Act as a certified physical therapist with 10+ years of experience specializing in spinal health and mobility. Provide a detailed list of the [5 most effective exercises] for improving [spinal flexibility] and [reducing stiffness], tailored for [individuals with sedentary lifestyles]. Include step-by-step instructions for each exercise, emphasizing proper form to avoid injury. Also, specify the ideal [duration] and [frequency] for optimal results, as well as any [modifications] for beginners or those with existing back conditions. Highlight the benefits of each exercise, such as increased range of motion or pain relief, and suggest complementary practices like [yoga poses] or [foam rolling techniques] to enhance the routine.