Rate of Force Development
Rate of Force Development (RFD) refers to the ability of the neuromuscular system (the coordination of our nervous system and muscles; this process enables the control, direction, and movement of the body) to increase the contractile force from a low or resting state, whereby muscle activation is performed as quickly as possible. In simple terms, it is the speed at which the contractile elements of the muscles can develop force.
We calculate the RFD by dividing the change in force measured in newtons by the time taken to achieve that change measured in meters per second. Therefore, RFD= Force (Newtons) / Time (seconds). RFD is an expression of explosive strength, measured in Newtons per second squared (N·s-1).
The RFD is one of the best ways to measure explosive strength. It calculates how fast an athlete can develop force, which is why it is called “The Rate of.” RFD is a pure expression of explosive strength. Explosive strength refers to the ability of a muscle or group of muscles to exert maximum force in a short amount of time. It highlights an athlete's capacity to quickly push or accelerate an object or body, demonstrating power and speed.
RFD can be calculated for all three movements: Isometric, Concentric (positive acceleration), and Eccentric (negative acceleration).
The RFD shows that force is expressed at slower velocities, allowing cross-bridges to form. This is also known as the sliding filament theory, which is responsible for skeletal muscle contraction. (CLICK for more about cross-bridge theory.) The best way to measure RDF is by the Peak or Maximal RFD, which measures the largest amount of explosive strength produced during a movement.
One would typically assign a range of sampling windows within a time frame of 5 milliseconds; for example, measure the peak RFD every 5 milliseconds and identify the largest RFD value out of those recorded. This value would then represent the peak RFD.
The most reliable sampling window reported for measuring peak RFD is 20 milliseconds, which is shown to be the most reliable. As seen in the chart below, you divide the change in force by the change in time to get the Peak RFD. The chart below demonstrates how to identify peak RFD during an isometric performance.
As seen below, if athlete A produced 180N in 0-0.5 milliseconds, you divide the change in force 180N by the time taken to achieve that 0.05, giving the Peak RFD = 3,6000N s-1.
Athlete A’s RFD chart above shows that their peak RFD measured was 8,400 Ns-1 during the 250-millisecond window. The ‘time to peak RFD’ for Athlete A was 100ms, and the average RFD was 5,600Ns-1, the average of all five RFDs in the final column.
This data will show an athlete’s ability to achieve maximal explosive strength (peak RFD). The athlete can then work on decreasing their time to achieve Peak RFD, allowing them to produce higher forces in shorter periods, thus increasing their explosiveness and overall performance.
What is Force
In physics, force (measured in newtons) is an influence capable of altering an object's motion. Forces can be characterised as either pushing or pulling and because they possess both magnitude and direction, they are vector quantities. To learn more about vector and scalar quantities, CLICK HERE for the blog post explaining them in detail.
The formula for force is derived from Newton's second law of motion, which states that the force acting on an object is equal to its mass multiplied by its acceleration. Thus, force is mass times acceleration (F = ma). Force is a vector because it has both magnitude (a unit of measurement to specify the size or intensity of an event or object having distance or quantity) and direction. When one force is stronger than another, it can cause objects to:
Accelerate
Decelerate
Change directions
Change shape
Force is a quantity that causes an object to change its motion, whereby motion is the change in the position of an object concerning its surroundings at a given interval of time and a change in direction.
Force, mass, and acceleration are all related! Here’s why - The more mass an object has, the more force you need to accelerate it, and the greater the force, the greater the object's acceleration. This means that an object's acceleration is directly proportional to the total force acting on it and inversely proportional to its mass. In other words, a larger net force acting on an object causes a larger acceleration, and objects with larger masses require more force to accelerate. To learn more about Force, CLICK HERE for a post going into more detail about force and the physics principle of Work.
The acceleration of an object depends directly upon the net force acting upon the object and inversely upon the object's mass. As the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased.
What is Velocity
In physics, velocity refers to the rate at which an object's position changes in relation to a particular frame of reference. Essentially, it is speed with a specified direction. It is a vector quantity like force because it possesses both magnitude and direction.
