CLINICAL TECHNIQUE

General Biomechanics: The HorseAs a Biological MachineJeff Moore

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ABSTRACT

The ‘‘Mechanical Mode’’ of a horse refers to the condi-tion of the musculature appropriate to the moment ortask—stretched, contracted, or slack. It can be becauseof posture or carriage, rigidity, as a result of defensive-ness, sustained contraction of musculature beyond theneed of the task at hand, general condition of the muscu-lature from moment to moment, and much more. Oneway to demonstrate this is to address the concept of sup-pleness. The mechanics of gait can be divided into purity,quality, and way of going. The mechanics of gait shouldbe well understood by the veterinarian so that when eval-uating the horse with the complaint of lameness they arebest able to integrate and differentiate the true lamenessfrom a mechanical, riding, or performance problem.

Keywords: Horse; Gait; Mechanics of gait; Lameness;Performance

INTRODUCTIONFor practical purposes, it is important to think of supple-ness as genetically determined or the innate range of mo-tion. Suppleness is determined by the configuration ofthe joints, the length of the tendons and ligaments, andthe contraction or relaxation of the muscles. Riders tendto deal with this in terms of ‘‘how the horse feels.’’ Whenthe horse does not perform optimally the rider thinks ofa problem of suppleness first; this is frequently incorrectand more often it is an issue related to mechanical mode.To demonstrate this idea, stand on the ground and touchyour toes. Left alone, you will reach whatever point yourbody allows in the optimum (natural) position. We lookat how far the person can reach toward the toes.

Now, arch or kink your back (lumbar spine) and arch orkink your neck (cervical spine) and try again. The differenceis dramatic. Usually a person is nearly paralyzed, and verylimited in the ability to bend. The potential range of motion(suppleness) does not change in 30 seconds, but the ability

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ournal of Equine Veterinary Science � Vol 30, No 7 (2010)

to demonstrate the potential suppleness is decimated. Thisis what happens to horses when they operate in a less-than-optimal mechanical mode or frame, carriage, and degree ofrelaxation. The rider is often at fault, may not realize it, andmay call the veterinarian to deal with a performance issue.

MATERIALS AND METHODSThe back acts as a bridge between the supports in front andthe ‘‘engine’’ behind—the hindquarters. From the stand-point of the rider, the back extends from the withers tothe point of the croup. The rib cage limits flexibility ofthe back. However, the back can still be flexed laterallyand longitudinally, and rotated to some degree. This ispart of what creates ‘‘bend,’’ ‘‘suppleness of the back,’’‘‘rising of the back,’’ and ‘‘carriage.’’

The head and neck constitute the major balancing mech-anism of the horse. These are the most defended parts ofthe horse when ridden or lunged and the most susceptibleto influence and displacement from external forces.

The neck is capable of considerable longitudinal flexion,lateral flexion, and rotation. This is part of what creates‘‘elevation’’ and ‘‘bend.’’ For example, in dressage, theshape/carriage of the neck, the physical development ofthe musculature of the neck, and the stretch/contractionrelationships are major considerations in judging whetherthe horse is moving ‘‘correctly’’ and fluently.

The legs act as static and active props. They store andrelease energy actively through the contraction of themuscles, or store and release energy passively throughstretch and recoil of the tendons. Both forelegs and hindlegs can adduct, abduct, and push against the groundduring the stance phase and swing phase.

The hind legs also load the forelegs, depending on thecurrent requirement at that moment in time. For efficiencyof speed, the hind legs push backward to propel the horseforward in a way that causes it to ‘‘roll over the forelegs.’’For a riding horse, we want the horse to use the pressureof the foreleg against the ground to push the body up or‘‘loading the hind legs.’’ This reduces efficiency for speed,and increases efficiency for nimbleness.

