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

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 R² = 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.