Gait Analysis: Normal and Pathological Function

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Perry also developed a stroke unit. Responsibility for persons disabled by a stroke forced her to expand her analysis process as the functional pathology of the hemiplegie is much more complex than that of polio. Because the standard clinical examination findings correlated poorly with the gait dysfunctions, they initiated a system of observational gait analysis. Developed in conjunction with a group of knowledgeable and dedicated physical therapists, the Rancho Los Amigos Observational Gait Analysis System became highly organized.

For the first time there was a means of cataloging the multiple dysfunctions that occur with the various types of pathology. For the past 15 plus years, they have taught this program nationwide. It is this program on which the organizational background of this book is based. A second development was the gait laboratory Its initial purpose was to document the improvement resulting from reconstructive surgery in patients who could not be returned to normal.

This system was designed to help ascertain whether or not surgery actually was the better alternative for these patients. Out of this beginning was developed a functional diagnostic system to be used for planning the reconstructive surgery of spastic patients. The emphasis of the program was, and still is, kinesiology electromyography because the primary disability of spastic patients is inappropriate musc1eaction errors in timing and intensity. Footswitches were developed to define the patient's stride characteristics, and an electrogoniometer, that accommodated for braces, was also developed.

Clinical service and research have had equal About the Author xxi emphasis from the beginning. Another novel emphasis has been on energy cost analysis of walking. An outdoor court was designed where habitual gait could be studied Dr. Waters spearheaded this. Today, the laboratory is fully equipped with automated motion analysis ViconP' and force plates, and force sensing walking aids are being added.

AlI types of disability have been studied over the years and continue to be seen as the clinical need increases cerebral palsy, hemiplegia, spinal cord injury, post polios, arthritis, joint replacement, amputees, myelodysplasia, and muscular dystrophy. At the Rancho Los Amigos Medical Center, current gait research is related to the effect of the new" energy storing" prosthetic feet for amputees.

Thus, Dr. Perry continues her lifelong dedication to the research and clinical application of gait. This publication encompasses the extensive work of Dr. Perry and her successful years as a therapist and a surgeon renowned for her expertise in human gait. Tables 1. Iliacus normal B. Iliacus continuous C. Iliacus inactive r xxix Examples Soleus , p. Isometric B. Isokinetic Waters Because each sequence involves a series of interactions between two multisegmented lower limbs and the total body mass, identification of the numerous events that occur necessitates viewing gait from several different aspects.

There are three basic approaches. Of these, the simplest system subdivides the cycle according to the variations in reciproc al floor contact by the two feet. A second method uses the time and distance qualities of the stride. The third approach identifies the functional significance of the events within the gait cycle and designates these intervals as the functional phases of gait. Reciprocal Floor Contact Patlems As the body moves forward, one limb serves as a mobile source of support while the other limb advances itself ta a new support site.

Then the limbs reverse their roles. For the transfer of body weight from one limb to the other, both feet 1'" 4 Gait AnalysisjPerry are in contact with the ground. This series of events is repeated by each limb with reciproc al tirning until the person's destination is reached. A single sequence of these functions by one limb is called a gait cycle GC. Hence, any event could be selected as the onset of the gait cyele.

Because the moment of floor contact is the most readily defined event, this action generally has been selected as the start of the gait cyele. Normal persons initiate floor contact with their heel i. As not all patients have this capability, the generic term initial contact lC will be used to designate the onset of the gait cycle. These often are called gait phases.

In this book the phases will identify the functional subdivisions of totallimb activity within the gait cycle.

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Stance is the term used to designate the entire period during which the foot is on the ground. Stance begins with initial contact Figure 1. The word swing applies to the time the foot is in the air for limb advancement. Swing begins as the foot is lifted from the floor toe-off. Stance Swing Figure 1. Clear bar represents the duration of stance. Shaded bar is the duration of swing. Limb segments show the onset of stance with initial contact, end of stance by raII-of!

Stance is subdivided into three intervals according to the sequence of floor contact by the two feet Figure 1. Both the start and end of stance involve a period of bilateral foot contact with the floor double stance , while the middle portion of stance has one foot contact Figure 1. Initial double stance begins the gait cycle. It is the time both feet are on the floor after initial contact. An alternate term is double limb support. This designation is to be avoided, however, as it implies an equal sharing of body weight by the two feet, which is not true during most of the double stance interval.

