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

Movement directly reflects neurological and musculoskeletal health, yet objective biomechanical assessment is rarely available in routine care. We introduce Portable Biomechanics Laboratory (PBL), a platform for fitting biomechanical models to handheld smartphone video. We validate PBL on over 15 hours of data synchronized to ground truth motion capture, finding joint-angle errors < 3∘ 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/ .