Concurrent Validity of a Portable Force Plate Using Vertical Jump Force–Time Characteristics

Abstract

This study examined concurrent validity of countermovement vertical jump reactive strength index modified and force-time characteristics recorded using a 1-dimensional portable and laboratory force plate system. Twenty-eight men performed bilateral countermovement vertical jumps on 2 portable force plates placed on top of 2 in-ground force plates, both recording vertical ground reaction force at 1000 Hz. Time to takeoff; jump height; reactive strength index modified; and braking and propulsion impulse, mean net force, and duration were calculated from the vertical force from both force plate systems. Results from both systems were highly correlated (r ≥ .99). There were small (d < 0.12) but significant differences between their respective braking impulse, braking mean net force, propulsion impulse, and propulsion mean net force (P < .001). However, limits of agreement yielded a mean value of 1.7% relative to the laboratory force plate system (95% confidence limits, 0.9%-2.5%), indicating very good agreement across all of the dependent variables. The largest limits of agreement were for jump height (2.1%), time to takeoff (3.4%), and reactive strength index modified (3.8%). The portable force plate system provides a valid method of obtaining reactive strength measures, and several underpinning force-time variables, from unloaded countermovement vertical jump. Thus, practitioners can use both force plates interchangeably.

Understanding the Key Phases of the Countermovement Jump Force-Time Curve

“…the peak negative COM displacement/deepest part of the squat) (10,11,18,19), as shown in Figure 3b. The braking phase has been called the "stretching phase" (11,20,27) and "eccentric phase" (17)(18)(19) in some previous studies, whereby it is assumed that the leg extensor muscle-tendon units are actively stretching to decelerate body mass but one cannot assume that all active muscles are stretching (i.e. medial gastrocnemius may actually shorten (12) during this phase).…”

“…when the athlete is standing still during the weighing phase). As a given net impulse can be achieved by applying a large force over a short time or a small force over a long time and variations in between, the shape of the net impulse produced in the braking phases will depend on the strategy employed by the athlete (17)(18)(19)(20)27). If the athlete attempts to minimize braking phase time, as may be the focus if they are instructed to perform the CMJ as fast as possible, they will have to produce a large braking force to match the unweighting phase net impulse and reduce momentum to zero (Figure 4).…”

“…If the athlete attempts to minimize braking phase time, as may be the focus if they are instructed to perform the CMJ as fast as possible, they will have to produce a large braking force to match the unweighting phase net impulse and reduce momentum to zero (Figure 4). Such a strategy will, therefore, be characterized by a taller (large force) and thinner (short time) active net impulse and a higher rate of force development in the braking phase (17)(18)(19)(20)27).…”

Effect of Landing Posture on Jump Height Calculated from Flight Time

“…Two methods of calculating jump height were used: (1) the vertical velocity of the COM wb at takeoff, and (2) flight time. The vertical GRFs (F ver ) were integrated by trapezoid rule integration to estimate the vertical velocity [6]. The vertical velocity at takeoff (V to ) was calculated using the following equation:…”

“…The force platform is one of the most widely used methods of vertical jump measurement and is considered the gold standard for determining the mechanical outputs of jumping [6]. Force platforms are used to measure the ground reaction force (GRF) and derive the velocity of the whole-body center of mass (COM wb ) using the impulse-momentum relationship.…”