Why does my back ache?

You repeatedly lifted your 30-pound grandchild over the weekend, and today you're paying for it — your lower back throbs. Welcome to a very large club: an estimated 84% of Americans cope with lower back pain at some point in their lifetime.

While nearly every major movement we make involves our back, not all back pain is created equal. Symptoms can vary widely, ranging from a dull, ongoing ache to intense, shooting pain or spasms. Sometimes we can pinpoint exactly when back pain struck — like after falling or hoisting a heavy object — but more often, nothing specific happened to trigger the pain.

"It's a bit complicated, because in many cases of back pain, we don't really know exactly where the pain is coming from," says Dr. Joerg Ermann, a rheumatologist at Harvard-affiliated Brigham and Women's Hospital. "But an important message is that it's very common, and in all likelihood it will ease over the course of days to weeks."

Acute vs. chronic back pain

If your back pain started within the past month, it's considered acute (short-term). Doctors often categorize low back pain as nonspecific, meaning they haven't identified an underlying reason for why it's happening. Nonspecific back pain is often muscular in nature, however. It means you've pulled or strained muscles or other soft tissues, such as tendons and ligaments.

Pulls and strains usually happen in a single moment, showing up as sudden, stabbing pain that worsens when you move or twist. You may also feel stiff and achy and have difficulty standing upright. If you've fallen or injured yourself in another way, bruising and swelling on a specific area of your back may be noticeable.

When it's not muscular in nature, back pain can be caused by a lengthy list of conditions that often stem from what are called mechanical problems with spinal discs (the shock absorbers between spinal bones) or, less commonly, inflammation in the spine. These conditions often result in long-lasting or recurrent pain. When the pain continues for more than three months, it's labeled as chronic. "It's an important distinction, because the types of problems that can cause acute or chronic back pain are different," Dr. Ermann says.

These non-muscular causes include

• herniated discs, which may press on surrounding spinal nerves
• osteoarthritis of a facet joint, which can narrow the space around the spinal cord
• degenerative disc disease, in which discs shrink and the spine becomes less flexible
• fractures of spinal bones (vertebrae)
• axial spondyloarthritis, an inflammatory condition of the spine, where back pain is typically worse in the morning and eases with movement.

"There are some bad things that can cause lower back pain, but in most cases, a sudden onset is from muscular strain or injury," he says.

Treatment approaches

Time and home-based treatments usually do the trick for most cases of acute back pain. While you may not feel like moving if you're in serious distress, short walks and gentle stretching can help keep muscles loose and speed your recovery. Other measures include taking anti-inflammatory pain relievers such as ibuprofen (Advil, Motrin) or naproxen (Aleve) and alternately applying heat and ice.

Consider seeing a doctor when back pain that came on suddenly just won't quit. "If after a couple of weeks it's still substantial, you need evaluation by a clinician, " Dr. Ermann says.

After symptoms ease, your doctor may also recommend physical therapy involving stretches and exercises. "We don't often do the things that our spine deserves, which include strengthening the core muscles that support it," he says.

If your lower back pain is accompanied by additional symptoms, the problem may be more serious. Alarming symptoms include fever and chills, unexplained weight loss, pain that spreads down the legs, bowel or bladder problems, and weakness, numbness, or tingling in one or both legs. These signs should prompt an immediate medical evaluation.

"It depends on how bad your symptoms are, but you definitely don't want to wait it out," Dr. Ermann says. "If you can't pee, go to the ER. But if you have shooting pain down your leg, going to urgent care the next day may be appropriate."

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