MINI-SYMPOSIUM: THE ELECTIVE FOOT: (i) Relevant foot ... · tarsometatarsal,metatarsophalangeal(MTP)andinter-phalangeal(IP)joints)of thefootmakeupitsfour segments:thehindfoot,themidfoot,theforefootand

Current Orthopaedics (2002) 16,165^179�c 2002 Publishedby Elsevier Science Ltd.doi:10.1054/ycuor.268, available online at http://www.idealibrary.com on

MINI-SYMPOSIUM:THEELECTIVEFOOT

(i) Relevant foot biomechanicsR. J. Abboud

Foot Pressure Analysis Laboratory/Clinic,University Orthopaedic & Trauma Surgery Department,TORTCentre,Ninewells Hospital & MedicalSchool,Dundee DD19SY, Scotland,UK

KEYWORDSfoot, ankle, biomechanics,gait, pressure

Summary In order to e¡ectively treat any part of the human musculoskeletal sys-tem, it is important to fully understand its biomechanics.Biomechanics is the study ofnormalmechanics (kinetics and kinematics) in the musculoskeletal system by analysingforces and their e¡ects on anatomical structures.Thenormalmechanics ofthe foot andankle result from the combined e¡ects of muscle, tendon, ligament, and bone. Thecoordinated and uni¢ed e¡ect of these tissues within the foot, ankle and lower extre-mityresults in themoste⁄cient force attenuation.To comprehend biomechanics of thefoot during standing and walking, it is important to understand gait. �c 2002 Published byElsevier Science Ltd.

INTRODUCTIONBiomechanics is the study of normalmechanics (kineticsand kinematics) in themusculoskeletal system by analys-ing forces and their e¡ects on anatomical structures. Inorder to e¡ectively treat any part of the human muscu-loskeletal system, it is important to fully understand itsbiomechanics.

FOOTBIOMECHANICSThe human foot is a complex multi-articular mechanicalstructure consisting of bones, joints and soft tissues,playing an extremely important role in the biomechani-cal function of the lower extremity and is controlled byboth intrinsic and extrinsic muscles. It is the only part ofthe body that acts on an external surface, providing sup-port and balance during standing and stabilizing thebody during gait. During the stance phase (STP), be-tween heel strike (HS) and toe o¡ (TO), it has to adaptto a changing pattern of loading as the centre of mass ofthe body moves. An equal and opposite reaction, theground reaction force (GRF), develops when the footcomes in contact with the ground. The GRF changes indirection and magnitude as the body propels itself for-wards (or backwards) (Fig.1).The foot must also be relatively compliant to cope

withunevenground, bothbare and shod,whilemaintain-ing its functional integrity. During ground contact, footfunction reverses the convention that a muscle is ¢xed

Correspondence to: RJA.Tel: +44-(0)1382 496276; Fax: +44-(0)1382496347; Email: [email protected]

at its origin and moves from its insertion. The conven-tional anatomical insertion is often ¢xed against theground, and the origin in theheel or legmoves in relationto that ¢xed point.This provides both £exibility and sta-bility during walking.The important mechanical structures of the foot

include:

1. The bony skeleton, which together with the ligamentsand arches, provides relative rigidity and the essentiallever arm mechanism required to maintain balanceduring standing and facilitate propulsion,

2. The joints which confer £exibility,3. The muscles and tendons which control footmovement.

The foot is the endpartof the lower kinetic chain thatopposes external resistance.1 Normal arthrokinematicsand proprioception within the foot and ankle in£uencethe ability of the lower limb to attenuate the forces ofweightbearing (static anddynamic).The lowerextremityshould distribute and dissipate compressive, tensile,shearing, and rotatory forces during the stance phase ofgait. Inadequate distribution of these forces can lead toabnormal movement, which in turn produces excessivestress which can result in the breakdown of soft tissueandmuscle.The normalmechanics of the foot and ankleresult in themost e⁄cient force attenuation.

FOOTSTRUCTUREThe 26 bones (seven tarsals, ¢ve metatarsals, and14 phalanges) and six joints (ankle, subtalar, midtarsal,

Figure 1 Normalgaitcycle: (A) fromthe Polygon Software (ViconMotion Systems Ltds, yellowarrowis GRF) and (B) multiple footprints fromthe GaitRite Systems (right (magenta) andleft (green)). RIC=right initial contact, LIC=left initial contact, RTO=right toe-o¡, LTO=left toe-o¡ and RHS=right heel strike, RFFL=right forefoot loading, RHO=right heel o¡ andRTO=righttoe o¡.

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RELEVANTFOOTBIOMECHANICS 167

tarsometatarsal, metatarsophalangeal (MTP) and inter-phalangeal (IP) joints) of the foot make up its foursegments: the hindfoot, the midfoot, the forefoot andthe phalanges (Fig. 2(A)).

