Normal Human Locomotion

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Normal Human Locomotion, Part 1: Basic Concepts and

Terminology

Ed Ayyappa, MS, CPO

ABSTRACT

Over the past several decades, the evolution of gait science has produced an array of terms and

concepts relating to observational gait analysis. Prosthetists and orthotists use various forms ofgait analysis on a daily basis as an important part of clinical care.

When the basic principles of normal walking are understood, a more penetrating grasp of

pathological gait becomes possible. The result is expanded ability to differentiate between

pathological and compensatory gait deficits. In addition, efficient interaction with the clinicteam demands a sound conceptual knowledge base of human locomotion and related

terminology. This will facilitate an optimal treatment plan for the patient and enhancecommunication and prescription recommendations to the physician and relevant payingagencies. This article is intended to be an introduction to gait science with these goals and

objectives in mind.

Introduction

Nearly a century ago, A.A. Marks, an American prosthetist, offered a precise qualitative

description of normal human locomotion when he illustrated and analyzed the walking process ineight organized phases and discussed the implications of prosthetic design on the function of

amputee gait (see Figure 1). Well ahead of his time, Marks praised "kinetoscopic" photography

as a potential diagnostic tool for the improvement of walking deficits (1).
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Figure 1. Marks described the walking process in eight organized phases and discussed the

relationship between prosthetic design and gait function.

Insight into normal walking patterns can help practitioners improve the efficiency of personswith gait-related pathologies. Such knowledge may assist the clinician in the selection of orthotic

or prosthetic componentry, alignment parameters and identification of other variants that may

enhance performance (2). Familiarity with gait terminology and function enables the prosthetistor orthotist to communicate effectively with other members of the medical team and contributes

to the development of a sound treatment plan.

The terminology of human walking began with descriptive phrases obtained as a result ofobservational and kinematic analysis of normal subjects. This approach yielded terms such as"push off" and "heel strike" (as differentiated from "foot flat"). The limitations of these terms for

clinical use became apparent as practitioners' understanding of normal locomotion increased and

was melded with a careful observation of pathological function. "Push off," for example, is amisleading term because in free-walk velocity during the last period of stance phase (preswing),

the posterior compartment musculature is quiescent. While a differentiated heel strike and foot

flat may describe normal function, these terms are woefully inadequate in describing thecommon clinical picture of an equinus stance phase. Many more contemporary terms describe

events and functions that were not apparent through observation but could be measured through

instrumentation in gait laboratories.

The separate contributions of Saunders et al. (3), Perry (4), Sutherland (5,6) and others haveincreased practitioners' understanding of gait science and terminology. Decades of work by

Jacquelin Perry, MD, have resulted in descriptive terms for the phases and functional tasks of

gait (7). These phases and tasks have received wide acceptance and serve as the descriptivemedium for this article. Contemporary terminology continues to evolve through dialogue within

professional organizations such as the North American Society of Gait and Clinical Movement

Analysis (8) and the American Academy of Orthotists and Prosthetists (AAOP) Gait Society (9).


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This article, presented as an introduction to the AAOP Professional Development Certificate

Program in Gait and Pathomechanics (10), attempts to reflect current contemporary usage of gait

terminology.

Divisions of Gait Cycle

Gait characteristics are influenced by the shape, position and function of neuromuscular andmusculoskeletal structures as well as by the ligamentous and capsular constraints of the joints.

The primary goal is energy efficiency in progression using a stable kinetic chain of joints and

limb segments that work congruently to transport the passenger unit-head, arms and trunk(HAT). The lower extremities and pelvis, which carry the HAT, are referred to as the locomotor

apparatus.

The gait cycle is the period of time between any two identical events in the walking cycle. Any

event could be selected as the onset of the gait cycle because the various events follow eachother continuously and smoothly. Initial contact, however, generally has been selected as the

starting and completing event.

By contrast, the gait stride is the distance from initial contact of one foot to the following initial

contact of the same foot.

