> ## 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.

# Peak Trunk Flexion

> Maximum forward trunk lean relative to vertical during the rise of each sit-to-stand repetition.

## Technical Details

* **Variable Type:** Discrete Metric
* **Data Type:** Float
* **Units:** degrees (°)
* **Column Name:** `peak_trunk_flexion`, `mean_peak_trunk_flexion`
* **Optimal Direction:** Middle — moderate values are optimal
* **Measurement:** Maximum trunk angle relative to vertical from [Seat Off](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/seat-off) through [Stand](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/stand)
* **Reference:** Same trunk angle convention as the squat [Max Trunk Angle](https://docs.uplift.ai/biomechanics/activities/squat/metrics/max-trunk-angle)

## Variants

* **Peak Trunk Flexion** (`peak_trunk_flexion`) (per repetition)
* **Mean Peak Trunk Flexion** (`mean_peak_trunk_flexion`) (summary): average Peak Trunk Flexion across all analyzed repetitions.

## Description

Peak Trunk Flexion is how far the person leans forward while standing up.
Forward lean moves the center of mass over the feet and peaks around seat off, which is why the measurement window starts at Seat Off.
Some forward lean is necessary to rise, but excessive lean can indicate a strategy that compensates for limited leg strength.

## Use Cases

* Movement strategy assessment
* Identifying compensation for lower extremity weakness
* Consistency of rise technique across repetitions


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