Rate of Force Development (Early versus Late)

Rate of force development (RFD) can be broken down into two stages. There is an early stage rate of force development and a late stage rate of force development.  Early stage RFD is typically measured from 0-100 ms while late stage RFD is anything after.

Importance of Early Stage RFD

Sporting movements are often required to be fast, reactive movements that occur over a small amplitude. For example a large countermovement jump can take between 500-1000ms, while a squat jump with no countermovement may take around 300 to 430ms (1). In sport, movement amplitude is going to be much more similar to that of a squat jump (zero to minimal countermovement) than to that of a large CMJ. At the same time, sprinting ground contact times can last as short as 100ms. With this in mind, it is easy to see how early RFD may play an important role in sporting movement, especially those covering a small amplitude over a short period of time (ranging from 100-430ms).

Importance of Late Stage RFD

As mentioned above, Late stage RFD is typically measured as anything above 100ms. However, depending on what papers you look at, this time variable may vary. Regardless, for the sake of consistency in this paper we are going to have late stage RFD start at 100ms (it is probably much more like a sliding scale than a concrete cutoff). Most sporting movements incorporate some aspects of late stage RFD. For example, a squat jump lasts 300-430ms, which means late stage RFD makes up the majority of this time interval. At the same time, a sprint is only 100ms, so late stage RFD my play a minimal role at top speed, but probably a much larger role during the acceleration phase where ground contact times are longer (1,2,3). Depending on the movement, late stage RFD’s role in performance can vary.

 

Force Displayed In Performance = Early Stage RFD + Late Stage RFD

 

Influence of physiology and neurology on stages of RFD.

The influences of physiology (contractile properties of the muscle) and the neurology (motor unit firing rates and recruitment) differ quite a bit depending on the time of the movement. For early stage RFD, the neurological aspects of motor unit firing rates, motor unit recruitment and speed of muscular stimulation play a much larger role than the contractile properties do. However, as the time frame becomes larger and moves past 100ms, late stage RFD and its associated contractile influences play a larger role (2,3).

 

Think of this in terms of a turning on a light. Early stage  RFD is synonymous with the time it takes to flip switch to the time it takes for light bulb to turn on. Depending on the wiring of the house, there might be a delay between the switch and light. Just like the wiring of the human body, a more fine tuned nervous system will have a better early stage RFD. Again, back to the light bulb, once the light turns on, some lights need about a second or two to reach max brightness. This is synonymous with the late stage RFD. The light bulb is like the contractile properties of the muscle. The better the light bulb, the quicker it will reach max brightness. The better suited the muscular adaptations are for later stage RFD, the more force the muscle will be able to produce.

Training implications

Since early and late RFD are governed by two different properties, it is logical that training for early and late RFD should be different. Late stage RFD might be more associated with the idea of having a “strength base”. For example, many of the exercises done to build muscular strength are also associated with increases in late stage RFD and to an extent, a small part of early RFD. However, once these contractile adaptation begin to become less pronounced from training, the benefits on performance will begin to decrease. This could be why it is often noted that highly trained athletes do not see the same benefits from heavy strength training that untrained athletes do.

However, this is not to be said training should be stopped once these benefits are longer being seen. Instead, training emphasis might need to be shifted. Being that the athlete may be proficient in late stage RFD, it could be time to focus more on early stage RFD. The process of early stage RFD development probably requires much more reactive, higher velocity, lower load, skill based exercises. The concern with this style of training is no longer geared towards hypertrophy and structural changes of the muscle and tendon, but is now focused on the nervous system. Due to the inability to directly measure the nervous system, such adaptations are hard to pinpoint and it could have do with aspects of discharge rates and cortical motor arousal with a subsequent reduction in inhibition.  Despite the fact that the exact neural adaptation cannot be pinpointed, it should not stop the coach from using what knowledge is available.

Highlighting Different Properties For Early and Late RFD

A simple stretching study did a great job of illustrating the different underlying mechanisms involved in early and late RFD. The study was done to investigate the affects of static stretching on RFD. It was found that late stage RFD was negatively effected by static stretching while early stage RFD was not. What does this tell us? Static stretching may have an effect the compliance of the muscle and tendon tissues. In other words, static stretching acutely alters the musculotendon unit. However, the static stretching did not effect the early stage RFD, which means that early RFD is not produced by the same mechanisms as late stage RFD, otherwise they would have been equally reduced. (4)

Another study looked at influence of high velocity isokinetic training on early and late stage RFD. If was found that the high velocity training increased early stage rate of force development, but did not increase late stage rate of force development, nor did it increase maximal voluntary contraction. In other words, early stage RFD is influenced by separate qualities than that of late stage RFD and maximal strength (5).

Conclusion

Early and late stage RFD differ in terms of neural and physiological mechanisms. Late stage RFD might be much more related to the contractile elements of the muscle while early stage RFD might have to do more with the nervous system. Depending on the state of the athlete, specific shifts in training should be considered in order to obtain the best results in performance.

 

Citations

  1. Van Hooren, B and Zolotarjova, J. The difference between countermovement and squat jump performance: A review of underlying mechanisms with practical applications. J Strength Cond Res. Submitted 2017.

  2. Folland, J. P., Buckthorpe, M. W. and Hannah, R. (2014), Human capacity for explosive force production: Neural and contractile determinants. Scand J Med Sci Sports, 24: 894–906. doi:10.1111/sms.12131
  3. Maffiluetti et al (2016) Rate of Force Development: Physiological and Methodological Considerations
  4. Morais et al (2012) The Rate of Force Development Obtained at Early Contraction Phase Is Not Influenced by Active Static Stretching
  5. de Oliveria et al. (2013) Are Early and Late Stage Force Development Influenced By Fast-Velocity Training

Images

  1. https://markogradyblog.files.wordpress.com/2014/01/cmj-image.jpg
  2. https://www.freelapusa.com/wp-content/uploads/2016/02/Sprinter-at-Starting-Line.jpg

 

 

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