> ## 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.

# Relative Power

> Mean concentric power of each sit-to-stand rise, normalized to body mass, with its mean and change across the capture.

## Technical Details

* **Variable Type:** Discrete Metric
* **Data Type:** Float
* **Units:** W/kg; percent (%) for Change
* **Column Name:** `relative_power`, `mean_relative_power`, `relative_power_change`
* **Optimal Direction:** Higher is better
* **Calculation:** `0.9 × 9.81 × (rise height / rise duration)`, where rise height is the [Rise Height](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/rise-height) and rise duration is the time from the repetition's lowest point to [Stand](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/stand)

## Variants

* **Relative Power** (`relative_power`) (per repetition)
* **Mean Relative Power** (`mean_relative_power`) (summary): average Relative Power across all analyzed repetitions.
* **Relative Power Change** (`relative_power_change`) (summary): percent change from the first to the last [valid](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-validity) repetition, `100 × (last − first) / first`. Negative values mean power dropped over the test. Requires at least two valid repetitions.

## Description

Relative Power estimates the average power produced to lift the body during the rise, per kilogram of body mass.
It follows the sit-to-stand power equations of Alcazar et al. (2018, 2020): mean force is approximated as 0.9 × body weight (body mass minus the shanks and feet), and mean velocity as the vertical rise divided by the rise duration.

The published method estimates the rise from body height and chair height and assumes the rise takes half of each repetition.
Uplift instead measures both directly for every repetition from the hip center (pelvis keypoint) trajectory, from the repetition's lowest point to Stand.
Because of this difference, Uplift values are not directly interchangeable with the published reference values.

Relative Power Change compares only valid repetitions, so a final partial stand does not distort the trend.

## Use Cases

* Lower extremity muscle power assessment
* Fatigue across repetitions
* Tracking strength and power changes over time

## References

* Alcazar J, et al. Relation between leg extension power and 30-s sit-to-stand muscle power in older adults: validation and translation to functional performance. *Sci Rep.* 2020;10:16337. [doi:10.1038/s41598-020-73395-4](https://doi.org/10.1038/s41598-020-73395-4)
* Alcazar J, et al. The sit-to-stand muscle power test: an easy, inexpensive and portable procedure to assess muscle power in older people. *Exp Gerontol.* 2018;112:38-43.


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