Each gait cycle is divided into two periods, stance and swing. Stance is the time when the foot is

in contact with the ground, constituting 62 percent of the gait cycle. Swing denotes the time

when the foot is in the air, constituting the remaining 38 percent of the gait cycle. In those cases

where the foot never leaves the ground, sometimes referred to as foot drag, the swing phasecould be defined as the phase when all portions of the foot are in forward motion.

Double support is the period of time when both feet are in contact with the ground. This occurstwice in the gait cycle-at the beginning and end of stance phase-and also is referred to as initial

and terminal double-limb stance (see Figure 2) . As velocity increases, double-limb support timedecreases. Running constitutes forward movement with no period of double-limb support.
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The third functional task is limb advancement. Four phases contribute to limb advancement:

preswing, initial swing, midswing and terminal swing. During these phases, the stance limb

leaves the ground and advances forward to posture itself in preparation for the next initialcontact. The preswing phase serves in both single-limb support and limb advancement.

Phases of Gait

The gait cycle can be described in the phasic terms of initial contact (IC), loading response (LR),

midstance (MSt), terminal stance (TSt), preswing (PSw), initial swing (ISw), midswing (MSw)

and terminal swing (TSw) (see Figure 3). The stance period consists of the first five phases:initial contact, loading response, midstance, terminal stance and preswing. The swing period

primarily is divided into three phases: initial swing, midswing and terminal swing. Preswing,

however, prepares the limb for swing advancement and in that sense could be considered acomponent of swing phase.

Initial Contact

Initial contact is an instantaneous point in time only and occurs the instant the foot of the leading

lower limb touches the ground. Most of the motor function that occurs during initial contact is in

preparation for the loading response phase that will follow.

Initial contact represents the beginning of the stance phase. Heel strike and heel contact serve as

poor descriptors of this period since there are many circumstances when initial contact is not

made with the heel alone. The term "foot strike" sometimes is used as an alternative descriptor.

Loading Response

The loading response phase occupies about 10 percent of the gait cycle and constitutes the periodof initial double-limb support. During loading response, the foot comes in full contact with the

floor, and body weight is fully transferred onto the stance limb.

The initial double-support stance period occasionally is referred to as initial stance. The term

foot flat (FF) is the point in time when the foot becomes plantar grade. The loading responseperiod probably is best described by the typical quantified values of the vertical force curve. The

ascending initial peak of the vertical force graph reveals the period of loading response(see

Figure 4) .
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Figure 4. The vertical force graph can identify with precision all five phases of gait occurring

during stance. The typical force curve of normal gait will show the individual exceeding body

weight at two intervals.

Midstance

Midstance represents the first half of single support, which occurs from the 10- to 30-percent

periods of the gait cycle. It begins when the contralateral foot leaves the ground and continues as

the body weight travels along the length of the foot until it is aligned over the forefoot. The

descending initial peak of the vertical force graph reveals the period of midstance (see Figure 4) .

Terminal Stance

Terminal stance constitutes the second half of single-limb support. It begins with heel rise and

ends when the contralateral foot contacts the ground. Terminal stance occurs from the 30- to 50-

percent periods of the gait cycle. During this phase, body weight moves ahead of the forefoot.

The term heel off (HO) is a descriptor useful in observational analysis and is the point during thestance phase when the heel leaves the ground. The ascending second peak of the vertical force

graph demonstrates the period of terminal stance (see Figure 4) .

Roll off describes the period of late stance (from the 40- to 50- percent periods of the gait cycle)

when there is an ankle plantarflexor moment and simultaneous power generation of the tricepssurae to initiate advancement of the tibia over the fulcrum of the metatarsal heads in preparation

for the next phase.

Preswing

Preswing is the terminal double-limb support period and occupies the last 12 percent of stancephase, from 50 percent to 62 percent. It begins when the contralateral foot contacts the ground

and ends with ipsilateral toe off. During this period, the stance limb is unloaded and body weight

is transferred onto the contralateral limb. The descending portion of the second peak of thevertical force graph demonstrates the period of preswing(see Figure 4).
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Terminal contact (TC), a term rarely used, describes the instantaneous point in the gait cycle

when the foot leaves the ground. It thus represents either the end of the stance phase or the

beginning of swing phase. In pathologies where the foot never leaves the ground, the term footdrag is used. In foot drag, the termination of stance and the onset of swing may be somewhat

arbitrary.

