The risk for a muscle strain may be increased by several factors. In general, your risk is increased if your work or hobbies require a lot of physical activity since your muscles are put to use more often. If you haven't warmed up or stretched properly before starting a physical activity, your muscles are tighter and you're at a higher risk for straining them. This is also the case if you haven't properly conditioned your muscles before starting vigorous physical activity. Also, if you're not very flexible to begin with, you may be more likely to strain your muscles. Muscles are more susceptible to injury if they're tired, so overuse or excessive physical activity may increase your risk for a muscle strain.