The tail is a ‘‘minor’’ balancing mechanism that is mobilein all directions. It is often considered, in dressage, anindicator of the muscular condition, and ‘‘activity’’ orswinging (alternating contraction and stretching) of theback. A relaxed horse, in the correct mechanical mode,

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should have the tail arched up and away from the body, withthe middle and end of the tail swinging. Tail carriage andswinging can be breed-specific; for example, someAppaloosas carry the tail curled upward or stick the tailstraight out, whereas Iberian horses and their relatives,the Pasos and Fresians, sometimes have a ‘‘dead’’ tail hang-ing straight down, even when the top line musculature isrelatively stretched, relaxed, and lively. An inactive,clamped, or stuck-out tail is often considered an indicatorof sustained contraction in the long back muscles, of defen-siveness, or of discomfort, but this may have to be consid-ered in terms of the breed under consideration, whetherthe condition changes over time, is different from that ofthe horse’s norm, or is different when being ridden orwhen shown ‘‘free.’’ If this changes for the worse, at somepoint in time, it can support the possibility of an injury,training problems (including riding style and the use of‘‘gadgets’’ such as running reins, tight tie-downs or sidereins, running W, etc.), discomfort, anxiety, or surgery tokeep the tail still.

The mechanics of the gait are best approached by think-ing of gait in terms of (1) purity; (2) quality; and (3) wayof going. The regularity of the horse’s gait is characterizedby the regularly repeated sequence and timing of footfallsand phases. In dressage, we address only the walk, trot,and canter. In other disciplines and in some breeds, thereare other gaits—pace, amble, foxtrot, running walk, tolte,rack, to name just a few. Some of these are natural to thebreed or individual, whereas some are artificially producedduring training. It may be of some interest to note that inplaces where horses are left to their own devices to repro-duce (as in Latin America) without much human interven-tion or selection, they frequently tend to revert to a certaintype or conformation, and to a certain type of movement.They also tend to consistently amble or nearly pace, ratherthan demonstrating a clear walk and trot. This is lessapparent in the feral horses in the US Southwest, theChincoteague ponies, and Prezwalski’s Horse.

The ‘‘3 basic gaits’’ of most breeds and disciplines, that is,walk, trot, and canter, have been presented using dressageas a basis for comparison in terms of purity, correctness,and quality.

PurityThe correct sequence and timing of footfalls and phases.

Sometimes apparent soundness issues are actually prob-lems of balance or mechanical problems such as tension,sustained co-contraction, or anxiety. These may be createdby riding and training techniques; the mysterious sound-ness issues sometimes fit in here. In dressage we refer to‘‘Reins lame,’’ a gait irregularity caused by mechanicalmode such as constraint or some other action by the riderthat makes the horse irregular by interference, even thoughit is sound in the veterinary medical sense.

CorrectnessThis is widely used to address purity, but is more appropri-ately applied to lateral deviations within the stride—wingingin or out, ‘‘rope-walking,’’ spraddling backward, etc.

QualityThe qualities that are valued vary by breed or discipline.Dressage values include reach and roundness of the trajec-tory of the limbs (amplitude), and springiness or elasticity.The desired qualities vary among different breeds, such asin Paso Finos (smooth, flat, and quick), Hackneys (highknee action, tight back muscles, and speed), draft horses(mainly intended to work at walk), and jumpers (speed,power, nimbleness, and jumping ability—gait quality isnot judged). ‘‘Quality’’ is determined by genetics, andcan be significantly affected by training, for better or worse.Dressage horses are often selected for extravagance ofmovement when viewed in profile, sometimes at theexpense of lateral mobility and nimbleness, and are there-fore highly susceptible to disruptive lateral influence andcertain kinds of injuries.

WalkWalk is distinguished by having four footfalls per stride,and eight periods of stance phase (number of feet on theground at any given time, – 2–3, 2–3, etc.). It is unique inthat it always has two feet on the ground. Walk is also distin-guished by the oscillation of the head and neck, whichimproves the reach of the forelegs, but within the limitationsof the horse’s natural capabilities. This forward and down-ward oscillation of the head and neck is easily lost when thehorse is distracted or constrained. ‘‘Quality’’ in the walk isan issue of elasticity, amplitude of the limb movements,carriage, and use of the neck. The ground is covered by thereach of the limbs, not by suspension. The most commonfault of ‘‘purity’’ in the walk is an unequal timing—leading,in an extreme version, to a pace-like gait or amble, eventhough the sequence of the footfalls remains correct.

