Methodological considerations in the calculation of the rate of force development scaling factor

Within this methodological study, we aimed to explore the effects of seven different commonly applied routines for rate of force development (RFD) calculation on the calculation of rate of force development scaling factor (RFD-SF) and r ² and the between session reliability of RFD-SF in three different muscle groups.

We tested grip force muscles, elbow extensors and knee extensors of 13 healthy subjects asking them to produce around 120 rapid isometric force pulses per muscle to varying submaximal amplitudes. Then, we applied different filtering and analysis procedures, such as different filtering cut-off frequencies (no filter, 5 Hz cut-off, 10 Hz cut-off) and time windows during which RFD was calculated (0-50 ms, 0-75 ms, 0-100 ms and time from onset of contraction to peak force (PF) value). RFD-SF and r ² were obtained from the relationship between the PF and the corresponding RFD.

Our results showed that the magnitude of the calculated RFD-SF and r ² values significantly differed among different RFD calculation methods (p  <  0.0005), but that r ² was high in most applied methods (RFD-SF ranged between 4.6 and 17.7, while r ² ranged between 0.63 and 0.98). Regardless of the tested muscle group and applied method to calculate RFD, the reliability of the calculated RFD-SF was moderate (0.5  <  ICC_3,1  <  0.75) to good (ICC_3,1  >  0.75). The highest r ², highest ICC, lowest SEM% and MDD% were observed in case of RFD calculated during first 100 ms and when cut-off filter at 5 Hz was applied, indicating those two methods as the most appropriate ones.

Our study contributes to the establishment of methodological suggestions that will help in maximizing the reliability of RFD-SF measurements, collecting normative data, and comparing results among different populations and lab settings.