Project Abstract This research aims to develop a novel motor unit number estimation (MUNE) technique that integrates two recent advances in electrophysiology, i.e. compound muscle action potential (CMAP) scan and high-density surface electromyography (HDEMG), to provide accurate and reliable estimation of functioning motor units in the muscle. The proposed technique will be used as a sensitive biomarker for quantifying or tracking motor unit population in clinical settings or to explore axon excitability and muscle rearrangements in response to medication, exercises or physical therapies in aging, amyotrophic lateral sclerosis (ALS) and other neurological disorders.