The cause of lower back pain while standing is often age-specific. Those who are younger (<55 years old) often have lower back pain when standing due to muscle cramping and back compression. Those who are older (65+ years old) often have lower back pain when standing due to nerve compression from arthritic narrowing of the spinal canal.
Lower back pain in ‘younger’ people
Younger people often have back pain when standing because they adopt standing postures that require the muscles in their lower back to be constantly contracted. This constant muscle contraction often leads to pain from muscle cramping and back compression. The standing postures that most commonly cause constant muscle contraction are a) standing with a slight forward lean, b) standing with a forward poking chin, c) standing with forward rounded shoulders, or d) standing with arms folded across the chest.
Relieving this pain is accomplished by adopting standing postures that keep the muscles of the lower back soft and relaxed. The four posture changes that do this are a) standing tall by trying to get the crown of your head as close to the ceiling as possible, b) standing with your chin pulled back so that your ears are above your shoulders, c) standing with your shoulders gently pulled back, and d) keeping your hands at your sides or clasped behind your back instead of folded across your chest. For most people, it is one or a combination of these posture changes that works.
An easy at-home-test to determine if this is the cause of your pain is as follows. Begin by assuming your normal standing posture and then place your hands on your hips with your thumbs pointing backward. Slide your hands toward the middle of your back until your thumbs are on the muscles that run up and down along the side of your spine. Determine whether those muscles feel hard or soft by ‘poking’ them with your thumbs. If they feel hard and contracted, try one or more of the posture changes above to find the one(s) that make your muscles soft and relaxed. Take note of how you are standing now and ensure you stand like that from now on to enjoy longer periods of standing without lower back pain.
Have a look at this video for a demonstration of how to feel if your back muscles are soft and relaxed or hard and contracted and the postural modifications you can make to keep your back muscles soft and relaxed while standing.