Rate of Force Development (RFD)
Rate of force development (RFD) is the ability of a muscle or muscle group to produce force quickly from a state of rest or low force. In other words, it is the rate at which force is generated during the early phase of a muscle contraction. RFD is an important measure of explosive strength and power in many athletic activities such as sprinting, jumping, and weightlifting.
RFD can be calculated as the slope of the force-time curve during the initial phase of muscle contraction. Typically, RFD is measured as the rate of force increase between 0 and 200 milliseconds (ms) from the onset of muscle contraction. This time frame is considered the "early phase" of muscle contraction and is where RFD is most relevant.
Reading Data Output from Force Plates
Force plates are instruments used to measure the forces exerted on the ground during various activities, including running, jumping, and weightlifting. Force plates can be used to analyze RFD by measuring the ground reaction forces (GRFs) that occur during the early phase of muscle contraction.
The data output from force plates typically includes the following measurements:
Vertical force (Fz): This is the force perpendicular to the ground and represents the weight of the object or individual being measured.
Anterior-posterior force (Fx): This is the force parallel to the ground and in the forward/backward direction.
Medial-lateral force (Fy): This is the force parallel to the ground and in the left/right direction.
By analyzing the force-time curve for each of these measurements, it is possible to determine the RFD.
Using Force Plates to Analyze RFD
To analyze RFD using force plates, the following steps can be taken:
Collect data using force plates during a specific activity, such as a vertical jump or a bench press.
Calculate the force-time curve for each of the GRFs measured by the force plates.
Identify the onset of muscle contraction (when the force begins to increase) and the peak force.
Calculate the RFD as the slope of the force-time curve between 0 and 200 ms from the onset of muscle contraction.
Repeat the analysis for multiple trials and take the average RFD.
Using Data from Force Plates to Determine Muscle Fiber Type and Training Needs
While force plates can be used to measure RFD, they cannot directly determine muscle fiber type. However, there is evidence to suggest that RFD is positively correlated with fast-twitch muscle fibers (type II fibers) (1).
To determine muscle fiber type, a muscle biopsy is typically performed and analyzed under a microscope. However, there are indirect measures that can be used to estimate muscle fiber type, such as electromyography (EMG) and velocity-based training.
EMG measures the electrical activity of muscle fibers and has been shown to be a reliable indicator of muscle fiber type (2). Velocity-based training involves measuring the speed of the bar during a lift and using this information to estimate the force produced by the lifter. This method has also been shown to be a reliable indicator of muscle fiber type (3).
Once muscle fiber type has been determined, training needs can be identified. For example, individuals with a higher proportion of fast-twitch muscle fibers may benefit from training methods that prioritize explosive power and strength, such as plyometrics and weightlifting.
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References:
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Aagaard P, Simonsen EB, Andersen JL, Magnusson P, Dyhre-Poulsen P. Increased rate of force development and neural drive of human skeletal muscle following resistance training. J Appl Physiol. 2002;93(4):1318-1326. doi: 10.1152/japplphysiol.00283.2002
Mann JB, Thyfault JP, Ivey PA, Sayers SP. The effect of velocity of movement on performance factors in resistance exercise. J Strength Cond Res. 2010;24(1):196-205. doi: 10.1519/JSC.0b013e3181c63ae5
Higbie EJ, Cureton KJ, Warren GL III, Prior BM. Effects of concentric and eccentric training on muscle strength, cross-sectional area, and neural activation. J Appl Physiol. 1996;81(5):2173-2181. doi: 10.1152/jappl.1996.81.5.2173
Sale DG. Neural adaptation to resistance training. Med Sci Sports Exerc. 1988;20(5 Suppl):S135-145. doi: 10.1249/00005768-198810001-00010
Bamman MM, Petrella JK, Kim J-S, et al. Cluster analysis tests the importance of myogenic gene expression during myofiber hypertrophy in humans. J Appl Physiol. 2007;102(6):2232-2239. doi: 10.1152/japplphysiol.01566.2006