The hindfoot

The hindfoot consists of the talus and calcaneus. Thethree parts of the talus (body, neck, andhead) are orien-tated to transmit reactive forces from the foot throughthe ankle joint to the leg. Lying between the calcaneus,and tibia, it communicates thrust from one to the other.The calcaneus is the largest and most posterior bone inthe foot andprovides a lever arm for the insertion of theAchilles tendon, which is the largest and one of thestrongest tendons in the body throughwhich gastrocne-mius and soleus impart powerful plantar£exion forces tothe foot. Its height,width and structure enable the calca-neus to withstand high tensile, bending and compressiveforces on a regular basis without damage.

Themidfoot

The navicular, the cuboid and three cuneiformsmake upthe midfoot.The navicular medial to the cuboid, articu-lates with the head of the talus anteriorly and is the key-stone at the top of the medial longitudinal arch. Thecuboid articulates with the calcaneus proximally and thefourth and ¢fth metatarsals distally. The three cunei-forms, are convexly shaped on their broad dorsal aspectwhilst the plantar surface is concave and wedge shapedso that the apexof eachbonepoints inferiorly.The cunei-forms articulate with the ¢rst, second and third meta-tarsals distally. This multi-segmental con¢guration inconjunctionwith connecting ligaments andmuscles con-tributes greatly to the stability of themidfoot.

The forefoot

There are ¢ve metatarsals in the forefoot, these all ta-pered distally and articulating with the proximal pha-langes. The ¢rst metatarsal is the shortest and widest.Its base articulates with the medial cuneiform and issomewhat cone shaped.The head of the ¢rstmetatarsaladditionally articulateswith two sesamoids on its plantararticular surface.The secondmetatarsal extends beyondthe ¢rst proximally, and articulates with the intermedi-ate cuneiform as well as with the medial and lateral cu-neiforms in a ‘key-like’ con¢guration which promotesstability andrenders the secondray the sti¡est andmoststable portion of the foot playing a key role in stabilizingfoot posture after hallux surgery.The third, fourth and ¢fth metatarsals are broad at

the base, narrow in the shaft and have dome-shapedheads. The ¢fth has a prominent styloid, laterally and

proximally at its base, onwhich the peroneus brevis ten-don inserts. Avulsion fracture of the styloid commonlyoccurs when the foot is inverted against the contractingperoneus brevismuscle.2

The phalanges

Phalanges constitute digits.The big toe (hallux) consistsof two phalanges, all other toes containing three. Theheads of the proximal and middle phalanges tend to betrochlear shaped allowing for greater stability. Function-ally, the toes contribute toweightbearing and loaddistri-bution and also e¡ect propulsion during the push-o¡phase of gait.

FUNCTIONJoints of the foot are controlledby extrinsic and intrinsicmuscles of the lower limb andprovide for themajormo-tion function, angulation and support of the foot. Aswith all joints, motion occurs by rotation about an axisin a plane of motion. The three planes of motion in thefoot are de¢ned as: sagittal plane (Sp), frontal plane (Fp)and transverse plane (Tp).The foot, or any part of the foot, is de¢ned as being

adducted when its distal aspect is angulated towardsthe midline of the body in theTp and deviated from theSp passing through the proximal aspect of the foot, orother speci¢ed anatomical reference point. Abductioniswhen the distal aspect is angulated away from themid-line (Fig. 3(A)).The foot is de¢ned as being plantar£exedwhen the distal aspect is angulated downwards in the Spaway from the tibia, and dorsi£exed when the distal as-pect is angulated towards the tibia in the Sp (Fig. 3(B)).The foot is described as being inverted when it is tiltedin the Fp, such that its plantar surface faces towards themidline of the body and away from theTp, and evertedwhen its plantar surface faces away from the midline ofthe body and away from theTp (Fig. 3(C)). The foot isconsidered to be supinatedwhen it is simultaneously ad-ducted, inverted and plantar£exed, and pronated whenit is abducted, everted and dorsi£exed (Fig. 3(D)).With the exception of the midtarsal, MTP and IP

joints, the three remaining major joints move in onlyone plane, i.e. one degree of freedom.The former threejoints have two degrees of freedom of motion occurringindependently of one another (adductionâbduction/dorsi£exion^plantar£exion).The ankle joint is the articulation between the distal

part of the tibia and the body of the talus, permittingdorsi£exion andplantar£exion of the foot around its axisof motion which passes obliquely in a lateral^plantar^posterior, to medial^dorsalânterior direction (Fig. 2(B)).The minimum range of ankle joint motion as necessaryfor normal locomotion is 101 of dorsi£exion and 201 of

Figure 2 Foot structure: (A) four segments: hindfoot (1, 2), midfoot (3^7), forefoot (8^12), phalanges (13^26), (B) ankle and subtalaraxes. 1Fcalacaneum, 8F¢rst metatarsal, 13F17Fproximal phalanges, 2Ftalus, 9Fsecond metatarsal, 18Fdistal phalange,3Fnavicular,10Fthirdmetatarsal,19^22Fmiddle phalanges and 4Fmedial cuneiform,11Ffourthmetatarsal, 23^26Fdistal pha-langes. 5Fintermediate cuneiform,12F¢fthmetatarsal,6Flateral cuneiform,7Fcuboid.

