Walking stresses the joints least, but jogging and running will burn more calories per minute. The treadmill surface can be hard. If you plan to run or jog, investing in a surface with more give is worth it. If you plan to walk only, you can get by with a narrower and shorter treadmill belt, but it should still be at least 5 feet long. Make sure the handrails can support your weight should you lose your balance.
Some treadmills come with resistance levers for upper-body exercise.
But they may feel awkward during a long workout. Holding the handrails reduces the workout intensity. Once you have mastered the technique, keep your hands off the rails whenever possible. When walking or running on a slant, maintaining good posture with only a slight bend at the waist is best for your back. Exercise bikes provide fairly intense workouts with little stress to the knees.