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Defining the Term Power in Strength and Conditioning

This is an excerpt from Developing Power-2nd Edition by NSCA -National Strength & Conditioning Association.

The colloquial use of the term power as a generic trait is commonly misunderstood and misinterpreted (36, 85). Power is, by definition, the rate of doing work. The unit of measure for work is the joule and the unit of measure for power is the watt (W), defined as 1 joule per second. Coaches often indicate that athletes are powerful by describing their movements as occurring at a high velocity relative to the force they must produce or the load they must overcome during the movement. Therefore, movements that occur at lower velocities because of external loads that must be moved (e.g., another person during a tackle or a weighted jump squat) may still be described as powerful because the velocity is high relative to the force required or the mass being accelerated. The colloquial use of the term powerful is likely a loose interpretation of the mathematical definitions of power. The following mathematical equations associated with power and work can be arranged several ways to derive the various equations for power.
Because work is a product of force and displacement, substitution leads to the following equation:

Simplified further (because velocity = displacement ÷ time), the equation can once again be rearranged to what is commonly used or expressed by strength and conditioning practitioners as the equation for power:
Power (W) = force (N) × velocity (m/s)

Power can be expressed as the mean, termed mean power (P_mean), or as the highest peak of instantaneous power, termed peak power (P_peak), over an entire movement or within a specific phase of a movement. As such, the P_mean will always be a lower value and represents the power across the entire movement or phase, whereas the P_peak is the highest power produced during a discrete time point. For example, the P_mean during a countermovement jump was reported as 765 W, while the P_peak was reported as 5,014 W (9). The discrete time point within which the P_peak occurs depends on the sample frequency of the device being used to assess power. Thus, if using a force plate sampling at 1,000 Hz (i.e., 1 sample recorded every 0.001 s), which is suggested to be the criterion approach (63), the P_peak reported simply represents the highest power value recorded within a 1-millisecond time frame of the movement being assessed. The P_mean and P_peak can also be reported at the level of the body (if performing a task involving body mass alone), an external implement (such as a barbell), or the entire system (if quantifying power applied to a combination of both the body and the external implement). Thus, the term system power is often used to describe power produced by athletes during loaded ballistic jumps and weightlifting variations. A representative force–time curve produced during the countermovement jump and its associated phases is shown in figure 2.1. In figure 2.2, the corresponding force–, velocity–, power–, and displacement–time curves

between the commencement of unweighting (i.e., the start of the countermovement jump) and takeoff that were obtained from the representative force–time curve shown in figure 2.1 are displayed, along with the individual occurrences of the peak value for each variable.

The current trend among strength and conditioning professionals to measure and report P_mean or P_peak has led to the development of ballistic assessments (e.g., bench press throw and vertical jumps) (54). During ballistic assessments, power is often calculated to understand the force–velocity profile of an athlete. However, ballistic assessments should not be considered the measurement of power. For example, jump height is often incorrectly assumed to be an indirect measure of leg power, but it is the net impulse (if deconstructing the jump using the impulse–momentum theorem) and the mechanical work (if deconstructing the jump using the work–energy theorem—see chapter 1) produced during the propulsive phase of the jump that dictates jump height (40). Previously reported moderate to large correlations between propulsion power and jump height are artificially inflated due to the almost perfect association (r = 0.83-0.94) between the propulsion velocity that coincides with the instant of the P_peak and the subsequent jump height (40). Accordingly, a higher propulsion velocity at the instant of the P_peak will lead to a greater velocity at takeoff, which in turn directly dictates vertical jump height (35, 45). Thus, instead, the P_mean or P_peak may be measured during ballistic activities. In fact, system power could technically be measured during any activity except for those that are isometric, where velocity is zero and power is therefore zero. Furthermore, when measuring power, it is critical to fully describe the methods of measurement (discussed in the next section) so that the results can be interpreted within the correct context. Other variables, such as force and velocity, should also be presented, because power is the mechanical construct of force and velocity (49). Therefore, to correctly interpret power as a measured variable, it is necessary to understand the combination of force and velocity changes that elicit the measured power output.

More Excerpts From Developing Power 2nd Edition

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