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.

Chapter four examined the potential for the intrinsic foot muscles to actively control LA compression and recoil that occurs due to the application and release of external load. This study tested the hypotheses that activation of AH, FDB and QP is associated with muscle stretch that occurs in response to LA compression produced by external loading on the foot, and that activation of these muscles (via electrical stimulation) will generate sufficient force to counter LA compression. Muscle tendon units (MTU) of AH, FDB and QP stretched in response to LA compression occurring due to external load. Recruitment of these muscles increased with increasing load beyond specific force thresholds. LA deformation and muscle stretch plateaued towards the maximum load of 150% body weight, when muscle activity was greatest. Electrical stimulation of the plantar intrinsic muscles countered the deformation that occurred due to the application of external load by reducing the length and increasing the height of the LA. These findings demonstrate that these muscles have the capacity to control LA deformation and may buttress the LA during foot loading.

Chapter five tested the hypothesis that AH, FDB and QP will actively lengthen and shorten during the stance phase of gait in response to variable loading of the foot that occurs during walking and running at different speeds. For both walking and running the LA compressed during the initial loading phase (early stance) and recoiled as the load subsided (late stance), with the magnitude of compression increasing with gait velocity and the associated increase in vertical ground reaction force. 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. MTU length change and peak muscle activity increased with gait velocity for all muscles. This thesis provides in-vivo evidence that the plantar intrinsic foot muscles actively lengthen and shorten during the stance phase of gait and are therefore capable of contributing to power dissipation and generation during gait. We suggest that the intrinsic foot muscles actively contribute to the foot spring mechanism and are regulated in response to the magnitude of load encountered.

In summary, this thesis has provided a detailed description of the function of the three largest plantar intrinsic foot muscles, AH, FDB and QP during postural and dynamic tasks. These muscles are activated in a highly co-ordinated manner in order to adjust the stiffness of the longitudinal arch in response to the loading demands encountered during postural activity and locomotion.