The termination of stance and the onset of swing is defined as the point where all portions of the

foot have achieved motion relative to the floor. Likewise, the termination of swing and the onsetof stance may be defined as the point when the foot ends motion relative to the floor. Toe off

occurs when terminal contact is made with the toe.

Initial Swing

The initial one-third of the swing period, from the 62- to 75-percent periods of the gait cycle (6),is spent in initial swing. It begins the moment the foot leaves the ground and continues until

maximum knee flexion occurs, when the swinging extremity is directly under the body and

directly opposite the stance limb.

Midswing

Midswing occurs in the second third of the swing period, from the 75- to 85-percent periods ofthe gait cycle (6). Critical events include continued limb advancement and foot clearance. This

phase begins following maximum knee flexion and ends when the tibia is in a vertical position.

Terminal Swing

In the final phase of terminal swing from the 85- to 100-percent periods of the gait cycle (6), the

tibia passes beyond perpendicular, and the knee fully extends in preparation for heel contact.

Temporal Parameters

The potential to assess gait through quantified measurement emerged with the sunrise-to-sunset

movement of a lone traveler on foot over a known distance or with the hailing chant of each

advancing step of a marching army. Such events would have enabled measurement of walkingvelocity, or distance traversed per unit of time, and cadence, or steps per unit of time. Gait

parameters related to time are referred to as temporal parameters.

Stride length, cadence and velocity are three important interrelated temporal parameters.

Commonly misused, the terms step length and stride length are not synonymous. As a dynamicmeasurement of gait, step length is the distance in meters from a given floor-contact point of the

ipsilateral (or originating) foot in stance to the same floor-contact point of the contralateral (or

opposite) foot in stance (see Figure 5) ; for example, the distance from right-heel contact to left-heel contact. The step period is the segment of time in seconds taken for one step to occur and is

measured from an event of one foot to the following occurrence of the same event with the other

foot.
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Figure 5. Stride length is comprised of two step lengths.

Stride length, on the other hand, contains both a left- and a right-step length(see Figure 5)(e.g.,

the distance from right-heel contact to the following right-heel contact). Stride length sometimes

is referred to as cycle length and is expressed in meters. A reduction in functional joint motion orthe presence of pain or muscle weakness may reflect a reduction in stride or step length.

Pathological gait commonly produces asymmetries in step length between limbs. Stride period or

cycle time is the period of time in seconds from initial contact of one foot to the following initial

contact of the same foot.

Cadence refers to the number of steps taken per unit of time and is the rate at which a person

walks expressed in steps per minute. Natural or free cadence describes a self-selected walking

rhythm.

Velocity combines stride length and cadence and is the resultant rate of forward progression.Velocity is the rate of change of linear displacement along the direction of progression measured

over one or more strides and is expressed in meters per second. It is the best single index of

walking ability.

Reductions in velocity correlate with joint impairments, amputation levels and many acutepathologies. Velocity may be quantitatively measured or qualitatively assessed using the terms

free, slow and fast. Free walking speed describes the normal self-selected walking velocity. Fast

walking speed describes the maximum velocity attainable by a subject with a pathological gait.

Slow walking speed describes a velocity below the normal self-selected walking speed. Fastwalk velocity for healthy subjects can increase by as much as 44 percent (11), but pathological

subjects have less buffer. Since velocity affects many parameters of walking, the typicaldescription of normal gait generally presupposes a comfortable self-selected velocity. With this

free walk velocity, the individual will naturally enlist both the mannerisms and speed that will

provide maximum energy efficiency.
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Temporal parameters historically have been obtained in a gait lab by means of microswitches

embedded in plantar foot pads taped to the bottom of the foot or shoe (12) (see Figure 6) . The

rollover patterns are recorded as the patient walks a measured distance, and the temporalparameters are calculated.