Normal Gait and Common Gait Abnormalities

Single limb support begins when the opposite foot is lifted for swing. In keeping with the terminology for the double contact periods, this should be and often is called single stance. Vertical dark bars are the periods of double limb stance right and leit feet. Horizontal shaded bar is single limb support single stance. Total stance includes three intervals: the initial double stance, single limb support and the next terminal double stance.

Swing is the clear bar that follows terminal double stance. Note that right single limb support is the same time interval as left swing. During right swing there is leit single limb support. The third vertical bar double stance begins the next gait cycle. During the single limb support interval the body's entire weight is resting on that one extremity. The duration of single stance is the best index of the limb's support capability. Terminal double stance is the third subdivision. It begins with floor contact by the other foot contralateral initial contact and continues until the original stance limb is lifted for swing ipsilateral toe-off.

The term terminal double limb support has been avoided, as weight bearing is very asymmetrical. Note that single limb support of one limb equals swing of the other, as they are occurring at the same time Figure 1. The precise duration of these gait cyele intervals varies with the person's walking velocity. The duration of both gait periods shows an inverse relationship to walking speed. That is, both total stance and swing times are shortened as gait velocity increases.

The change in stance and swing times becomes progressively greater 6 Gait AnalysisjPerry Table 1. Among the subdivisions of stance a different relationship exits. Walking faster proportionally lengthens single stance and shortens the two double stance intervals.! The reverse is true as the person's walking speed slows. This pattern of change also is curvilinear. Having an interval when both feet are in contact with the ground for the limbs to exchange their support roles is a basic characteristic of walking.

When double stance is omitted, the person has entered the running mode of locomotion. I Stride and Step The gait cycle also has been identified by the descriptive term strider Occasionally the word step is used, but this is inappropriate Figure 1. Stride is the equivalent of a gait cycle. It is based on the actions of one limb.

Step length is the interval beIWeen initial contact of each foot. Stride length continues until there is a second contact by the same foot. Gait Cycle 7 The duration of a stride is the interval between two sequential initial floor contacts by the same limb i. Step refers to the timing between the two limbs. There are two steps in each stride or gait cycle. At the midpoint of one stride the other foot contacts the ground to begin its next stance period. The interval between an initial contact by each foot is a step i. The same offset in timing will be repeated in reciprocal fashion throughout the walk.

References 1. J Biomech 10 4 , Mann R: Biomechanics. Philadelphia, W. Saunders Company, , pp. J Bone Joint Surg 46A 2 , Bull Prosthet Res 18 1 , Chapter 2 Phases of Gait I norder to provide the basic functions required for walking, each stride involves an ever-changing alignment between the body and the supporting foot during stance and selective advancement of the limb segments in swing. These reactions result in a series of motion patterns performed by the hip, knee and ankle. Early in the development of gait analysis the investigators recognized that each pattern of motion related to a different functional demand and designated them as the phases of gait.

Further experience in correlating the data has progressively expanded the number of gait phases identified. It now is evident that each stride contains eight functional patterns. Technically these are sub phases, as the basic divisions of the gait cyc1eare stance and swing, but common practice also calls the functional intervals phases. In the past it has been the custom to use normal events as the critical actions separating the phases.

While this practice proved appropriate for the amputee, it often failed to accommodate the gait deviations of patients impaired by paralysis or arthritis. For example, the onset of stance 10 Gait AnalysisjPerry customarily has been called heel strike; yet the heel of a paralytic patient may never contact the ground or do so much later in the gait cyele. Similarly initial floor contact may be by the whole foot ifoot flat , rather than having forefoot contact occur later, after a period of heel-only support. To avoid these difficulties and other areas of confusion, the Rancho Los Amigos gait analysis committee developed a generic terminology for the functional phases of gait.

The phases of gait also provide a means for correlating the simultaneous actions of the individual joints into patterns of total limb function. This is a particularly important approach for interpreting the functional effects of disability. The relative significance of one joint's motion compared to the other's varies among the gait phases. Also, a posture that is appropriate in one gait phase would signify dysfunction at another point in the stride, because the functional need has changed. As a result, both timing and joint angle are very significant. This latter fact adds to the complexities of gait analysis.

