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 of gait 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 clinic team demands a sound conceptual knowledge base of human locomotion and related terminology. This will facilitate an optimal treatment plan for the patient and enhance communication and prescription recommendations to the physician and relevant paying agencies. This article is intended to be an introduction to gait science with these goals and objectives in mind.
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 in eight 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).
Insight into normal walking patterns can help practitioners improve the efficiency of persons with 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 prosthetist or 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 of observational 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 a misleading 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 the common 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 have increased 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 descriptive medium 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). 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.
Gait characteristics are influenced by the shape, position and function of neuromuscular and musculoskeletal 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 each other 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 phase could 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 occurs twice 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 time decreases. Running constitutes forward movement with no period of double-limb support.
In normal walking, initial double-limb support takes up about 12 percent of the gait cycle, and terminal double-limb support occupies 12 percent as well. Generally, the two periods of double-limb support represent 25 percent of the gait cycle.
Single support is the period of time when only one foot is in contact with the ground. In walking, this is equal to the swing phase of the other limb. The term ipsilateral is used to describe the same side of the body, and the term contralateral is used to describe the opposite side of the body or the opposite limb. The direction of walking is referred to as the line of progression.
A complete gait cycle can be viewed in terms of three functional tasks of weight acceptance, single-limb support and limb advancement (see Figure 3) .
The first functional task is weight acceptance. Two phases of the stance period, initial contact and loading response, are involved in the performance of weight acceptance. The demand for immediate transfer of body weight onto the limb as soon as it contacts the ground requires initial limb stability and shock absorption while simultaneously preserving the momentum of progression. When the functional task of weight acceptance has been achieved, the individual is said to demonstrate a stable kinetic chain.
The second functional task is single- limb support. Primarily, two phases are associated with single-limb support: midstance and terminal stance. In addition, preswing is a transitional phase that could be considered part of single-limb support as well. During this period, the contralateral foot is in the swing period, and total body weight is exclusively supported on the stance limb. Forward progression of body weight over the stationary foot while maintaining stability is accomplished.
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 initial contact. The preswing phase serves in both single-limb support and limb advancement.
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 a component of swing phase.
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.
The loading response phase occupies about 10 percent of the gait cycle and constitutes the period of 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 response period 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) .
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 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 the stance 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 triceps surae to initiate advancement of the tibia over the fulcrum of the metatarsal heads in preparation for the next phase.
Preswing is the terminal double-limb support period and occupies the last 12 percent of stance phase, 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 the vertical force graph demonstrates the period of preswing (see Figure 4) .
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 foot drag 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 onset of 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.
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 occurs in the second third of the swing period, from the 75- to 85-percent periods of the 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.
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.
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 walking velocity, 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 dynamic measurement 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.
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 or the 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 acute pathologies. 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. Fast walk 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 typical description 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.
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 temporal parameters are calculated.
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 within the 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 record those 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 replace microswitch technologies in the future and be increasingly accessible to prosthetists and orthotists for clinical use.
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.
Saunders et al. defined walking as the translation of the center of mass through space in a manner requiring 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 expenditure and 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 limits wastes 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, the unsuccessful 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 center of 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).
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 the reciprocating 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.
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.
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) .
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 nears midstance; 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 displacement savings 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, the vertical 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 whose intersections 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.
The most important mechanism to smooth this pathway is foot and ankle motion. At initial contact, 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 of pretibials and triceps surae, smooth the pathway of the center of mass during stance phase (see Figure 11) .
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. Foot and ankle motion thus facilitate the path of the CG, keeping it relatively horizontal throughout stance phase.
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 relative extension. Responding to a plan- tar-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 in terminal 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 knee extends, 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.
To avoid extraordinary muscular and balancing demands, the pelvis shifts over the support point of 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 narrows the 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 other although it often is described as the measurement from heel center to heel center.
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 forward in normal human locomotion.
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 to diagnose and communicate orthotic and prosthetic gait deficits.
The first rocker is referred to as the heel rocker. The momentum generated by the fall of body weight 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 body weight 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 metatarsal heads 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.
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)
The author would like to express appreciation to Ken Hudgens, program manager of the prosthetic and orthotic department, California State University - Dominguez Hills, for illustrations 4, 10, 11 and 12.