Full length articlePartitioning ground reaction forces for multi-segment foot joint kinetics
Introduction
In clinical gait analysis and human movement research, traditional single segment foot models are increasingly being replaced by models that subdivide the foot into several segments (e.g. [[1], [2], [3], [4]]). To date, these multi-segment foot (MSF) models have primarily been confined to the analysis of joint angles. Expanding these kinematic-only models to also allow for kinetics analysis may provide additional insights into foot function [[5], [6], [7], [8], [9]], but requires several additional parameters. In addition to segment orientations, inverse dynamics based kinetics calculations also rely on identification of joint centers of rotation, estimation of segment inertial properties, and measurements of ground reaction forces (GRFs) under each segment. Of these, measuring segment GRFs is perhaps the most difficult hurdle from a technological standpoint, as commercial devices capable of measuring both segment vertical and shear forces are not yet commonplace [10], and the use of multiple adjacent force plates [[5], [11], [12]] requires targeted walking which may not be clinically feasible.
A method that can accurately partition the GRFs from a single force plate is attractive because it would allow MSF joint kinetics to be calculated from commonly employed equipment already found in gait and movement analysis clinics and laboratories. Two potential methods of GRF partitioning have been developed previously. The first method quantifies joint kinetics from a single force plate only when the location of the center-of-pressure (CoP) passes anterior to the joint, i.e. the entire GRF is applied to adjacent segments sequentially. This technique has only been used to examine kinetics of the metatarsophalangeal joints [[13], [14], [15]], but could theoretically also be applied to other joints in the foot, such as the midtarsal joint. The second method employs an additional pressure mat secured to the top of the force plate. The segment vertical forces from the pressure mat are then used to help partition the shear forces from the force plate using an assumption of proportionality [[7], [16]]. The accuracy of this latter method on segment GRFs alone has been evaluated [[9], [17], [18]], but neither method has been validated in terms of application to inverse dynamics based MSF joint kinetics.
The purpose of the present study was to assess the accuracy of potential GRF partitioning methods on the calculation of MSF joint kinetics (i.e. moments, powers, and work), using a previously published kinetic multi-segment foot model [[5], [19]]. This was accomplished by comparing estimates obtained from the partitioning methods to those from a multiple force plate approach that isolated forces under two segments at a time. By studying the potential errors inherent in these methods, we hope to better understand their validity and applicability. The ability to calculate foot joint kinetics from a single force platform would provide researchers and clinicians with a new tool with which to study foot muscle function, better understand foot pathologies, and evaluate potential treatment interventions.
Section snippets
Participants
We re-analyzed data that were previously collected [[5], [17], [19]], consisting of 13 healthy pediatric participants(9 M/4F; age 13.1 ± 3.1; height 156 ± 18 cm, weight 51 ± 18 kg). Four of the original 17 subjects were excluded due to concerns over foot placement accuracy. All participants were volunteers and signed consent forms approved by the local Institutional Review Board.
Protocol
The employed marker set, associated multi-segment foot model, and collection protocol have all been previously
MTP joint moments (Fig. 2)
In the sagittal plane, the CPcross method resulted in a substantial delay in the onset of the MTP extension moment, thereby missing a large portion of the early signal. The onset of GRF under the hallux segment varied substantially among subjects, occurring on average at 18% of stance (Table 1). The CoP, on the other hand, did not pass the MTP joint until 71% of stance. At this point, the extension moment increased rapidly, then overshot the 2Plate peak by 17% (CPcross: 0.145 ± 0.032 Nm/kg;
Discussion
The purpose of this study was to evaluate the accuracy of two different single-plate GRF partitioning methods on MSF joint kinetics. Overall, the results show that 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. Each method may have utility when applied to specific joints, planes, and conditions, provided the potential errors are well understood. Discussion below focuses on understanding the
Conflicts of interest
No conflicts of interest to report.
Acknowledgements
This work was supported by funding from the Center for Research in Human Movement Variability of the University of Nebraska at Omaha and the National Institute of Health (P20GM109090) to KZT.
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