Stuart Currie DC,
Director of Research, Sole Supports.
INTRODUCTION
The foot is the primary interface with the ground allowing smooth locomotion through space: a simple enough concept perhaps, but how is forward movement really accomplished? Through a complex system of pivots, vaults, and rockers, the foot absorbs force and enables movement – a truly brilliant design. One question that is often asked is does the foot push, pull, or act as a spring?
DISCUSSION
The following is a presentation of what we know about how downward directed forces are converted into sagittal plane forward momentum. Dananberg stated that the amount of sagittal plane motion of the foot required during gait is 500% that of the frontal and transverse planes the significance of which cannot be minimized1. Dananberg’s article is considered a critical treatise on sagittal plane biomechanics as it applies to the foot. A summary of points made in the article is as follows:
- Working models of “muscle-less” walking machines have been constructed, tested and proven accurate.
- Bipedal gait is based inherently on the integration of gravity and momentum.
- The power for walking is predominantly by the swing leg, pulling the body over the standing limb.
- The stance limb is passive.
- Restrictions in this system (through a loss of range of motion) cause compensations.
- The magnitude of forces involved is sufficient to create deformity.
- Pathologic ligamentous laxity or neurologic absence of the muscle guarding response can contribute to an accommodation in which the body forms its own rocker deformity of the foot.
- Failure of one joint to move properly alters the motion of the entire structure.
- The power to propel the body forward is extrinsic to the foot.
From Kapandji, IA, The Physiology of the Joints, Vol.II, p.199
It becomes evident when evaluating these concepts, that the foot can function as a relatively passive rocker in the transmission of movement and that this rocker is affected directly by its posture. The three key contact points at the base of this vaulted posture are the first metatarsal head, the fifth metatarsal head, and the heel. It is this posture that defines the stabilized position of the foot that is so important in maintaining supination against body weight.
In her chapter on basic functions, Jacqueline Perry describes three rockers of the foot: the heel rocker, the ankle rocker and the forefoot rocker.2 The heel rocker preserves forward momentum of the body as weight drops onto the leg by rolling forward. The ankle rocker is a fulcrum for continued propulsion as the tibia continues to advance and is dependent on the soleus muscle to allow forward progression of the tibia. As the heel rises and the center of pressure advances past the metatarsals, the forefoot rocker allows continued unimpeded forward progress as the phalanges extend on the metatarsal heads. During the stance phase, while the foot is on the ground there is also a pivot shift as weight transfers from the lateral aspect of the foot to the medial side just before toe-off. The overall function of the rocker system is to convert the momentum of the moving body at heel strike into an efficient forward roll.
Sagittal restrictions cause blockages in forward motion. These restrictions can include ankle and meta-tarso-phalangeal joint limitations, both of which can be addressed with manual therapy techniques and manipulation.
The various arches of the foot, medial longitudinal, lateral and transverse provide selective planar stability to the foot. As a whole, they can be considered a plantar vault. Any one of these arches is only a 2D slice of the 3D architecture of the foot. The stability of the vault and rockers is related to the degree of re-supination the foot can achieve. The rockers of the foot can be made or broken by foot posture – i.e. a well formed plantar vault. If a proper plantar vault is not present in a pathologic foot it is benefitted by an orthosis that is casted in a corrected position providing a spring mechanism for gait that the foot did not have otherwise. A study investigating the effects of an orthotic designed to support the plantar vault in a specific corrected position has shown that these foot orthoses can increase the economy of gait.3
For the clinician it is important to realize that in order to see sagittal plane compensations properly, you must evaluate your patients from the side during walking. Restrictions in flexion and extension at the knee, hip and foot can only be visualized from the side. This is important because Dananberg also showed these blockages in hip extension and toe dorsi-flexion (hallux limitus) are also causative factors in chronic low back pain.4
It is generally accepted in the realm of orthopedics that joint loads are affected by posture, position and motion. It is therefore logical to assume these concepts are at work in the foot, and the morphology that is observed can have a profound effect on the transmission of forces through the body during gait.
Reference List
- Dananberg HJ. J Am Podiatr Med Assoc 2000 January;90(1):47-50.
- Perry J. Gait Analysis: Normal and Pathologic Function. Thorofare: SLACK Inc.; 1992.
- Trotter LC, Pierrynowski MR. J Am Podiatr Med Assoc 2008 November;98(6):429-35.
- Dananberg HJ. J Am Podiatr Med Assoc 1993 August;83(8):433-41.