Figure 6. Microswitches embedded in plantar pads record the timing of foot-floor contact over ameasured distance, providing temporal gait data

Although microswitches have been the standard for some time, perhaps the most promising

measurement tools for collecting temporal data are pressure-sensing arrays. A thin plastic sheet

of film can slip nearly unnoticed between the plantar surface of the foot and the orthosis withinthe shoe (see Figure 7). This array, connected to a computer via a lead wire, can measure

dynamic pressure patterns and record critical events throughout the walking cycle. A prosthetic

version can provide pressure measurements at 60 individual sites within a socket and recordthose measurements during multiple events of the gait cycle. The current clinical relevance lies

in identifying critical gait events and skin-loading pressure patterns. Because of the ease in

collection of plantar pressure readings and relative modest cost, this approach may well replacemicroswitch technologies in the future and be increasingly accessible to prosthetists and

orthotists for clinical use.

Figure 7. A thin array can be inserted within the shoe to capture pressure data.
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Microswitch technologies enable the clinician to record tendencies toward excessive inversion,

eversion or prolonged heel-only time and can suggest modifications to alignment or

componentry of prostheses or orthoses to normalize such patterns.

Time-distance parameters have enormous potential for setting outcome goals. Variations in time-

distance values often are pathology-specific. Asymmetries in hemiplegics, for example,obviously are greater than in most other pathologies; this technology is uniquely suited for

quantifying those asymmetries.

As basic temporal technologies develop and become increasingly affordable and as mean

pathology-specific values are obtained, these time-distance parameters, captured from

microswitch or piezoelectric film pressure technology, may become the baseline for measuring

functional outcomes.

Determinants of Gait

Saunders et al. defined walking as the translation of the center of mass through space in a mannerrequiring the least energy expenditure. They identified six determinants or variables that affect

that energy expenditure (3). Variations in pelvic rotation, pelvic tilt, knee flexion at midstance,

foot and ankle motion, knee motion, and lateral pelvic displacement all affect energy expenditureand the mechanical efficiency of walking.

As a functional basis for understanding energy efficiency in gait, these principles have stood the

test of time (13-15). These determinants of gait are based on two principles: 1) Any displacement

that elevates, depresses or moves the center of mass beyond normal maximum excursion limitswastes energy, and 2) Any abrupt or irregular movement will waste energy even when that

movement does not exceed the normal maximum displacement limits of the center of mass. A

successful long-distance runner intuitively takes advantage of these principles. By contrast, theunsuccessful runner lumbers from side to side and lurches up and down in a vicious spiral of

exhaustion followed by increased energy expenditure.

Of the six determinants of gait, three provide mechanical advantages that limit vertical

displacement of the center of mass. The term center of mass is synonymous with the term centerof gravity (CG). Without these mechanical advantages that limit displacement, the center of mass

would displace vertically 7.5 cm (3 inches) on a person of average height. Resulting from these

three determinants, the center of mass is said to displace vertically only 5 cm (2 inches).

Pelvic Rotation

The trailing extended weight-bearing limb is elastically linked through the joints of the pelvis

with the advancing swing limb. Ligamentous constraints and muscular activity combine with

forward momentum of the advancing swing limb to position the pelvis into four degrees of

rotation from the line of progression prior to initial contact(see Figure 8). During thereciprocating contralateral swing phase, the pelvis rotates in the opposite direction, first returning

to its neutral position and then continuing to rotate an additional 4 degrees. Thus the total range

of pelvic rotation is 8 degrees.


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In the act of pelvic rotation, both the trailing and advancing limbs are effectively lengthened

through the rotation that uses the pelvic width to extend both support points. At the very time

when the center of mass would otherwise drop excessively, this rotation prevents .95 cm (3/8-inch) of downward displacement of the center of mass.

Figure 8. Pelvic rotation effectively extends the trailing and advancing support points.

Pelvic Tilt

At midstance, the center of mass reaches its highest point as the body vaults over a planted leg. It

would be even higher were it not for the pelvis, which tilts down toward the swing side 5 degrees

from vertical (positive Trendelenburg sign) and thus depresses the center of mass .5 cm (3/16-inch) in an efficient method of energy conservation. This is referred to as pelvic list or pelvic tilt

and is possible only in conjunction with adequate limb clearance in swing phase(see Figure 9).