TrotTrot is distinguished as having two footfalls (diagonal pairs)per stride, and four phases—two stance phases and twophases of suspension or number of feet on the ground (2–0, 2–0, etc.). The most common fault of ‘‘Purity’’ in trotis when the diagonal pair of hooves does not strike and/or leave the ground simultaneously. This is referred to as‘‘Dissociation’’ or ‘‘Diagonal Advanced Placement.’’ How-ever, it is not very common. ‘‘Quality’’ in the trot (for a dres-sage horse) is determined mainly by amplitude of the limbmovements, elasticity/springiness, and marked phase ofsuspension. The ground is covered by the reach of the limbsand during the phases of suspension, although some horsesdo not reach much with the limbs, but rather cover theground mainly during the phase of suspension. Conversely,

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some horses cover more ground by ‘‘sprawling’’—largerange of motion of the nongrounded limbs during a pro-longed stance phase.

CanterCanter is distinguished as having three footfalls, one phaseof suspension per stride, and five periods of stance phase.The number of feet on the ground is 1–3–2–3–1–0, 1–3–2–3–1–0. Racing gallop, a very short, or lazy, or collectedcanter all have different phasing. Canter is unique in beingnaturally asymmetrical. It has a natural rocking action as thehorse rolls over its feet, and by having only one hoof on theground during the stride (twice). The most common faultin canter is negative dissociation (nonsimultaneous land-ing) of the diagonal pair of hooves (the forefoot of the diag-onal lands before its diagonal hind foot). In a collected orslow canter, the phase of suspension may not be apparentto the naked eye, or in some cases may not even exist.

Engagement and balance are affected more by the reachof the outside legs as compared with the inside legs. Incanter, the leg that actively supports the weight most isthe outside hind leg. The inner foreleg also supportsweight, but more as a rollover action rather than as anactive or upthrusting function. The emphasized beat ina balanced canter is the landing of the diagonal pair. Themore the leading foreleg seems to be the emphasizedbeat, the more the horse is ‘‘on the forehand.’’

The mechanics of any gait can be affected significantly bytraining, use of gadgets, and state of mind of the horse.When evaluating a horse under saddle, it is important totake into account the biomechanical effect of the rider(weight displacement, rigidity, or accommodation/har-mony of the rider), interference through the reins suchas constraint versus slackness, or defensiveness on thehorse’s part.

Mechanics of the MovementsMovements may be considered as exercises or tricks. Indressage, some of the movements considered include thefollowing: ‘‘shoulder in,’’ ‘‘haunches in,’’ ‘‘piaffe,’’ ‘‘pas-sage,’’ ‘‘half pass.’’.

This is an area with many choices and includes ‘‘way ofgoing,’’ execution of ‘‘tricks’’ or movements, changes ofspeed, and balance.

Way of going refers to how the horse moves over theground in each gait—stiff, elastic, long, short, with or with-out suspension, lateral and longitudinal balance, carriageor outline, narrowness of the base of support, straightness,bending, verticality over the legs, duration and placementof stance phase, etc. Also included is how the horse turns,changes direction, goes on curved lines, and speeds up andslows down.

Execution of ‘‘tricks’’ or movements may be in the con-text within which the rider perceives a problem. This can be

a difficult evaluation for the veterinarian because it is onlyapparent under saddle, and the veterinarian may not bean expert in how certain movements are optimally executedor perceived, for example, spin in western riding, pirouette,or half-pass in dressage.

Balance is the achievement or maintenance of equilib-rium. It is thought of as equilibrium in stability (stable orstabile balance) and as equilibrium in instability (unstableor labile balance). An example of stable balance is a sumowrestler on hands-and-knees—very hard to dislodge thewrestler in this position because of a low center of massand a broad base of support. In contrast, a balance-beamgymnast is an example of unstable balance—very easy todislodge because of a high center of mass and a narrowbase of support; however, both have a good balance orequilibrium.

A draft horse pulling a huge load needs stable balance(relatively long and wide base of support). In contrast,a dressage horse or jumper needs unstable balance (rela-tively short and narrow base of support)—for nimblenessand influence-ability by the rider.

Mystery ProblemsThey are peculiarities that might cause a rider or trainer tocall the veterinarian other than obvious pathologies, forexample, injuries, swellings, and limping. When the tradi-tional veterinary lameness examination does not identifythe source of the gait abnormality, there may remain issuesin the way of going and influence of the rider.

Some examples of the issues faced include the following:

Crookedness in Canter. This is a natural phenomenon,which occurs because of the unique asymmetry of thegait. It can be exacerbated by the ‘‘corrections’’ of the rider(pulling the neck/nose in, sitting toward the outside, orgeneral constraint).