168 CURRENT ORTHOPAEDICS

Figure 3 (A) adductionâbduction, (B) plantar£exion^dorsi£exion, (C) inversionêversion and (D) supinationFpronation.


plantar£exion.The ankle joint also has slight movementin theTp during plantar£exion, causing instability of thejoint in this position.3

The subtalar joint includesboth the talocalcaneal jointand the talocalcaneal part of the talocalcaneonavicularjoint, i.e. it is a composite terminology for the two jointsbeneath the talus. Its axis of motion passes through thesubtalar joint obliquely at approximately 421 from theTp

and 161 from the Sp and resultant motion in Fp3

( Fig. 2(B)); thesemotions occur simultaneously.The nor-mal motions exhibited by this joint are supination andpronation.The talonavicular and the calcaneocuboid joints to-

gether form the midtarsal joint.This joint has two axesof motion, an oblique axis and a longitudinal axis whichare con¢ned to the talonavicular joint and the calcaneo-


cuboid joint, respectively. Each axis allows movement inone plane only, but because it forms angles to the threebody planes, supination/pronation of the forefoot re-sults. The interfaces between the posterior aspect ofthe metatarsal bones and the lesser tarsus produce thetarsometatarsal jointswhich have a very limitedrange ofgliding action.The exception to this is the joint betweenthe ¢rst metatarsal bone and the medial cuneiformwhere considerable movement is possible. At the MTPjoints, the roundedheads of themetatarsal bones are lo-cated in the shallow cavities of the phalanges.Up to 901of extension is possible at these joints, but only a few de-grees of £exion. All of the IP joints allow extension,which is related to abduction, and £exion, which is re-lated to adduction, of the foot.

ARCHESOF THEFOOTThe foot has to both support body weight whilst stand-ing and to act as a lever to propel the body during loco-motion. It must be able to conform to even and unevensurfaces, and thus be capable of making good contactwith almost any supporting surface, forming a rigid plat-form that will not collapse under body weight. This ismade possible by a series of bony longitudinal and trans-verse arches (TAs),maintainedby ligaments andmuscles.Themedial arch (MA) comprises the calcaneus, talus, na-vicular, the three cuneiforms and their three metatar-sals. The pillars of the arch are the tuberosity of thecalcaneus posteriorly and the heads of the medial threemetatarsal bones anteriorly. The lateral arch (LA) con-sists of the calcaneus, the cuboid and the lateral twometatarsal bones.The MA and LA are relatively rigid instanding but become more compliant during walking;the MAbeing themore £exible of the two.A series of TAs exist around theMTP joints, forming a

convex curve in the direction of the dorsumwhen look-ing at the plantar surface of a non-weight bearing foot.This series of TAs disappear and £atten to varying de-grees, during weight bearing.The integrity of the archesis supported by the ligaments, muscles and tendonswhich provide the combined strength, £exibility andmovement necessary for normal function.Their relativeimportance di¡ers in the three arches; whilemuscles areindispensable to the maintenance of the MA, ligamentsare a relativelymore important part in the LA.

MUSCLESOF THEFOOTThe muscles of the foot are essential to maintain theshape of the functional foot.They can be divided into ex-trinsic muscles arising from the lower leg, and intrinsicmuscles arising within the foot itself. These can in turnbe divided into dorsal and plantar groups. During loco-motion, all of the muscles of the lower limb are actively

providing stability and balance during standing, and astrong lever arm e¡ect during propulsion.

BIOMECHANICALINSTRUMENTATIONRecent advances in computer technologyhave furtheredthe understanding of the biomechanical aspects of thehuman musculoskeletal system by measuring the kine-matic and kinetic variables. Kinematics is related to themeasurement of motion irrespective of the forces in-volved using cine/cameras to observe the inter-segmen-tal relationship of the trunk and limbs. Kineticsconcentrates on the study of forces associatedwithmo-tion using force plates, pressure platforms and/or inshoesensorsproviding a directdescription/orientation of footposture.

GAITANALYSISGait analysis is used by researchers and clinicians to de-scribe an individual’s pattern of walking. In modern reha-bilitation, there is an increasing need for objective andquantitative measurement of the relevant aspects ofgait.This should ultimately lead to a better understand-ing of inter-related foot/limb function.Gait analysis alsoinvolves the measurement of muscle activity, and bothkinetic and kinematic elements during gait. Most of theproblems associated with foot disorders are in one wayor another related to the weight-bearing process at thefoot^ground or foot/shoe^ground interface.

Gait cycle (GC)

During normal walking, one full GC is referred to as thetime interval between two consecutive heel strikes ofthe same foot (Fig. 1(A):1^12 and 1(B):RIC^RIC) on theground. This time interval is known as the stride time.Stages of a GC are shown in Fig. 1(A and B) and aremarked for the right side: heel strike (RHS), forefootloading (RFFL), midstance (RMS), heel o¡ (RHO) and toeo¡ (RTO). The GC is divided into two major parts: thestance phase (STP) and the swing phase (SWP), these re-presenting the weight and the non-weight-bearing peri-ods for the foot and on average last for about 60% and40% of the gait cycle, respectively. The double supportperiod, which on average lasts 10% of GC and occurstwice in anyoneGC, indicates thatboth feet are in touchwith the ground.

