The present investigation has been designed to confirm the effect of instruction (hard-and-fast instruction compared to fast instruction) upon maximal voluntary isometric force (MVF) and maximal rate of force development (MRFD) in muscle groups which differ with regards to muscle mass and usage. In addition, we took advantage of the force data collected during unilateral and bilateral leg extension, to compare the instruction effects on the indices of the bilateral deficits (BI, the differences between the data collected during bilateral extensions and the sum of the data collected during unilateral left and right extensions) with regard to MVF (BIMVF) and MRFD (BIMRFD). Force-time curves were recorded during maximal isometric contractions of the elbow flexors, the leg extensors of the take-off and lead legs and during bilateral leg extension in 26 healthy young male volunteers from the track-and-field national team of Tunisia. In the first protocol, the subjects were instructed to produce MFV as hard-and-fast as possible (instruction I). In the second protocol (instruction II) the subjects were instructed to provide MFRD, that is the most explosive force, by concentrating on the fastest contraction without concern for achieving maximal force. The present study confirmed the importance of an appropriate instruction for the measurement of MRDF The MRFD (F = 40.8, P < 0.001) were significantly higher when measured after instruction II compared to instruction I. The effect of the instruction upon MRFD were similar for muscle groups with different volumes, cortical representations and uses. The same results (F = 52.1; P < 0.001) were observed when MRFD was related to MVF [MRFD% = (MRFD/MVF) x 100]. On the other hand, MVF was similar following both instructions (ANOVA, F = 0.562; P = 0.454). Moreover, the results of the present study suggested that the effect of instruction was significantly larger for BIMRFD than for BIMVF.