The correct answer is the release of free radicals. In accidental injuries such as a pulled muscle, there will be a subsequent inflammation and the inflammatory process involves the release of inflammatory mediators which then will increase the migration of inflammatory cells (i.e. neutrophils, lymphocytes) on the site of inflammation. This influx of inflammatory cells will produce a large amount of free radicals (i.e. hydrogen peroxide, superoxide ions) which then will damage DNA.