An Olympic sprinter readies for a race. After the sound of the starting gun, he propels himself forward from the starting block and immediately grimaces in pain, grabbing for the back of his thigh. Within 48 hours he begins noticing extensive bruising on the back of his thigh extending into the back of the knee. He now has difficulty rising from a seated position and flexing his knee. Bending at the waist generates more pain. Which muscle is likely injured?
A) Biceps femoris
B) Miopsoas
C) Gluteus maximus
D) Popliteus

The sprinter grabbed the back of his thigh because it starts right on top of the posterior end of the pelvis and extends almost midway into the thigh.

The functions of the Gluteus Maximus include but are not limited to:

It keeps the leg straight at the hip;
Most importantly, when the body stoops or bends over, it helps the body regain an upright erect position. It does this by (in conjunction with other muscles) pulling the pelvis backwards.

Because the sprinter injured the GM, he or she will be unable to resume upright position or flex his knee (which exerts pulling stress on the GM) without feeling a great deal of pain.

Cheers!