Effects of weighing phase duration on vertical force-time analyses and repeatability

Area of Science:

• Biomechanics
• Sports Science
• Human Movement Analysis

Background:

• Force plate analysis is crucial for evaluating athletic performance and movement.
• Accurate calculation of kinetic, temporal, and kinematic metrics relies on defining the force plate's weighing phase (WP).
• The impact of varying weighing phase durations on data reliability remains under-investigated.

Purpose of the Study:

• To investigate the effect of different weighing phase durations on the agreement of vertical force-time variables.
• To assess the repeatability of average force (WPav) and standard deviation of force (WPsd) across varying durations.
• To determine optimal weighing phase durations for accurate force plate analysis.

Main Methods:

• Compared weighing phase durations of 0.5, 1.0, and 1.5 seconds against a 2.0-second reference.
• Calculated limits of agreement (LOA) for system weight, onset threshold, onset time, net impulse, take-off velocity, and displacement.
• Assessed WPav and WPsd repeatability using repeatability coefficients across 137 jumps (counter-movement, squat, single leg).

Main Results:

• Shorter durations yielded minimal WPav differences (< ±0.25%) but led to significant accumulated errors in net impulse (±2%) and take-off displacement (±23%).
• Using 5xWPsd as an onset threshold resulted in substantial differences (±25% to ±72%) and affected onset time (±6% to ±18%).
• WPav demonstrated high repeatability, but integration effects necessitate WPs longer than 2.0 seconds; WPsd showed poor repeatability.

Conclusions:

• Weighing phase duration significantly influences the accuracy of integrated force-time variables in jump analysis.
• A weighing phase duration longer than 2.0 seconds is recommended for reliable force plate data, especially when using onset thresholds.
• Further research is needed to validate the use of WPsd due to its poor repeatability.