In more detail, velocity equals displacement (s) divided by the change in time (Δ t), and the abbreviated equation you’ll see in physics for velocity is s/Δ t. Put, velocity is the speed at which something moves in one direction. The delta symbol (Δ) is a shorthand for the “change” in some unit of measure. To learn more about displacement, CLICK HERE.
Velocity is the rate at which an object changes position or the displacement of an object over an amount of time. Velocity is a vector with both magnitude and direction; for example, Dave travels 80mph northeast or 80mph towards the beach.
Therefore, the speed must have some direction attached to qualify as velocity. Velocity measures an object's motion's speed and direction and is expressed in units as m/s (meters per second), which is its official SI measurement.
In weightlifting, the faster the velocity, the lower the relative intensity (weight load). As the load increases, the velocity at which the weight moves decreases. In weightlifting, you will sometimes see the phrase “Peak Velocity” or maximum velocity, which is the highest velocity reached during a specific phase of an action.
For example, the peak velocity in weightlifting is the fastest point of the barbell speed during a concentric lift and can be seen on a velocity-time graph for each repetition. If you want to increase an athlete’s velocity, you either increase the time over which force is applied or the amount of force produced in a given time. Regarding sports, we want to increase the amount of force produced in the least amount of time, which is the impulse we will discuss in the next chapter.
Force-Velocity Curve
Firstly, let's define the relationship between force and velocity. Velocity is the speed of the movement in meters per second, and force is the power produced to move the object (mass) measured in Newtons. Velocity is a vector, so it is measured in seconds (time) and meters (displacement). Force is a scalar, which is why it is measured in Newtons. The Force-Velocity curve seen below is commonly used in sports to show the relationship (the curve) between an athlete's strength (force) and speed (velocity).
The Force-Velocity curve demonstrates that the higher the force required, the decrease in velocity is seen. For example, a 100% 1RM maximal strength deadlift will require large amounts of force, but the velocity of the movement will be incredibly slow. Similarly, a Speed-Strength Deadlift with 30% of 1RM requires less force production, as seen on the Y-axis, but it's performed at near maximum velocity. To simplify the inverse correlation, the other decreases simultaneously as one factor increases. As the force increases, velocity decreases, and as velocity increases, the force decreases.
The force-time curve is a graphical representation that illustrates the relationship between the force applied to an object and the duration of that force over time. It helps visualise how force develops during a specific movement, highlighting the changes in force output as the movement progresses. In athletic performance, the force-velocity curve provides insights into an athlete's ability to generate force quickly (rate of force development) and the maximum force they can produce.
The force-velocity curve, pictured below, visualises the inverse relationship between force and velocity. This relationship states that if someone generates maximal force in an exercise, they will generate minimal velocity. Similarly, if someone generates maximal velocity in an exercise, they will generate very little force.
What is Power?
Because power combines force and velocity, the force-velocity curve shows athletic power (power = force * velocity). Power can also be expressed in a formula as P=w/t, meaning it equals work divided by time elapsed.
Power is the ability to exert maximum force in the shortest time, requiring both force and speed. Power is how fast a given force can move an object. Power is the rate of doing work, whereby work is defined as the energy needed to apply a force to move an object a particular distance, where force is parallel to the displacement.
A high power value equals a significant force that creates a relatively large motion. It's important to note that power doesn’t always result in maximum velocity. Power is relative to the load; movements at slow velocities due to external loads that must be moved can be described as powerful as the velocity is high relative to the force required.
To learn more about Power, CLICK HERE for my blog post explaining power in greater detail.
Force-Velocity Curve Applied to Weightlifting
Applying this to powerlifting, the interaction is between the amount of force needed to overcome the weight on the barbell in kilograms and how fast the barbell is moving in meters per second. A powerlifter will compare the weight on the barbell to how fast they can move it for the three lifts: squat, bench press and deadlift.
