Shoulder weakness can be caused by a rotator cuff tear, strained muscles or tendons, joint stiffness, arthritis, or a problem with nerves that travel from the neck into the arm. A rotator cuff problem often makes lifting or rotating the arm painful or difficult, while nerve problems may also cause tingling or numbness. Because these conditions can overlap, a clinician may need to examine the shoulder, neck, and arm.
How can a shoulder problem lead to weakness?
The shoulder relies on muscles, tendons, nerves, and joints working together to move and stabilize the arm. The rotator cuff is a group of muscles and tendons that helps keep the upper arm bone in its socket and allows controlled shoulder movement. Nerves carry signals from the brain and spinal cord to the muscles, allowing them to contract.
Weakness can occur when an injury damages a muscle or tendon, when a nerve cannot transmit signals properly, or when pain and stiffness make normal movements difficult. Sometimes, a person feels unable to lift the arm because movement hurts; in other cases, the muscle itself has lost strength. These differences can help guide a clinical assessment, although symptoms alone cannot reliably identify the cause.
What are the common causes of shoulder weakness?
1. Rotator cuff injury or tear
The rotator cuff helps lift and rotate the arm. Its tendons can become irritated, damaged, or torn through gradual wear or a sudden injury. A tear may make it difficult to raise the arm, reach overhead, lift objects, or reach behind the back. Shoulder pain may worsen at night, although some rotator cuff injuries cause little or no pain.
According to Mayo Clinic, rotator cuff injuries may develop gradually with repeated overhead movements or heavy lifting, or occur after a fall or accident. The likelihood of these injuries increases with age, and they are common in adults older than 50. However, they can also occur in younger people, particularly during sports or other activities that stress the shoulder.
2. A pinched nerve in the neck
A nerve that leaves the spinal cord in the neck can become compressed or irritated. This is called cervical radiculopathy. Although the problem begins in the neck, it can affect the shoulder, arm, or hand because the involved nerve supplies these areas.
The American Academy of Orthopaedic Surgeons (AAOS) explains that a pinched nerve may cause pain that travels from the neck into the arm, muscle weakness, numbness, or tingling. Age-related changes in the spine, such as arthritis, can contribute to nerve compression. A herniated disc, in which part of a disc pushes outward and presses on a nerve, is another possible cause.
Some people notice neck discomfort along with arm weakness, while others mainly experience symptoms in the shoulder or arm. Nerve-related symptoms can overlap with shoulder tendon problems, so an examination of both regions may be necessary.
3. Injury to the nerves around the shoulder
The brachial plexus is a network of nerves that runs from the neck through the shoulder into the arm and hand. These nerves help control movement and carry sensory information. An injury to this network can interfere with muscle function and may affect more than one part of the arm.
According to the Cleveland Clinic, brachial plexus injuries can occur when the neck and shoulder are forcefully pulled apart, the arm is stretched suddenly, or trauma damages the nerves. Symptoms may include weakness, pain, reduced sensation, or difficulty moving the arm or hand. The effects depend on which nerves are involved and how the injury occurred.
Other nerves near the shoulder can also be affected by injury or compression. The location of the nerve problem can influence which muscles become weak and whether numbness or tingling occurs alongside the weakness.
4. Muscle strain or tendon inflammation
A strained shoulder muscle or an irritated tendon can make lifting and rotating the arm uncomfortable. These problems may follow unfamiliar exercise, heavy lifting, repetitive work, or a sudden movement. Pain can limit how much force a person can comfortably use, creating a feeling of weakness even when the muscle is not completely damaged.
Symptoms may improve as the injured tissue recovers, but ongoing pain or a persistent reduction in strength deserves medical evaluation. A strain and a tendon tear can produce similar complaints, so the extent of an injury should not be assumed from symptoms alone.
5. Joint stiffness, arthritis, or a shoulder injury
Problems affecting the shoulder joint can also interfere with arm movement. Frozen shoulder causes stiffness and restricted motion, while arthritis can contribute to pain and difficulty moving the joint. A shoulder dislocation or fracture may cause sudden pain and an inability to use the arm normally after an injury.