Ground reaction force

The GRF magnitude, direction, point of application, andthe way in which it is spatially distributed over the plan-tar surface of the foot during gait is of great relevance to


both the assessment and any subsequent treatment planfor the lower limbs. The GRF is counteracted and con-trolled by the function of the lower limb muscles which,in conjunction with the bones, joints and tendons of thefoot, controls the kinetic and kinematic progression offoot with the ground.

The foot in gait

During locomotion, lower limb muscles act in concertwith each other as either synergists or antagonists andhave primarily two functions: to stabilize, and acceler-ate/decelerate the foot during both the weight-bearingand propulsion phases of gait. Some muscles have morethan one function. For example, the long extensors ¢rststabilize the joints of the toes during propulsion, thenserve as accelerators in ankle jointdorsi£exion followingtoe o¡, and ¢nally assist as decelerators of the foot atHS.To give an example, the tibialis anterior deceleratesthe foot following HS, and then accelerates to assist inankle joint dorsi£exion followingTO. Consequently, thedi¡erent phases of gait are therefore described in rela-tionship to lower limbmuscle activity.

Heel strike

At HS, and for a very short period immediately there-after known as the transient period, the GRF is anteriorto both the ankle andknee joint and lastsbetween10 and20ms4 (Fig. 1(A:1)); this can be measured using a forceplate and linked video camera array, e.g. the Vicon Sys-tem.5 The location of the GRF then changes immediatelyafter HS to a location posterior to both joints(Fig.1(A:2)), creating an external plantar£exionmomentaround the ankle joint. At this instant, the tibialis ante-rior, with assistance from the extensor hallucis longusmuscle, contracts eccentrically, producing an internaldorsi£exionmomentwhich decelerates the rate of anklejoint plantar£exion.6^8 The combined synergistic actionof these two muscle groups allows the foot to passivelyplantar£ex in a smooth, regulatedmanner such that an-kle joint plantar£exion is virtually stopped synchro-nously with the forefoot making contact with theground. This control e¡ect avoids the sudden ‘slapping’of the forefoot on the ground, which can, in some cases,produce considerable forefoot trauma.3 The tibialisanteriormuscle has been a favoured object of study overthe years and the use of cinematography9 has demon-strated that the subtalar joint is inverted and the ankleis dorsi£exed at themoment of HS and the leg is intern-ally rotated. More recently, the tibialis anterior musclewas demonstrated to be subject to a statistically signi¢-cant delay of180ms (Po0.001) in diabetic subjects whencompared to a normal control group.10 This late ¢ringmeans that the normal modulating role of the muscle in

lowering the foot to the ground after HS is disturbed,leading to forefoot slap and subjecting it to high plantarpressure. If the HS can be delayed and foot slap pre-vented, a restoration to the normal foot^ground ap-proach can be achieved and would be compellingevidence of the true aetiology of high forefoot plantarpressures in diabetic subjects.This would also provide asimple and e¡ective preventivemethod of reducingmor-bidity in diabetic patients.

Forefoot contact andMS

During the initialmovement of the forefoot only the lat-eral side of the forefootmakes contact with the ground.Asweight is transferred to the forefoot, the e¡ectof theGRF on the lateral side of the forefoot tends to evert theforefoot against the resistance caused by the contrac-tion of tibialis anterior.The controlled relaxation of thetibialis anterior facilitates smooth progressive loading ofthe forefoot from lateral to medial locations. This isachievedbygradually decreasing the resistance to prona-tion of the forefoot around the longitudinal axis of themidtarsal joint. The tibialis anterior then relaxes andgoes silent until the TO phase of gait. Therefore, theGRF gradually everts the forefoot until full foot contactis achieved; loading of the forefoot is then transferredfrom the lateral tomedial side.7 FollowingHS and beforeforefoot contact, the calf muscles, i.e. the tibialis poster-ior, with soleus and gastrocnemius, begin to contract.Thesemuscles, which function to collectively deceleratesubtalar joint pronation and internal leg rotation, con-tinue to contract throughout the MS phase and relax atHO or very shortly thereafter6,8,9 (see Fig. 4). Duringforefoot loading, the GRF maintains its posterior direc-tion (Fig.1(A:3^4)), butwith an increasingmagnitudeun-til MS, where it begins tomove anteriorly along the foot.This causes theGRF to become smaller and to ultimatelyreverse in direction (Fig. 1(A:5^7)). At this point, theGRF is still posterior to theknee jointbutbecomes ante-rior to the ankle joint (Fig. 1(A:5)). At the beginning ofthe MS period, the posterior calf muscles become primemovers which initiate subtalar joint supination and ex-ternal leg rotation.Both the tibialis posterior and the so-leus have attachments which create signi¢cant leverarms relative to the axis of motion of the subtalar joint.The forward momentum of the tibia over the foot

which is ¢xed on the ground produces ankle joint dorsi-£exion. After forefoot contact, the ankle is plantar-£exed, and following this, the ankle joint begins todorsi£ex as the tibia moves forwards, over the foot.The tibia continues to move forward, causing ankle jointdorsi£exion throughout the MS period until the point ofHO. Deceleration of forward tibial momentum also ex-tends theknee in preparation forHO.Themuscles whichdecelerate the forwardmomentum of the tibia to assistin knee extension are those which have signi¢cant lever

Figure 4 Timing of sixmuscles activityduringwalking.GC=gastrocnemius,SL=soleus,PT=tibialis posterior,PL=peroneus longus,PB=peroneusbrevis and AT=tibialis anterior.