Each of the eight gait phases has a functionalobjectiveand a critical pattern of selective synergistic motion to accomplish this goal. The sequential combination of the phases also enables the limb to accomplish three basic tasks. Weight acceptance begins the stance period and uses the first Table 2.

Single limb support continues stance with the next two phases of gait mid stance and terminal stance. Limb advancement begins in the final phase of stance pre-swing and then continues through the three phases of swing initial swing, midswing and terminal swing. Task A: Weight Acceptance This is the most demanding task in the gait cyele. Three functional patterns are needed: shock absorption, initial limb stability and the preservation of progression.

The challenge is the abrupt transfer of body weight onto a limb that has just finished swinging forward and has an unstable alignment. Two gait Phases are involved, initial contact and loading response Table 2. The joint postures present at this time determine the limb's loading response pattern. Objective: The limb is positioned to start stance with a heel rocker. The phase begins with initial floor contact and continues until the other foot is lifted for swing. Objecti ves: Shock absorption Weight-bearing stability Preservation of progression Task B: Single Limb Support Lifting the other foot for swing begins the single limb support interval for the stance limb.

This continues unti1 the opposite foot again contacts the floor. During the resulting interval, one limb has the total responsibility for supporting body weight in both the sagittal and coronal planes while progression must be continued. Two phases are involved in single limb support: mid stance and terminal stance. They are differentiated primarily by their mechanisms of progression.

The hip is flexed, the knee is extended, the ankle is dorsiflexed to neutral. Floor contact is made with the heel. Shading indicates the reference limb. The other limb clear is at the end of terminal stance. Loading Response Figure 2. Body weight is transferred onto the forward limb shaded. Using the heel as a rocker, the knee is flexed for shock absorption.

Ankle plantar flexion limits the heel rocker by forefoot contact with the floor. The opposite limb clear is in its pre-swing phase. It begins as the other foot is lifted and continues until body weight is aligned over the forefoot. It begins with heel rise and continues until the other foot strikes the ground. Throughout this phase body weight moves ahead of the forefoot. During the second half of support, the limb shaded advances over the stationary Ioot by ankle dorsifiexion ankle rocker while the knee and hip extend.

The opposite Iimb clear is advancing in its mid swing phase. The knee increases its extension and then just begins to f1ex slightly. Increased hip extension puts the Iimb in a more trailing position. The other Iimb clear is in terminal swing. Task C: Limb Advancement To meet the high demands of advancing the limb, preparatory posturing begins in stance.

Then the limb swings through three postures as it lifts itself, advances and prepares for the next stance interval. Four gait phases are involved: pre-swing end of stance , initial swing, mid swing and terminal swing. It begins with initial contact of the opposite limb and ends with ipsilateral toe-off. Weight release and weight transfer are other titles some investigators give to this phase. While the abrupt transfer of body weight promptly unloads the limb, this extremity makes no active contribution to the event. Instead, the unloaded limb uses its freedom to prepare for the rapid demands of swing.

Floor contact by the other limb clear has started terminal double support. The referenee limb shaded responds with increased ankle plantar flexion. The opposite clear limb is in Loading Response. Initial Swing Figure 2. The foot is lifted and limb advanced by hip flexion and increased knee flexion. The ankle only partially dorsiflexes. The other limb clear is in early mid stance. Hence, the term pre-suiing representative of its functional cornrnitment. It begins with lift of the foot from the floor and ends when the swinging foot is opposite the stance foot.

GC This second phase of the swing period begins as the swinging limb is opposite the stance limb Figure 2. The phase ends when the swinging limb is forward and the tibia is vertical i. Limb advancement is completed as the leg shank moves ahead of the thigh. Mid Swing Figure 2.

Advaneement of the limb shaded anterior to the body weight line is gained by further hip flexion. The other limb elear is in late mid slance. Terminal Swing Figure 2. The hip mainlains earlier flexion. A change in direction increases the requirements. Stairs and rough terrain further the demand. Running and the various sports present even greater needs. Despite these variations in complexity, there are underlying functional patterns common to alI. Body Subdivisions During walking the body functionally divides itself into two units, passenger and locomotor Figure 3. While there is mot ion and muscle action occurring in each, the relative intensity of these functions is markedly different in the two units.

