PURPOSE: To determine the effects of age on (i) the onset of vastus medialis obliquus (VMO) activity relative to that of vastus lateralis (VL) and (ii) the onset of quadriceps activity in terminal swing relative to heel-strike, during stair descent. METHODS: Sixty-six healthy men and women with no known knee pathology were recruited from the community and the University of Melbourne. Thirty-three older people aged over 50 (mean (SD) 68 (8) years) and 33 younger people aged under 40 (mean (SD) 25 (4) years) participated. Surface electromyography (EMG) was used to record muscle activity during stair descent. Muscle onset was identified from individual EMG traces with a computer algorithm and validated visually. RESULTS: Both younger and older participants demonstrated a relatively synchronous onset of VMO and VL (i.e. within 10ms of each other) with no differences between the age groups evident (P=0.262). However older participants activated their quadriceps significantly earlier than the younger group (P<0.001) even when differences in body mass and speed of stair descent were accounted for. CONCLUSION: Age-related changes in quadriceps activity are apparent. The clinical implications of such changes are unknown, but increases in joint loading are possible. Earlier quadriceps activation may compensate for balance and strength impairments in older people during challenging activities.