The foot core system: a new paradigm for understanding intrinsic foot muscle function Br J Sports Med bjsports-2013-092690Published Online First: 21 March 2014

The foot is a complex structure with many articulations and multiple degrees of freedom that play an important role in static posture and dynamic activities. The evolutionary development of the arch of the foot was coincident with the greater demands placed on the foot as humans began to run. The movement and stability of the arch is controlled by intrinsic and extrinsic muscles. However, the intrinsic muscles are largely ignored by clinicians and researchers. As such, these muscles are seldom addressed in rehabilitation programmes. Interventions for foot-related problems are more often directed at externally supporting the foot rather than training these muscles to function as they are designed. In this paper, we propose a novel paradigm for understanding the function of the foot. We begin with an overview of the evolution of the human foot with a focus on the development of the arch. This is followed by a description of the foot intrinsic muscles and their relationship to the extrinsic muscles. We draw the parallels between the small muscles of the trunk region that make up the lumbopelvic core and the intrinsic foot muscles, introducing the concept of the foot core. We then integrate the concept of the foot core into the assessment and treatment of the foot. Finally, we call for an increased awareness of the importance of the foot core stability to normal foot and lower extremity function.

Evaluation and Retraining of the Intrinsic Foot Muscles for Pain Syndromes Related to Abnormal Control of Pronation (Bahram Jam, Mphty (Manip), BScPT, FCAMT)

Abstract: Little clinical research exists on the contribution of the intrinsic foot muscles (IFM) to gait or on the specific clinical evaluation or retraining of these muscles. The purpose of this clinical paper is to review the potential functions of the IFM and their role in maintaining and dynamically controlling the medial longitudinal arch. Clinically applicable methods of evaluation and retraining of these muscles for the effective management of various foot and ankle pain syndromes are discussed.Key Words: intrinsic foot muscles, medial longitudinal arch, pronation, exercises

Effect of plantar intrinsic muscle training on medial longitudinal arch morphology and dynamic function

Edward P. Mulligan, Patrick G. Cook, May 4, 2013
Manual Therapy; In Press

A specific training program emphasizing the neuromuscular recruitment of the plantar intrinsic foot muscles, colloquially referred to as “short foot” exercise (SFE) training, has been suggested as a means to dynamically support the medial longitudinal arch (MLA) during functional tasks. A single-group repeated measures pre- and post-intervention study design was utilized to determine if a 4-week intrinsic foot muscle training program would impact the amount of navicular drop (ND), increase the arch height index (AHI), improve performance during a unilateral functional reaching maneuver, or the qualitative assessment of the ability to hold the arch position in single limb stance position in an asymptomatic cohort. 21 asymptomatic subjects (42 feet) completed the 4-week SFE training program. Subject ND decreased by a mean of 1.8 mm at 4 weeks and 2.2 mm at 8 weeks (p < 0.05). AHI increased from 28 to 29% (p < 0.05). Intrinsic foot muscle performance during a static unilateral balancing activity improved from a grade of fair to good (p < 0.001) and subjects experienced a significant improvement during a functional balance and reach task in all directions with the exception of an anterior reach (p < 0.05). This study offers preliminary evidence to suggest that SFE training may have value in statically and dynamically supporting the MLA. Further research regarding the value of this exercise intervention in foot posture type or pathology specific patient populations is warranted.

The architecture and contraction time of intrinsic foot muscles

a) The University of Queensland, School of Biomedical Sciences, Department of Anatomy & Developmental Biology, St. Lucia 4072, Australia
b) Department of Kinesiology, San Francisco State University, San Francisco, CA 94132, USA

Although critical for effective human locomotion and posture, little data exists regarding the segmentation, architecture and contraction time of the human intrinsic foot muscles. To address this issue, the Abductor Hallucis (AH), Abductor Digiti Minimi (ADM), Flexor Digitorum Brevis (FDB) and Extensor Digitorum Brevis (EDB) were investigated utilizing a cadaveric dissection and a non-invasive whole muscle mechanomyographic (wMMG) technique. The segmental structure and architecture of formaldehyde-fixed foot specimens were determined in nine cadavers aged 60–80 years. The wMMG technique was used to determine the contraction time (Tc) of individual muscle segments, within each intrinsic foot muscle, in 12 volunteers of both genders aged between 19 and 24 years.

