Chapter 10, Problem 2CCS

In Chapters 7 and 8, you were introduced to Kayla Tanner, a 45-year-old mother of four who had suffered a dislocated right hip in the bus accident on Route 91. Six weeks after the injury, Mrs. Tanner reported that she was still unable to walk or run without hip pain, and had weakness in her hip, knee, and ankle. Mrs. Tanner walked with a limp that her doctors attributed to weaknesses in flexion at the knee, inversion of the foot, and plantar flexion.

Electromyography (which measures muscle electrical activity) and nerve conduction studies (which measure the speed of nerve impulses) revealed that Mrs. Tanner’s sciatic nerve had been damaged as a result of her injuries—most likely as a result of the nerve being compressed when the hip was dislocated. This large nerve innervates a large number of lower limb muscles. Since her surgery, Mrs. Tanner has been undergoing intense physical therapy and has shown significant improvement.

In order to assess the function and strength of a specific muscle, a physical therapist will often apply resistance (push against the moving limb) to mimic the action of an antagonist muscle. What is an antagonist muscle, and why would the therapist mimic its action?