# Sit‐to‐Stand Power From 2D Pose Estimation as an Indicator of Muscle Strength in Older Adults

## TL;DR

This study shows that measuring power during a sit-to-stand test using 2D pose estimation is a strong indicator of muscle strength in older adults.

## Contribution

The study introduces a cost-effective 2D pose estimation method to assess muscle strength during the sit-to-stand test.

## Key findings

- Peak STS power strongly correlates with muscle strength and mass but weakly with physical performance.
- 5x-STS time and 30-s STS repetitions correlate more with physical performance than with strength or muscle mass.
- 2D pose estimation shows high agreement with reference motion capture and force-plate systems.

## Abstract

The sit‐to‐stand (STS) test is a crucial tool for sarcopenia assessment. However, the two most widely used diagnostic frameworks differ in their conceptualization of the test. The European Working Group on Sarcopenia in Older People 2 views the STS test primarily as a proxy for muscle strength, while the Asian Working Group for Sarcopenia 2019 (AWGS 2019) classifies it as a measure of physical performance. This discrepancy poses challenges to conceptual clarity in both research and practice. Recent advancements in pose‐estimation algorithms allow for kinematic assessment using a standard handheld devices, providing a simple and cost‐effective alternative to conventional motion‐analysis.

A total of 129 community‐dwelling older adults (mean age 71.8 years) who could ambulate independently underwent assessments of muscle strength (knee extensor and handgrip strength), physical performance (usual walking speed, Timed Up and Go [TUG] velocity and Short Physical Performance Battery [SPPB]) and muscle mass (appendicular skeletal muscle index by bioelectrical impedance analysis). Kinematic analysis of the 30‐s sit‐to‐stand (30‐s STS) was conducted using 2D pose estimation to derive peak STS power and joint angles. For validation, a subset of 20 participants completed 30‐s STS trials that were recorded concurrently using 2D pose estimation, optical motion capture and force‐plate recordings. The correlation between peak STS power, five times sit‐to‐stand (5x‐STS) time, and 30‐s STS repetitions with clinical indicators was examined.

Peak STS power correlated strongly with muscle strength and mass (knee extensor strength r = 0.64; handgrip r = 0.64; ASMI r = 0.70; all p < 0.001) but only weakly with physical performance (usual walking speed: r = 0.19, p = 0.030; TUG velocity and SPPB: p > 0.05). In contrast, the 5x‐STS time and 30‐s STS repetitions showed moderate to strong associations with physical performance (e.g., SPPB r = −0.79 and 0.52, respectively; both p < 0.001) but did not correlate with strength or muscle mass. Despite systematic biases between pose estimation and the reference, the agreement was high (intraclass correlation coefficient for peak STS power = 0.94; joint‐angles = 0.83–0.89). In sex‐stratified, within‐subject mixed‐effects models, changes in posture and timing explained modest variance in ΔPeak STS power (marginal R
2 = 0.024 in men; 0.138 in women).

Peak STS power correlated more strongly with muscular strength, while 5x‐STS and 30‐s STS tests were more closely related to physical performance. These findings align better with and provide stronger support for the AWGS 2019 framework.