Irregularity in Lateral Movements. Problems seen onlyin lateral movements are usually rider issues. The horsemust be evaluated under saddle because the problem canbe caused by the rider. The rider may cause unfortunateeffects when trying desperately to achieve a certain ‘‘headposition’’ or ‘‘carriage.’’ Many of the techniques used forachieving ‘‘carriage’’ generate negative mechanical effectsthat work against the mechanics of the gait. Contributingfactors include the following:� Excessive abduction;� Too prolonged a stance phase;� Active constraint by rider; or� Irregular steps in turns and lateral movements—lurching,

often an issue of abduction (too much angle of thelimb laterally away from the body’s midline againstthe ground, causing a lateral reach and falling onto theinside leg).

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Lack of springiness or suspension in trot and canter maybe a balance problem. Too much of the horse’s energy isreleased as speed and not enough is used for suspensionor carriage.

This requires pressure against the ground by the forelegsto push the thorax upward, at which point the thoracic slingmuscles are activated to maintain the thorax between theforelegs. This may also be an important issue of sore feet!

Sore Back. Deep muscle soreness is usually not easily man-aged by extra padding or ‘‘riding lightly’’ in cases where thecause is a problem rooted in suboptimal ‘‘mechanicalmode’’ in which the horse should but does not stretchthe top line musculature by contraction of the ventral mus-culature acting on relatively relaxed back muscles. This isseen clinically as one of the following:� Active depression of the back—sustained contraction of

long back muscles can cause soreness, even without therider on the horse’s back.

� Passive depression of the back (sag)—lack of contractionof opposing (ventral) muscles. The back muscles aremore or less ‘‘relaxed.’’ This is a small problem and iseasier to manage.

� Poor saddle fit can cause either active or passive depres-sion of the back.

� Poor mechanical mode can cause either active or passivedepression of the back.

� Negative influence of the rider causing an irregularity.

One can sometimes alleviate a sore back by sitting‘‘lightly,’’ in cases where improved padding and a new sad-dle have not solved the problem. This entails changing themechanical mode of the horse by stretching the top line sothat the back operates upwards (bow-and-string—contrac-tion of the abdominal musculature acting on the relaxedmuscles of the top line). Usually, the rider’s biomechanicsalso need to be changed; otherwise, the problem may mostlikely reoccur.

With the growth of dressage riding, the additional prob-lem of lameness and other related problems that the veter-inarian has to manage become more evident to theexamining clinician. Some of these problems include the‘‘mystery’’ problem of the sore back—forever ongoing, be-cause of the seemingly ‘‘unfixable’’ nature. In dressage,riders often force the horse into a head and neck carriage(calling it ‘‘on the bit’’), which contracts the top line anddepresses the back. This style of training can cultivate themechanical mode that exacerbates the contraction anddepression of the back. No amount of padding can mitigatethe effects of the training and the inappropriate mechanicalmode.

Irregular Steps in Piaffe or Passage. This may appear asan excessive/increased backward thrust of the posterior

phase of the most posterior hind limb when the horse is‘‘driven’’ or beginning the piaffe/passage.

Irregularity in piaffe and passage is often an issue concern-ing the backward push of the posterior phase of the mostposterior hind limb, rather than a quick lift of the hindlegs (thus, momentarily loading the inside hind leg). Thisis often a rider error or misperception, not a lameness issue.

Causes may include the following:� Driving into a fixed or pulling hand;� Timing of lift, rather than lameness; or� Backward push against the ground of the farthest back

hind leg, rather than momentarily loading the insidehind leg, to allow a quick lift of the leg.

Switching Leads Behind. Some breeds (for example, Ara-bian breed) are more susceptible to this than others How-ever, the problem can be exacerbated by:� The horse that defensively blocks a strong or fixed action

of the inner rein or lunge line.� A rider with a rigid back and forward-pushing seat

against a fixed hand (not accommodating the naturalrocking action of the canter).

� A ‘‘lunger’’ that acts on the line at the wrong moment ofthe stride.

Lateral Deviation of the Haunches. � The biomechanics

of the rider can cause the haunches to swing in or out byblocking the stretch of the musculature of the inner orouter side. � Constraint on the outside rein during turns shortens the

musculature of the outer side (mechanical mode again).

