It’s time for leg power

Keeping one leg straight, kick the other one up. Aim for a horizontal leg – i.e. perpendicular to your trunk. If you don’t fully get there, that’s OK. You will improve every time you try it.
Kicks will build muscles in the leg and hips, which will be great for your back. Do 15-20 kicks. A heated swim spa in your own backyard makes these exercises really convenient.