This study examined the neuromuscular adaptations of young and old adults in response to a challenge to balance during gait. 12 young (22.2 ± 2.0 yr.) and 10 old adults (78.8 ± 6.3 yr.) performed five level walking trials with and without a challenge to balance which consisted of adding a load bilaterally to the participants' ankles and increasing gait speed (1.5 m/sec. and 1.7 m/sec.). Onset, duration, and amplitude of muscle activity in the vastus lateralis, rectus femoris, biceps femoris, and semimembranosus were calculated. Old adults responded to the added load by increasing the activation intensity of their vastus lateralis and semimembranosus disproportionately when compared with young adults. Also, old adults activated their knee flexors earlier and all muscles for a longer period of time than young adults. These findings suggest old adults alter both magnitude of activation and duration of lower extremity musculature in response to increased inertial and velocity demands, and these response characteristics differ from those of young adults.