Force and Time is All We Need: Interpreting Force Platform Data Effectively

Speaker Profile: Jake Cowan, Sports Performance Officer at the Tasmania Institute of Sport and PhD candidate at the University of Tasmania.
The Problem of Metric Overload: Force platforms generate an overwhelming volume of data. For the Countermovement Jump (CMJ) alone, there are over $100$ available metrics.
Methodological Risks: The abundance of data often leads practitioners to:
- Get lost in lists of values, measures, correlations, and ratios.
- Engage in "cherry-picking," where they select only the metrics that show a desired change or support a specific narrative.
Presentation Goal: To provide a streamlined framework that identifies the most important metrics and cuts through the "bullshit" by focusing on fundamental data.

Lachlan James' Research: Based on a paper by Lachlan James from La Trobe University, factor analysis was used to determine how many unique constructs actually exist within CMJ data.
Construct Limitations: At most, there are only about $2$ to $4$ distinct constructs represented by the vast array of available metrics.
Proportionality of Metrics: Many variables are highly related; a change in force often results in a proportional change in power. Therefore, different metrics do not necessarily provide different physiological information.

Direct Measures: Only two things are directly measured by a force plate:
1. Force ( $F$ ): Measured as pressure on the plates (essentially a "fancy bathroom scale").
2. Time ( $t$ ): Recorded by the computer during data capture.
Calculated Derivatives: Every other metric is a mathematical calculation derived from these two variables. As calculations move further from the raw data, they include:
- Acceleration ( $a$ )
- Velocity ( $v$ )
- Power ( $P$ )
- Jerk
- Snap
- Relative Rate of Power Development
Compounding Error: The further a metric moves from direct measurement, the more the potential for error is compounded. This obscures the "raw picture" of the athlete's performance.

Rate of Force Development (RFD):
- Validity: High. It represents the amount of force produced over a specific time, which is directly relevant to performance.
- Reliability: Low. There is a significant amount of "spread" and inherent error in RFD measurements, making it difficult to determine what constitutes a meaningful change.
Peak Force:
- Reliability: High. It provides consistent results over time.
- Validity: Questionable. Most force plates record at $1000 \text{ Hz}$ ( $0.001\text{ s}$ intervals). Identifying the force at a single millisecond removes almost all contextual information regarding the rest of the force-time curve.
Golden Rules for Metric Selection:
1. Avoid Distance from Direct Measure: Do not use metrics that are too many steps removed from force and time.
2. Understand Raw Data: Always look at and understand the raw trace before zooming in on specific metrics.

Power ( $P$ ) Calculation: Power is defined as: $P = Force \times Velocity$
Velocity ( $v$ ) Calculation: Velocity is the integral of acceleration over time: $v = \int a \, dt$
Acceleration ( $a$ ) Calculation: Acceleration is force divided by mass ( $a = \frac{F}{m}$ ).
Expanded Power Formula: When broken down to its fundamental components, power is: $P = F \times \int \frac{F}{m} \, dt$
Relative Power: To make this relative to body mass, the formula becomes even more complex: $P_{\text{relative}} = \frac{F \times \int \frac{F}{m} \, dt}{m}$
Implication: Even a small error in the initial force reading is magnified through multiple steps of integration and division. Chasing derivatives is like moving further away from a painting; you lose the "nuance" and specific details required to adjust training stimuli.

Communication Strategy: Athletes often respond better to simple explanations rather than complex data points. Cowan focuses on three conceptual "toddler-level" tools:
1. Size: How big the force is.
2. Shape: The specific profile of the force-time curve.
3. Bubbles: A metaphor for the area under the curve (impulse).
Impulse ( $I$ ): Defined as force multiplied by time ( $I = F \times t$ ). It represents the area underneath the curve; the bigger the "bubble," the higher the athlete jumps.

Standing Phase: A flat, straight line representing the athlete's static body weight.
Unweighting Phase: The "Jesus take the wheel" moment where the athlete unlocks their joints and begins to descend; the force drops below body weight.
Braking Component: The portion where the athlete attempts to stop the downward movement.
Eccentric Deceleration: The force produced above body weight while the athlete is still moving downward.
Transition Point: The point where velocity becomes positive (the bottom of the jump).
Concentric Phase: The drive upward from the transition point until takeoff.
Signatures: Just like a signature, every athlete has a unique curve shape. The objective is to maximize the space under the curve in the shortest time possible.

Subject: A netball athlete monitored over a four-week gym block.
Baseline Observation: Comparing the right leg (orange curve) and left leg (blue curve) against the combined total (gray curve).
Phases Identified:
- Blue shaded area: Blue shading represents the eccentric portion (unweighting, braking, and eccentric deceleration).
- Yellow shaded area: Represents the concentric portion of the jump.
Intervention Strategy: The goal was to "pinch and lift" the concentric portion of the curve by implementing a training block with a concentric and late-RFD bias.
Results Analysis:
- Metric: Concentric Impulse.
- Finding: After four weeks, the concentric impulse showed a change greater than the Smallest Worthwhile Change (SWC).
- Conclusion: While metrics confirmed the change, simply looking at the raw force-time curves (on the same scale) clearly illustrated the improvement. Understanding the bottom levels of the "pyramid" (Force and Time) makes high-level metrics less critical.