MedlinePlus, a service of the US National Library of Medicine, lists sprains, strains, dislocations, fractures, frozen shoulder, arthritis, and torn rotator cuffs among common shoulder problems. These conditions do not all affect strength in the same way, which is why the history of the symptoms and the physical findings matter.
What symptoms may occur with shoulder weakness?
The experience of shoulder weakness depends on the structures involved. A person may notice reduced strength, pain with movement, restricted motion, or changes in sensation. Examples of symptoms that may accompany shoulder weakness include:
- Difficulty lifting the arm overhead or reaching for items on a shelf.
- Pain when lifting, rotating, or reaching behind the back.
- Aching around the shoulder, sometimes more noticeable at night.
- Pain that begins in the neck and travels into the shoulder or arm.
- Tingling, numbness, or changes in sensation in the arm or hand.
- Difficulty controlling arm movements or using the hand normally.
- Stiffness that limits how far the shoulder can move.
These experiences can occur with several different conditions and are not a reliable way to distinguish a tendon tear from a nerve problem. For example, both conditions can interfere with raising the arm, and some rotator cuff injuries cause little pain. The timing of the weakness, how it began, associated symptoms, and examination findings provide important additional information.
How do doctors distinguish a rotator cuff tear from a nerve problem?
A clinician usually begins by asking when the weakness started, whether it followed an injury, which movements are affected, and whether symptoms are changing. Information about neck pain, tingling, numbness, previous shoulder injuries, work activities, and sports may also be relevant.
During the examination, the clinician may assess shoulder movement, compare strength on both sides, check tenderness, and examine the neck, arm, sensation, and reflexes. Certain movements may help identify which structures need closer assessment, but no single finding necessarily establishes the cause.
The AAOS describes imaging and nerve tests that may help when cervical nerve compression is suspected. Depending on the findings, a clinician may consider the following investigations:
- X-rays: May help identify fractures, joint changes, or abnormalities in bone alignment.
- Ultrasound or MRI: May be used to examine soft tissues, including rotator cuff tendons, when further assessment is needed.
- MRI of the neck: May help identify a disc problem or other changes associated with nerve compression.
- Electromyography (EMG) and nerve conduction studies: May help assess nerve and muscle function when the source of weakness is uncertain.
Not everyone needs imaging or nerve testing. The choice depends on the medical history, examination findings, suspected cause, and whether the results are likely to influence care.
When should shoulder weakness be checked urgently?
Seek prompt medical assessment if weakness begins suddenly after a fall, collision, or other shoulder injury, particularly if you cannot raise or use the arm normally. A visibly deformed shoulder, substantial swelling, or intense pain after an accident also warrants urgent assessment because a fracture or dislocation may be present.
Mayo Clinic recommends immediate medical attention for certain shoulder injuries, including those involving a deformed joint or an inability to move the arm away from the body. Weakness that persists, worsens, or interferes with everyday activities should also be evaluated rather than repeatedly worked through.
Sudden weakness affecting one arm, especially when accompanied by facial drooping, difficulty speaking, confusion, or sudden balance problems, may indicate a stroke. The US Centers for Disease Control and Prevention (CDC) advises calling emergency services immediately when stroke symptoms occur. Do not assume that sudden arm weakness is simply a shoulder injury.
Shoulder discomfort accompanied by chest pressure, difficulty breathing, or unusual sweating can also signal a medical emergency. Seek emergency help rather than trying to determine whether the symptoms originate in the shoulder.
What happens after the cause is identified?
Management depends on whether the weakness comes from a tendon injury, nerve compression, muscle strain, joint disease, or another problem. Some conditions improve with activity adjustments, appropriate pain management, and a tailored rehabilitation plan. Other injuries, including certain traumatic tendon tears or significant nerve injuries, may need specialist assessment and additional treatment.
Until the cause is clearer, avoid repeatedly lifting heavy objects or forcing movements that worsen the weakness or pain. Do not begin strenuous shoulder exercises simply to test your strength, especially when symptoms started suddenly or after an injury. A clinician or physical therapist can help determine which movements are appropriate and whether further investigation is necessary.
The key point is that shoulder weakness can arise from structures within the shoulder or from nerves connecting the neck and arm. Identifying the source helps guide appropriate care, particularly when weakness is new, persistent, or getting worse.