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arms for ankle joint plantar£exion. These muscles are:tibialis posterior, soleus, and £exor digitorum longus,with also some late MS assistance fromperoneus longus.Gastrocnemius is also an ankle joint plantar£exor; how-ever its femoral origin provides it with an antagonisticfunction relative to the other calf muscles.The bones of the lesser tarsus are stabilized

during the MS period as follows: in early MS, soleus,tibialis posterior, peroneus longus, and peroneus brevismuscles are responsible for this stabilizing e¡ect.The lat-eral side of the foot must be stabilized against theground before the individual bones of the lesser tarsuscan be stabilized in theTp. The soleus maintains the Spstability of the cuboid in order to serve as an e¡ectivepulley for the function of peroneus longus. Peroneuslongus pulls the ¢rst ray laterally against the lesser tar-sus, whilst the tibialis posterior pulls the lesser tarsalbonesmedially.Collectively, the e¡ects of these opposing actions

serve to compress the lesser tarsus in theTp, thusprodu-cingmedio-lateral stability.The magnitude of the adduc-tion force generated by the tibialis posterior exceedsthat of the abduction force exerted by the peroneuslongus. Consequent to this, peroneus brevis exerts anadditional abduction force upon the lesser tarsus tothereby neutralize the otherwise existing imbalance.Therefore, peroneus brevis complements the functionof peroneus longus to help in maintaining the transversestability of the lesser tarsus during the stance phase.Theforefoot cannot abduct at themidtarsal jointbeyond thenormal lockingpointof that joint,which is reachedwhenit is fully pronated. Further abduction of the forefoot isfacilitatedbypronation of the subtalar joint. In disorders,such as cerebal palsy, the entire foot may be pronatedinto a £at foot position; this is described clinically as theperoneal spastic £atfoot.11

Heel-o¡ and propulsion phase

At HO, also known as heel rise, the GRF is anterior inrelation to both ankle and knee joints creating an exter-nal dorsi£exormomentwhich is opposedbyboth the so-leus andgastrocnemiusmuscles (Fig.1(A:6^8)).Thepeakactivity of these muscles coincides with the peak exter-nal moment occurring just after HO. The metatarsalsmust become stable before they can assume theirweight-bearing function during propulsion. FollowingHO, the footmomentarily bears full body weight alone.Simultaneously, the vertical GRF generated exceeds thatof bodyweight and, moreover, the ball of the foot is alsosubjected to torque and high shear forces. The majorfunction of the intrinsic muscles during the second halfof theMSphase is devoted to providing the tensile forcesnecessary in stabilizing the bones of themetatarsus andlesser tarsus transversely and posteriorly against oneanother.When the foot is abnormally pronated the in-

trinsic musclesmust function longer and stronger duringthe MS period. Tension forces are increased when thefoot is abnormally pronated, and this tension increaseswhen the joints of the skeleton are also subluxed.There-fore, economy of intrinsic muscle function, during theMS and propulsive periods is dependent upon supinationof the foot.12

Heelriseresults froman interactionbetween forwardmomentum of the body, deceleration of the tibia, andpassive knee £exion. At the end ofMS, the trunk is direc-ted forwards relative to the foot, and the body is fallingforwards over the weight-bearing foot. Forward trunkmomentum carries the thigh and leg with it. The kneeextends during MS, but immediately prior to heel risethe knee begins to £ex. Thereafter, the tibia continuesto move forward while the heel rises, as evidenced bymaintaining ankle dorsi£exion into the early propulsionperiod.The muscles which are primarily responsible forthe tibial deceleration are tibialis posterior, gastrocne-mius and soleus with later assistance from the £exor di-gitorum longus and peroneus longus.3 Just beforecommencement of heel rise, contraction of the gastro-cnemius muscle halts knee extension and then begins to£ex it.During the propulsion phase, the ankle initially dorsi-