Basically, the passenger unit is responsible only for its own postural integrity. Normal gait mechanics are so efficient that the demands on the passenger unit are reduced to a minimum, making it virtually a passive entity that is carried by the locomotor system. Alignment of the passenger unit over the limbs, however, is a major determinant of muscle action within the locomotor system. During walking, the upper body is a relatively passive passenger unit that rides on a motor system.

Passenger Unit The head, neck, trunk and arms are grouped as a passenger unit, because they are carried rather than directly contributing to the act of walking. Elftman introduced the tenn HAT to represent this mass, that is, a structure on top of the locomotor apparatus.? Muscle action within the neck and trunk serves only to maintain neutral vertebral aIignment with minimal postural change occurring during normal gait. Arm swing involves both passive and active elements, but the action does not appear essential to the normal gait pattern.

Experimental restraint of the arms registered no measurable change in the energy cost of walking. This presents a long lever that is 33cm 12in above the level of the hip joints in an average height man l84cm Figure 3. Locomotor Unit The two lower limbs and pelvis are the anatomical segments that form the locomotor system. Eleven articulations are involved: lumbosacral, bilateral hip, knee, ankle, subtalar, and metatarsophalangeal joints Figure 3.

In a man of average height cm this point is 33cm above the hip joint. This means the pelvis is dually considered a part of the passenger unit and the locomotor system. Setween the base of the spine and the toes, 11 joints are involved Iumbosacral and both hips, knees, ankles, subtalars, and metatarsophalangeal groups. The bony segments pelvis, thigh, shank, foot and toes serve as levers. As a multisegmented unit, each limb alternately assumes the responsibility to support the passenger unit in a manner that also carries it forward Figure 3.

Then, after being relieved of body weight, the limb rapidly swings itself forward to a new position and prepares to provide progressional support again Figure 3. The pelvis has a dual role. As part of the locomotor system it is a mobile link between the two lower limbs Figure 3. In addition, the pelvis serves as the bottom segment of the passenger unit that rides on the hip joints.

Locomotor Functions As the locomotor unit carries the body to its desired location each weight-bearing limb accomplishes four distinct functions. The accomplishment of each function depends on a distinct motion pattern. Each represents a complex series of interactions between the body mass and the two multisegmented Iower limbs. During walking these blend into a singIe, three-dimensional pattern.

Standing Stability Stability in the upright position is deterrnined by the functional balance between the alignrnent of the body and muscle activity at each joint. Each body segment is a weight that will fall toward the ground through the pull of gravity unless it is restrained. Within each segment there is a point, the center Table 3. Figure 3. The boxes identify the relation of the pelvis to the reference limb terminal stance, mid swing, terminal swing. There is passive stabiIity when the CjG of the upper segment is aligned directly over the center of the supporting joint.

The security of this position depends on the quality of the supporting surface and the nature of any external forces. In the body three anatomical situations challenge standing stability. First is the top-heavy relationship between the passenger unit and the locomotor system. Second is the multisegmented nature of the supporting limbs.

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The third factor is the contours of the lower limb joints. Alignment of body weight is the dominant factor. During standing and walking the effect of body weight is identified by the ground reaction force vector GRFV or body vector Figure 3. That is, as body weight falls toward the floor, it creates a force in the floor of equal magnitude but opposite in direction. This can be captured by appropriate instrumentation and represented as a mean line, the body vector. By reI ating the alignment of the body vector to the joint centers, the magnitude and direction of instabiIity are defined.

This indicates the muscle and ligament forces required to establish stability. The ligamentous skeleton is built for mobility rather than stability. The bones are long and the joint surfaces rounded. Hence, controlling forces are required. If the limb segments were shaped like a cube, force demand would be minimal. The supporting surfaces would be broad and flat and mass center low Figure 3.

