BACKGROUND: Biologic changes are expected to occur prior to disability. Compared with physical disability measures, measures of muscle impairment may be an earlier indicator of functional decline. The purpose of this study was to describe a new approach of measuring muscle impairment during a functional task. METHODS: Right quadriceps muscle activity was recorded using surface electromyography (sEMG) from 160 older women (age 73.9 +/- 3.9 years, mean +/- SD). Specific patterns of muscle activity during the chair stand task were determined using an exploratory principal components factor analysis (PCFA). Muscle activity parameters were validated by comparison to the Physical Performance Test, gait speed, and the Functional Status Questionnaire. RESULTS: The PCFA indicated two factors (magnitude and timing) that represented important components of quadriceps muscle activity during chair stand, explaining 68.6% of the variance in performance. The slope of the rise of muscle activity represents a combination of the magnitude and timing components of muscle activity. Compared with women with a slope <1, women with a slope > or = 1 walked faster (1.17 m/s vs 1.09 m/s; p = .02) and reported less difficulty with activities of daily living (ADL) (98.6 vs 95.8; p = .003) and instrumental ADL (97.3 vs 92.2; p = .001). CONCLUSIONS: Quadriceps muscle activity recorded during chair stand is a valid and reliable measure of muscle performance during a functional task. As a biologic measure of muscle activation, sEMG may identify muscle impairment, which could indicate functional decline earlier than measures of functional status.