> ## Documentation Index
> Fetch the complete documentation index at: https://docs.uplift.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# X-Factor Zero Crossing

> The instant the hip-shoulder separation (X-Factor) angle crosses zero following its steepest rise, used to locate the Top of Swing.

## Technical Details

* **Variable Type:** Event
* **Data Type:** Integer
* **Units:** Frame number
* **Column Name:** `x_factor_zero_crossing_frame`
* **Required for QA:** False
* **Measurement:** Zero crossing of the X-Factor angle nearest the point of its steepest rate of change
* **Timing:** Used internally to identify P4 Top of Swing

## Description

X-Factor Zero Crossing marks the instant hip and shoulder rotation have no separation, following the steepest rise in the X-Factor signal — the point where the shoulders are stacked directly on top of the hips. It is used internally to bound the search for the Top of Swing event.

## Use Cases

* Top of Swing detection