With less tilt, the mass of the cube would falI back to its usual resting position once the displacing force was relaxed. Even this margin of stability is not available in the normal skeleton, as the rounded joint surfaces of alI the bones oHer no stabilizing edges Figure 3. Consequently, whenever the segments' centers of gravity are not in line, the upper segment will falI, unless there are controlling forces.

Gait analysis: normal and pathological function (2nd edition) | The Bone & Joint Journal

Three forces act on the joints: falling body weight, ligamentous tension and muscular activi ty. The hip and knee can use a balance between ligamentous tension and the body vector as a source of passive stability when the joints are hyperextended. At the knee there is the posterior oblique ligament. The hip is limited anteriorly by the iliofemoralligament Figure 3. Hyperextension of these joints allows the body weight line to pass anterior to the center of the knee and posterior to the hip joint axis.

In this position the joints are locked by two opposing forces: the body weight vector on one side of the joint and ligamentous tension on the other. At the ankle there is no similar source of passive stability. The ankle and subtalar joints each have a significant range of motion beyond neutral in both directions. Also, the ankle joint is not located at the middle of the foot. It is far closer to the heel than the metatarsal heads Figure 3. Heellength is further restricted by the support area being the calcaneal tuberosities rather than the posterior tip of this bone.

The apex of these rounded tuberosities is aImost in line with the posterior margin of the ankle joint. Hence, the margin for security Basic Functions 25 Figure 3. The height of the vector is proportional to the magnitude of the GRF. Anteriorly the mid- and forefoot extend the foot lever to the metatarsal heads, thus providing a much longer segment about lOcm. The midpoint between the calcaneal tuberosities and metatarsal heads would lie about 5cm anterior to the transverse axis of the ankle.

Quiet Standing. With the body erect and weight evenly distributed Figure 3. In theory quiet standing balance can be attained without any muscle action. This necessitates aligning the center of the passenger unit anterior margin of the eleventh thoracic vertebra exactly over the axis of the hip, knee, ankle and subtalar joints. Stability, however, is lacking, as none of the joints are Iocked. Consequently, the sIightest sway can unbalance every segment. Even the force of a heart beat might be sufficient.

During quiet standing, balance beam measurements showed the body vector extends downward from the center of the head ear canal , passes lcm anterior to the L4 vertebral body and rests in the foot 1. The standard deviations of 2cm also confirm considerable variability in the resting location of the center of pressure.

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Variations in mobility of the ankle and knee, as well as relative strength of the gastrosoleus muscle groups, would determine the different alignments. With knee extension limited to zero, stable alignment of the body vector requires ankle dorsiflexion. Persons having a range of knee hyperextension can Basic Functions 27 Figure 3. The stabilizing forces are ligamentous tension on one si de and the vector on the opposite side of the join!.

The ankle lacks passive stability. Hence, the heellever is much shorter than the forefoot lever, which extends to the metatarsal heads. The normal easy stand ing" position uses on1y a minimal margin of stability, with the body's center of gravity being just O. In the coronal plane the width of the foot support area is determined by the distance between the lateral margins of the feet. Mean distance between the centers of the feet averages 3 inches. In reality, the normal quiet stand ing posture tends to be shifted slightly to the right of midIine O.

Paired scales showed a mean 5. In contrast, force plate measurements registered a O. Recordings of postural sway reveal that quiet standing is not totally stationary. In both planes sagittal and coronali there is a sIow, but continuaI, shifting of body weight between the two limbs. Two mechanisms contribute to this subtle body instability: cardiac dynamics and the lack of absolute proprioception. This can be considered a measure of postural versatility.

It passes slightly anterior to the thoracic spine, just anterior to the knee and barely posterior to the hip joint. Basic Functions 29 Figure 3. Limb posture during quiet stand ing is similar to that used in mid stance. Hence, the person's ability ta stand is a preliminary test of his or her ability ta walk. The alignment needed is a functional balance of proprioception, joint mobility and musc1e control. Dynamic Stability. During wa1king, the body moves from behind to ahead of the supporting foot.

At the same time the area of support changes from the heel to flat foot and then the forefoot. These two variables mean that the body lacks passive stability throughout stance. Only in the midpoint of the stance period does body alignment approximate that of a stable quiet standing posture Figure 3. As the limb is loaded at the beginning of stance, the foot is ahead of the trunk. This places the body vector anterior to the hip and posterior to the knee Figure 3. A flexion torque is created at both joints, necessitating active extensor muscle response to restrain the fali of body weight.

