Rate of force development (RFD) is merely a change in force with time.
The change in force over time can be expressed either as an equation: difference of Force / difference of time between two given instants or plotted on a graph with F on the y-axis and t on the x-axis.
Therefore it is the slope of the Force Time curve.
When calculating the numbers, force is routinely measured in Newtons (N) and time in seconds (s). Hence, RFD is measured in Newtons per second (N/s).
RFD is mainly related to neural activation and the firing frequency of the muscular fibres seems to be the most important factor contributing to it, muscle size and fibre type are secondary factors.
As in most duels, domination of the opponents doesn’t come by expressing the most absolute strength.The key in this battle is not Force rather Time.
In combat sports the guy reaching a specific strength threshold in the shortest amount time is the first one to move the opponent. Think rugby, american football, sumo, wrestling….
In other disciplines the guy expressing the greatest amount of force in a shortest time will be the first crossing the end line. See sprinting where ground contact times reach .25” at most and ground forces are up to 4x the athletes bodyweight.
“Peak Rate Of Force Development, a measure of Explosive Strength and Peak Force, a measure of maximal strength are key and main indicators of Vertical Jump Height” by CP McLellan, J Strength and Conditioning Research
Rate of Force Development also has a great influence in injury prevention and prehab together with Reaction Time : once your brain recognises danger it will react firing maximum frequencies signals to the interested muscle fibres. Some time is necessary to achieve the desired contraction and that span may be the difference between life and death!
Explosiveness is vital for athletes, how can you practically measure it?
Peak rate of Force Development is therefore the maximum steepness of the force-time curve.
Even if it is very important for so many movement, usually Peak RFD happens close to the easiest angles of the rep or during eccentric to concentric inversion of the movement, thanks to the elastic energy stored in the tendons during descent and it may be sometimes tricky to analyse because it considers only the best moment in the repetition.
Think about it also as your maximum potential of explosiveness for the repetition.
Peak Force and Time to peak Force —> Index Of Explosive Strength
In order to consider the whole repetition, the mean value for the RFD could be considered. An equivalent and more used value is the Index Of Explosive Strength, which consider the Peak Force divided by the Time to Peak Force.
Consider your repetition quite explosive when your Average RFD is more than half of your Peak RFD.
Index of Starting Strength or S-Gradient
Peak and Average don’t give us any specific information about the very first instants of the rep.Here comes the Index of Starting Strength or Starting Strength or S-Gradient (ISS or S-Grad) defining the mean explosiveness in the very first part of the repetition (0.0″ to 0.3”).
If your ISS is anyway close to a half of your Peak RFD… well, we may be glad to become your teammates or coaches!!!
Also, positive numbers will indicate true concentric only movements (this is for you box squatters!!), negative numbers will tell some kind of countermovement starting the rep (watch out wanna be box squatters!)
By the way, you can find mathematical definitions for these and all the other indexes on the mighty “Supertraining” by Yuri Verkhoshansky and Mell Siff or in “Science and Practice of Strength Training” by Vladimir Zatsiorsky.
In a paradise-like situation, a truly gifted athlete is born expressing Peak RFD in the very first moment of the lift, therefore being equal to ISS and greater than IES.
Combos of different relationships among these indexes shape different Force -Time curves which are also called Strength Signature.
The best way to improve RFD is definitely training with moderate loads focusing on maximum accelerating effort. This is also known by many as The Dynamic Effort.
Pay Attention: when training with resistances there is a limit to the acceleration that can be impressed to reach the end of a specific movement without detaching from the resistance. This forces the final part of many lifts to a quick deceleration and it is exactly the opposite phenomenon we’re looking for.
For this reason Compensatory Acceleration Training (using a combo of Weights and Chains or Elastic Bands),Ballistic Training with Med Balls or Olympic Weightlifting, Plyometrics or Shock Training are the best means to improve RFD.
CAT adapts the load to the range of motion increasing the resistance in the final part of the rep and therefore avoiding deceleration. It is useful when trying to increase general Explosiveness. Some exercises like Box Squats or Deadlifts may be good to check on Explosiveness in the first phase of the movement (check for S-Grad).
Ballistic Training just let you release the load (actually it is better when you throw it), therefore is the best method to give it all out, whether using light medballs to increase RFD at high speed or moderate to heavy barbells for squat jumps/bench press throws to work at slower speed. Check for Peak RFD here
Olympic Weightlifting is the most versatile. S-Grad and IES won’t be of great help here as Peak RFD will be the go to value here.
Research from Hernandez-Davo and Sabido offers a great overview of all the different methods to train RFD.
Plyometrics is often considered king for explosiveness,simply because of the short time constraint they impose to the exercise (ground contacts should last no more than .2” to .3” for the rep to be truly plyometric). Check for IES while jumping like a rabbit around the gym.
Eventually high loads and increasing Maximum Strength will come back to help increase maximum voluntary contraction and RFD once all these methods have no more effect.