Rate of force development (RFD) is the rate at which force rises during a muscle action. It is calculated as the change in force divided by the change in time and is usually expressed in newtons per second. RFD is often described as a measure of explosive strength.
RFD matters because many sporting actions allow very little time to produce force. Sprint ground contacts, rapid changes of direction, and balance recoveries may occur in well under 250 milliseconds, often too little time to reach maximal force. In these situations, how quickly force can be produced may matter as much as maximal strength.
How RFD Is Measured
- With a force plate during jumps or isometric pulls
- With an isokinetic or isometric dynamometer at a single joint
- Over defined time windows from force onset, such as 0 to 50, 0 to 100, or 0 to 200 milliseconds
- As peak RFD, the steepest slope found anywhere in the force-time curve
Early RFD is thought to depend strongly on neural factors such as how quickly motor units are activated, while later RFD is more closely related to maximal strength. Results are sensitive to testing protocol, the method used to detect force onset, and data filtering, so values from different tests should be compared with care.
Observing It in Video
Standard video does not measure force, so it cannot measure RFD. It can, however, provide related timing information. Frame-by-frame review can estimate ground contact time, the duration of a countermovement, or time to takeoff, and side-by-side comparison can show whether an athlete reaches key positions sooner than in an earlier recording.
Limits and Pitfalls
- Faster movement in video does not prove higher RFD; technique, depth, and body position also change timing
- Time-based estimates are limited by frame rate and the precision of event identification
- Comparing RFD values across devices or protocols can be misleading
- Video-based timing should be described as a timing measure, not a force measure
RFD is best assessed with force-measuring equipment, with video used to add context about technique.