The Forefoot. what does it do, and what is its role in injury?
“Anatomy is destiny”
– Sigmund Freud (1856 – 1939) : Collected Writings (1924)
It has been established that by the time an individual reaches 30 years of age, they will have taken in excess of 60 million steps in course of their normal daily activities.
This does not include any activities outside those associated with day to day living, for example sporting endeavors. Without question, the foot is remarkable.
It is the structure responsible for the initiation and execution of the characteristic that sets homo sapiens apart from every other species on earth: Bipedal upright locomotion.
The foot is the platform from which, through the coupling mechanisms of the joints, all structures proximally must be dependent.
As an exercise in engineering, it is without peer in terms of function, integration and complexity.
It is this very complexity that limits our absolute understanding of its mechanical functioning.
We do however now understand that the foot is responsible for much more than just allowing forward propulsion.
The sensitive nerve endings and proprioceptors in the foot feed afferent signals to the brain that control many of the delicate adjustments at any level required for effective gait.
The brain is constantly monitoring the harmonics of the vibrations caused by impact and forward progression. Many researchers currently believe interference with the frequencies of these vibrations play a key role in injury, because they “scramble” the normal or “correct” message the brain expects to receive.
In this instance, it is proposed that injury can occur secondary to altered muscle firing and therefore abnormal loading patterns.
In sport, it is often quoted that impact forces on the foot during running approach three to four times body weight (Lillich and Baxter, 1986). In addition, the average runner will strike the ground 480 to 1,200 times per kilometre (Brody 1980).
Whilst these figures reflect a significant load on the foot, its engineering is such that it should easily be able to withstand such load.
It is becoming increasingly clear that injury is not precipitated by the load as such, but rather by a caveat to the load, in terms of altered mechanical axes, altered muscle function or changed psychophysical or physiological status.
Whatever the cause of injury, a thorough understanding of the anatomy, physiology and biomechanics of the foot is mandatory for those attending sports injuries.
Some Anatomical and Biomechanical Considerations of the Forefoot
The forefoot is classically defined as those structures distal to LisFranc’s joint, and includes the metatarsals and the phalanges. See figure 9.1. Whilst the foot can be regionalised, it cannot be compartmentalised, since each region has an intimate and interdependent relationship with the other.
The forefoot is primarily responsible for executing the propulsive phase of gait, and, as such, it is required to control the body’s forward progression in a sustained and balanced manner for in excess of 40% of the stance phase of gait.
Figure 9.1: The zones of the foot. A, the rearfoot, B the midfoot, and the forefoot represented in grey as the zone distal to LisFranc’s joint
Figure 9.2 illustrates pressure and force data for a normal forefoot during gait.
Figure 9.2: Normal pressure and force data for the forefoot.
These pressure and force maps can be radically altered by faulty foot mechanics, as demonstrated in figure 9.3
Figure 9.3: Abnormal pressure maps under the forefoot indicated by red and purple tracings.
The forefoot comprises five metatarsals and fourteen phalanges, as well as the associated joints, ligaments and muscles.
The metatarsophalangeal (MPJ or MTP), proximal interphalangeal (PIPJ), and distal interphalangeal joints (DIPJ) are condyloid and hinge joints (Anderson and Hall 1995) and have a close packed position in full extension.
In addition, the 1st metatarsophalangeal joint has two sesamoid bones insinuated in the tendon of flexor hallucis brevis.
These sesamoids function as anatomical pulleys to the flexor hallucis brevis muscle, as well as protecting the tendon of flexor hallucis longus from trauma as it passes between the metatarsal head and the base of the proximal phalanx.
The 1st metatarsophalangeal joint therefore comprises the articular facets of four bones within a single synovial joint capsule (Dykyj 1989). The 1st metatarsophalangeal joint has motion available in two planes, sagittal and tranverse, because of its condylar nature.
The motion of the 1st metatarsophalangeal joint is therefore more complex than it may appear, with Heatherington et al (1989) describing the joint as a “dynamic acetabulum”.
The joint is stabilized medially and laterally by the combined strengths of the collateral ligaments, the suspending sesamoid ligament, and the plantar sesamoid ligament. See figure 9.4.
Further stability is provided by a strong plantar band of tissue comprising the deep transverse metatarsal ligament and associated tissues which prevent separation of the joints under weight-bearing stress.
Figure 9.4: The 1st MPJ is a complex joint with many supporting and complementary components.
The forefoot is highly adapted to the enormous shear loads it must accept during running and walking.
These forces are especially evident in sport, with stopping and direction changes. The forefoot therefore contains structures which protect against these shear forces, and concomitantly protect the delicate nerves, vessels and tendons.
The encapsulated fat pads found in the forefoot and heel have a dynamic protective function superior to anything created in the laboratory.
Many athletic footwear companies have attempted to reproduce these properties of human fat pad tissue, and to date have not succeeded. The structure of the plantar fat pad is reproduced in figure 9.5.
The protective properties of this tissue are determined by its unique and ingenious framework of fatty tissue contained within tough bands of tissue running in vertical tranverse and sagittal directions.
There is a slow degeneration in the plantar fat pad in terms of its size and the quality of collagen, with advancing age. This is coupled to a decreasing load through the 1st metatarsophalangeal joint which leaves the lesser metatarsal head somewhat vulnerable to injury as we grow older.
Altered forefoot mechanics and a general tendency towards increase body weight with advancing years may also conspire toward forefoot injury with age.
Figure 9.5: The plantar fat pad is a complex honeycomb arrangement of fatty tissue contained within strong septa. It provided a very important protective role for the underlying boney and other soft tissues of the rearfoot. Fat pads under the metatarsal heads and pads of the digits provide a similar role in these high weight-bearing locations.
The 1st metatarsophalangeal joint undergoes two periods of dorsiflexion during gait. The first, an active dorsiflexion, occurs just before heel contact, allowing the digits to clear the ground during late swing phase.
This dorsiflexion is primarily achieved through the action of extensor hallucis longus. Immediately after heel lift, the digits plantarflex, from lateral to medial, with the 1st metatarsophalangeal joint plantarflexing to the ground in late contact phase.
The second phase of 1st metatarsophalangeal dorsiflexion is passive, and occurs immediately after heel lift, signaling the commencement of the propulsive phase of gait.
There remains some controversy over the required range of 1st metatarsophalangeal joint dorsiflexion.
Valmassey (1996) reports that unassisted dorsiflexion of the 1st metatarsophalangeal joint is on average 77°, while average assisted dorsiflexion is 82°.
In dynamic studies, Perry (1992) determined the necessary dorsiflexion of the 1st metatarsophalangeal joint during gait was between 50° and 60°.
The lesser digits are stabilised against the ground from propulsion at the lesser metatarsophalangeal joints predominantly by the action of flexor digitorum longus and flexor digitorum brevis.
The tendons of these muscles insert into the distal and middle phalanges respectively.
Next...Injuries to the 1st metatarsophalangeal joint
[This piece is reprinted with permission from Bartold, S.J., 2014, The Foot and Leg in Sport, Amazon Publishing, pages 250-255]
Director of Bartold Clinical