The authors did not measure maximal expiratory pressure (Pemax). Since Pemax correlates with PEFR in patients with ALS,2the interpretation of the PEFT data in the absence of respiratory muscle strength measurements creates more questions than it gives answers. The PEFT reflects in large part the ability of the expiratory muscles to develop rapid (explosive) force and is related to the rate of pressure rise. It is also related to the Pemax because the capacity of skeletal muscles for rapid force development declines in proportion to the ability to generate maximal force.3 If this is not the case for ALS patients, the measurement of Pemax would establish the superiority of PEFT in monitoring respiratory function in these patients. If, on the other hand, changes in PEFT relate to Pemax, then the measurement of Pemax is preferable to that of PEFT. PEFT, as for all indexes of rapid force development, is less reliable and less reproducible than Pemax.5 In addition, the measurement of PEFT will require the standardization of the forced expiratory maneuver and, specifically, the speed of inspiration prior to exhalation, which was not controlled for in the study of Wilson et al.1 A fast inspiration to total lung capacity will prestretch (eccentric contraction) the expiratory muscles, which will then develop greater pressure (and a greater rate of pressure rise) during the subsequent forceful (concentric) contraction.6–7 The property of skeletal muscles to produce greater force when a concentric contraction is immediately preceded by an eccentric contraction is known as the stretch-shortening cycle. Therefore, standardizing the expiratory maneuver used for measuring PEFT will help to minimize the variability of the PEFT.