Portable biomechanics laboratory enables clinically accessible movement analysis from a handheld smartphone

across patients with neurological injury, lower-limb prosthesis users, pediatric inpatients, and controls. Across 1021 videos recorded in prospective clinical deployment, PBL was easy to implement, yielded reliable gait metrics (ICC > 0.9), and detected clinically relevant differences in movement. For cervical myelopathy patients, its gait quality measures correlated with modified Japanese Orthopedic Association (mJOA) scores and were responsive to clinical intervention. Handheld smartphone video can therefore deliver accurate, scalable, and low-burden biomechanical measurement, enabling greatly increased monitoring of movement impairments. We release the first clinically validated method for measuring whole-body kinematics from handheld smartphone video at https://IntelligentSensingAndRehabilitation.github.io/MonocularBiomechanics/.