> ## 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.

# Contact Phase Trunk Flexion

> Forward or backward trunk flexion at apex (ball contact).

## Technical Details

* **Variable Type:** Discrete Metric
* **Data Type:** Float
* **Units:** degrees (°)
* **Column Name:** `contact_phase_trunk_flexion`
* **Measurement:** Sagittal-plane trunk flexion at [Apex](/biomechanics/activities/tennis/events/apex) (ball contact)

## Description

Contact phase trunk flexion is the forward/backward bend of the trunk at ball contact. Positive values indicate forward flexion, negative values indicate backward extension. It reflects posture at the moment of impact and can be compared with [Finish Phase Trunk Flexion](/biomechanics/activities/tennis/metrics/finish-phase-trunk-flexion) to track trunk motion through the rest of the serve.

## Use Cases

* Posture at contact
* Trunk control and consistency
* Comparison with finish-phase trunk flexion