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.
References:
- 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.
Max,
I will be starting the last phase of the off-season with my soccer team next week. If you would like the data i’d be more than happy to share. Currently my indicators are max chin ups, 10 yrd sprint, and max goblet squat. Also, your article is really helpful. Been trying to come up with creative way to increase RFD without heavy squats. (Only 2 squats rack are on the wght. room.)
Would be great to hear from you.
-Alex
Alex,
If you are limited on space, I would advise thinking about ways you can incorporate bands. Pretty much anything you can do with a barbell, you can do with a dumbbell. However, if loading becomes an issue you can always transition from double leg to single leg work. The bands can be used as accommodating resistance in conjunction with the free weights. There are some creative contrast methods you can use with free weights and bands to emphasize RFD. You could also use rack pulls and other “in rack” isometrics. This will allow you to transition from athlete to athlete relatively quickly because it eliminates the need to load up the bar and change the weights each time an athlete goes. Hopefully one of these methods will spark an idea. Once you understand the concept of RFD, you are only limited by your creativity, try something new and see how it works
Why do our muscles contract at 65% maximal velocity?
For example, is it OK to contract as early as possible?
It was just an example used to illustrate the significance of rate of force development. The 65% is not based on any science, it was just to show that during a movement you will never have a contraction that is of maximal velocity and maximal force, it will fall somewhere in between. If a movement has to be performed at a specific velocity (like in the example 65% vmax) in order to be completed in the time framed allowed (0.4 seconds in the example) anything lower than 65% vmax won’t be performed fast enough. It will be different for each person based on their anthropometrics (body type and limb lengths) and how their muscular system and nervous system works.
I’ve read a few of you articles and I like them, good info and straight forward language. I have a question on this one though, maybe I missed something but the section where you said RFD (F/t) should be redefined to make time the independent variable, isn’t time already the independent variable as it should be? The language muddies it a little, but “50% max force divided by 50% the time to reach max force”, is simply 50% change in force over 50% change in time and isn’t that what you’re arguing for? Outside of targeting specific time frames but that also wouldn’t change the variables.
Thank you for the kind words, much appreciated. In regards to your question, rate of force development is often measured “using 50% max force divided by time to reach 50% max force”. Time is dependent on how long it takes to develop 50% RFD (force is independent). If time was independent, we would measure the most amount of force in a specific time frame and force would be dependent on the specific time frame. Hopefully that helps clear it up.
Example: time dependent force independent max force = 100lbs
rfd = 50% max force (50lbs) divided by time taken to reach 50% max force
Example: time independent force dependent max force = 100lbs
rfd = force developed in given time frame / given time frame