If someone shoves my shoulders and stretches my rectus abdominis muscles (anterior abdominal muscles), then my stretch reflex will occur causing... O contraction of my rectus abdominis and relaxation of antagonist muscles in my back contraction of my rectus abdominis and also of antagonist muscles in my back o relaxation of my rectus abdominis and contraction of antagonist muscles in my back relaxation of my rectus abdominis and also of antagonistmuscles in my back

Contraction of my rectus abdominals and relaxation of antagonist muscles in my back.  

Explanation:

When the stretch reflex occurs, the rectus abdominals, which are the agonists' muscles, contract themselves, and the muscles in the back, which are the antagonists, relax allowing, the moving of our spine and abdomen backward and preventing an incorrect movement were both muscular groups contract at the time.

This reflex allows muscles to stretch up to a specific length preventing injuries and a rapid reaction regarding the antagonists and agonists muscles because the information does not have to travel up to the brain. The sensory information travels up to the spinal cord, to a sensory neuron, then a motor neuron at this level gives a response, which is the stretching of the abdominal rectus up to a certain point and the second set of neurons make are responsible for the contraction of the antagonists' muscles.