Surface integrated electromyograms (iEMG) of agonist and antagonist muscles were studied during the rising phase of maximal isometric efforts (elbow flexion, unilateral and bilateral leg extension) to explain the difference in maximal rate of force development (MRFD) with a hard-and-fast instruction (instruction I) and a fast instruction (instruction II ). Force and EMG were simultaneously recorded in 24 athletes and iEMG were computed at MRFD and during different phases of force development (P(0-25), P(25-50), P(50-75), P(75-90) and P(90-100)). A two-way ANOVA for repeated measures (muscle x instruction) showed that the value of iEMG at MRFD was significantly higher with instruction II for elbow flexion, unilateral and bilateral leg press exercises (F>4.9; P<0.04). The effect of instruction upon iEMG of the agonist muscles corresponding to the different phases of the force development was significant for elbow flexion (F=4.2; P<0.05 ) unilateral (F>6.4; P<0.02) and bilateral leg extension (F>9 and P<0.006 for soleus and vastus lateralis; but F=3.2 and P=0.08 for vastus medialis). There was a significant interaction between instruction and phase of force development (F>2.6; P<0.05 ): iEMG was significantly higher with instruction II at the beginning of force development (P(0-25)) for all the muscles (except the soleus muscle during the bilateral leg exercise) but not at higher force (P(75-90) and P(90-100)). The steeper force development with instruction II can be explained by the better activation of the agonist muscles at the beginning of force development. A lower co-activation of the antagonist muscles does not explain the improvement in MRFD as the iEMG of the antagonist muscles was not lower with instruction II but was proportional to the activation level of the agonist muscle.