Figure 9. Pelvic tilt reduces vertical displacement of the center of mass

Knee Flexion During Midstance


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The stance limb enters initial contact with the knee in nearly full extension. It then flexes as the

foot shifts to a plantar-grade position and continues moving into flexion until it reaches

approximately 15 degrees. The knee then begins to extend but retains some flexion as it nearsmidstance; due to a relatively less extended knee as the tibia reaches verticality when the center

of mass is at its peak, the summit of the CG is depressed in its elevation by 1.1 cm (7/16-inch).

To summarize, the .95-cm displacement savings from pelvic rotation, .5-cm savings from pelvic

tilt and 1.1-cm savings derived from knee flexion at midstance result in a combined displacementsavings of 2.1 cm (approximately 1 inch). Without these three determinants, pelvic rotation,

pelvic tilt and knee flexion at midstance, the vertical displacement of the center of mass is

thought to be 7.5 cm (3 inches). With the 2.1 cm savings derived from these determinants, thevertical displacement of the center of mass has been reduced to approximately 5 cm (2 inches). If

these three determinants were the only mechanisms affecting the progression of the center of

mass as it traverses through space, the CG pathway would consist of a series of arcs at whoseintersections an abrupt shift in the direction of the CG would occur as it reached its lowest point.

However, both foot and ankle motion as well as knee motion serve to smooth the pathway of the

CG.

Foot and Ankle Motion

The most important mechanism to smooth this pathway is foot and ankle motion. At initialcontact, the ankle is elevated due to the heel lever arm but falls as the foot becomes plantar

grade. At heel rise, the ankle again is elevated, which continues through terminal stance and

preswing. These ankle motions, coordinated with the knee and controlled by muscle action ofpretibials and triceps surae, smooth the pathway of the center of mass during stance phase (see

Figure 11) .

Figure 11. Foot and ankle motion smooths the pathway of the center of mass.

The controlled lever arm of the forefoot at preswing is particularly helpful as it rounds out the

sharp downward reversal of the center of mass. Thus it does not reduce a peak displacement

period of the center of mass as the earlier determinants did but rather smooths the pathway. Footand ankle motion thus facilitate the path of the CG, keeping it relatively horizontal throughout

stance phase.

Knee Motion

Knee motion is intrinsically associated with foot and ankle motion. At initial contact before the

ankle moves into a plantar-grade position and thus is relatively more elevated, the knee is in
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relative extension. Responding to a plantar-grade posture (when the ankle is depressed), the

knee flexes. Passing through midstance as the ankle remains stationary with the foot flat on the

floor, the knee again reverses its direction to one of extension. As the heel comes off the floor interminal stance, the ankle again is elevated, and the knee flexes. In preswing, as the forefoot rolls

over the metatarsal heads, the ankle moves even higher in elevation as flexion of the knee

increases (see Figure 12) . Generally, at periods when the ankle center is depressed, the kneeextends, and at periods when the ankle is elevated, the knee flexes. Knee motion, intimately

associated with foot and ankle motion, smooths the pathway of the center of mass and thus

conserves energy.

Figure 12. Knee motion coordinated with foot and ankle motion smooths the pathway of the

center of mass.

Lateral Pelvic Displacement

To avoid extraordinary muscular and balancing demands, the pelvis shifts over the support pointof the stance limb. If the lower extremities dropped directly vertical from the hip joint, the center

of mass would be required to shift three to four inches to each side to be positioned effectively

over the supporting foot. The combination of femoral varus and anatomical valgum at the knee

permits a vertical tibial posture with both tibias in close proximity to each other. This narrowsthe walking base to 5-10 cm (2-4 inches) from heel center to heel center. This reduces the lateral

shift required of the center of mass to 2.5 cm (1 inch) toward either side (see Figure 13) . The

walking base or stride/step width typically is measured from one ankle joint center to the otheralthough it often is described as the measurement from heel center to heel center.


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Figure 13. Lateral pelvic displacement improves the position of the center of mass over the

support limb.

A wide walking base may increase stability-but at a cost of energy efficiency-and the center of

mass remains in a box two inches tall and two inches wide as the individual ambulates forwardin normal human locomotion.

