To treat arthritis in the lower back with physical therapy, begin with gentle stretching exercises like pelvic tilts and knee-to-chest stretches to improve flexibility and reduce stiffness. Strengthen core muscles with exercises such as bridges and modified planks to support the spine. Incorporate low-impact aerobic activities, like walking or swimming, to enhance mobility without stressing the joints. Heat therapy before exercise can reduce stiffness, while cold therapy afterward helps manage inflammation. Gradually increase exercise intensity as strength improves.