Novel Multi-Segment Foot Model Incorporating Plantar Aponeurosis for Detailed Kinematic and Kinetic Analyses of the Foot With Application to Gait Studies

Abstract

Kinetic multi-segment foot models have been proposed to evaluate the forces and moments generated in the foot during walking based on inverse dynamics calculations. However, these models did not consider the plantar aponeurosis (PA) despite its potential importance in generation of the ground reaction forces and storage and release of mechanical energy. This study aimed to develop a novel multi-segment foot model incorporating the PA to better elucidate foot kinetics. The foot model comprised three segments: the phalanx, forefoot, and hindfoot. The PA was modeled using five linear springs connecting the origins and the insertions via intermediate points. To demonstrate the efficacy of the foot model, an inverse dynamic analysis of human gait was performed and how the inclusion of the PA model altered the estimated joint moments was examined. Ten healthy men walked along a walkway with two force plates placed in series close together. The attempts in which the participant placed his fore- and hindfoot on the front and rear force plates, respectively, were selected for inverse dynamic analysis. The stiffness and the natural length of each PA spring remain largely uncertain. Therefore, a sensitivity analysis was conducted to evaluate how the estimated joint moments were altered by the changes in the two parameters within a range reported by previous studies. The present model incorporating the PA predicted that 13%-45% of plantarflexion in the metatarsophalangeal (MTP) joint and 8%-29% of plantarflexion in the midtarsal joints were generated by the PA at the time of push-off during walking. The midtarsal joint generated positive work, whereas the MTP joint generated negative work in the late stance phase. The positive and negative work done by the two joints decreased, indicating that the PA contributed towards transfer of the energy absorbed at the MTP joint to generate positive work at the midtarsal joint during walking. Although validation is limited due to the difficulty associated with direct measurement of the PA force in vivo, the proposed novel foot model may serve as a useful tool to clarify the function and mechanical effects of the PA and the foot during dynamic movements.

Keywords: foot kinematics; foot kinetics; healthy adults; inverse dynamics; motion analysis; multi-segment foot model; plantar fascia; walking.

FIGURE 1

Multi-segment foot model incorporating the plantar aponeurosis (PA) for detailed kinematic and kinetic analyses of the foot. (A) Marker placements and the definitions of the coordinate systems of the phalanx, forefoot, and hindfoot segments. (B) The model of PA. The PA was modeled as five linear springs connecting the origin, via point, and insertion. (C) Segmentation of the phalanx, forefoot, and hindfoot segments. (D) Forces applied to the origin, via point, and insertions of the PA.

FIGURE 2

Free body diagram of the phalanx (A), forefoot (B), and hindfoot segment (C). See Table 2 for notations.

FIGURE 3

Mean joint angle profiles of the foot during walking. Mean (solid line) ± standard deviation (red band). Means and standard deviations of the joint angles during quiet standing were also plotted on the right side of each graph.

FIGURE 4

Mean plantar aponeurosis length and force profiles during walking. Mean (solid line) ± standard deviation (red band).

FIGURE 5

Mean joint moment (A) and joint power (B) profiles during walking. Mean (solid red line = with PA, dotted blue line = without PA) ± standard deviation (red and blue bands, respectively). Color bar below each graph shows the results of the SPM analysis.

FIGURE 6

Comparison of the joint moment (A) and joint power (B) profiles between models with (blue) and without the plantar aponeurosis (red).

FIGURE 7

Comparisons of the mean joint moment profiles when the inertial parameters were altered.

FIGURE 8

Comparisons of the mean peak moments (A) and %plantar aponeurosis (PA) contribution (B) at toe-off when the PA stiffness parameters were altered.