> ## 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.

# Twist Trunk Arm Speedup

> Speedup factor of peak twist angular velocities from the torso to the upper arm.

## Technical Details

* **Variable Type:** Discrete Metric
* **Data Type:** Float
* **Units:** N/A (ratio/factor)
* **Column Name:** `twist_trunk_arm_speedup`
* **Measurement:** Ratio (or factor) of peak upper arm twist angular velocity to [Peak Torso Twist Angular Velocity](/biomechanics/activities/tennis/metrics/peak-torso-twist-angular-velocity)—how much the upper arm twist speeds up relative to the torso

## Description

Twist trunk arm speedup quantifies the increase in peak twist angular velocity from the torso to the upper arm during the serve.
A value greater than 1 indicates the upper arm reaches a higher peak twist angular velocity than the torso, supporting efficient [Twist Kinematic Sequence](/biomechanics/activities/tennis/metrics/twist-kinematic-sequence).

## Use Cases

* Twist kinetic chain and energy transfer
* Technique quality (proximal-to-distal speedup)
* Coaching and training feedback