Meaning: His muscles feel very hot and painful as he runs.
Choice of Words: "Burn" means to be on fire or to feel very hot. "Muscles" are the body parts that allow you to move. "Fire" is the process of burning something.
Alternative Expressions
My legs are burning with fire.
Meaning: This sentence describes a feeling of extreme pain or discomfort in my legs.
Choice of Words: My legs are the lower limbs of my body that I use to walk and run. Burning with fire is a metaphor to describe the intense pain or discomfort I feel in my legs. Fire is a symbol of heat and destruction, so burning with fire suggests that the pain is severe and almost unbearable.
The muscles in my legs are on fire.
Meaning: This sentence also describes a feeling of extreme pain or discomfort, but it specifically refers to the muscles in my legs.
Choice of Words: Muscles are the tissues in my body that allow me to move. In this sentence, the muscles in my legs are the specific muscles that I am referring to. On fire is a metaphor to describe the intense pain or discomfort I feel in my muscles. Fire is a symbol of heat and destruction, so on fire suggests that the pain is severe and almost unbearable.
My muscles feel like they're burning with fire.
Meaning: This sentence describes a feeling of extreme pain or discomfort in my muscles, and it uses a simile to compare the pain to the burning sensation of fire.
Choice of Words: My muscles are the tissues in my body that allow me to move. Feel like they're burning with fire is a simile to describe the intense pain or discomfort I feel in my muscles. A simile is a figure of speech that compares two things using the words like or as. In this case, I am comparing the pain in my muscles to the burning sensation of fire. Fire is a symbol of heat and destruction, so burning with fire suggests that the pain is severe and almost unbearable.