Interplay of upper and lower motor neuron degeneration in amyotrophic lateral sclerosis.

Objective: We studied motor unit recruitment to test a new method to identify motor unit firing rate (FR) variability.

Methods: We studied 68 ALS patients, with and without upper neuron signs (UMN) in lower limbs, 24 patients with primary lateral sclerosis (PLS), 13 patients with spinal cord lesion and 39 normal subjects. All recordings were made from tibialis anterior muscles of normal strength.