One manifestation of advancing age altering movement control is that old vs young adults execute movements with higher agonist-antagonist co-activation. An analysis of muscle-to-muscle coherence, the similarity between a pair of signals in the frequency domain, could reveal if age modifies the common synaptic input from spinal and cortical sources, respectively, to muscle pairs in the alpha (∼10 Hz) and beta (∼30 Hz) frequency bands, and mediate the increased co-activation. PURPOSE: To determine if there are changes in the EMG coherence between agonist and antagonist leg muscles of young and old adults during walking. METHODS: In this pilot study, 2 young (age 23 and 24) and 2 old adults (age 79 and 81) walked at 2 and 3 mph on a treadmill. Surface EMG activity (16-500 Hz) was collected at 1 kHz from the vastus lateralis, biceps femoris, gastrocnemius lateralis, and tibialis anterior using a telemetric system. Cross-correlation, coherence, and its 95th confidence limit were computed in a 200-ms window before and after heel strike for 300 steps. RESULTS: Averaged in BF/VL and TA/GS muscle pairs and the 2 speeds, co-activation in old vs young was 26% and 11 %. Both old but neither young adults exhibited significant (p < 0.05) coherence at ∼10 Hz between the VL and BF in the stance and swing phase at 2 and 3 mph. All subjects showed coherence at ∼10 Hz between the GS and TA but old adults had more coherence in the swing phase at both speeds. In control experiments in young subjects there was coherence (p < 0.05) across the frequency spectrum when EMG was recorded during an isometric contraction from 2 electrodes placed on the same muscle. Thus, cross-talk between muscle pairs probably did not contaminate the coherence observed during walking. CONCLUSION: These initial results indicate that age may modify the common synaptic input from spinal and cortical sources to muscle pairs and this adaptation may underlie the increased co-activation. Supported by NIH grants AG024161, NS049783.