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Objective: An inability of the nervous system to fully activate the muscle is one factor that can contribute to age-related muscle weakness. Superimposed electrical stimulation can be used to determine voluntary muscle activation (VA). The aim of this study was to assess VA of the quadriceps muscle in healthy older and younger subjects. Design: Electrical stimulation causes moderate discomfort in younger subjects, but no study has assessed discomfort in older subjects. The quadriceps muscle In 20 moderately active older subjects (mean age, 75 yrs) and 12 younger subjects (mean age, 25 yrs) was stimulated during two maximal voluntary contractions using a 100-Hz pulse train. A visual analog scale for pain (VAS-pain) was used to evaluate discomfort. Results: Ability to activate the quadriceps muscle was generally very high, and there was no significant difference between the older (mean, 0.96) and younger (mean, 0.98) subjects. Discomfort did not differ between the older (mean VAS-pain score, 41 mm) and younger (mean VAS-pain score, 37 mm) subjects. Conclusions: Our results indicate that healthy, moderately active older subjects have the ability to almost complete VA of the quadriceps muscle and that discomfort during electrical stimulation is generally moderate. (Less)