Active regulation of longitudinal arch compression and recoil during walking and running

Abstract

The longitudinal arch (LA) of the human foot compresses and recoils in response to being cyclically loaded. This has typically been considered a passive process, however, it has recently been shown that the plantar intrinsic foot muscles have the capacity to actively assist in controlling LA motion. Here we tested the hypothesis that intrinsic foot muscles, abductor hallucis (AH), flexor digitorum brevis (FDB) and quadratus plantae (QP), actively lengthen and shorten during the stance phase of gait in response to loading of the foot. Nine participants walked at 1.25 m s⁻¹ and ran at 2.78 and 3.89 m s⁻¹ on a force-instrumented treadmill while foot and ankle kinematics were recorded according to a multisegment foot model. Muscle-tendon unit (MTU) lengths, determined from the foot kinematics, and intramuscular electromyography (EMG) signals were recorded from AH, FDB and QP. Peak EMG amplitude was determined during the stance phase for each participant at each gait velocity. All muscles underwent a process of slow active lengthening during LA compression, followed by a rapid shortening as the arch recoiled during the propulsive phase. Changes in MTU length and peak EMG increased significantly with increasing gait velocity for all muscles. This is the first in vivo evidence that the plantar intrinsic foot muscles function in parallel to the plantar aponeurosis, actively regulating the stiffness of the foot in response to the magnitude of forces encountered during locomotion. These muscles may therefore contribute to power absorption and generation at the foot, limit strain on the plantar aponeurosis and facilitate efficient foot ground force transmission.

Figures

Figure 1.

Compression and recoil of the longitudinal arch (LA). The LA angle is defined as the sagittal plane rotation of the metatarsals relative to the calcaneus. An increase in LA angle indicates compression of the LA which is calculated by subtracting LA angle at foot contact from peak LA angle, which generally occurred at mid-stance. Group mean LA angles are presented at foot contact (a), peak LA angle (b) and toe-off (c) when running at 3.89 m s−1 with data indicating that the LA compresses and recoils during stance phase. (Online version in colour.)

Figure 2.

Depiction of the muscle–tendon unit (MTU) pathways (top row) and anatomical pathways (bottom) for abductor hallucis (AH, red), flexor digitorum brevis (FDB, blue) and quadratus plantae (QP, green). Filled circles indicate origin and insertion points for each MTU, while open circles indicate tether points. The MTU length changes for AH and FDB will be due to a combination of rotations occurring about the longitudinal arch (LA) and metatarsophalangeal (MTP) joints, while QP MTU length changes will occur owing to changes in the LA angle. (Online version in colour.)

Figure 3.

Raw data collected from a representative participant while running at 3.87 m s−1. Vertical and horizontal forces are calculated from the force-instrumented treadmill. Longitudinal arch (LA) angle is calculated based on multisegment foot kinematics and intramuscular electromyography (EMG) recordings are collected from the abductor hallucis (AH) (top), flexor digitorum brevis (FDB) (middle) and quadratus plantae (QP) (bottom). Shaded areas indicate stance phase. (Online version in colour.)

Figure 4.

Group mean ensembles ± standard error of the mean for vertical ground reaction force, longitudinal arch (LA) angle (degrees, °), electromyography (EMG) root mean square (RMS) signal amplitude and changes (Δ) in muscle–tendon unit (MTU) length for abductor hallucis (AH, red circles), flexor digitorum brevis (FDB, blue squares) and quadratus plantae (QP, green triangles). Group mean ensembles are defined from toe-off (TO) to ipsilateral toe-off for the right foot. Data recorded during walking at 1.25 m s−1 and running at 2.78 and 3.89 m s−1. For each muscle EMG data are normalized to the maximal amplitude recorded for all trials. Change in MTU length and LA angle is calculated by offsetting the MTU length and LA angle at heel contact in the 1.25 m s−1 condition, respectively. Vertical ground reaction force (GRF) data are normalized to body mass. FC, foot contact. (Online version in colour.)

Figure 5.

Group mean data for longitudinal arch (LA) compression (a), peak muscle–tendon unit (MTU) strain (b) and electromyography (EMG) root mean square (RMS) amplitude (c) during stance for abductor hallucis (AH, red circles), flexor digitorum brevis (FDB, blue squares) and quadratus plantae (QP, green triangles). LA compression is calculated by subtracting the LA angle at heel strike in the 1.25 m s−1 condition from the peak angle occurring during stance, at each gait velocity. EMG RMS values are normalized to the maximal amplitude recorded during all trials. * Denotes significant difference, with all values increasing with increasing gait velocity (all p ≤ 0.05). (Online version in colour.)

Figure 6.

(a) Changes in FDB muscle–tendon unit (MTU) length (blue line), metatarsophalangeal (MTP) joint flexion/extension (green circles) and longitudinal arch (LA) angle (red squares) during stance phase of running at 2.78 m s−1. Data shows that MTU length recoils rapidly during late stance in parallel to LA recoil. This recoil happens despite the opposing influence of MTP joint extension occurring at the same time that should presumably lengthen the MTU. (b,c) The large moment arm of FDB across the LA, compared with its relatively small moment arm across the MTP joints, thus providing a biomechanical rationale for why MTP extension has minimal effect on overall length changes of the MTU. (Online version in colour.)