£exes slightly, and then plantar£exes until TO.Shortly after heel rise, ankle dorsi£exion is stoppedconsequent to the weight no longer passing throughthe heel. At this time, the calf muscles, which previouslydecelerated the forward momentum of the tibia andrate of ankle dorsi£exion, now begin to plantar£ex theankle joint. During the early stage of the propulsionphase, gastrocnemius, soleus, peroneus longus andpossibly tibialis posterior all contribute to ankle jointplantar£exion. During propulsion, only one muscle, thetransverse pedis, more properly called the transversehead of adductor hallucis, appears to have the neces-sary requirements for the transverse stabilization ofthe forefoot.Shortly after heel rise, the lateral side of the foot lifts

from the ground.Weight is transferred to themedial sideof the forefoot where it normally concentrates on thegreat toe during the ¢nal stages of propulsion.7 The twomuscleswhich primarily involved in this propulsive eventare: peroneus longus and brevis (Fig. 5).During normal propulsion, the lesser toes are stabi-

lized against the ground, and each toe is extended ateach of the IP joints. The long and short plantar£exorsof the lesser digits stabilize the toes against the ground,but they cannotdo so unless the toes are ¢rst convertedinto rigid beamsby the extensormechanism.The follow-ing factors in£uence propulsion phase stability of thegreat toe: stability andplantar£exion of the ¢rstray, nor-mal sesamoid function, normal strength and function ofthe muscles responsible for hallux, and ¢rst MTP jointstability.


Reduced e⁄ciency of any of the preceding factors re-duces the e¡ective potential for the hallux to bearweight during propulsion.The hallux must be completelystable before it can bear its share of the load during pro-pulsion.When it is not stable, either part or all of theload it normally supportsmust be supported by the sec-ond or thirdmetatarsal heads.The hallux cannot be sta-bilized adequately unless the ¢rst ray is also stable andthe sesamoids are normally positioned under the ¢rstmetatarsal heads.The hallux must also be able to dorsi-£ex 65^701 on the ¢rstmetatarsal,13 during ¢nal propul-sion, or it will sublux at the ¢rst MTP joint and becomeunstable. This full dorsi£exion range of the hallux canonly be achieved when the ¢rst ray plantar£exes duringpropulsion.

Toe-o¡ and swing phase

Just prior to TO, the GRF is still anterior to the anklejoint but has moved posterior to the knee joint.The ti-bialis anterior begins to contract immediately beforeTOuntilmidswing, thus creating a rather smaller dorsi£exormoment around the ankle joint which helps in clearingthe foot o¡ the ground (Fig.1(A:9)).The foot then entersthe SWP and the GRF disappears until the following HS(Fig.1(A:12)).

Gait analysis instrumentation

Gait analysis laboratories use re£ective markersand/or high-technology equipment such as powerfullow/high frequency cameras along with compatible soft-ware for the analysis and presentation of motion (e.g.Vicon System5). They have not been proven to be coste¡ective and have been the topic of debate in recent na-tional and international conferences.14,15 Additionally, thetype of analysis is dependent on the type of equipmentavailable, the expertise of the user and the space avail-able (130m2 to accommodate the cameras in a formatto produce three-dimensional gait analysis is required).On average, upto 112 h is needed per subject to recordthe gait data.In the last decade or so, a new portable gait

analysis system (e.g. GaitRite16) appeared on themarket providing the user with an automated meansof measuring the spatial and temporal parametersof gait. These systems complement the formergeneration and are portable, relatively cheap, easyto use and provide an almost complete analysis instanta-neously.They are very useful when dealing with patientswith ‘shu¥ing’ gait and low assessment tolerance, forexample those su¡ering from Parkinson’s disease.The primary disadvantage is however their in-competence in providing inter-segmental relationship

which is sometimes crucial when assessing patients withcerebral palsy.

Pressure analysis systems

As shown in Fig. 1(A), the GRF changes in direction andmagnitude as the body propels itself forward.This forceis proportional to in¢nite discrete areas on the plantarsurface of the foot when in contact with the groundand is described as foot pressure.Gait and pressure sys-tems have often been thought to be the same but this isnot the case. In the latter case, the body weight is dis-tributed over in¢nite discrete points on the plantar sur-face of the foot, and indicates gait cycle (mainly theinshoe systems), pressure points, gait pattern, foot printandposture, and centre ofpressure.The formerprovidesinter-segmental relationships between the various partsof the body (foot, lower leg, thigh, spine, etc.) but givesno indication of pressure distribution over the plantarsurface of the foot.Over the last two centuries, many attempts have

been made to develop a suitable technique to measurethe pressure distribution underneath the plantar surfaceof the foot.The range of techniques and equipment cur-rently available vary from the cheap and simple (e.g. Har-ris&Beathmat and the Podotrak) to extremelycomplexand expensive devices.Most are capable of only measur-ing thevertical pressure despite ongoing work invariouscentres to develop the ultimate shear plus vertical sen-sor.17 The choice of a system should depend heavilyon the precise measurements needed, bearing in mindthe ¢ve most important characteristics when choosinga system: range, reliability, reproducibility, frequencyresponse and resolution, not to mention being user-friendly.As an add-on to gait and pressure measurement de-

vices in understanding the biomechanics of the foot/an-kle, the following devices can also be considered ofimportance: portable electromyography,8^10 goniome-try,18 and energy expenditure.

