Stretching is usually part of warm-up routines in many sports, but it affects the subsequent muscle force; therefore, it could negatively influence post-activation po- tentiation (PAP), one of the warm-up's main effects. The aim of this study was to evaluate the acute effects of pas- sive stretching on PAP and fibre conduction velocity (CV). Seven subjects underwent 2 experimental sessions, con- trol (C) and stretching (S), each consisting of 2 series (7 min resting) of 3 maximal voluntary contractions (MVC) of biceps brachii (5 s isometric contraction, 10 s recov- ery). During the resting phase of the S session, the biceps brachii was passively stretched (5×45 s stretches, 15 s re- covery). Root mean square (RMS), mean frequency (MF) and CV were calculated from electromyography. Peak torque (pT) and half-contraction time (½CT) were mea- sured and normalised by the arm muscular area (pTn). After C, pTn increased and ½CT decreased (p<0.05); moreover, MF and CV increased (p<0.05). After S, ½CT increased (p<0.05) and RMS decreased (p<0.05). Passive stretching could blunt the effects of PAP, presumably due to mechanical and neuromuscular changes. The observed changes in CV suggest a possible decrease in Ca 2+ sensi- tivity in contractile proteins. Therefore, the use of passive stretching in warm-up routines remains questionable.