Why are upper motor neuron signs difficult to elicit in amyotrophic lateral sclerosis?

It is often difficult to identify signs of upper motor neuron lesion in the limbs of patients with amyotrophic lateral sclerosis, in whom there is neurogenic muscle wasting of varying severity. The reasons for this are complex and not related simply to the degree of lower motor neuron muscle wasting but, rather, depend on the pathophysiological abnormalities that develop in response to damage to descending motor pathways and to motor neurons and interneurons in the ventral horns of the spinal cord. The different mechanisms underlying the clinical phenomenology of the functional motor defect in amyotrophic lateral sclerosis, that lead to difficulty in detecting classical upper motor neuron signs, are discussed.