FOOT/ANKLEPROPRIOCEPTIONFoot/ankle proprioception is the awareness of the posi-tion andmotion of the foot joints in spacewith the abilityto accuratelymatch reference joint angles without visualfeedback. Any excess or limitation of motion in the sub-talar complex will tend to have implications proximally,for the lower limb, and distally for the foot.18 Ankle inju-ries are among the most common injuries in physicallyactive people and vary from mild ligamentous sprain tosevere disruption of the articular surfaceswith extensivesoft tissue damage.Of all the time lost through injury inrunning andjumping sports, 20^25% involves theankle.19

In considering ruptures of the ¢bular collateral ligament,


it was suggested that functional instability was due tomechanical instability, peroneal weakness, tibio-¢bularstrain and proprioceptive de¢cit. Injuries occur when aforce, applied to the foot, is transmitted through the ta-lus displacing it beyond the normal elasticity of the ankleligaments. If the force is axial in nature, fractures of thetibial plafond occur (the so-called pilon fracture) withvariable degrees of compression. If the force involves lessaxial load butmore rotationalmovement of the talus onthe tibia, malleolar fractures occur. Hence, traumaticdisorders of the ankle occur as the result of one, or acombination of two, of the following foot movements:internal rotation, external rotation, abduction and ad-duction.20

Various extrinsic and intrinsic factors have been con-sidered to be potential risks for ankle injuries. Extrinsic

Figure 5 E¡ectof highheel shoes on feet (‘+’sign indicates activ

factors include training errors, type of activity (e.g.sport), exercise time, equipment andenvironmental con-ditions. Among the intrinsic factors, it was found thatheight, weight and previous history of ankle sprain had asigni¢cant relationshipwith the incidence of lateral anklesprain.21 In addition, individuals withmuscle strength im-balance in the form of elevated eversion-to-inversionstrength ratio and greater plantar£exion strength had ahigher incidence of inversion ankle sprain.22 Recently, adirect method for quantitative measurement of ankleproprioception using a three-dimensional electromag-netic goniometer system, the Isotraks, with a view toassessing the role of proprioceptive de¢cits and variousrisk factors in the aetiology of ankle injuries was devel-oped.18 Measurements were based on the ability of sub-jects to accurately match reference joint angles without

emuscle function during standing).


visual feedback.Further work, yet unpublished, was car-ried out showing that footwear alters normal volunteersfoot/ankle proprioception and this was also demon-strated in patients who had su¡ered previous ankle inju-ries. Ankle injuries are fewer in unshod populationssuggesting shoes may alter the ability of the protectivemechanisms around the ankle inpreventingdamage. Thismaybe due to increased leverage at the ankle con¢rmedby the unnaturally high heel and reduction of foot/ground sensation. Equally, it may be that a foot held in a¢rmheel countermightmodifyproprioceptive sensationaround the ankle/heel complex. As foot/ankle injuriesoccur mainly under dynamic activities, current work isnow being focussed on measuring ankle proprioceptionduring walking. The deceptive advertising of expensiveathletic shoes to safeguard feet through ‘cushioning im-pact’ which accounted for123% greater injury frequencyof the lower extremity than the cheapest shoes washighlighted in a recent article.23

CASEREPORTSIn addition to clinical and biomechanical assessments,knowledge of functional orthoses and footwear me-chanics is important to provide a comprehensive treat-ment modality, not to mention that common senseshould prevail. Since its inception in 1993, most patientsseen at the Foot Pressure Analysis Clinic (FPAC) in Dun-dee, regardless of howminor or complex their problemwas,wereusing ill-¢tting footwearwithdiscrepancies in-shoe width and size when compared to their feet. Insome cases, there was a di¡erence of up to 3 UK sizes

Figure 6 Case A: (A) A/P weight-bearing X-ray, (B) Med/Latwe

and 4 cm inwidth across themetatarsal head area, need-less to say causing abnormal biomechanical forcesthrough the foot joints. The cumulative damage causedby footwear over theyears goes inmost casesunnoticedandgets ignoreddespite clear signs of pain anddorsal cal-lus formation, the latter can only develop as a result offriction with the inner shoe.The damage becomes mul-ti-compound when using high-heel shoes causing an in-stant forward shift of the centre of mass, resulting inincreasedpressure under themetatarsal heads and toes,abnormal joint andmuscles function within the foot andlower leg and ultimately balance, proprioception andbody posture (Fig. 5). Below are three cases recentlytreated at the FPAC.

Case A. A patient su¡ered fractures of the proximalphalanges of the second and third toes of the right footand thirdmetatarsal shaft.On reviewing at one year, thefractures were well healed (Fig. 6(A and B)) but left thepatient with forefoot pain on the plantar and dorsal as-pect of the thirdmetatarsal head during walking.As the pain was present both bare and shod, this ex-

cluded any e¡ect the shoes might have had in causingpain. Assessment was focussed on the foot biomecha-nics.While theweight-bearing X-rays did not in the ¢rstinstance show any clinical abnormality, foot pressure as-sessment (Fig. 6(C)) showed that there was relativelyvery low pressure underneath the thirdmetatarsal head(circle 5) when compared to the adjacent second andfourth leading to the conclusion that the third shafthealed in an abnormally dorsi£exed position (Fig. 6(B)red arrow) which caused a neuroma and hence the pain.The neuroma was excised and the right foot was sup-ported with a custom-made insole to compensate for

ight-bearing X-ray and (C) dynamic footpressure print.


the abnormal pressure distribution and abnormal thirdmetatarsal shaft.The patient is currently seen for insolereplacement, otherwise discharged fromregular clinical/biomechanical review.

