Characters with this fear are afraid of losing physical mobility and becoming trapped in their own body. It can arise from a number of events, such as being diagnosed with a degenerative condition, a close friend being paralyzed in an accident, or a mental health condition that causes the character to obsess over such a possibility. For a character with this fear, a loss of bodily control may be inevitable (if they've received a devastating diagnosis) or something they worry could happen in the future.

Self-checking the body for abnormalities
Creating a living will
The character taking good care of themselves (exercising regularly, eating healthy foods, avoiding cigarettes, scheduling regular check-ups, etc.)
Being active
Having tests conducted that can predict the presence of certain genetic conditions 
Being fiercely independent
Having claustrophobic tendencies
Being overly cautious when it comes to health and personal risk
Strictly following safety guidelines and road laws
Avoiding activities that could cause an accident (diving, climbing, etc.)
Driving themselves everywhere (instead of trusting someone else to do it)
Avoiding anyone who has lost their mobility (Lou Gehrig's disease patients, people who are paralyzed, etc.)
High anxiety; worrying that something is wrong and obsessing over possible diagnoses
Researching symptoms online and haunting forums but not seeking medical attention