Rate Of Force Development Part 2: Training and Increasing RFD
Last post, I went over some of the terms and definitions of rate of force development (RFD). I also mentioned motor units (MU) and if, at this point, you have no clue what I’m talking about, go back and read it. It’s right here. Why should you care about increasing your rate of force development? Answer: power sports (which is every sport to some degree) are dependent upon the ability to produce high levels of force at any given moment, like running away from a T-Rex.
There are two main ways research and experience backs up to train RFD: explosive strength training (Newton et al. Med. Sciences Sports Exer. 1999) and maximal load training, i.e. picking up heavy stuff. (McBride et al, J. Strength and Conditioning Research 2002). It should be noted that most of the research has been done with isolated muscles/movements (it’s a lot easier to test the quadriceps muscle in a leg extension machine than the various muscle groups in a deadlift) and so it can be a little tricky to apply to real life. However, where science has holes, the experience of coaches fills the gap!
First: force = mass x acceleration Keep this in mind…
Explosive training (speed work) is taking a sub-max load (say, 50% of your one rep max) and moving it as fast as possible, with good form obviously, for 1-3 reps per set. That’s key- as fast as possible. Those high threshold motor units, the ones that produce the most force, are recruited to move that weight quickly by contracting quickly. Even though the load is light, the acceleration is high. By challenging your system to move loads supa fast (actual speed measurement), we can increase the force production by increasing the acceleration part of the equation. This is one way to train and increase RFD, by working on the "speed" (or "velocity" for the nerds) part of the equation.
Typically at SAPT, we program 1-3 reps for 6-8 sets with a strict :45-:60 rest period. Why the rest parameters? We want to keep the nervous system “primed” and if the rest period is too long, we lose a bit of that ability to send rapid signals to the muscles.
Maximal load training, aka picking up some freakin’ heavy weight, will typically be above 90% of your one rep max, likewise we keep the rep range between 1 and 3 (mainly because form can turn to utter poo very quickly under heavy loads if the volume is too high). This untilizes the other part of the force equation, mass. If the acceleration is low, the mass has to be high in order to create a high force production. Once again, neural drive is increased and those high threshold MU’s are activated. The threat of being crushed beneath a heavy bar can do that.
Bottom line: As the an athlete's RFD increases –> the recruitment threshold of the more powerful motor units decreases –> more force is produced sooner in the movement –> heavier weights can be moved/athlete becomes more explosive in sport movements.
Think back on poor lifter B from last post who had a really low RFD during his 400lb deadlift attempt. Being the determined young man that he is, he trained intelligently to increase if RFD through practicing speed deadlifts (to get the bar off the floor faster) and maximal training, (to challenge the high threshold units to fire). Pretty soon, instead of taking 3 seconds to even get the bar off the floor, it only takes 1 second of effort and instead fo straining for 5 seconds just to get the bar to his knees, he’s able to accelerate through the pull and get it to lock out in just under 4 seconds. Success!
For sake of the blog post, we could assume he always had the capability of producing enough force to pull 400lbs, but could produce it fast enough before his body pooped out. Now, with his new and improved RFD, 400lbs flies up like it’s nothin.’
Another thing to keep in mind is the torque-angle relationship during the movement. Right… what?
All that means is the torque on the joints will change depending on their angles throughout the movement, thus affecting the amount of force the muscles surrounding those joints must produce. For example, typically* the initial pull off the floor in a deadlift will be harder than the last 1-2 inches before locking out due to the angle of the hip and knees (at the bottom, the glutes are in a stretched position which makes contracting a little tougher than at the top when they’re closer to their resting length.) Same concept applies to the bench press, typically** the first 1-2 inches off the chest are more difficult than the last 1-2 inches at lockout. The implication of all this being the muscles will have different force-production demands (and the capability to meet those demands) throughout the exercise.
Knowing this, we can train through the “easier” angles and still impose a decent stimulus to keep those higher threshold motor units firing the whole time. How?
With chains and bands! Yay!
Aside from looking totally awesome, chains provided added resistance during the “easier” portions of the exercise to encourage (read: compel) muscles to maintain a high force output throughout the movement. Watch Conrad, The Boss, deadlift with chains:
At the bottom, when the torque-angle relationship is less favorable, the weight is the lightest and as he pulls up, the weight increases as glutes must maintain a high level of force output to complete the deadlift. No lazy glutes up in hea’! Bands produce a similar effect. Check out the smashingly informative reverse band bench post Steve wrote here.
