The EMG can only detect lower motor neuron dysfunction (as Zaphoon told you), because it's the lower motor neurons that directly innervate the skeletal muscles and therefore directly affect them. The upper motor neurons innervate the lower motor neurons, so they indirectly affect the skeletal muscles. Lower motor neuron dysfunction will cause things such as weakness, muscle atrophy, muscle twitching, diminished reflexes . . . although many other things can cause those same symptoms. Upper motor neuron dysfunction will cause brisk reflexes, muscle spasticity and cramping, primitive reflexes (e.g. Babinski and Hoffman) . . . although once again . . . many other conditions can cause those same symptoms. Now please go relax.