Swimming is an excellent form of low-impact aerobic conditioning that is easy on your back and spine. The buoyancy on the water supports your body weight, reduce stress on your joints and spine while allowing for greater range of motion. Before starting swimming or any new exercise program, it is a good idea to get all clear from your doctor first. https://www.nhs.uk/live-well/exercise/swimming-for-fitness/
Two important advice to consider when swimming with lower back pain:
(1) Avoid shear force
One problem with swimming is that many strokes and approaches create shear force across the structure in your lower back through repetitive rotation of spine and hips. Shear force contributes to the breakdown of the discs and other sensitive structures in your lower back. To avoid this kind of force consider the following (a) getting a coach or a trainer to perfect your stroke such as learning to keep your shoulders in line with your hips while swimming and (b) use a mask and snorkel to eliminate the need to arch your lower back while lifting your head up for air or as you rotate your lower back to turn your head to take a breath.
(2) Focus on strokes that does not cause stress on your spine
The position and movement of various strokes will affect your spine in different ways:
> Free and back stroke do not force your back to arch, however you run the risk of developing or worsening the pain related to your discs and other structures in your lower back due to repetitive rotation in the lower back.
> Butterfly and breaststroke force your lower spine to arch backward during the stroke. These movements add stress to the facet joints in the back of your spinal column and can lead to problems or worsening pain over time.
The bottom line is, when it comes to back pain there is no best or safe stroke with swimming. It all depend on a number of factors, including the underlying cause of your back pain and your swimming ability, mechanics and workout intensity. Because of potential risks involved, if you have back pain it’s best to get advice from your doctor and/or physical therapist before starting a swimming exercise program. Also getting a coach or a trainer will be a good idea in order to help you to perfect the swimming technique best for your needs.