In-vivo function of human plantar intrinsic foot muscles

Abstract

This thesis investigates the in-vivo function of the plantar intrinsic foot muscles. Though much speculation has been made of the function of these muscles, scant detail exists pertaining to their function. This thesis provides a novel description of the function of these muscles in providing active support for the longitudinal arch (LA) during postural tasks and locomotion. Furthermore, the following chapters provide evidence of an active mechanism to stiffen the LA, primarily provided by the graded activation of these muscles in response to increasing load. This mechanism may have important implications for how energy is stored and released within the foot. Chapter one provides a general overview of the existing literature pertaining to the function of these muscles. Chapters two, three, four and five contain the individual manuscripts from each experiment performed as part of this thesis. Chapter six provides a summary of the findings from the thesis and some general suggestions for the direction of future research in this field.Chapter two investigates the role of the plantar intrinsic foot muscles in providing postural support for the foot during quiet standing. Intra-muscular electromyographic (EMG) activity was recorded from abductor hallucis (AH), flexor digitorum brevis (FDB) and quadratus plantae (QP) while participants performed two balance tasks of graded difficulty. Each task was performed while standing on a force plate, allowing appraisal of any relationship between loading, postural sway and intrinsic foot muscle activity. Intrinsic foot muscle activation increased in response to postural demand, with these muscles displaying highly correlated inter-muscular activation patterns in response to medial postural sway. Contrary to previous thoughts, these muscles are clearly important in postural control and are recruited in a highly co-ordinated manner to stabilise the foot and maintain balance, particularly during single leg stance, in the medio-lateral direction.The purpose of Chapter three was to investigate if the neurophysiological properties of the largest intrinsic foot muscle (abductor halluces) are matched to its suggested postural function. A highly selective, quadrifilar arrangement of fine wire EMG electrodes was employed to describe the discharge properties of AH motor units during ramp and hold isometric contractions, as well as during a submaximal, constant force, fatiguing contraction. Abductor hallucis motor units displayed small rate coding ranges, relatively low peak discharge rates and were largely resistant to fatigue. This muscle is comparatively fatigue resistant and appears to rely predominantly on recruitment to generate force, optimizing the use of slow twitch, fatigue resistant fibres to generate moderate to large amounts of force for sustained periods of time. These properties appear well matched to AH's postural function that involves providing stabilisation of the LA during weight-bearing tasks. 3Chapter four examined the potential for the intrinsic foot muscles ...

Effects of Foot-Core Training on Foot-Ankle Kinematics and Running Kinetics in Runners: Secondary Outcomes From a Randomized Controlled Trial

“…The MLA should have the capacity to be flexible in response to running loads, allowing foot-joint adjustments to dampen impacts through multiple mechanisms, including stiffness and power absorption, but it must also be rigid enough to allow propulsion in the push-off phase ( McDonald et al, 2016 ). Our foot-core training may have increased the ability of the plantar intrinsic muscles to provide force-dependent alterations in the MLA stiffness and to facilitate efficient foot-to-ground contact during running ( Kelly, 2015 ; Kelly et al, 2018 ). An actively restricted MLA may help to decrease the mechanical demand on the soft tissues of the foot, such as ligaments, fascia, and tendons, and may result in fewer injuries in these structures, such as plantar fasciitis, which derives from repetitive abnormal strain and loading of the plantar fascia and flattening of the MLA ( Wearing et al, 2006 ; Chang et al, 2014 ).…”