The Velocity x-axis in weightlifting terms represents muscle contraction velocity or velocity of movement measured in meters per second. The y-axis represents force; weightlifting represents muscle contractile force, or the amount of ground reaction force produced measured in Newtons. The curve demonstrates a clear trade-off between force and velocity, meaning you cannot have high force and high velocity or low force and low velocity.
Force-Velocity Weightlifting Zones
Below, the graph has plotted five training zones against the inverse force-velocity curve.
Absolute/Maximum Strength (90-100%1RM)
The maximum amount of force a person can produce through a specific movement. For example, a 1RM deadlift represents the maximum force a lifter could produce during that exercise. This training zone is classified as 90%+. It measures an individual's overall strength capacity and is typically assessed through exercises like squats, deadlifts, or bench presses, where the focus is on lifting the heaviest possible load.
Strength-Speed (80-90%1RM)
The intensities are relatively high (80-90%1RM) and lean more towards strength than speed. It requires athletes to produce optimal force in a shorter time frame than maximum strength, thus reducing the amount of force produced. It achieves higher velocities than the maximal strength zone but less peak force.
Peak Power (30-80%1RM)
Peak power is the “sweet spot” between strength speed and speed strength. It's where both force and velocity coincide. It’s the point where you can create the most force and velocity simultaneously without sacrificing too much of either. The peak power zone covers two traits/zones: Speed-Speed (30-60% 1RM) and Strength-Speed (80-90% 1RM). Peak power is the zone where the weightlifter exerts maximal speed and force.
Remember, Power is relative to the load. For example, if a lifter performs a back squat with 80% of 1RM at a slower velocity than a 30%1RM Squat, it can be described as powerful if the velocity and speed of the movement are high relative to the load.
Speed-Strength (30-60%1RM)
As comparatively high velocities are employed within this zone (30-60% of 1RM), it emphasises speed rather than strength—hence the term 'speed-strength.' This zone is best for speed-training and dynamic effort training whereby the focus is maximal velocity explosiveness rather than maximal strength.
Maximum Velocity <30%1RM
This is the maximal movement velocity or muscle contractile velocity an individual is able to produce through a specific movement. This training zone intensity is approximately < 30 % of 1RM.
How to Achieve Peak RFD
The goal of a weightlifter in power training is to shift the force-velocity curve to the right, as demonstrated below. This results in an athlete being able to move larger loads at higher velocities and, therefore, becoming more explosive. Shifting the curve to the right represents an improved RFD, i.e., how fast an athlete can develop force. An athlete with more explosiveness can develop larger forces in a shorter period of time.
That said, the ratio of force and velocity at the two ends of the spectrum contributes to power, and this ratio needs to be balanced to optimise power output. In practice, this means that a weak but fast athlete might improve power by working within the max strength range, and a strong but slow athlete might improve power by working within the max speed range.
If a lifter trains one section of the Force-velocity curve, such as maximum strength, the athlete will only improve their performance in that section of the paradigm. If the athlete only trains maximal strength with maximal effort lifts (80-100% 1RM), this will lead to improvements in force production, but it will most likely result in a reduction in muscle contractile velocity.
Here comes the trade-off theory again. This is why sensible advanced athletes use periodisation and undulating training programs to ensure their training is complete and contains all types of training demonstrated on the Force-velocity curve, whether in blocks, phases, or undulated. Examples of training for Peak RFD are as follows: focusing on maximal velocity and maximal force production.
Maximal Velocity/Speed-Strength
Dynamic Effort Training (using Accommodating resistance)
The dynamic effort method in strength training involves lifting a submaximal weight at high velocity or "maximal speed." This approach improves an athlete's rate of force development. By incorporating dynamic effort training, athletes enhance their ability to generate significant force in a short amount of time. As a result, lifters experience improved bar speed, and athletes can perform better in their respective sports.
Dynamic effort training with accommodating resistance involves bands or chains that make a lift heavier at the top and lighter at the bottom. This method encourages the lifter to generate force throughout the entire range of motion rather than easing off as it approaches lockout.
