Lower back muscles get very stiff even in the best of office chairs. Forward bending is the most fundamental method of stretching and lengthening them.
I recommend using active isolated stretching over static stretching, especially in cases where muscles are extremely stiff and tight.
Active isolated stretching is more effective because it does not provoke the protective stretch reflex.
One of the chief causes of lower back pain is tightness in the gluteal muscles. Here is a simple, yet extremely effective technique you can do anytime during the day.