Evaluating horses can obviously be done in several wayssuch as in hand with a halter or bridle, under saddle, on thelunge line, or free in a round pen or arena. The horse’s behav-ior and the trainer’s competence may make a difference,when choosing techniques that are most useful to the veter-inarian; in many cases several approaches need to be tried.

In hand evaluation can be made difficult by a ‘‘frolic-some,’’ noncompliant horse. An inexperienced handlercan also make it difficult.

Few trainers or owners lunge horses well enough to makethis a reliable option for evaluating an obscure lameness orperformance issue. Sometimes the horse is simply ‘‘out ofcontrol’’ on the lunge line. The veterinarian might say‘‘lunge bit-by-bit and quietly,’’ but for the horse who hasbeen locked in a box stall for 10 days, or an inexperiencedowner not trained to this level, can make this a challengingexercise. The evaluation may fail because poor lungingcauses more torque on the legs. Similar to ‘‘get him toeat this medicine’’ lunging is sometimes problematic.

When using a free arena or round pen controlling the gaitanalysis is at best difficult and solely dependant on the

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individual horse and their disposition. It is recommendedto use this venue only when other means are not available.

Evaluating a horse under saddle is greatly affected by thequality of the rider. Trainers often say, ‘‘of course he lurches,you pull outward and downward with the rein, and at thewrong moment of the stride.’’ It may not be withinthe scope of some veterinarian’s comfort zone to criticizethe owner or trainer when the veterinarian needs their sup-port and interest in helping solve the performance problem.

Riding factors that may affect a horse’s movementinclude the following:� Posting—riders may post heavily or rigidly enough to

make a horse with a sagging back appear irregular/lame.� Sitting—riders who sag and wallow, or hold themselves

rigid can make a horse with a sagging back or tightly heldmuscles appear irregular/lame.

� Rein effects—sometimes the rider’s efforts to make thehorse go in a certain style make it difficult for the veter-inarian to make a complete evaluation. Telling a rider ‘‘tostay in two-point position’’ decreases or eliminates thenegative effects on the horse’s back. Having the ridermaintain a loose rein does the same for rider’s potentialnegative effects on the head/neck. Sometimes that is notpossible because of the horse’s behavior, or the precon-ceptions or anxieties of the rider.

� Displacement of the center of mass by an unbalancedrider displaces the shared center of mass of the ‘‘centaur’’and can cause apparent irregularities in what is seen dur-ing the evaluation.

� Previous training issues (including devices) can affect theregularity of performance.

DISCUSSION AND CONCLUSIONThere are few guidelines that have been suggested for theequestrian rider to obtain the best outcome for their ‘‘notright’’ horse and to permit the sport medicine clinician

the best chance of helping the horse with a performanceproblem:� Call the veterinarian—don’t guess.� Tell the veterinarian what you believe to be the problem,

even if it is unclear and hard to explain.� Tell the veterinarian about any ‘‘new’’ issues that have

been noticed but that may or may not be believed tobe directly or indirectly contributing to the problemsuch as carrying the tail in a different way, wincing, anx-iety issues, oinks, panics, shying, ‘‘shutting’’ down, orany unusual behavior or reaction.

� Accept the fact that riders may not be able to do exactlywhat is asked by the veterinarian, such as lunging slowly,or leading in hand.

� Do not try to out-guess the veterinarian, or self-medicate ‘‘on the side’’—it only confuses the issuesand affects the conclusions.

� Ask, but do not advise, the veterinarian about othertreatments that appeal to rider (‘‘How about tryingthis?’’).

� Tell the veterinarian if you have used massage, chiro-practic, acupuncture, or psychic healing and alsowhat the outcomes were after the treatment. Do notexpect the veterinarian to embrace everything; how-ever, they might be interested in some of the sugges-tions offered.

� If there are shoeing concerns, the veterinarian and farrierworking in partnership is the best solution.

The variables and complications of evaluation and com-munication are enormous. Identifying the cause of an ap-parent problem, investigating ‘‘mystery ailments,’’ anddetermining the contribution of the handler and rider tothe problem are all challenges. All these issues can bemade simpler, and may result in better outcomes whenall the ‘‘players’’ understand their role and work responsi-bly as a team for the benefit of the horse.