Research Review By Dr. Demetry Assimakopoulos©

Audio:

Download MP3

Date Posted:

September 2015

Study Title:

The Foot Core System: A New Paradigm for Understanding Intrinsic Foot Muscle Function

Authors:

McKeon PO, Hertel J, Bramble D & Davis I

Author's Affiliations:

Department of Exercise and Sport Sciences, School of Health Sciences and Human Performance, Ithaca College, New York; Department of Kinesiology, Curry School of Education, University of Virginia; Department of Biology, University of Utah, Salt Lake City, Utah; Department of Physical Medicine and Rehabilitation, Spaulding National Running Center, Harvard Medical School, Massachusetts, USA.

Publication Information:

British Journal of Sports Medicine 2015; 49: 290-299.

Background Information:

While the plantar foot is controlled by both the intrinsic and extrinsic foot muscles, the authors propose that the “central core” of the plantar foot is chiefly influenced by the plantar intrinsic foot muscles.

The authors liken the foot core to the lumbopelvic core. Both systems contain smaller local ‘stabilizers’ and larger global ‘movers’. The stabilizers are muscles with smaller cross-sectional area and moment arms. They are able to increase intersegmental stability and create a stable base for larger muscles to anchor upon and produce gross motion. In this paper, the authors describe the foot core subsystems, as well as assessment and rehabilitation methods using this well-known analogy.

Summary:

The Foot Core System:

The foot is controlled by both local stabilizers and global movers, much like the lumbopelvic complex. The global movers are muscles which attach proximally to the lower leg. They travel distally, cross the foot/ankle and attach onto the foot. The extrinsic muscles are responsible for producing global movement, while also contributing (to a lesser degree) to arch stability. The local stabilizers, on the other hand, are the four plantar intrinsic muscular layers, which function to stabilize the arches. When the intrinsic muscles are not functioning properly, the arch can become unstable and mal-aligned, leading to abnormal movement and injury.

The Foot Core System is rooted in the functional interdependence of the passive, active and neural subsystems, which were originally and famously proposed by Panjabi in the spine (1).

Passive Subsystem:
  • Includes the bony and articular structures – ligaments (see picture below) and joint capsule.
  • The functional configuration of the bony foot anatomy results in four distinct arches: the medial and lateral longitudinal arches, and the anterior and posterior transverse metatarsal arches (2).
  • These arches have been viewed as a functional ‘half-dome’ (see picture below), which is adaptable to load changes during dynamic activities (3). This ‘half-dome’ is centred (theoretically) on the talus.
  • The half-dome is predominantly supported by passive structures, such as the plantar aponeurosis and plantar ligaments. It also has local dynamic support from the intrinsic and extrinsic foot muscles.
foot ligaments
foot domes
Active Subsystem:
  • Consists of contractile muscles and tendons that attach on the foot.
  • The local stabilizers are the plantar intrinsic muscles. These muscles support the foot arches, are activity and load dependent and work together to provide dynamic arch support during gait. They also act as a platform for standing, while providing a lever for dynamically propelling the body forward during activity (4).
  • Global movers are the muscles that attach proximally on the lower leg, cross the ankle and distally attach on the foot. They create global motion.
Neural Subsystem:
  • Includes the sensory receptors in the plantar fascia, ligaments, joint capsules, muscles and tendons.
  • The intrinsic foot muscles likely provide sensory information about changes in the foot dome posture via the stretch response. They can be modulated through training.
  • Fatigue of these muscles has been shown to increase navicular drop during standing in healthy people and negatively affect joint position sense in other areas of the lower extremity.
Foot Core System Assessment:

No gold standard assessment method for the quality or capacity of the intrinsic foot muscles exists. Some studies have evaluated the use of toe flexion strength, but these methods are unable to separate the efforts of both intrinsic and extrinsic foot flexors. They also do not assess their ability to support the foot arches, which is their more important function.

One test has been posited to test a patient’s ability to maintain a neutral foot posture and medial longitudinal arch height during single limb stance. It is performed as follows:
  1. Place the patient’s hindfoot in subtalar neutral. The calcaneus and metatarsal heads should be on the ground.
  2. Ask the patient to dorsiflex their toes – this is meant to engage the windlass effect, and exaggerate the medial longitudinal arch.
  3. Ask the patient to lower their toes to the ground, while maintaining the medial longitudinal arch for 30 seconds.
  4. The clinician should watch for gross changes in navicular height and overactivity of the extrinsic muscles (what Janda described as a ‘noisy foot’).
To date, there is some evidence showing this test can detect improvements in foot core function after rehabilitation in patients with lower extremity injury (6, 7).

Some studies attempted to use EMG testing of the abductor hallucis as a surrogate for all the medially located intrinsic foot muscles, with limited success (5, 6). Fine-wire EMG using real-time guided ultrasound has assessed the activation of the plantar intrinsics in healthy subjects. Still, more clinical studies using these methods to assess plantar intrinsic muscle function in those with lower extremity injury are needed.

MRI analysis has discovered less total volume of the forefoot plantar intrinsic muscles in patients with unilateral plantar fasciitis and diabetic neuropathy (9).

