Rate of Force Development (RFD)

Summary:

The rate of force development (RFD) is a measure of explosive strength, or simply how fast an athlete can develop force. Athletes with higher rates of force development have been shown to perform better during numerous physical performance tests. This therefore highlights the potential importance this value has in the role of athletic development. Whilst many forms of training have been shown to improve the rate of force development in untrained individuals, only resistance and ballistic training have shown to enhance this quality in trained athletes. Lastly, though there are multiple ways to measure the rate of force development, the time-interval sampling windows appear to be the most reliable.

What is the Rate of Force Development (RFD)?

The rate of force development (RFD) is a measure of explosive strength, or simply how fast an athlete can develop force – hence the ‘rate’ of ‘force development’. This is defined as the speed in which the contractile elements of the muscle can develop force (1). Therefore, improving an athlete’s RFD may make them more explosive as they can develop larger forces in a shorter period of time. Developing a more explosive athlete, may improve their sporting performance. In fact, higher RFDs have been directly linked with better jump (2-8), sprint (9), cycling (10), weightlifting (5, 6), and even golf swing performances (11).

The RFD is commonly believed to be manifested during the stretch-shortening cycle (SSC). Depending upon the duration of the stretch-shortening cycle (SSC), exercises are classified as either slow- (≥250 milliseconds) or fast-SSC (≤250 milliseconds) movements (12). For example, a countermovement jump (CMJ) is classified as a slow-SSC movement as the duration of the SSC lasts approximately 500 milliseconds (3). On the other hand, sprinting is classified as a fast-SSC movement as duration of the SSC last between 80-90 milliseconds (13). Table 1 displays the SSC durations of some common exercises.

Because the movement is slower, exercises with slow-SSC have a longer timeframe to develop force than those with a fast-SSC, this means slow-SSC exercises can typically create higher peak forces (7, 19). However, as there is typically less urgency to develop force during the slow-SSC movements, they often do not develop force as quickly as fast-SSC movements. This means that exercises with a slow-SSC produce lower RFDs than fast-SSC movements (7, 19). Therefore, slow-SSC exercises produce higher peak forces, but lower RFD than fast-SSC movements.

On the other hand, as it takes 140-710 milliseconds to develop peak force during various jump exercises (7, 5, 20), fast-SSC exercises may struggle to produce peak forces because the SSC simply does not last long enough. Whilst they may not be capable of producing peak forces, they can produce large a RFD due to the speed of the movement (Table 2).

It is suggested that exercises characterised by larger joint displacements (i.e. work through a larger range of movement), are typically categorised as slow-SSC movements. Whereas exercises with smaller joint displacements are commonly referred to as fast-SSC movements (21). For example, a CMJ (slow-SSC movement) experiences larger joint displacements than sprinting (fast-SSC) (Figure 1). This helps to dissociate between what are slow-SSC movements and what are fast-SSC movements when no research has determined what classification they belong too.

What causes an increase in Rate of Force Development?

Improvements in RFD are likely to be the result of increases in muscle-tendon stiffness (22, 23), enhanced muscle force production via changes in muscle fibre type or type area (from type I to type IIA) (24, 25), and increases in neural drive during the early phase of the SSC (<100ms) (26, 27). In contrast, RFD appears to be negatively affected by changes in muscle fibre type (from type IIX to type IIA) (28), and increases in fascicle length (causing reduced muscle stiffness) (29).

Why is the Rate of Force Development important for Sports?

As power is a key determinant in the performances of many sports, optimising an athlete’s explosiveness may be of great importance (30-35). Research has identified that the RFD has been directly linked to performances during jumping (2-8), weightlifting (5, 6), cycling (10), sprinting (9), and even during the golf swing (11) – suggesting a better RFD can lead to a better athletic performance. Moreover, elite sprinters have been shown to possess greater RFD than well-trained sprinters (9). Collectively, this information suggests that RFD may be an important contributor to athletic performance.

• RFD linked to better jump, sprint, cycling, weightlifting and golf swing performances
• Elite-level sprinters have better RFD than well-trained sprinters
• Power trained athletes have greater RFD than non-power trained athletes (36).
• Power trained athletes have greater RFD than endurance athletes (36).

How to Calculate the Rate of Force Development

As RFD is an expression of explosive strength, it is measured in Newtons per second squared (N·s^-1). The RFD can be calculated for isometric, concentric and eccentric muscle contractions, with the latter two otherwise referred to in the research as the ‘positive’ and ‘negative’ acceleration phases of the SSC (2, 3). In fact, one study suggests that eccentric RFD is a better predictor of jump performance than concentric RFD because it summarises several intrinsic properties of muscle and tendons during a key moment (2). However, this is yet to be validated by other research.

Multiple measures of RFD have been developed in order to measure various components of performance during both isometric and dynamic movements:

1. Average RFD or IES (Index of Explosiveness) (2, 3, 5, 6, 8, 9, 11, 37, 38)
2. Time-interval RFD (11, 39)
3. Instantaneous RFD (40, 41)
4. Peak or maximal RFD (42, 4, 5, 7, 10, 11, 36)
5. Time to peak RFD (11, 7)

Average RFD: This value is identical to the IES discussed by Zatsiorsky (37), and is calculated by dividing the peak force by the time to achieve peak force (39). However, this form of measuring average RFD has been shown to have a lower levels of reliability in comparison to time-interval RFD and peak RFD (39). These lower levels of reliability may be associated to each athlete’s time to achieve peak force, as not all athletes can achieve peak force in the same time frame. Therefore, measuring RFD using predetermined time-intervals can accommodate for these variances.

