A thigh strain is a tear to your quadriceps (thigh muscle).
It is a common injury sustained in sports like football that involve kicking or sprinting.
Your quadriceps are located at the front of the thigh and are responsible for straightening your knee. The Quadriceps muscle is made up of four muscles: Rectus Femoris, Vastus Lateralis, Vastus Intermedius and Vastus Medialis.
Any of these muscles can strain (or tear) but the most common is the Rectus femoris. This is because it is the only one of the four muscles which crosses both the hip and knee joints. This make it more susceptible to injury.