The American Academy of Orthopaedic Surgeons says you're more likely to pull a muscle if:

• You have tight muscles that haven't been properly stretched or conditioned.
• You have muscles that work together, such as the quadriceps and hamstrings, but one muscle in the tandem is weaker than the other. This is known as muscle imbalance.
• You have a fatigued muscle that is no longer able to properly absorb energy when you run or play sports.