How to calculate Average RFD

Example – Calculating Average RFD

Average RFD [N·s^-1] = Peak Force [N] / Time to achieve peak force [s][/toggle]

Time-Interval RFD: Though this measure of RFD is effectively the same as average RFD, it is calculated at various time-intervals (e.g. 0-30, 0-50, 0-90, 0-100, 0-150, 0-200, and 0-250 milliseconds [39]). This value simply represents a change in force divided by a change in time. It is calculated by dividing the force at the end of the time interval by the duration of the time interval (39) (Table 3). Just note that when calculating RFD, the time should be calculated in seconds, not milliseconds.

How to calculate Time-Interval RFD

Example – Calculating RFD with a 0-30 millisecond time-interval:

RFD [N·s^-1] = Change in Force [N] / Change in Time [s]

RFD [N·s^-1] = Force [N] at 30 milliseconds / 0.03 second time-interval [s]

RFD [N·s^-1] = 50N / 0.03s

RFD [N·s^-1] = 1,666

The strength and conditioning coach should select the time-intervals they wish to use based upon the nature of the exercise. For example, if the exercise has a fast-SSC movement (e.g. Sprinting – 80-90 milliseconds), then it may be suggested that time-intervals of ≤100 milliseconds may be most appropriate (e.g. 0-50, 0-90, and 0-100 milliseconds). Furthermore, the early time-intervals (<100ms) are often referred to as ‘early phase’ RFD, whilst >100s time-intervals are referred to as ‘late phase’ RFD.

Instantaneous RFD: This value is measured by using the maximal tangential slope between two adjacent data points. In other words, the data is recorded using 1-milliseconds time-intervals, and from this, the change in force is divided by the change in time at every 1-millisecond time-interval. As this value of RFD is calculated every 1-millisecond, it provides a very precise measure of RFD.

Peak or Maximal RFD: This value of RFD is realistically as simple as it sounds, it is the largest amount of RFD produced during the movement. Most commonly, the value is identified by measuring the peak RFD during numerous sampling windows of: 1, 2, 5, 10, 20, 30, and 50 (40, 39). For example, if the strength and conditioning coach selected a sampling window of 5 milliseconds, they would measure peak RFD every 5-milliseconds (e.g. 0-5, 5-10, and 10-15 milliseconds and so forth). They would then simply identify the largest RFD value out of those recorded – this value is then the peak RFD. Whilst all of these sampling windows have been reported as reliable measures of peak RFD, the 20-millisecond sampling window has been shown to be the most reliable (39). Table 4 demonstrates how to identify peak RFD during an isometric performance.

From this data the coach is able to calculate the athlete’s peak RFD, time to peak RFD, and average RFD. These variables are useful tools for comparing a group of athletes to identify who the best and worst individuals are at developing force quickly.

Time to Peak RFD: As displayed in Table 4, this value of RFD is extremely straight-forward. It is a useful tool for measuring performance as it provides the coach with information on how quickly the athlete is able to achieve their maximal explosive strength (i.e. peak RFD). Decreasing an athlete’s time to achieve peak RFD will allow them to produce higher forces in shorter periods of time, and therefore may increase their explosiveness and overall athletic performance.

Validity and Reliability

The RFD and its various measures (Average RFD, Time-Interval RFD, Peak RFD, and Time to Peak RFD) have all been shown to be a valid and reliable tool for assessing explosive strength (11, 39). However, the most reliable measures for assessing RFD appears to be any of the time-interval sampling windows (i.e. 0-30, 0-50, 0-90, 0-100, 0-150, 0-200, and 0-250 milliseconds), and peak RFD using 20-millisecond windows (39). It is therefore recommended that these two variables should be preferable when measuring RFD.

Practical Application: How to improve the Rate of Force Development

Increasing the RFD whilst simultaneously reducing the time in which peak RFD occurs, will result in a left and upward shift in the force-time curve (Figure 2). This left and upward shift enables the athlete to produce greater forces in a shorted period of time, ultimately improving their explosiveness.

Figure 2 – Shift in the force-time curve after a successful training programme.

It is often suggested that athletes must train at various sections along the force-time curve if improvements in RFD are to occur. By only training on one part of the force-time curve (e.g. maximum strength), it is likely that the athlete will only improve their performance at that section on the paradigm. For example, only training maximal strength may lead to improvements in force production, but it may also result in a reduction in the time to achieve that force (Figure 3). As training programmes which combine strength and power training have been repeatedly shown to improve athletic performance more than strength or speed training alone (43), there is no surprise that most exercise professionals commonly use an all-rounded approach within their programming.

Figure 3 – Force-time curve after training specific elements.

The following types have training have all shown to improve RFD:

• Resistance training (44, 45, 46, 47, 48)
• Ballistic training (49, 50, 51, 52)
• Olympic Weightlifting (53)
• Plyometric training (54, 55, 50, 56)
• Balance training (57, 58)

However, only those listed below have shown RFD improvements in trained or athletic subjects:

• Resistance training (59, 60, 61, 62, 63)
• Ballistic training (51, 52)

So whilst numerous training methods have been shown to improve RFD in untrained and elderly males and females, little research has shown RFD improvements in trained or athletic subjects.

Conclusion

RFD is a reliable measure of explosive strength, with higher RFDs have been linked with better athletic performance. Improvements in RFD are likely to be the result of increases in muscle-tendon stiffness, enhanced muscle force production via changes in muscle fibre type (from type I to type IIA), and increases in neural drive. The most reliable values for measuring this component of performance appear to be calculating RFD at various time-intervals and peak RFD using 20-millisecond sampling windows. Though various methods of training have been shown to improve performance (resistance, ballistic, Olympic Weightlifting, plyometrics, and balance training), only resistance and ballistic training have been proven to increase RFD in trained and athletic populations.

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