Outcome measures in amyotrophic lateral sclerosis (ALS) clinical trials rely mostly on changes in lower motor neuron (LMN) function. Quantitative and sensitive measures of disease progression in upper motor neuron (UMN) predominant ALS or Primary Lateral Sclerosis (PLS) are lacking. Recently, the Accurate Test of Limb Isometric Strength (ATLIS) was validated and shown to be more sensitive than either vital capacity or the revised ALS Functional Rating Scale (ALSFRS-R) but its utility in monitoring disease progression in UMN predominant ALS or PLS is unknown. To evaluate markers of disease progression in UMN predominant ALS and PLS using ATLIS we are analyzing parameters in addition to peak force in this population. To date, we have enrolled 126 patients diagnosed with motor neuron disease as part of a longitudinal ATLIS study at Cedars-Sinai Medical Center. Forced vital capacity, ALSFRS-R, and ATLIS scores were obtained longitudinally for most of those patients. A retrospective analysis of the 126 subjects has so far identified 8 patients diagnosed with PLS at the time ATLIS was first performed, 7 UMN predominant ALS patients, and 11 with primary muscular atrophy (PMA) or lower motor neuron predominant ALS. Comparisons of different ATLIS parameters between UMN ALS and/or PLS and LMN ALS and/or PMA are underway, including time to peak force and rate of ascent to peak force as potential markers for UMN involvement. Sensitive and objective markers of disease progression including ATLIS have the potential to detect treatment responses across different ALS variants.