While the pattern of segmentation and segmental –architecture (e.g. fibre length) and –Tc of individual muscle segments within the same muscle were similar, they varied between muscles. Also, the average whole muscle Tc of FDB was significantly (p < 0.05) shorter (faster) (Tc = 58 ms) than in all other foot muscles investigated (ADM Tc = 72 ms, EDB Tc = 72 ms and ABH Tc = 69 ms). The results suggest that the architecture and contraction time of the FDB reflect its unique direct contribution, through toe flexion, to postural stability and the rapid development of ground reaction forces during forceful activities such as running and jumping.

Fatigue of the plantar intrinsic foot muscles increases navicular drop  J Electromyogr Kinesiol. 2008 Jun;18(3):420-5. Epub 2007 Jan 8,Headlee DLLeonard JLHart JMIngersoll CDHertel J.

Abstract: Our purpose was to assess the effect of foot intrinsic muscle fatigue on pronation, as assessed with navicular drop, during static stance. Twenty-one healthy young adults participated. Navicular drop was measured before and after fatiguing exercise of the plantar foot intrinsic muscles. Surface electromyography of the abductor hallucis muscle was recorded during maximum voluntary isometric contractions (MVIC) in order to find the baseline median frequency (MedF). Subjects then performed sets of 75 repetitions of isotonic flexion contractions of the intrinsic foot muscles against a <st1:metricconverter productid="4.55 kg" w:st="on"><st1:time hour="4" minute="55" w:st="on">4.55</st1:time> kg</st1:metricconverter> weight on a custom pulley system. After each set an MVIC was performed to track shifts in MedF. After a MedF shift of at least 10%, navicular drop measurements were repeated. Subjects exhibited 10.0+/-3.8mm of navicular drop at baseline and 11.8+/-3.8mm after fatigue (p<0.0005). The change in navicular drop was significantly correlated with change in MedF (r=.47, p=.03). The intrinsic foot muscles play a role in support of the medial longitudinal arch in static stance. Disrupting the function of these muscles through fatigue resulted in an increase in pronation as assessed by navicular drop.

A Comparison of Muscle Activities in the Lower Extremity between Flat and Normal Feet during One-leg Standing 

Ju-Eun Lee , PT1), Ga-Hyeon Park, PT1), Yun-Seop Lee , PT, PhD1), Myoung-Kwon Kim, PT, PhD1)*1) Department of Physical Therapy, Youngsan University: San 150, Joonam-dong,Yangsan, Kyeongsangnam-do, Republic of Korea. TEL: +82 55-380-9367, FAX: +82 55-380-9305

Abstract. [Purpose] This study examined the differences in muscle activation between flat and normal feet in the one-leg standing position which delivers the greatest load to the lower extremity. [Subjects] This study was conducted with 23 adults, 12 with normal feet and 12 with flat feet, with ages ranging from 21 to 30 years old, who had no neurological history or gait problems. [Methods] The leg used for one leg standing was the dominant leg of the subjects. The experimenter instructed the subjects to raise the non-dominant leg with their eyes open, and thesubjects maintained a posture with the non-dominant leg’s knee flexed at 90° and the hip joint flexed at 45° for six seconds. In the position of one-leg standing, a horizontal rod was set at the height of the waist line of the subjects who lightly placed two fingers of each hand on the rod to prevent inclination of the trunk to one side. Measurements were taken three times and the maximum value was used. A surface electromyogram (TeleMyo 2400T, Noraxon Co., USA) was used to measure muscle activities. [Results] We compared muscle activities between flat and normal foot, and the results show a significant difference between normal and flat feet in the muscle activity of the abductor hallucis muscle. [Conclusion] The subjects with flat feet had relatively lower activation of the abductor hallucis muscle than those with normal feet during one leg standing. We infer from this that the abductor hallucis muscle of flat foot doesn’t work as well as a dynamic stabilizer, compared to a normal foot, during one leg standing.

Key words: Flat foot, Electromyography, One-leg standing

Recruitment of the plantar intrinsic foot muscles with increasing postural demand

Luke A. Kelly a,b, Sami Kuitunen c,d, Sebastien Racinais a,b, Andrew G. Cresswell b,⁎

Background: The aim of this study was to determine the difference in activation patterns of the plantar intrinsic footmuscles during two quiet standing taskswith increasing postural difficulty.Wehypothesised that activation of these muscles would increase with increasing postural demand and be correlated with postural sway.

