Aging effects on motor units in the human thyroarytenoid muscle

Objectives: To determine whether age differences are present in the human laryngeal thyroarytenoid muscle that would indicate that different normative values would be needed for identifying motor unit abnormalities. Study Design: Twenty-six consecutively recruited healthy subjects between the ages of 21 and 72 years participated in a laryngeal electromyography study. Methods: Bipolar needle electrodes were used to record motor unit action potentials from several locations in the right and left thyroarytenoid muscles of each subject. The duration of a motor unit was measured when at least 10 firings of the same motor unit could be identified. On the average, four units were measured per muscle. Results: In the subjects less than 60 years of age, motor unit duration did not increase significantly with age. However, motor units from subjects greater than 60 years of age had longer durations than those from subjects less than 60 years of age (P < .00005), and 25% of the units measured in subjects greater than 60 years of age had longer durations than any of the units measured in subjects less than 60 years of age. Further, the older subjects differed from each other in their mean unit durations (P < .0001). In subjects less than 60 years of age, significantly longer durations were found for units innervated by the longer, left-side recurrent laryngeal nerve in comparison with the right-side nerve (P = .005). Conclusions: Different mean and SD values should be used for patients less than and greater than 60 years of age and for the right and left sides, when evaluating motor units in the thyroarytenoid muscles.