What are the effects of induced toe flexor weakness on foot kinematics? A study protocol and preliminary results

Abstract

The foot core is supported by active subsystems like intrinsic foot muscles(1). Weakness of these muscles can lead to a decrease in the medial longitudinal arch(MLA), resulting in altered foot mechanics, function, and increasing the risk of injuries(1,2). Intrinsic muscle strength is compatible with toe flexor strength and has been found to be lower in flat feet (3,4). It is challenging to determine the isolated effects of intrinsic muscle weakness in foot kinematics while walking(4) which can provide valuable insights for clinical reasoning. What are the effects of induced toe flexor weakness on foot kinematics? 4 adults (3 female,1 male;24.75±2.98 y.o.) with typical foot posture (Foot-Posture-Index-6 score: <5) participated into the pilot study. Toe flexor muscle strength of the dominant foot was assessed with a dynamometer (Lafayette Instrument Company, USA) while sitting before and after the fatigue procedure (Figure-1:a1-a2) (5). A 3D-printed foot arc heightening device (AHD) with 4 kg resistance spring was used to generate fatigue in the toe flexor muscles (Figure-1:2). The participants were required to complete 75 reps. for each set by a metronome at 45 BPM under the discomfort level (6/10) until achieving 10% muscle force-drop(Figure-1:c1-c2). Heel-rising and extrinsic muscle activation were not allowed. The Oxford Foot Model was used to analyze three trials of walking kinetics and kinematics. Wilcoxon test was used for statistical non-parametric paired analysis (p<0.05).Download : Download high-res image (148KB)Download : Download full-size image To achieve >10% muscle weakness each participant completed varying numbers of sets (3-5 sets). The decrease of great toe and toe flexor muscle strength was 19.57%±7.01 and 19.01%±3.58 after the procedure respectively. Some of the effects of the procedure remained after analyses were completed (15.67%±13.34 and 12.3%±11.31). The mean velocity, temporospatial parameters, kinematic parameters of pelvis, hip and knee joints, ankle power and arch height were not different before and after the procedure (p>0.05). Peak hindfoot plantarflexion was lower and peak hindfoot inversion was higher significantly after the procedure. The sagittal and frontal plane range of the hindfoot relative to the tibia decreased (p<0.05, Graph-1: I,II,III) The pilot study protocol was effective enough to induce temporary toe flexor muscle weakness. Although the isometric muscle force reduced for intrinsic muscles after the procedure, controversially to the literature (2), increased hindfoot inversion was found which may be related to increased motor unit activation or proprioceptive alterations which should be studied in detail. The device was more efficient in great toe grasping compared to other toes, which might result in differential level muscle weakness among the toes. Comparison studies with a larger sample size are needed to conclude to describe the effects of fatigue procedure.