The Role of Arch Compression and Metatarsophalangeal Joint Dynamics in Modulating Plantar Fascia Strain in Running

Abstract

Elastic energy returned from passive-elastic structures of the lower limb is fundamental in lowering the mechanical demand on muscles during running. The purpose of this study was to investigate the two length-modulating mechanisms of the plantar fascia, namely medial longitudinal arch compression and metatarsophalangeal joint (MPJ) excursion, and to determine how these mechanisms modulate strain, and thus elastic energy storage/return of the plantar fascia during running. Eighteen runners (9 forefoot and 9 rearfoot strike) performed three treadmill running trials; unrestricted shod, shod with restricted arch compression (via an orthotic-style insert), and barefoot. Three-dimensional motion capture and ground reaction force data were used to calculate lower limb kinematics and kinetics including MPJ angles, moments, powers and work. Estimates of plantar fascia strain due to arch compression and MPJ excursion were derived using a geometric model of the arch and a subject-specific musculoskeletal model of the plantar fascia, respectively. The plantar fascia exhibited a typical elastic stretch-shortening cycle with the majority of strain generated via arch compression. This strategy was similar in fore- and rear-foot strike runners. Restricting arch compression, and hence the elastic-spring function of the arch, was not compensated for by an increase in MPJ-derived strain. In the second half of stance the plantar fascia was found to transfer energy between the MPJ (energy absorption) and the arch (energy production during recoil). This previously unreported energy transfer mechanism reduces the strain required by the plantar fascia in generating useful positive mechanical work at the arch during running.

Fig 1. Modified foot marker set.

Fig 2. OpenSim Plantar Fascia Model.

The tissue displayed represents the first slip of the plantar fascia with an origin on the medial process of the calcaneal tuberosity and an insertion at the base of the proximal phalanx. A wrapping surface was also created around the metatarsophalangeal joint. This image was developed using OpenSim 3.0.1.

Fig 3. Estimated plantar fascia strain during stance phase.

Presented as the combined effect of arch compression and metatarsophalangeal joint (MPJ) angle, the effect of arch compression only, and the effect of MPJ angle only (mean ± sd). Strain values ≤ 0 represent lengths at which the PLF is estimated to be slack.

Fig 4. Estimated plantar fascia strain during the stance phase of unrestricted shod running for forefoot strike (FFS) and rearfoot strike (RFS) runners (mean ± sd).

Fig 5. Metatarsophalangeal joint angle, net moment and power traces during stance phase (mean ± sd).

Fig 6. Metatarsophalangeal joint dynamics.

(A) Peak flexion angle, (B) peak and average net extension moment, (C) peak positive and negative power, and (D) positive and negative work, during stance phase (mean ± sd). * Significantly different from ‘Barefoot’ condition (p<0.0167). ** Significantly different from ‘Shod’ condition (p<0.0167).

Fig 7. Plantar fascia energy transfer mechanism.

Plantar fascia (PLF) net power is represented by the evenly broken line; PLF contribution to arch power is represented by the unbroken line; and power that the PLF absorbs at the metatarsophalangeal joint (MPJ) is represented by the unevenly broken line. The shaded region represents the energy absorbed at the MPJ during the propulsive phase of ground contact.