The inability to lift your arm usually comes from a problem in the shoulder joint itself, most commonly a rotator cuff injury, shoulder impingement, or frozen shoulder. Less often, it signals a nerve issue in your neck or, in rare but urgent cases, a stroke. The cause matters because some of these need immediate attention while others improve gradually with the right approach.
Rule Out a Stroke First
If your arm weakness came on suddenly, especially on one side of your body, treat it as a medical emergency until proven otherwise. Stroke causes sudden numbness or weakness in the face, arm, or leg, and it almost always affects just one side. Other warning signs include sudden confusion, trouble speaking, difficulty seeing, loss of balance, or a severe headache with no clear cause.
Use the F.A.S.T. test: ask the person to smile (does one side of the face droop?), raise both arms (does one drift downward?), and repeat a simple phrase (is the speech slurred?). If any of these are present, call 911 immediately. Do not drive to the hospital. Paramedics can begin treatment on the way.
If your arm trouble developed gradually over days or weeks, or if it’s clearly connected to shoulder movement and position, a stroke is unlikely. The remaining causes below are far more common.
Rotator Cuff Injuries
The rotator cuff is a group of four muscles and tendons that hold your shoulder joint together and control arm movement. The supraspinatus muscle, the one most frequently injured, is responsible for the first 30 degrees of lifting your arm away from your body. When it’s damaged, that initial lift becomes painful or impossible.
Rotator cuff problems exist on a spectrum. Tendon irritation (tendinopathy) causes pain with movement but the tendon is still intact. Partial tears damage some of the tendon fibers. Complete tears mean the tendon has fully separated, and you may not be able to hold your arm up at all. If someone lifts your arm out to the side for you and you can’t slowly lower it back down on your own, that’s a strong indicator of a significant tear.
How these injuries happen depends on your age. In younger people, a sudden force like a fall or heavy lift can cause an acute, complete tear. In people over 40, years of repetitive overhead motion gradually wear down the tendon, creating degenerative tears that worsen over time. Many people with partial tears don’t realize they have one until a minor event tips it into something more painful.
Treatment for Rotator Cuff Tears
The American Academy of Orthopaedic Surgeons published updated guidelines in 2025 confirming that both physical therapy and surgery produce significant improvement for small to medium full-thickness tears. For partial tears, physical therapy is the recommended starting point, with surgery reserved for people who still have pain and limited function after a proper rehab program.
One important consideration with the non-surgical route: while physical therapy improves how the shoulder feels and functions, the tear itself doesn’t heal. Over 5 to 10 years, the tear size, muscle wasting, and fat replacement in the muscle tissue can progress. This doesn’t mean everyone needs surgery, but it’s worth discussing the long-term picture with an orthopedic specialist, particularly if you’re younger or very active.
When surgery is performed, the results are encouraging. In a recent study comparing surgical techniques, patients who started with an average abduction (side-lifting ability) of about 94 degrees improved to 141 degrees at six months, a gain of nearly 47 degrees. The majority of patients achieved good to excellent functional results.
Shoulder Impingement
Impingement happens when the tendons and fluid-filled cushion (bursa) in your shoulder get pinched in the narrow space beneath the bony roof of the joint. The hallmark symptom is a “painful arc,” meaning your arm feels fine at your side and fine once it’s fully overhead, but there’s a zone of sharp pain between about 60 and 120 degrees of lifting. If you can push through that middle range, impingement is more likely than a tear.
The pain tends to be worst with overhead reaching, putting on a coat, or sleeping on the affected side. Impingement often involves either inflammation of the bursa (bursitis) or irritation of the supraspinatus tendon (tendinitis), and sometimes both at the same time. Bursitis tends to cause more diffuse, swollen-feeling pain, while tendinitis makes it harder to hold your arm in certain positions against resistance.
Most cases of impingement respond well to rest from aggravating movements, ice, anti-inflammatory medication, and physical therapy focused on strengthening the muscles that pull the shoulder blade into better alignment. This gradually opens up the cramped space where the pinching occurs.
Frozen Shoulder
Frozen shoulder (adhesive capsulitis) is different from the conditions above because the joint capsule itself thickens and tightens, physically restricting movement in every direction. You can’t lift your arm, but you also can’t rotate it outward to reach behind your head or inward to reach behind your back. The stiffness affects both active movement (what you can do yourself) and passive movement (what someone else can do with your arm), which distinguishes it from muscle or tendon problems.
It develops in three stages. The freezing phase lasts 2 to 9 months and is the most painful period, with pain that worsens at night and steadily increasing stiffness. The frozen phase lasts 4 to 12 months, during which the pain actually eases but the stiffness becomes the dominant problem, severely limiting daily activities. Finally, the thawing phase brings a gradual return of movement.
The total timeline from onset to resolution can stretch well beyond a year. Frozen shoulder is more common in people with diabetes, thyroid disorders, or after prolonged immobilization from a different injury. Physical therapy during the frozen and thawing phases helps speed recovery, and corticosteroid injections can reduce pain during the freezing phase.
Nerve Problems in the Neck
Sometimes the problem isn’t in your shoulder at all. Cervical radiculopathy occurs when a nerve root in the neck is compressed, usually by a herniated disc or bone spur. When the C5 nerve root is affected, it weakens the deltoid muscle (the main muscle for lifting your arm) and the biceps. You might notice arm weakness along with pain that radiates from the neck into the shoulder and down the arm.
The key difference from a shoulder injury is the pattern: a nerve problem often causes weakness without the same sharp, localized shoulder pain during movement. You may also notice tingling, numbness, or a dull ache that follows a line from your neck to your hand. Turning or tilting your head may reproduce or worsen the symptoms.
Most cervical radiculopathy improves with physical therapy, activity modification, and time. The nerve compression often resolves as inflammation decreases over several weeks to months.
How Doctors Figure Out the Cause
A physical exam can narrow down the diagnosis surprisingly well before any imaging. Your doctor will watch how you move your arm through its full range, testing both what you can do on your own and what happens when they move it for you. They’ll check specific positions: if you can’t slowly lower your arm from a raised position without it dropping, that points to a supraspinatus tear. If lifting your arm with the thumb pointed down reproduces your pain, that suggests impingement.
An MRI is the standard imaging test when a rotator cuff tear is suspected, as it shows soft tissue detail that X-rays miss. X-rays are still useful for ruling out fractures, arthritis, or bone spurs. For suspected nerve problems, an MRI of the cervical spine or nerve conduction testing can confirm the diagnosis.
If your inability to lift your arm developed after a specific injury, came on with significant weakness, or has persisted for more than two weeks without improvement, getting evaluated sooner rather than later helps preserve your options, especially with rotator cuff tears where delays can allow the muscle to deteriorate.