Smartphone video-based knee extension moments during chair rise relate to MRI measures of muscle force-generating capacity.

Preserving muscle function is essential for maintaining independence during aging, but muscle force-generating capacity is not commonly measured clinically due to a lack of accessible, sensitive tools. Gold standard approaches include magnetic resonance imaging (MRI), which quantifies muscle volume and microstructure, and dynamometry, which measures joint moments during voluntary contraction. Both modalities are time-consuming and costly, so low-fidelity measures like five times sitto- stand time (5xSTS) are often used instead. OpenCap, which quantifies musculoskeletal dynamics from smartphone videos, may provide an accessible assessment of muscle function. We evaluated whether OpenCap-derived moments during chair rise relate to MRI-based measures of quadriceps muscle volume and microstructure. We collected quadriceps MRI, dynamometry, and 5xSTS time with concurrent OpenCap for healthy adults of various ages (n=19, 30-78 y). We computed quadriceps volume and radial diffusivity (a microstructural correlate of fiber size and type) from MRI. We estimated peak knee extension moment using OpenCap and dynamometry. The OpenCapderived knee moment was related to quadriceps volume (r=0.63, p=0.014), radial diffusivity (r=0.61, p=0.016), and a composite metric combining the two (r=0.77, p=0.002). Isometric and concentric isokinetic dynamometry moments correlated with volume (r=0.66-0.75, p=0.002-0.009), but not radial diffusivity (r=0.04-0.52, p=0.054-0.91). 5xSTS time was not associated with any MRI-derived measures (r=-0.16-0.01, p=0.72-0.97). In summary, OpenCap smartphone-derived joint moments relate to muscle size and microstructure, unlike the current clinically-accessible measure-5xSTS time. OpenCap offers a rapid, scalable assessment of muscle force-generating capacity without specialized equipment, enabling higher-fidelity assessments in clinics and large-scale studies where imaging and dynamometry are impractical.