> ## 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.

# Forward Trunk Arm Speedup

> Speedup factor of peak forward 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:** `forward_trunk_arm_speedup`
* **Measurement:** Ratio (or factor) of [Peak Upper Arm Forward Angular Velocity](/biomechanics/activities/tennis/metrics/peak-upper-arm-forward-angular-velocity) to [Peak Torso Forward Angular Velocity](/biomechanics/activities/tennis/metrics/peak-torso-forward-angular-velocity)—how much the upper arm speeds up relative to the torso in the serve direction

## Description

Forward trunk arm speedup quantifies the increase in peak 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 forward angular velocity than the torso, supporting efficient [Forward Kinematic Sequence](/biomechanics/activities/tennis/metrics/forward-kinematic-sequence) (torso → upper arm → forearm).

## Use Cases

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