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The Science of Speed Part 2 – Early Phase Rate of Force Development

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Muscle force is controlled by the level of motor unit recruitment and the rate at which signals are discharged to the motor unit telling it to contract (1). In part 1 of this series I covered how motor unit recruitment is enhanced by training and what this means for performance. In this article I will try my best to explain rate of force development (RFD), with a particular focus on early phase rate of force development. Early phase RFD is very important for performance in fast sporting actions like the golf swing.

Rate of Force Development (RFD) is simply a measure of how quickly somebody can produce force in a given movement (whichever movement being tested). RFD is usually broken down into two phases.

Early phase rate of force development – Force produced in first 100ms of contraction
Late phase rate of force development – Force produced after the first 100ms of contraction
There are some papers that use different time periods*

This is an important concept to understand in speed training. In very fast activities like swinging a golf club there is not enough time available to reach our maximal force. It can take approximately 300-500ms from the onset of maximal effort contraction to reach maximal muscle force. (2)

In the golf swing, from the start of the downswing to impact can take approx 200 to 250ms, but for force to make it out to the club head in time for impact it needs to be produced earlier (based on my biomechanics understanding). This *might* mean that the ability to generate as much force as possible in time frames of 100 to 200ms is very important for club head speed.

Something I am very curious about but do not currently have an answer for is how the backswing and transition influences this. Downswing time is counted from when the club starts moving in the downswing, but we have of course started to transition from backswing to downswing with our lower body, pelvis, and torso before this. The golf swing is a multi joint, full body movement, which utilizes the stretch shortening cycle (SSC). i.e It is a dynamic exercise with a substantial wind up phase. A lot of RFD testing in studies is done with isometric (static) contractions and / or single joint exercises.

These are different types of tasks which means there are limitations for inference to sporting actions. My thinking is that because the golf swing has quite a long counter movement / stretch phase in the backswing and transition (backswing is about 750ms), that we may have more time than what solely looking at downswing time frames would suggest to produce force. This will vary for different parts of the body. The legs will have more time to produce force, while the arms will have less.

The SSC is muscle actions that involve quick lengthening / stretching before a shortening contraction. Actions involving the SSC allow greater force outputs. Example of a non SSC exercise would be a vertical jump where you go down to the crouch position, hold for a few seconds, then explode upwards from there. When we jump in this fashion we can’t jump as high as when we do the downward phase and then immediately change direction into the upward phase. A golf swing without the SSC would be eliminating the backswing and starting the downswing from a stationary position. Obviously we wouldn’t do this as it would be very difficult to produce force. (The golf swing actually has many SSC’s because so many muscles are being stretched and shortened).

Research into the effect of high velocity and traditional strength training shows interesting effects on early and late phase RFD. Early phase RFD is largely influenced by a nervous system function called Rate Coding, also known as Motor Unit Firing Frequency. This is the rate at which signals are sent to the motor unit telling it to contract. The technical term for these signals are “action potentials”. Rate coding plays a very important role in the initial burst of force at the onset of contraction in a very fast, explosive activity (first 50-75ms) (3). With higher rate coding we can produce more force in the early stages of a very fast time limited movement. This is very beneficial, as we don’t have a lot of time to produce force in these activities.

As the time from onset of contraction goes on, our maximum force capability, which is highly correlated with size of the type IIa and IIx muscle fibers, begins to become very important. Maximum force (strength) does not rely on high levels of RFD. This is because in measures of maximum force we have all the time we need to build up to it. Think of a one rep max or pushing against an immovable object as hard as you can. In very light load, fast exercises, like swinging a club, the movement is usually over before we have had time to apply all of our force. We get to apply a certain proportion of our maximal force, so we want to ensure we have access to very high forces, but also that we can ramp up towards our maximum very quickly.

My current understanding of how this ties in with developing the ability to swing a golf club faster is that both early and late phase rate of force development, and the factors that influence each are very important.

This is why I believe in striving to get stronger with heavy strength training, and training to produce force as fast as possible against light weights. Examples of this include maximum speed swings, jumps, throws, rapid band work etc. As I have touched on in previous articles, from a training perspective, heavy strength work and maximum speed swings are the most important areas to invest time and energy into. Swing technique is also critical.

It seems that light weight high velocity training primarily increases early phase rate of force development whilst not affecting late phase rate of force development (4). Heavy strength training may have the capacity to improve both early and late phase RFD, but it all depends on the intent during the training. The training status of the person would be very important here too.
In untrained people, it is more likely that heavy strength training done with maximum intent to move the weight as quickly as possible will improve early and late phase RFD, as well as maximum force. Generally as someone becomes more experienced in training and the easy “newbie” gains wear off, training needs to become more specific to the quality you are trying to improve. This would suggest that very fast movements would be necessary to improve early phase RFD in people who have more strength training experience.