Plyometric Training
Plyometric training is an explosive movement exercise designed to increase power, speed, and strength. It typically includes exercises that exert maximum force in short intervals, such as jump squats, box jumps, and depth jumps. The primary goal of plyometric training is to improve the force development (RFD) rate. The force-time curve is a graphical representation that illustrates the relationship between the force applied to an object and the duration of that force over time. It shows how force develops as a function of time during a muscle contraction or movement.
On the curve, the x-axis typically represents time, while the y-axis represents the force in Newtons. The curve rises as an athlete exerts force, indicating that force is being generated. The area under the curve can represent the total amount of work done or the impulse delivered over that period. The shape of the force-time curve can vary based on factors such as the athlete's training (e.g., speed training vs. strength training) and the specific movement performance. It helps visually compare the rate at which different athletes develop force, highlighting differences in explosiveness and strength capacity by enhancing the muscular system's ability to generate explosive force quickly.
Compensatory Accelerative Training (CAT)
CAT, or Compensatory Acceleration Training, refers to a technique used in barbell training to enhance movement velocity, improving force production and power output. This method emphasises that the athlete should intentionally maximise the acceleration of the barbell throughout the entire concentric phase of a lift, regardless of the weight being lifted. The goal is to produce maximum force by moving the weight as quickly as possible, even when using lighter loads. This approach is based on the principle that increasing acceleration allows an athlete to compensate for lighter weights and still generate significant force, enhancing overall power and explosiveness in their movements.
Ballistic Training
Ballistic training is a type of exercise that focuses on explosiveness and power through high-velocity movements. This training method involves performing exercises that require the athlete to exert maximum force in short, quick bursts, often with minimal resistance. The primary goal of ballistic training is to enhance an athlete’s ability to generate rapid force, thereby improving their overall explosiveness and performance in their sport. This involves tuck jumps, kettlebell swings, broad jumps, sprints, Medicine Ball Throws, and even Olympic lifts like cleans and push Jerks.
Overwhelming research states the importance of training explosiveness and maximum strength to improve RFD. It is essential to train all sections along the force-time curve to improve RFD, both maximum strength and explosive strength. The athlete will only enhance their performance at that section of the paradigm by training only one part of the force-time curves. Maximal Strength training with heavier loads will increase neural drive to the muscle, which can improve RFD.
Higher RFD has been linked to better athletic performance, improved muscle-tendon stiffness, enhanced muscle force production via muscle fibre type recruitment changes, and increased neural drive. The graph below is another excellent example of peak RFD and how it demonstrates high levels of explosiveness in an athlete. The explosive athlete (Athlete A), an elite sprinter, can achieve higher levels of force in 200 ms than the untrained and heavy resistance-trained athlete (Athlete B), a powerlifter.
Although Athlete A does not achieve the same maximum strength as Athlete B, Athlete A has more maximal peak force and, therefore, is subsequently more explosive. Athlete A can generate more force at 200 ms than the heavy resistance-trained athlete.
Absolute Maximal Strength/Strength-Speed Training
Maximal Training
This form of training is quite simplistic. You use the compound main lifts: Squat, Bench Press, and Deadlift. You lift close to your one-rep maximum with low volume, sets typically of 1-3 repetitions and no more. The goal is lifting as close as possible to your maximum with proper form and technique.
Overload Training
“Overload” refers to any method that enables you to lift heavier weights than you usually would for a specific lift. This technique often involves de-loading the weight during the lower part of the repetition, where you are working hardest against gravity. The load becomes progressively heavier as you near lockout, which is the opposite of a conventional lift.
Overload can also be achieved through a reduced range of motion, eliminating the portion that proves most challenging. Overload training can exceed 100% of a lifter's 1RM due to the overloading training tools applied to the main three lifts. Examples of them are as follows that allow a lifter to lift heavier weights for more repetitions than without the overload training tools:
Squat:
Reverse Bands (Lightened Method)
Chains
Hatfield Squats
Pin Squats
Bench Press:
Slingshot
Reverse Band
Block Press
Pin Press
Deadlift:
Reverse Bands
Block Pull
Trap Barbell
Rack Pull Deadlift
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