Hi, I'm Dr. Aria Yarmand, and this is safe cardio for a back strain. Walking is the best low impact exercise and does not negatively affect the lower back. Running on the other hand may cause undue stress on the disks of the lower back. When you run in a healthy spine there's pressure put on the disks. Normally your body can handle this. However if you've had previous lower back pain the pressure from running may aggravate your symptoms. Stationary bikes are another example of a low impact exercise which is safe for the lower back. Keeping in mind that during stationary biking you should always keep an arch in the lower back. Another safe exercise for muscle strain is the plank. Resting on your elbows and keeping your toes on point make sure that the spine is straight during this exercise. Hold this position for between 10 to seconds. This helps strengthen the core and stabilizes the spine. Another safe exercise for back strain is the McKenzie exercise. Lying flat on your stomach do a modified push-up by extending your arms upwards. Making sure your pelvis is flat on the ground when you are doing it. Repeat this between eight to 12 times for about 15 seconds each repetition.