The most common cause is from a muscle or tendon strain. The outside of the thigh is made up of the following structures: the vastus lateralis muscle, the tensor fascia latae muscle, and the iliotibial tract (commonly known as the IT band). The biceps femoris also runs down the side of the thigh close to the knee. If any of these become injured or stretched past their normal limits, muscle pain may arise.
The IT band for runners, in particular, is problematic. In the running gait, the IT band may become aggravated from overuse or too much, too soon. If you’re a running experiencing muscle on the side of your thigh, an IT band injury is a possible cause. For runners, the greater trochanter and other structures at the hip may also become irritated from repetition and overuse.
More serious conditions may also be the issue. For instance, meralgia paresthetica is defined by pressure on the lateral femoral cutaneous nerve. When this nerve is compressed, numbness, burning, and tingling may result on the side of the thigh. Blood clots are further a possibility. Although, blood clots are more rare in the upper leg than in the lower leg.