Paired associative stimulation (PAS) based on Hebbs law of association can induce plastic changes in the intact human. The optimal interstimulus interval (ISI) between the peripheral nerve and transcranial magnetic stimulus is not known for muscles of the lower leg. The aims of the current study were to investigate the effect of PAS for a variety of ISIs and to explore the efficacy of PAS when applied during dynamic activation of the target muscle. PAS was applied at 0.2 Hz for 30 minutes with the tibialis anterior (TA) at rest. The ISI was varied randomly in seven sessions (n=5). Subsequently PAS was applied (n=14, ISI=55 ms) with the TA relaxed or dorsi-flexing. Finally an optimized ISI based on the subject SEP latency plus a central processing delay (6ms) was used (n=13). Motor evoked potentials (MEPs) were elicited in the TA before and after the intervention and the size of the TA MEP extracted. ISIs of 45, 50 and 55ms increased and 40ms decreased TA MEP significantly (p=0.01). PAS during dorsi-flexion increased TA MEP size by 92% (p=0.001). PAS delivered at rest resulted in a non-significant increase, however when the ISI was optimized from SEP latency recordings, all subjects showed significant increases (p=0.002). No changes in MEP size occurred in the antagonist. Results confirm that the excitability of the corticospinal projections to the TA but not the antagonist can be increased following PAS. This is strongly dependent on the individualized ISI and on the activation state of the muscle.