Partitioning ground reaction forces for multi-segment foot joint kinetics.

BACKGROUND Kinematic multi-segment foot models have been increasingly used to study foot function. The addition of kinetics to these models may enhance their utility; however, this been hindered by limitations in measuring ground reaction forces (GRFs) under individual foot segments. PURPOSE To determine the accuracy of partitioning segment GRFs from a single force platform on foot joint kinetics. METHODS Two potential partitioning methods were applied to a previously published three-segment kinetic foot model. The first method calculated joint kinetics only when the center of pressure crossed anterior to a joint (CPcross). The second method utilized a virtual pressure mat and a proportionality assumption to partition GRFs from the force platform (PRESS). Accuracy was assessed by comparing joint moments and powers obtained from each partitioning method to those obtained from a dual force plate approach that isolated forces under two segments at a time (2Plate). Thirteen healthy pediatric subjects walked in a controlled manner so as to isolate the kinetics acting at the metatarsophalangeal (MTP) joint and, subsequently, the midtarsal joint. RESULTS The PRESS method was generally more accurate than the CPcross method, and both methods were more accurate at the midtarsal joint than at the MTP joint. At the MTP joint, sagittal plane moment peaks, power peaks, and work done were slightly overestimated, more so by CPcross than PRESS. At the midtarsal joint, sagittal plane moments were captured well by PRESS, while CPcross missed the early portion of the moment, but both methods captured power profiles fairly accurately. SIGNIFICANCE Analysis of kinetics in multi-segment foot models may provide insight into foot function, pathologies, and interventions. Partitioning accuracy and generalizability is promising for analysis of the midtarsal joints but has limitations at the MTP joint.