Bad backs are equally likely to be due to strains, caused by lifting heavy weights, as they are inactivity such as sitting for long periods in one position in the office or in a car.
When you feel able, try the following. Please don't do them if they cause pain but if you are able to, they will help strengthen both the back muscles and those in the abdomen which provide balance in the core.
I've personally found them excellent for preventing recurrences (and later have discovered they are exercises used by physios for bad backs):
- On your hands and knees, arch up your back, aiming to rock slightly back and forwards. This helps stretch the upper and lower back after periods of inactivity (maybe do it when you get in from work?).
- Staying on your hands and knees, raise your left arm and right leg so that they are straight, then bring them both underneath you so the elbow and knee almost touch. Repeat this for 30 seconds, rest, then do the same with the right arm and left leg.
- Lying on your back with your legs bent and feet on the floor, raise one bent leg so the knee approaches your hands, then grab behind the knees and pull towards your torso. Repeat on the other side and then try with both legs.
- Finally, the best thing I ever did for a bad back was sit-ups. Always start with bent legs and tuck your toes under the edge of a chair to provide a brace and lift the shoulders only (it isolates the abdominal muscles more). When you get good, lie on you back, then lift both your shoulders and straightened legs off the floor, balancing on your bottom, hands on your ears. Then pull both the legs in and torso up so the knees meet with the elbows. Don't pull the head with your hands as you might strain your neck.
The abdominal muscles oppose those of the lower back and when the latter get stronger, it causes the lower back to arch in an excessively concave fashion. Stronger abdominals will help pull that part of the torso straighter again.
For now, though, do try some Atrogel® 2-4 times a day and those stretching exercises.