Electromyography in patients with peripheral nerve lesions does not show denervation potentials in muscles of similar myotomes innervated through a different nerve. The distribution of arm weakness due to a peripheral nerve lesion is usually very different than that produced by an upper motor neuron lesion. The differentiation between an upper motor neuron lesion and peripheral nerve injury can usually be made on clinical grounds. Electromyography and nerve conduction velocities are abnormal in peripheral nerve lesions.