Maximal voluntary force and rate of force development in humans - importance of instruction

The present investigation has been designed to confirm the effect of instru ction (hard-and-fast instruction compared to fast instruction) upon maximal voluntary isometric force (MVF) and maximal rate of force development (MRF D) in muscle groups which differ with regards to muscle mass and usage. In addition, we took advantage of the force data collected during unilateral a nd bilateral leg extension, to compare the instruction effects on the indic es of the bilateral deficits (BI, the differences between the data collecte d during bilateral extensions and the sum of the data collected during unil ateral left and right extensions) with regard to MVF (BIMVF) and MRFD (BIMR FD). Force-time curves were recorded during maximal isometric contractions of the elbow flexors, the leg extensors of the take-off and lead legs and d uring bilateral leg extension in 26 healthy young male volunteers from the track-and-field national team of Tunisia. In the first protocol, the subjec ts were instructed to produce MFV as hard-and-fast as possible (instruction I). In the second protocol (instruction II) the subjects were instructed t o provide MFRD, that is the most explosive force, by concentrating on the f astest contraction without concern for achieving maximal force. The present study confirmed the importance of an appropriate instruction for the measu rement of MRDF. The MRFD (F=40.8, P<0.001) were significantly higher when m easured after instruction II compared to instruction I. The effect of the i nstruction 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)x100]. 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.