How to Calculate the Rate of Force Development
As RFD is an expression of explosive strength, it is measured in Newtons per second squared (N·s-1). The RFD can be calculated for isometric, concentric and eccentric muscle contractions, with the latter two otherwise referred to in the research as the ‘positive’ and ‘negative’ acceleration phases of the SSC (2, 3). In fact, one study suggests that eccentric RFD is a better predictor of jump performance than concentric RFD because it summarises several intrinsic properties of muscle and tendons during a key moment (2). However, this is yet to be validated by other research.
Multiple measures of RFD have been developed in order to measure various components of performance during both isometric and dynamic movements:
- Average RFD or IES (Index of Explosiveness) (2, 3, 5, 6, 8, 9, 11, 37, 38)
- Time-interval RFD (11, 39)
- Instantaneous RFD (40, 41)
- Peak or maximal RFD (42, 4, 5, 7, 10, 11, 36)
- Time to peak RFD (11, 7)
Average RFD: This value is identical to the IES discussed by Zatsiorsky (37), and is calculated by dividing the peak force by the time to achieve peak force (39). However, this form of measuring average RFD has been shown to have lower levels of reliability in comparison to time-interval RFD and peak RFD (39). These lower levels of reliability may be associated with each athlete’s time to achieve peak force, as not all athletes can achieve peak force in the same time frame. Therefore, measuring RFD using predetermined time-intervals can accommodate for these variances.
How to calculate Average RFD
Example – Calculating Average RFD
Average RFD [N·s-1] = Peak Force [N] / Time to achieve peak force [s][/toggle]
Time-Interval RFD: Though this measure of RFD is effectively the same as average RFD, it is calculated at various time-intervals (e.g. 0-30, 0-50, 0-90, 0-100, 0-150, 0-200, and 0-250 milliseconds ). This value simply represents a change in force divided by a change in time. It is calculated by dividing the force at the end of the time interval by the duration of the time interval (39) (Table 3). Just note that when calculating RFD, the time should be calculated in seconds, not milliseconds.
How to calculate Time-Interval RFD
Example – Calculating RFD with a 0-30 millisecond time-interval:
RFD [N·s-1] = Change in Force [N] / Change in Time [s]
RFD [N·s-1] = Force [N] at 30 milliseconds / 0.03 second time-interval [s]
RFD [N·s-1] = 50N / 0.03s
RFD [N·s-1] = 1,666