Let’s dig into the results of an actual training study to get a better understanding of what happens in response to strength training done with the intent to lift and lower the weights in a controlled fashion, avoiding any sudden acceleration. This would be the typical “slow and controlled advice” that many would receive in gyms.

15 healthy but untrained males with an average age of 24 underwent a 14 week training study.
The focus of the study was on the lower body, and the training exercises were leg press, hack squat, leg extension, and leg curl. The subjects did the program 3 days per week.

For the first 5 weeks they performed 4 sets of 8-10 reps in each exercise. For the middle 5 weeks they performed 4 sets of 8-10 reps in each exercise. For the last 5 weeks they performed 5 sets of 6-8 reps in each exercise. Of course, as the reps per set went down, the weight lifted went up. From what I can tell, they used a load that resulted in failure within the prescribed rep range on each set. This is a pretty hard training program!!

At the end of the study the subjects saw no increase in early phase rate of force development, an 11% increase in rate of force development at 250ms, and an 18% increase in maximum force. The percentage distribution of type IIa fibers increased from 31% to 41%, while the percentage distribution of type IIx fibers decreased from 11% to 4%. For a primer on muscle fiber types, check out part 1 of this series.

Fiber type shifting will be the topic of another article, and is very interesting. There are some important tweaks we can make to our strength training to try and maintain as many type IIx as possible (which is very very valuable for performance in high speed activities).
Maximum Force increased by an average of 18% but early phase RFD remaining unchanged. This means the early phase RFD relative to the new maximum force decreased (by 10-18%).  The decrease in type IIx fiber type distribution plays a big role in this. (5)

What does this actually mean? RFD at at 250ms and maximum force increased. This is great. Early phase RFD relative to the new maximum force decreased. This is largely due to the shifting of type IIx to type IIa fibers. We can reduce or eliminate this fiber type shifting by moving the weights as fast as possible (especially in the upward phase) rather than in a controlled fashion, stopping 2-3 reps shy of failure in each set, and doing slightly lower volume. This would likely be a better overall option for those training to be very explosive in fast sporting actions. Chris Beardsley has covered this a lot.

In contrast to the results of the study explained above, heavy strength training programs with the intent to move the weights as quickly as possible (i.e generate as high an RFD as possible), have been shown to increase both early and late phase RFD. This is in spite of the fact that because the weights are heavy, they are in reality moving quite slowly. (5)

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Key Takeaway Point:

The intent to move the weight / implement as explosively as possible is absolutely essential for improving early phase RFD. This is the case whether the weight is light or heavy. This intent has also been shown to enhance maximal strength gains. This is very different to the slow and controlled tempo that is often described in body building / general fitness instruction.

Remember, training must transfer to the task we are trying to improve. When training to improve our ability to produce lots of force in very small time frames, this is the intent we should have in the vast majority of our training.

A blend of light and fast exercises (maximum speed swings the most important), and heavy exercises that load our major muscle groups, trying to move as fast as possible (even though they may actually be moving slowly), is likely the best way to maximize your results. This is exactly how all the programs on the Fit For Golf App are set up.

I hope you enjoyed this article and please let me know if you have any questions.


References:

1) Enoka, R.M. and Duchateau, J. (2017) “Rate coding and the control of Muscle Force,” Cold Spring Harbor Perspectives in Medicine, 7(10). Available at: https://doi.org/10.1101/cshperspect.a029702.

2) Aagaard, P. et al. (2002) Increased rate of force development and neural drive of human skeletal muscle following resistance training, Journal of Applied Physiology. Available at: shorturl.at/jwGOX. (Accessed: December 30, 2022).

3) Maffiuletti, N.A., Aagaard, P., Blazevich, A.J. et al. Rate of force development: physiological and methodological considerations. Eur J Appl Physiol 116, 1091–1116 (2016). https://doi.org/10.1007/s00421-016-3346-6

4) de Oliveira, F.B., Rizatto, G.F. and Denadai, B.S. (2013) “Are early and late rate of force development differently influenced by fast-velocity resistance training?,” Clinical Physiology and Functional Imaging, 33(4), pp. 282–287. Available at: https://doi.org/10.1111/cpf.12025.

5) Andersen, L.L. et al. (2010) “Early and late rate of force development: Differential Adaptive Responses to resistance training?,” Scandinavian Journal of Medicine & Science in Sports, 20(1). Available at: https://doi.org/10.1111/j.1600-0838.2009.00933.x.