Methods: Intra-muscular electromyographic (EMG) activity was recorded from abductor hallucis, flexor digitorumbrevis and quadratus plantae in 10 healthy participantswhile performing two balance tasks of graded difficulty (double leg stance and single leg stance). These two standing postures were used to appraise any relationship between postural sway and intrinsic foot muscle activity.

Findings: Single leg stance compared to double leg stance resulted in greater mean centre of pressure speed (0.24 m s−1 versus 0.06 m s−1, respectively, P≤0.05) and greater mean EMG amplitude for abductor hallucis (P≥0.001, ES=0.83), flexor digitorum brevis (P≤0.001, ES=0.79) and quadratus plantae (P≤0.05, ES=0.4). EMG amplitude waveforms for all muscles were moderate to strongly correlated to centre of pressure (CoP) medio-lateral waveforms (all r≥0.4), with muscle activity amplitude increasing with medial deviations of the CoP. Intra-muscular EMG waveforms were all strongly correlated with each other (all r≥0.85).

Interpretations: Activation of the plantar intrinsic footmuscles increases with increasing postural demand. These muscles are clearly important in postural control and are recruited in a highly co-ordinated manner to stabilise the foot and maintain balance in the medio-lateral direction, particularly during single leg stance.

© 2011 Elsevier Ltd. All rights reserved.



Abstracts sensorimotor function 

Distribution and behaviour of glabrous cutaneous receptors in the human foot sole

Paul M. Kennedy and J. Timothy InglisSchool of Human Kinetics, University of British Columbia, Vancouver, Canada

To document the activity of cutaneous mechanoreceptors in the glabrous skin of the foot sole, tungsten microelectrodes were inserted through the popliteal fossa and into the tibial nerve of thirteen healthy human subjects. A total of 104 cutaneous mechanoreceptors were identified in the glabrous skin of the foot. This sample consisted of 15 slow adapting type I (14 %), 16 slow adapting type II (15 %), 59 fast adapting type I (57 %), and 14 fast adapting type II units (14 %). The location of the receptors and the outline of the receptive fields were determined by using nylon monofilaments perpendicularly applied against the surface of the skin. This revealed that the receptors were widely distributed without an accumulation of receptors in the toes. There were also larger receptive fields predominantly isolated on the plantar surface of the metatarsal-tarsal region of the foot sole. Furthermore, with the foot in an unloaded position, there was no background discharge activity in any of the cutaneous receptors in the absence of intentionally applied stimulation. These findings suggest that skin receptors in the foot sole behave differently from those receptors found on the glabrous skin of the hand. This may reflect the role of foot sole skin receptors in standing balance and movement control.


The Human Balance System— A Complex Coordination of Central and Peripheral Systems

By the Vestibular Disorders Association, with contributions by Mary Ann Watson, MA, and F.

Owen Black, MD, FACS 

Good balance is often taken for granted. Most people don’t find it difficult to walk across a gravel driveway, transition from walking on a sidewalk to grass, or get out of bed in the middle of the night without stumbling. However, with impaired balance such activities can be extremely fatiguing and sometimes dangerous.

Symptoms that accompany the unsteadiness can include dizziness, vertigo, hearing and vision problems, and difficulty with concentration and memory. 


Effect of vision, proprioception and the position of the vestibular organ on postural sway

Eva Ekvall Hansson RPT, PhD, Anders Beckman MD, PhD, Anders Håkansson Professor Lund University, Department of Clinical Sciences in Malmö/Family Medicine/General Practice


Conclusion: When measured together, it seems that vision and proprioception as well as position of the vestibular organ affect postural sway, vision the most. Mediolateral sway does not seem to be influenced by the position of the vestibular organ.

Objective: To investigate how postural sway was affected by provocation of vision, by the position of the vestibular organ and by provocation of proprioception, when measured together.

Method: Postural sway was measured by using a force plate. Tests were performed with eyes open and eyes closed, with head in neutral position and rotated to the right and to the left and with ead maximally extended, both standing on firm surface and on foam. Measures of mediolateral (ML) speed (mm/sec), anteriorposterior (AP) speed (mm/sec) and sway area (SA) (mm2/sec) were analysed using multilevel approach.

Results: The multilevel analysis revealed how postural sway was significantly affected by closed eyes, standing on foam and by the position of the vestibular organ. Closed eyes and standing on foam both significantly prolong the dependent measurement, irrespective of whether it is ML, AP or SA. However, only AP and SA were significantly affected by vestibular position, i.e. maximal head movement to the right and extension of the head.

Keywords: Postural control, balance, vestibular system.