Pain in the inner thigh can be caused by a strained adductor muscle, explains North Shore-LIJ Orthopaedic Institute. A strained thigh muscle is categorized as a grade I, II or III strain, with a grade I strain being relatively minor and a grade III strain being the most severe.
There are four primary adductor muscles in the thigh: the adductor longus, adductor brevis, adductor magnus and gracilis muscles, notes MassageTherapy.com. If the pain is in the upper portion of the inner thigh, the person's injury likely involves an injured tendon in one of these four muscles. Pain nearer to the mid-thigh suggests injured muscle fibers, and pain in both areas suggests injuries to both the tendons and the muscle fibers.
Grade I thigh-muscle strains are commonly acquired while stretching, notes North Shore-LIJ Orthopaedic Institute. This type of muscle strain involves only minimal tearing of the muscle. A grade II strain involves partial tearing of the muscle and is more severe than a grade I strain. A grade III strain occurs when part of the affected muscle group completely ruptures.
In addition to pain, there are some other symptoms that suggest a thigh muscle is strained. Bruising, swelling or redness following a thigh injury are common signs, notes North Shore-LIJ Orthopaedic Institute. A popping or snapping sensation of the muscle may also occur, and the muscles or tendons may be weaker than usual.