During mid stance the body advances to a position over the supporting foot Figure 3. This reduces the flexion torques to zero. Continued advancement of the body over the supporting foot gradualiy introduces passive extension at the hip and knee. At the same time body weight moves ahead of the ankle and thus introduces a new area of postural instability. Now active control by the plantar flexor muscles is needed to restrain the forward fali of body weight Figure 3.

Thus, throughout stance, muscle action is directed toward decelerating the influences of gravity and momentum that create flexion torques at the hip and knee and dorsiflexion torques at the ankle, ali of which threaten standing stability. Faster wa1king speeds increase the demands on the decelerating muscles, as the body vector becomes greater with acceleration. Conversely, within a limited range, the required intensity of muscular activity can be reduced by walking more slowly.

The limitation in this saving is the need for sufficient gait velocity ta preserve the advantages of momentum, which is used as a substitute for direct extensor muscle action. Loading Response: the vector is anterior to the hip and posterior to the knee and ankle. Mid Stance: At the onset 01this phase early the body weight vector is slightly behind the knee but anterior to the ankle.

By the end 01 the phase late the vector has moved lorward 01 the ankle and the knee. At the hip the vector has moved posteriorly. Terminal Stance: The vector is posterior to the hip, anterior to the knee and maximaily lorward 01the ankle. As one foot is lifted for swing, this balance is lost abruptly. Now the center of the HAT is aligned medial to the supporting limb, and the cormecting link is a highly mobile hip joint.

Two preparatory actions are essential to preserve standing balance over a single limb. These are lateral shift of the body mass and local muscular stabilization of the hip joint to keep the pelvis and trunk erect Figure 3. During quiet standing the lateral shift places the center of the trunk over the foot. Both foot and knee valgus are used. For walking, less stability is sought since the swinging limb will be prepared to catch the falling body at the onset of the next step and knee valgus is less.

Progression The basic objective of the locomotor system is to move the body forward from the current site to a new location so the hands and head can perform their Basic Functions Figure 3. Instability is avoided by a shift of the body vector toward the stance limb and strong contraction of the hip abductors to support the unstable pelvis. To accomplish this objective of the locomotor system, forward fall of body weight is used as the primary propelling force Figure 3. Mobility at the base of the supporting limb is a critical factor in the freedom to fall forward. Throughout stance, momentum is preserved by a pivotal system created by the foot and ankle.

In serial fashion the heel, ankle and forefoot serve as rockers that allow the body to advance while the knee maintains a basically extended posture Figure 3. Progression occurs because the ankle musc1es yield as well as restrain the joint. Forward swing of the contralateral limb provides a second pulling force Figure 3.

This force is generated by accelerated advancement of the limb and its anterior alignment. The sum of these actions provides a prope1ling force at the time residual momentum in the stance limb is decreasing. It is particularly critical in mid stance to advance the body vector past the vertical and again create a forward fall position.

At the end of the step the falling body weight is caught by the contralateral swing limb, which by now has moved forward to assume a stance role. In this 32 Gait AnalysisjPerry Figure 3. The Initial Step. From a quiet standing posture with weight on both feet, three actions are used to begin walking. Then all weight is transferred laterally to the continuing stance limb. Lastly, weight moves forward on the stance limb as the body is allowed to falI forward and the swing foot is lifted Figure 3.

Ankle control of the supporting limb is modified to allow the forward faII. From the common stance posture of slight ankle dorsiflexion, the soleus merely reduces its holding force and the tibia increases its forward tiIt. Body weight follows the change in limb alignment. When the person stands with a hyperextended knee and the ankle is slightly plantar flexed, the pretibial muscles tibialis anterior and long toe extensors contract to actively pull the tibia forward.

Once the vertical axis has been passed, the limb is in a position to fall passively under soleus control in the usual manner. Hence, regardless of the initial standing posture of the limb, initiation of a step begins with a shift in body weight and anterior displacement at the ankle of the supporting limb. Lifting the swing limb uses the change in body posture for the propulsion. Hip flexion and ankle dorsiflexion lift the swing limb, creating an anterior force that further disturbs standing balance.

