Rate of Force Development (RFD)

The Rate of Force Development measures the time until an athlete reaches peak force. It’s an important strength test, used to track fitness progress for explosive athletes. It’s one of the important metrics in velocity based training.

 

To express massive amounts of force is important, but unless you are a powerlifter it’s not as important as the ability to express massive amounts of force at a high rate. It’s not as easy as it might seem, especially since force is most optimally expressed at slower velocities allowing for maximal actin-myosin cross bridges. However, there’s more to producing force than simply looking at the muscle length-tension relationship, and that is the elastic tension created at maximal lengths and speeds. We will take a look at all those relationships a bit later. 

For now, you need to know that a high rate of force development is attributed to faster sprints, higher jumps, and more powerful movements in general. You will also be interested in the fact that most sports require a high rate of force development to succeed at top levels including: golf, weightlifting, sprinting, baseball, tennis, and just about every other sport. Therefore, this is an article that all of you should pay close attention to and probably take to heart.

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Muscle-Length Tension

Ok I am going to make this as basic as possible to ensure that all of you get the meaning of the point I am trying to make. This isn’t anatomy class, so if you are a professor, give me a break. The main point is that the sarcomere (actin and myosin) which is the structural unit of the muscle cell, can’t form cross-bridges (can’t contract the muscle fiber) when completely lengthened. Opposingly, when a muscle fiber is fully contracted, the proteins within the sarcomere are too bunched together to create any more force

Rate of Force Development (RFD): Muscle-Length Tension

https://en.wikibooks.org/wiki/Structural_Biochemistry/Protein_function/Myosin

 

The key phrase here is “for the sarcomere”, but this doesn’t take into account elastic components of the body, especially the most underrated protein filament titin, connective tissue, and nerves. The stretch shortening cycle is where we clearly see that myofilaments are only a small part of the equation because passive force production more than makes up for actin and myosin’s inability to create force. Titin is the largest human protein, and it’s job is  to keep actin and myosin in place to resist stretching. It’s the structure component, but acts  like a spring. The thicker and tighter it is results in more passive force during a stretch. It’s like a rubber band creating massive force production. The faster the stretch also results in a greater resulting amount of passive force production. 

Rate of Force Development (RFD): Muscle-Length Tension image 2

Odegard, Gregory, et al., 2009

You also have to take into account the nerves, especially the muscle spindles and the golgi tendon organ (GTO). Muscle spindles run parallel to muscle fibers, and their job is to resist stretching as well. When they realize the muscle is being stretched, they counter that stretch with a passive contraction. If you have ever received a physical from your doctor, you’ve probably had the little hammer looking object pounded against your knee. Hopefully that pounding caused your lower leg to kick out. If you noticed, the doctor swung the hammer rather quickly. If he or she had swung the tool slowly, you probably wouldn’t have kicked. The faster a muscle is being lengthened , the greater and more rapid passive force reaction. The GTO is a nerve component located in the tendon, and its job is to react to high forces in the tendon that could result in injury or a tear. The GTO reacts opposite of the muscle spindles, inhibiting the muscles from contracting to minimize force. Basically, the GTO’s shut off a muscle to prevent possible tears. 

Finally, tendons and all connective tissue acts like the titin protein, but in a much bigger way. The thicker a tendon is and the tighter it becomes with its matrix will create more and more of a passive force production along with a higher rate of the force production. Yes, this is also known as a higher rate of force production. Now let’s put all of these components together, see what it does, and figure out if we can improve these components.


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Principle of Specificity

Now the more an athlete practices dynamic movements like depth jumps, bounding, sprints, and massive squats, the result is a more dramatic impact from the muscle spindles and less inhibition from the GTOs. This is where the principle of specificity steps in and creates a better athlete. Basically, the elastic components get better at creating passive force production at higher and higher rates. It’s with these elastic components that rate of force development (RFD) is thought to be expressed. 

Strength coaches should consider the principle of specificity when creating their programs for athletes. If rate of force development (RFD) is the main concern, then slow grinding squats might not be the best solution especially all the time. Adding elastic bands to the barbell during squats and other movements might be a better answer especially during preseason and in-season because the bands will cause increased eccentric velocity due to the added resistance on top of gravity interacting with the mass loaded on the barbell at 9.81m/s² (acceleration of gravity). Remember, the higher eccentric (aka stretch) velocity results in greater reactions of the elastic components: muscle spindles, titin, and tendons. 

Measuring Rate of Force Development

Measuring the rate of force development is simply finding the peak force (Newtons) and the time it takes to create that peak force in milliseconds(convert into seconds). For example, if you create 440N if .05 seconds (50 milliseconds), the rate of force development would be 8800Ns⁻¹(440/.05). The easiest way to gather the most information is to measure a given activity in different time intervals like every 50 milliseconds for example to create a vertical leap. It will probably look like this:

Rate of Force Development (RFD): measurement

Here you find that the Peak RFD is 8800Ns⁻¹. You also find the time to peak RFD, and you find the average RFD to be 5840Ns⁻¹(1460/.25). However, there is an easy way to determine rate of force development for particular movements like back squat or clean. 

With GymAware, you can easily measure peak force and the time it takes to reach that force. That means you can also track an athlete’s improvement. You can even measure average RFD with a depth jump, which is even more specific to athletic endeavors. For example, I performed a bench press today at 3982N and it took me .76 seconds to reach that peak, so my average RFD is 5239Ns⁻¹. If I were coaching myself, I might use bands, plyometrics, and high velocity presses to improve that in hopes of producing a higher peak force with the same weight at a faster time. I can measure the lift again in four weeks to see the improvement. 

Motor Unit Recruitment and Rate Coding

Remember what I said about the principle of specificity? The more I perform a particular movement will result in me getting better at that movement. A motor unit is an alpha motor neuron and all the muscle fibers that it innervates. When I lift with maximal effort and maximal velocity, I recruit more and more of these high threshold motor units. When we move our body throughout the day, we use small threshold motor units that don’t really innervate many muscle fibers. Obviously, if I want to produce maximal force, I need high threshold motor units because they innervate thousands of muscle fibers. 

The more I perform these movements, I also get better at rate coding, which is the speed of the signal from the spine to the muscles. If I want to get better at RFD, I need to perform movements at high velocities. Due to the muscle length-tension relationship and the principle of specificity, the body gets better at rate coding during high velocity movements because there is a high detachment rate of actin-myosin cross bridges making force production a lot harder. The body compensates for the high detachment rate with faster rate coding allowing the athlete to get better at producing force at higher velocities, not to mention the other passive forces from the elastic components that we already talked about. This makes velocity measurement imperative, and one might consider adding bands like I said before. 

Conclusion

In summary, Rate of Force Development (RFD) is a measurement that has to be considered when coaching athletes. If you want homerun hitters, Olympic level sprinters, and Olympic level weightlifters like me, it’s imperative to both measure and work towards improving. VBT is a tool for doing just that. Hopefully, I have given you several ideas for improving your athletes’ performance, and several ideas for measuring that improvement. Let me know if you have any questions at Travis@GymAware.com.

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Coach Travis mash

Travis Mash

Being a World Champion in powerlifting, Travis competed at a world-class level in Olympic weightlifting and has coached professional Olympic weightlifters alongside Don McCauley and Glenn Pendlay at Team MDUSA. Now Travis coaches the most successful weightlifting team in the USA.