Why One Shoulder Is Stiffer Than the Other (and Whether to Worry)

Most of us favour one side. You may reach for things with your dominant arm, carry a bag on one shoulder, or sleep on one side. Over time, the side you use less may feel tighter.

Sleep position is one habit worth noticing. Some studies report that people with one-sided shoulder pain often sleep on the painful side. The cause is not known, so this is an association, not a proven mechanism [R29].

• Joint sense matters. Shoulder disorders go with reduced joint position sense, and rehabilitation tends to move from range of motion to strength, then to control and coordination [R16].
• Both sides learn from each other. In one study of the wrist, training that challenged joint sense improved the trained wrist and the untrained opposite wrist [R15]. The sides of the body can share some learning.
• Two-arm training has a reason behind it. In stroke rehabilitation, training both arms together improved shoulder movement more than single-arm training [R14]. That evidence comes from stroke patients, not healthy adults. It is a reason the idea is interesting, not proof that any device works.

1. Check for warning signs first. Look at the red flags below. New, sudden or worsening stiffness deserves an assessment.
2. Map your range gently. Sitting tall, slowly reach each arm forward, out to the side and overhead, only as far as feels comfortable. Compare the two sides.
3. Seated option: assisted reach. Use your stronger hand to help gently guide the stiffer arm through a comfortable range. Keep it light and controlled.
4. Experiment with sleep. Try a different position for a few nights, such as a pillow between your arms. Notice what changes.
5. Move daily in small amounts. Short, regular sessions are easier to keep up than rare, long ones.