There are other ways and other aspects to discuss (like the fore-velocity curve... but that is a tale for another day!), but quite frankly, this blog post is reaching saga-like proportions so I’m going to cut it here. And remember kids:
*unless your name is Kelsey Reed and you have a torso 6 inches long… but can’t lock the pull out.
** unless your arms crazy long.
Rate of Force Development: What It Is and Why You Should Care
No, sorry, this is not a post on how to become a Jedi by increasing your rate of using the Force. Shucks.
The Rate of Force Development (RFD) we're going to talk about is that of muscles and is *kinda* important (read: essential to athletic performance). Today's post will enlighten you as to what RFD is and why one should pay attention to it. Next post will be how to train to increase RFD. So grab something delightful to munch on (preferably something that enhances brain function, like berries.) Caveat: There is a lot of information and other stuff that I’m not putting into this post, sorry, this is just a basic overview of why RFD is important for everyone.
What is RFD?
It is a measurement of how quickly one can reach peak levels of force output. Or to put it another way, it’s the time it takes a muscle(s) to produce maximum amount of force.
For example, a successful shot put throw results when the shot putter can exert the most force, preferably maximal, upon the shot in order to launch it as far as humanly possible. She has a window of less than a second to produce that high force from when she initiates the push to when it's released from her hand. Therefore, it is imperative that the shot putter possess a high rate of force development.
Where does RFD come from?
Well, let me introduce you to a little somethin’ called a motor unit. Motor units (MU) are a motor neuron (the nerve from your brain) and all the muscle fibers it enervates. It can be anywhere from a 1:10 (neuron:fiber) ratio for say eyeball muscles, which have to produce very fine, accurate movements. Or 1:100 ratio of say a quad muscle which produce large, global movements.
There are two main types of MUs: low threshold and high threshold. The low threshold units produce less force per stimulus than the high units. For example, a low unit would be found in the postural muscles as they are always “on” producing low levels of force to maintain posture. A high unit would be in the glutes, to produce enough force to swing a heavy bell or a baseball bat (even though the bat is light, the batter has to move that thing supa fast in order to smack a home run).
Also note the different stimuli required for the different units: small posture adjustments vs. a powerful hip movement. A low stimulus activates low threshold units and a high stimulus activates the high units.
Now, MUs are not exclusively low or high; MUs throughout the body are more like a ladder, low MUS at the bottom, with each successive rung being a higher threshold MU than the one below. And, like a ladder, you can just all of the sudden find yourself at the top of the ladder without having to climb the lower rungs. Unless of course, you’re a cat:
High MUs rarely (if ever) activate without the lower MUs activating first. So, the rate of force development is dependent upon how quickly the lower rungs of the MU ladder can be turned on to reach the highest threshold units (which produce the most force per contraction)… Not only that, but all those units working together produce more force than just the higher ones by themselves, so it's a good thing that the lower ones must activate too. The muscular force produced is the sum of all the motor units.
Why Care About RFD?
Since those higher threshold units won’t be active until the lower ones are on, force production will remain low until the higher ones can get their rears in gear, therefore, going up the MU ladder faster will result in more force produced sooner in any sort of movement.
Let’s take the example of two lifters, A and B. Both are capable of producing enough force to deadlift 400lbs. However, lifter A has a higher RFD than lifter B. Lifter A can produce enough force to get the bar off the ground in about 2 seconds and lock out (complete the lift) in about 3-4 seconds. Lifter B takes 3 seconds to get the bar off the floor and another 5 to get it near his knees. For those who don't know, a deadlift should be roughly 4-5 seconds TOTAL (typically, most people's muscles give out around then if the lift hasn't been completed). B-Man is going to fail the lift before he gets that bar to lock out and will hate deadlifting forever. Bummer.
Or, utilizing a Harry Potter for my analogy for this post, it is analogous to the rate of spell development; how quickly and how powerfully a wizard's spell is performed. In a duel, the faster and more forceful wizard will win. For example, when Professor Snape totally pwns Gilderoy Lockhart:
Hence, if one wants to get stronger, increasing the rate of force development is essential! Moving heavy weights is good (and high RFD helps with that as we saw with Lifters A and B from above); moving heavy weights FAST is even better when it comes to stimulating protein synthesis aka: muscle building. Possessing a high RFD is vital in order to move those bad boys quickly.
Next post, we’ll delve into training methods that can help increase the RFD so you won’t be these guys and skip deadlifting because your rate of force development is less than stellar…
