Contrast bath and  stretching exercise of spine. Avoid long sitting front of computer.

Chiropractic Mobilization will help. Do the cat/cow stretch. Get on all fours, with your arms straight and your hands directly under your shoulders; your knees hip-width apart.

Sitting/standing for prolonged period (>30mins) puts stress on your spine and back muscles that is why you feel the stiffness. Perform back stretches and core strengthening exercises followed by some hot fomentation. Take breaks and pace your activities so as to avoid sitting for long time. And most importantly correct your posture.

If you will sit long period definitely Core  muscles will get strain and also get stiffness. Do Core strengthening exercises and stretching exercises. Do Hot Water formantation.

Long sitting or standing cause stiffness in muscles, specially when posture is not correct. Better u should maintain proper posture while sitting or standing. Take frequent breaks n stretch ur self.