> ## 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.

# Max Shoulder ER

> The instant of maximum shoulder external rotation between ball release and apex.

## Technical Details

* **Variable Type:** Event
* **Data Type:** Integer
* **Units:** Frame number
* **Column Name:** `max_shoulder_external_rotation_event`
* **Required for QA:** False
* **Measurement:** Frame at which shoulder external rotation is maximum (between ball release and apex)

## Description

Max Shoulder ER marks the instant of maximum shoulder external rotation during the serve, typically occurring in the [Cocking](/biomechanics/activities/tennis/overhand_serve#phases) phase (between [Low Arm](/biomechanics/activities/tennis/events/low-arm) and ball contact).
It defines the end of the cocking phase and the start of the [Acceleration](/biomechanics/activities/tennis/overhand_serve#phases) phase.

## Use Cases

* Cocking phase duration and shoulder load
* [Cocking Phase Max Shoulder External Rotation](/biomechanics/activities/tennis/metrics/cocking-phase-shoulder-external-rotation) timing
* Acceleration phase start (max shoulder ER to ball contact)