Foot and Ankle Function: The Rocker Mechanisms

Perry has described the function of the heel, ankle and forefoot rocker mechanisms in normal

gait (4). Understanding the natural mechanics of these rockers greatly improves the abilities todiagnose and communicate orthotic and prosthetic gait deficits.

The first rocker is referred to as the heel rocker. The momentum generated by the fall of bodyweight onto the stance limb is preserved by this heel rocker. Normal initial contact is made by

the calcaneal tuberosity, which becomes the fulcrum about which the foot and tibia move. The

bony segment between this fulcrum and the center of the ankle rolls toward the ground as bodyweight is dropped onto the stance foot, preserving the momentum of forward progression.

The second rocker is the ankle rocker. The pivotal arc of the ankle rocker advances the tibia over

the stationary foot.

The third rocker is referred to as the forefoot rocker. During terminal stance, as the body vector

approaches the metatarsal-phalangeal joint, the heel rises and the phalanx extend. The metatarsalheads serve as an axis of rotation for body weight advancement.

The location of the ground-reaction force during preswing and concurrent loading on the

contralateral limb enables passive knee flexion, which prepares the ipsilateral limb for the

clearance demands of swing phase.


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Conclusion

Normal bipedal gait is achieved with a complex combination of automatic and volitional postural

components. Normal walking requires stability to provide antigravity support of body weight,

mobility of body segments and motor control to sequence multiple segments while transferring

body weight from one limb to another. The result is energy-efficient forward progression. (5)

Acknowledgements

The author would like to express appreciation to Ken Hudgens, program manager of theprosthetic and orthotic department, California State University - Dominguez Hills, for

illustrations 4, 10, 11 and 12.

References:

1. Marks AA. Manual of artificial limbs. New York: A.A. Marks Inc., 1905:17-20.

2. Ayyappa E, Mohamed O. Orthotics and prosthetics in rehabilitation. In: Lusardi M, ed.Clinical assessment of pathological gait. Newton, Mass.: Butterworth Heinemann,

manuscript submitted for publication, September 1996.

3. Saunders JB, Inman VT, Eberhart HD. The major determinants in normal andpathological gait. JBJS 1953; 35-A:543-58.

4. Perry J. Gait analysis; normal and pathological function. Thorofare, N.J.: Slack, 1992.

5. Sutherland D. Development of mature walking. Philadelphia: MacKeith Press, 1988.6. Sutherland DH, Kaufman KR, Moitoza JR. Kinematics of normal human walking. In:

Rose J, Gamble JG, eds. Human walking, 2nd ed. Baltimore: Williams & Wilkins,

1994;2:23-45.7. Pathokinesiology Service, Physical Therapy Department. Normal and pathological gait

syllabus. Downey, Calif.: Professional Staff Association of Rancho Los Amigos Hospital,1977.

8. Ounpuu S, ed. Terminology for clinical gait analysis (Draft #2). Prepared by AmericanAcademy of Cerebral Palsy Developmental Medicine Gait Lab Committee and

distributed at North American Clinical Gait Lab Conference, Benson Hotel, Portland,

Ore., April 6-9, 1994.9. Ayyappa E, ed. Words about words: the terminology of human walking, bipedal

exchange. Monograph of the American Academy of Orthotists and Prosthetists Gait

Society, Volumes 1-2, 1994.10. Ayyappa E, ed. American Academy of Orthotists and Prosthetists Gait Society, Gait and

Pathomechanics Syllabus, Certificate Program in Professional Development-Final

Report, August 1996.11. Finley FR, Cody K, Finizie R. Locomotive patterns in elderly women. Arch Phys Med

Rehab 1969;50:140-6.

12. Ayyappa E. Gait lab technology: measuring the steps of progress. O&P Almanac

1996;45:2:28,29,41,42,5613. Inman V. Ralston HJ, Todd F. Human walking. Baltimore: Williams & Wilkins, 1981

14. Bowker JH. Kinesiology and functional characteristics of the lower limb. In: Atlas of

limb prosthetics. St. Louis: CV Mosby, 1981;261-71


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15. Rose J, Gamble J. Human walking, 2nd ed. Baltimore: Williams & Wilkins, 1994.

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