A young kid falls from her bicycle.  An X-Ray reveals a comp…

A young kid falls from her bicycle.  An X-Ray reveals a complete fracture of her left Radius.  The doctor places a cast on her arm, which immobilizes her arm for 8 weeks.  After 8 weeks the cast is removed, however the muscles or the forearm are noticeably smaller when compared to the right forearm.  Which of the following would help explain the decrease in the size of her forearm muscles: