It is estimated that 60%-70% of all back pain is the result of muscle strain. Your musculo-skeletal system needs a basic level of strength and endurance in order to protect itself against back pain. Your muscles need a good supply of blood and energy in order to be maintained at peak performance. Poor posture or muscle spasms can deprive the cells and tissues in your back muscles of a good supply of blood and oxygen.
For example, when you sit and bend forward (i.e., typing at a computer) two of the three muscle groups which support your back (abdominal and hip flexors) are inactive and, therefore, being detrained (weakened). If this posture is maintained, muscles will become painful, weak, less elastic, and shortened.