Case B. A female patient with metatarsalgia who hadpreviously been treatedwith ¢ve di¡erent sets of insoles(Fig. 7(A)) was randomly selected from theOrthoticDe-partment database. She agreed to participate in a retro-spective foot pressure measurement study. She hopedthat themeasurements would improve themanagementof pain under the ball of her feet as the insoles she hadreceived over the years had aggravated rather than re-lieved her symptoms. Her symptoms started 20 yearsago shortly after bilateral hallux fusion surgery which in-evitably shortened the ¢rst rays (Fig. 7(B)). Despite thepain felt under the ball of her feet, there were no signsof callus formation. This coincided with the relativelylow dynamic pressure values recorded with the OpticalDynamic Pedobarograph. However, when compared tothehallux and ¢rstmetatarsal headpressurevalues, footpressure analysis showed higher pressure values underthe lesser toes and their respective metatarsal head on

Figure 7 Case B: (A) insoles provided, (B) patient’s feet and (C) p

both feet (Fig. 7(C)).This is a clear indication that atpusho¡ the forefoot is supinating as a compensatorymeasureto the shortening of the ¢rst rays.The insoles previously provided incorporated a bar or

domewhich aggravated the pain. Shewas treated symp-tomatically rather than for the primary causewhichwasincreased pressure under the lesser toes.The dome/but-ton elevated the second, third and fourth metatarsalshaftswhich further extended the lesser toes and in turnincreased the pressure under them, hence the failure inthe past orthotic management.The insole should incorporate internal transverse

and longitudinal rockers with cushioning under thelesser toes. The transverse rocker facilitates thetransfer of load from the lateral to medial aspects of thefoot whilst the longitudinal rocker compensates forthe ¢rst ray shortening and assists at the push o¡ stage(Fig. 8).

Case C. A patient aged 52 years was referred for bio-mechanical and foot pressure assessment in late 2000with ‘di¡use midfoot pain a¡ecting the plantar aspect ofhis rightmedial arch’ (Fig. 9(A)).He had had tarsal tunnel

atient’s dynamic pressure prints andpressure^time graphs.

Figure 8 Schematicof a composite insole incorporatinganin-ternal transverse and longitudinal rockers.Greycolour indicatesthe semi-£exible carbon ¢brebase andthe shadedpinkareare-presents the transverse rockerconsistingof soft (gelly) material.

Figure 9 Case C: (A) area of pain, (B) pressure^time graphand (C) dynamic pressure print.


surgery exploration in 1999 which failed to improve hissymptomswhich started in1992.Following biomechanical assessment and foot pres-

sure studies, the latter showingno signs of abnormalities,the problem was localized around the midpoint of the£exor hallucis longus (FHL) tendon, which upon pressurecaused severe pain shooting up the leg through the FHLtendon/muscle. In addition, it was interesting to notethat he experienced minimal or no pain when walkingbarefoot or when using his working boots which incor-porated a dorsal metal cup and a plantar steel bar, the

latter providing an external rocker mechanism. His painwas only experienced when wearing formal leathershoes size 10 UK compared to his foot sizes of 912 (left)and 1012 (right). The shoes being smaller than his rightfoot caused abnormal longitudinal forces along the ¢rstray in addition to the irritation caused by the built-inarch.As a temporary solution, the built-in arch was re-

moved and a semi-£exible carbon ¢bre insole insertedin his current shoes to provide himwith an internal rock-er to overcome the action of the FHLuntil the 8 years ofaccumulated in£ammation had eased o¡.Hewas advisedto purchase a sturdypair of shoesmatchinghis foot sizes,preferably with a rubber sole, and to transfer the carboninsole over.


Four days later, an e-mail was received from the pa-tient saying ‘after my visit to your clinic, I immediatelybought a pair of boots and transferred the temporaryinsole. The discomfort has drastically reduced and thepain has subsided. . ..’If the question ‘do you have pain when walking bare-

foot?’ had been asked over the eight years period of in-vestigation, it could have saved the patient pain and anunnecessary surgery, and the health servicemoney!

CONCLUSIONANDPRACTICEPOINTS

It is very important to complement the clinical assess-ment by a biomechanical one:

K Clinical assessmentK Biomechanical assessmentK Appropriate treatmentK Avoid TRIAL & ERRORmanagementK Always try to ¢nd the primary cause of theproblem

K Never treat the secondary symptoms withoutknowing the primary cause

K Try not to ignore any detail, no matter howirrelevant youmight think

ACKNOWLEDGEMENTSThe author wishes to thank Mr Ian Christie for theillustrations and Mrs Sheila Gibbs and DrTim Drew forediting thismanuscript.

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Documents

MINI-SYMPOSIUM: THE ELECTIVE FOOT: (i) Relevant foot ... · tarsometatarsal,metatarsophalangeal(MTP)andinter-phalangeal(IP)joints)of thefootmakeupitsfour segments:thehindfoot,themidfoot,theforefootand