This study investigated whether changes in lower limb muscle activity occured in anticipation of a possible perturbation in 11 young (mean age 27 years) and 11 older (mean age 68 years) adults. Altered muscle activity could affect tripping responses and consequently the ecological validity of experimental results of studies on tripping. It was hypothesized that anticipatory muscle activity would be present immediately after a trip, and decrease after several subsequent unperturbed (forewarned) walking trials. Electromyograms of lower limb muscles were measured in 3 conditions: during normal walking, during forewarned walking immediately after a trip, and during forewarned walking several trials after a trip had occurred. Small but statistically significant differences in averaged muscle activity over a stride were found among conditions. Young adults showed slightly increased activity immediately after tripping (co-contraction) in hamstrings, quadriceps and tibialis anterior muscles. This increased activity diminished after several unperturbed trials, although it did not return to the baseline activity levels during normal walking. In older adults, an increased muscle activity among conditions was only discerned in tibialis anterior and soleus muscles. This suggested that older adults prefer to avoid contact with the obstacle over joint stiffening. Yet, for both age-groups, the increases in muscle activity were very small when compared to tripping responses reported in the literature. Therefore, anticipatory effects are not expected to jeopardize the validity of experiments in which subjects are perturbed more than once. © 2005 Elsevier Ltd. All rights reserved.