Falls are a major problem in older adults. Many falls are due to stumbling. The aim of the present study is to investigate stumbling reactions of older adults and to compare them with young adults. While subjects walked on a treadmill, a rigid obstacle unexpectedly obstructed the forward sway of the foot. In general, older adults used the same movement strategies as young adults ('elevating' and 'lowering'). The electromyographic responses were categorized according to latencies: short-latency (~45 ms, RP1), medium-latency (~80 ms, RP2), and long-latency responses (~110 ms, RP3; ~160 ms, RP4). Latencies of RP1 responses increased by ~6 ms and of RP2 by 10-19 ms in older adults compared to the young. Amplitudes of RP1 were similar for both age groups, whereas amplitudes of RP2-RP4 could differ. In the early-swing elevating strategy (perturbed foot directly lifted over the obstacle) older adults showed smaller responses in ipsilateral upper-leg muscles (biceps femoris and rectus femoris). This was related to shorter swing durations, more shortened step distances, and more failures in clearing the obstacle. In parallel, RP4 activity in the contralateral biceps femoris was enhanced, possibly pointing to a higher demand for trunk stabilization. In the late-swing lowering strategy (foot placed on the treadmill before clearing the obstacle) older adults showed lower RP2-RP3 responses in most muscles measured. However, kinematic responses were similar to those of the young. It is concluded that the changes in muscular responses in older adults induce a greater risk of falling after tripping, especially in early swing.