lead in and out of the spinal cord. back spasm is a sudden and involuntary contraction, or uncontrollable tightening, of a muscle in response to strain, overuse, weakness, or muscle pain related to injury or a disorder.