> ## 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.

# Sit to Stand

> Biomechanical analysis of repeating sit-to-stand movements, including repetition events, per-repetition metrics, and summary metrics such as total time and relative power.

## Overview

The sit-to-stand test measures how quickly and powerfully a person can rise from a chair and sit back down over multiple repetitions.
It is widely used to assess lower extremity strength, power, and functional mobility, particularly in older adults and rehabilitation settings.
The most common variant is the five times sit-to-stand (5xSTS), but Uplift can analyze any number of repetitions from 1 to 25, so other multi-repetition protocols can be captured with the same movement.

Each repetition is analyzed in a cyclical pattern: Uplift detects a seat off, stand, and sit for every repetition, reports metrics for each repetition, and summarizes them across the capture.

## Instructions

Sit on an armless chair with your back upright, feet flat on the floor about hip-width apart, and arms crossed over your chest or stretched out in front.
Stand up fully, then sit back down fully, as quickly as possible, for the required number of repetitions.
Count reps as you return back to the sitting position.
Keep your arms crossed or outstretched throughout the test, and don't use your arms to assist in any way.
Start and finish seated: complete the final sit before the capture ends.

## Dimensions

Required inputs for processing:

* **repetitions**

## Output Variables

### Generic Outputs

Every capture includes session metadata, standard joint angle/velocity time series, and 3D keypoint positions common to all movements. See [Generic Outputs](https://docs.uplift.ai/biomechanics/generic-outputs.md) for the full list.

### Events

Events are recurring: each event occurs once per repetition, and its column lists one frame per detected repetition.

| Event | Description | Column Name |
| - | - | - |
| [Seat Off](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/seat-off) | the pelvis leaves the seated position at the start of the rise | `seat_off_frame` |
| [Stand](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/stand) | the pelvis reaches its highest point while standing | `stand_frame` |
| [Sit](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/sit) | the pelvis returns to the seated position | `sit_frame` |

### Phases

| Phase | Start | End | Description |
| - | - | - | - |
| Rise | [Seat Off](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/seat-off) | [Stand](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/stand) | moving from seated to standing |
| Descent | [Stand](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/stand) | [Sit](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/sit) | moving from standing back to seated |
| Seated | [Sit](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/sit) | next [Seat Off](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/events/seat-off) | seated between repetitions; excluded from per-repetition times |

### Per-Repetition Metrics

One value per repetition. Each value is stored at that repetition's Seat Off frame (or its Stand frame if Seat Off was not detected), and every other frame is `-1`.

| Metric | Units | Description | Column Name |
| - | - | - | - |
| [Repetition Number](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-count) | count | Repetition index within the capture (1, 2, 3, ...) | `repetition_number` |
| [Repetition Time](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-time) | seconds | Time from Seat Off to Sit | `repetition_time` |
| [Rise Time](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/rise-time) | seconds | Time from Seat Off to Stand | `rise_time` |
| [Descent Time](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/descent-time) | seconds | Time from Stand to Sit | `descent_time` |
| [Peak Rise Velocity](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/peak-rise-velocity) | m/s | Maximum upward velocity of the body center of mass during the rise | `peak_rise_velocity` |
| [Peak Trunk Flexion](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/peak-trunk-flexion) | degrees | Maximum trunk flexion relative to vertical during the rise | `peak_trunk_flexion` |
| [Relative Power](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/relative-power) | W/kg | Mean concentric power of the rise, normalized to body mass | `relative_power` |
| [Rise Height](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/rise-height) | meters | Vertical hip center travel from the lowest point to Stand | `rise_height` |
| [Valid Repetition](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-validity) | N/A | Whether the repetition reached a full stand (1) or not (0) | `valid_repetition` |

### Summary Metrics

| Metric | Units | Description | Column Name |
| - | - | - | - |
| [Repetition Count](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-count) | count | Number of repetitions detected | `repetition_count` |
| [Total Time](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/total-time) | seconds | Time from the first Seat Off to the last repetition's Sit | `total_time` |
| [Time to Final Stand](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/total-time) | seconds | Time from the first Seat Off to the last repetition's Stand | `time_to_final_stand` |
| [Mean Repetition Time](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-time) | seconds | Average repetition time | `mean_repetition_time` |
| [Repetition Time CV](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-time) | % | Coefficient of variation of repetition time | `repetition_time_cv` |
| [Repetition Time Change](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-time) | % | Percent change in repetition time from the first to the last repetition | `repetition_time_change` |
| [Mean Rise Time](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/rise-time) | seconds | Average rise time | `mean_rise_time` |
| [Rise Time Change](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/rise-time) | % | Percent change in rise time from the first to the last valid repetition | `rise_time_change` |
| [Mean Descent Time](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/descent-time) | seconds | Average descent time | `mean_descent_time` |
| [Mean Peak Rise Velocity](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/peak-rise-velocity) | m/s | Average peak rise velocity | `mean_peak_rise_velocity` |
| [Mean Peak Trunk Flexion](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/peak-trunk-flexion) | degrees | Average peak trunk flexion | `mean_peak_trunk_flexion` |
| [Mean Relative Power](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/relative-power) | W/kg | Average relative power | `mean_relative_power` |
| [Relative Power Change](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/relative-power) | % | Percent change in relative power from the first to the last valid repetition | `relative_power_change` |
| [Invalid Repetition Count](https://docs.uplift.ai/biomechanics/activities/stability/sit_to_stand/metrics/repetition-validity) | count | Number of repetitions that did not reach a full stand | `invalid_repetition_count` |

## Notes

* Events are detected from the vertical trajectory of the hip center (pelvis keypoint).
* Up to 25 repetitions are analyzed per capture. Captures with no detected repetitions or more than 25 fail QA.
* QA also fails if any repetition is missing its Seat Off or Sit, if events are out of order, or if Total Time is implausible for the number of repetitions. Finish the capture seated so the final Sit is detected.
* Repetitions that rise less than 80% of the capture's largest rise are flagged as invalid. They are still included in the per-repetition output and averages, but are skipped by Rise Time Change and Relative Power Change.
* Per-repetition values that cannot be calculated (for example, because an event was not detected) are `-1`. Summary metrics that cannot be calculated are empty (NaN).


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