Point-mass biomechanical model of the upper extremity during Lofstrand crutch-assisted gait

We propose a point-mass biomechanical model to estimate the forces and moments supported by the upper extremity during Lofstrand crutch-assisted gait. This model is based on the Newtonian classical mechanics and the angular momentum theorem. The system arm-crutch is divided into three segments: 1) crutch, 2) wrist-elbow, and 3) elbow-shoulder. The theoretical model was experimentally validated with a disabled person with spinal cord injury. Two crutch-assisted gait patterns have been chosen to carry out the experimental validation: two-point reciprocal gait and swing-through gait. Six position markers (three placed on the arm and three placed on the crutch) and two force sensors (placed on the crutch) were used in experiments for testing the model. The results were compared with a distributed-mass model based on studies previously published, concluding that the relative mean difference between models is less than 3% Body Weight and 1% Body Weight times Height when forces and moments are estimated, respectively. Some advantages of using a point-mass model are summarized: simple formulation, easy to understand; require less numerical calculation reducing the computational cost; requires less position markers placed on the subject, increasing therefore the comfort of the subject.