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Walking is a simple and inexpensive way to start becoming more active. Taking a brief walk of just 15 to 20 minutes most days of the week will help strengthen your lower back as well as the rest of your body. Try walking with a buddy to help motivate you and make your walks more fun. If you're walking by yourself, you may want to listen to music, a podcast, or an audio book. If you have lower back pain to the point that you feel more comfortable sitting than standing, cycling is a good option for cardiovascular exercise. A stationary bike inside typically will be better for you than the bumpy, uneven road terrain. Cycling is low-impact and easy on your joints, and will strengthen your legs, hips, and lower back while also giving you a good cardiovascular workout. Mixing your cardio and strength training together creates an effective workout that will strengthen your lower back without increasing any lower back pain you may have. You can find beginner interval training workouts, including a lot of videos, online for free. Doing 3-5 minute intervals of high-intensity cardio between strength-training intervals will help you get and keep your heart rate up without putting too much strain on your lower back. If you have convenient access to a pool, swimming for just 20 to 30 minutes two or three days a week is a good way to strengthen your entire back. To avoid making any back problems worse, join a class or hire a coach to perfect your technique.  Swimming is a low-impact exercise and the water helps support you, making it great exercise if you have joint problems or are overweight. If you're new to swimming, start slow with 10-minute swims. Every week or so, increase your time in the water by five minutes until you're swimming for a half hour or longer each session. If you're not up for swimming, walking or jogging in water provide some resistance to help strengthen your legs and lower back, without you having to worry about your breath. Over the course of your day, you should aim to walk at least 10,000 steps. A pedometer secured to your waist can keep track of those steps for you. Some models also connect to the internet and have apps that allow you to keep track of your progress over time.  Choose a pedometer that you can use easily and that will help you achieve your goals. You can get a very basic model, or one with a lot of extra features. If you're new to a more active lifestyle, set smaller goals to start and work your way up to 10,000 steps. Introduce walk breaks into your day by doing things such as parking further away when you go shopping, or taking the stairs instead of the elevator. Sitting for long periods of time can cause your lower back muscles to atrophy. Prevent this by getting up and walking around every 30 minutes or so if possible, and try to reduce the number of hours you sit overall.  For example, if you spend most of your work day sitting, try to remain standing when you go home, rather than sitting down on the couch to watch television. You also can invest (or ask your boss to invest) in a standing desk, so that you can stand for periods as you work throughout the day.
Go for walks regularly. Take up cycling. Exercise in intervals. Try swimming. Buy a pedometer. Maintain an active lifestyle.