Foot Core Training:

Exercises such as towel curls and marble pick-ups are typical exercises used in rehabilitation settings to activate the plantar intrinsic muscles. While they do activate these muscles, they also activate the extrinsic muscles. The short-foot exercise has been described as a way to isolate the intrinsic muscles (8, 10, 11). The patient is taught to pull the first MTP joint towards the calcaneus, while elevating the medial longitudinal arch (i.e. foot doming). The patient should be guided to sense subtalar neutral, with the toes neither flexed nor extended. This can be performed in a progression from sitting, to bipedal, to unipedal, and later integrated into squats, or single leg hops. They should initially be guided to establish control/contraction of the intrinsics, before increasing capacity. Training in this way has been shown to reduce arch collapse and improve balance ability, in superior fashion to towel curl exercises (12). It has also resulted in improved self-reported function in chronic ankle instability exercises after 4 weeks of training (13).

Barefoot Training:

Minimal/barefoot footwear use during walking and/or running may be used as a training tool to strengthen the foot core system. This training modality has shown an increase in the cross-sectional area of some of the foot core muscles in runners who trained for 5 months in minimalist footwear (14). Barefoot training also increases the sensory input to the plantar foot, which is important in improving postural stability, dynamic gait patterns and foot function.

Clinical Application & Conclusions:

The authors here suggest a paradigm shift in the way manual therapists and rehabilitation professionals treat the foot. While temporary support might be needed in the form of an orthotic device, it should be replaced as soon as possible with a strengthening program. The intrinsic foot muscles play a critical role in the foot core system as local stabilizers and direct sensors of foot deformation. Foot core assessment can provide insight into the foot’s ability to cope with functional demands. The short foot methods and exercises outlined above can be used to potentially increase the capacity and control of the intrinsic foot muscles.

Study Methods:

This is a narrative review. As such, no description of a search strategy or keywords were included.

Study Strengths / Weaknesses:

Strengths:
  1. The authors attempted to describe how the foot core system relates to the lumbopelvic core, as stipulated by Panjabi.
  2. They discussed how imaging modalities might be used in the assessment of the core, although they do not explicitly discuss whether these tools have any clinical relevance.
  3. They also pragmatically discussed how to improve foot core capacity with exercises which preferentially (theoretically) activate the plantar intrinsics.
Weaknesses:
  1. The authors did not discuss how the foot core relates to the foot/arch kinematics during functional tasks. Do short foot exercises decrease the rate of pronation in injured runners? If so, does it really matter?
  2. The authors did not discuss how the foot core relates to the kinetic chain, and whether or not changing foot core capacity improves lower extremity kinematics and/or pain.
  3. These authors have published extensively on barefoot exercise (mostly running). However, their conclusion that this training modality might be beneficial could be considered a stretch (no pun intended) at this point, given the limited evidence presented. Perhaps progressive barefoot training might decrease the perception of threat the foot (and therefore brain) experiences. We simply don’t know yet!

Additional References:

  1. Panjabi MM. The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement. J Spinal Disord 1992; 5: 383–389.
  2. Gray H, Standring S, Ellis H, et al. Gray’s anatomy : the anatomical basis of clinical practice 39th edn. New York: Elsevier Churchill Livingtone, 2005.
  3. McKenzie J. The foot as a half-dome. Br Med J 1955; 1: 1068–9.
  4. Soysa A, Hiller C, Refshauge K, et al. Importance and challenges of measuring intrinsic foot muscle strength. J Foot Ankle Res 2012; 5: 29.
  5. Fiolkowski P, Brunt D, Bishop M, et al. Intrinsic pedal musculature support of the medial longitudinal arch: an electromyography study. J Foot Ankle Surg 2003; 42: 327–333.
  6. Headlee DL, Leonard JL, Hart JM, et al. Fatigue of the plantar intrinsic foot muscles increases navicular drop. J Electromyogr Kinesiol 2008; 18: 420–425.
  7. Drewes LK, Beazell J, Mullins M, et al. Four weeks of short foot exercises affect lower extremity function, but not alignment, in patients with lower extremity injuries. J Athl Train 2008; 43: S105.
  8. Mulligan EP, Cook PG. Effect of plantar intrinsic muscle training on medial longitudinal arch morphology and dynamic function. Man Ther 2013; 18: 425–430.
  9. Janda V, Vavrova M, Hervenova A, et al. Sensory motor stimulation. In: Liebenson C. ed Rehabilitation of the spine: a practitioner’s manual. 2nd ed. Lippincott Williams & Wilkins, 2006.
  10. Chang R, Kent-Braun JA, Hamill J. Use of MRI for volume estimation of tibialis posterior and plantar intrinsic foot muscles in healthy and chronic plantar fasciitis limbs. Clin Biomech 2012; 27: 500–505.
  11. Greenman P. Exercise principles and prescription. In: Butler J. ed Principles of manual medicine. 2nd ed. Baltimore: Williams & Wilkins, 1996: 449–525.
  12. Sauer LD, Beazell J, Hertel J. Considering the intrinsic foot musculature in evaluation and rehabilitation for lower extremity injuries. Athl Train Sports Health Care 2011; 3: 43–47.
  13. Lynn SK, Padilla RA, Tsang KK. Differences in static- and dynamic-balance task performance after 4 weeks of intrinsic-foot-muscle training: the short-foot exercise versus the towel-curl exercise. J Sport Rehabil 2012; 21: 327–333.
  14. Sauer LD, Saliba SA, Ingersoll CD, et al. Effects of rehabilitation incorporating short foot exercises on self-reported function, static and dynamic balance in chronic ankle instability patients. J Athl Train 2010; 45: S67.
  15. An KN, Linscheid RL, Brand PW. Correlation of physiological cross-sectional areas of muscle and tendon. J Hand Surg (Edinburgh, Scotland) 1991; 16: 66–7.