Quadripulse stimulation (QPS) has been shown to induce bidirectional motor cortical plasticity. Although the interindividual variability of any rTMS protocols is highly recognized, QPS aftereffects seem to be less variable among individual. It has been assumed that recruiting not late I-wave, but early I-wave by monophasic, posterior-toanterior directed current might be a reason for less variable response by QPS compared with other conventional biphasic rTMS. Another idea is based on the fact that 15 min, short version of QPS with identical total number of pulses compared with 30 min, ordinary QPS produced less plasticity, and therefore, 30 min application, rather than short duration of rTMS, might be an important factor to reduce variability of QPS. If the former is correct, it is possible to hypothesize that short duration of QPS with voluntary contraction, targeting early I-wave more effectively, may produce plasticity. We therefore tested this modified version of QPS with voluntary contraction, but the results showed that no clear response was obtained by modified QPS.