What Is Absolute Force?
Absolute force, also know as absolute strength, is the most amount of force one can produce with no limit to the amount of time required to produce the force. Highest levels of absolute force can only be reached during an isometric or eccentric contraction. However, because an isometric contraction requires no lengthening or shortening of the muscle fibers(actually extremely minuet changes in length) and no movement of the limb it is not very practical to use (hard to quantify) and neither is eccentric maximal force due to the supra-maximal requirements (again, hard to measure). Instead, in this article absolute force will be synonymous with maximal concentric force. Absolute force can be measured by using a one-rep max. In such a case, the limiting factor in completion of movement will be the concentric force of a movement, not a single muscle group.
What Is Rate of Force Development?
Rate of force development (RFD) is the most amount of force one can produce in a given amount of time. It is often measured by dividing maximal force by time taken to reach maximal force.
RFD = Max Force / Time to reach Max Force
Depending on the task at hand, RFD may or may not have a large influence. For example, an olympic weightlifter might not be limited by time in the same way a basketball player jumping for a rebound might be. However, in both cases RFD is the limiting factor to the amount of force one can display. This is why RFD is far too vague. To better understand RFD, it needs to be reduced into specific categories.
Explosive RFD has been measured using 50% max force divided by time to reach 50% max force
Absolute RFD has been measured using 100% max force divided by time to reach 100% max force
In my opinion, this is a little bit of a backwards way of looking at RFD. We should not be making the percentage of RFD the independent variable and time the dependent. Instead, we should flip it around and make RFD dependent on time. This way we can analyze the properties of RFD in sport specific time frames.
Example: If jumping requires an average of .4 seconds then we should look at the percentage of max force someone can produce within .4 seconds instead of seeing how long it takes to develop a specific percentage of force
The force-velocity curve
The force-velocity curve depicts the relationship between the amount of force one can produce and velocity at which the force is produced at (Zatsiorsky). The higher the force of a movement is the slower the movement will be performed. When velocity reaches 0m/s the contraction becomes isometric.
Rate of Force Development and The Force Velocity Curve
As stated earlier, RFD is the amount of force one can produce in a given amount of time. What determines how fast a movement is performed is the velocity of contraction. The faster your muscles contract, the faster your limbs will move, and the faster you will be able to complete a movement. Movements such as jumping or throwing a baseball require a high velocity of contraction, which is why the amount of force that can be produced is limited. In order to think of this in sport-specific terms, imagine that every time you jump it takes roughly .4 seconds. In this example, in order to move at .4 seconds your muscles have to contract at 65% maximal velocity to complete the movement. This is key, because anything that falls below 65% maximal velocity cannot be used (too slow).
If you were to draw a dotted line on the force-velocity curve stemming from 65% and up, anything to the left of 65% could not be used during the jump. If the movement is performed at 55% maximal velocity (to the left of 65% maximal velocity) then the movement may be to slow for the given task. This is why you can never produce near maximal concentric force during high velocity movements. In order to produce a near maximal force, your velocity of contraction would have to be slower to allow for greater force production.
- Image 1: https://publi.cz/books/52/09.html
- Image 2: https://www.elitefts.com/education/training/using-the-force-velocity-curve-to-build-better-athletes/
- Image 3:https://www.elitefts.com/education/training/sports-performance/the-force-velocity-curve/
- Zatsiorsky V, Kraemer J (2006) Science and Practice of Strength Training. Champaign, Illinois: Human Kinetics.
- Siff, M.C. (2000). Supertraining. Denver: Supertraining Institute.