A more rapid hip flexion adds acceleration that augments the effect. Then it shifts to the stance limb and forward to the forefool. With the tibia back and the knee relatively hyperextended, the tibialis anterior and other pretibial muscles acts to advance the tibial. The falling body weight is caught by the contralaterallimb, which by now has completed its forward swing and is ready to assume a stance i.

Floor contact is made with the heel to continue the progression in stance. In this manner a cyele of progression is initiated and then serially perpetuated by reciprocal action of the two limbs. The Progression Cycle. Advancement mobility. As body weight is dropped of the body depends an stance limb onto the limb, the farce is primarily Basic Functions 35 directed toward the floor. Advancement of the body depends on redirecting some of this force in a manner that combines progression and stability.

The essential element for progression over the stance limb is rocker action by the foot and ank1e. Full ranges of passive extension at the knee and hip are the other critical factors. Floor contact is made by the rounded surface of the calcaneal tuberosities. The bony segment between this point and the center of the ankle joint serves as an unstable lever that ro11stoward the ground as body weight is dropped onto the foot. Action by the pretibial musc1es to restrain the rate of foot drop also creates a tie to the tibia that draws the leg forward. This progressional effect is transferred to the thigh by the quadriceps Figure 3.

While acting to restrain the rate of knee flexion, the quadriceps musc1e mass also ties the femur to the tibia. In this manner, the heel rocker facilitates progression of the entire stance limb. As a result the force of falling, rather than being totally directed toward the floor, has a significant portion realigned into forward momentum.

Heel rocker. As body Heel Rocker Figure 3. Pretibial muscles, as they decelerate the loot drop, also draw the tibia forward. Limb progression preserved. Once the forefoot strikes the floor, the ankle becomes the fulcrum for continued progression. With the foot stationary, the tibia continues its advancement by passive ankle dorsiflexion in response to the momentum present Figure 3.

During this period the body vector advances along the length of the foot to the metatarsal heads. A critical aspect of the ankle rocker is the yielding quality of the soleus muscle action. As it contracts to make the tibia a stable base for knee extension, the soleus muscle, assisted by the gastrocnemius, also allows tibial advancement.

Hence, there is graded intensity of plantar flexor muscle action. This is a prime example of selective control. Forefoot rocker. As the base of the body vector center of pressure reaches the metatarsal heads, the heel rises. The rounded contour of the metatarsals serves as a forefoot rocker Figure 3. Progression is accelerated as body weight falls beyond the area of foot support. This is the strongest propelling force during the gait cyele. The body mass is a passive weight at the end of a long lever, and there is no force restraining the fall. The forefoot rocker also serves as the base for accelerated limb advancement in pre-swing.

Pre-suring knee [lexion. An anterior propelling force is created through the Ankle Rocker Figure 3. The rate of tibial progression is decelerated by the soleus musele. Forefoot Rocker Figure 3. Both gaslrocnemius and soleus act vigorously 10 decelerate the rate of tibial advancement. Basic Functions 37 complex interaction of ankle, knee and hip mechanics that initiates limb advancement in pre-swing Figure 3. The base of the body vector is at the metatarsal heads and then passes through the center of the knee joint, so there no longer are stabilizing forces acting on the foot or knee.

Also, the limb is being rapidly unloaded by a transfer of body weight to the other foot. Residual gastrosoleus muscle action pivots the foot about the metatarsophalangeal MP joint. The result is simultaneous ankle plantar flexion and knee flexion. At the same time the adductors acting to restrain medial fan of the body also flex the hip.

Continued hip flexion in initial swing results in rapid advancement of the thigh, and needed momentum is added to the progressional system. Swing phase hip flexion. The progressional effect of the forward swinging limb is used by the weight-bearing limb during the early portion of its mid stance phase. At this time an added force is needed to draw the body mass and supporting limb forward and upward from the relatively low loading position acquired in the initial double support interval Figure 3.

Swing phase knee extension. The combination of knee extension and further thigh advancement in mid swing adds tibial weight ta the limb mass that is forward of the stance limb axis. This change in swing limb alignment continues the pulling force at a time when the stance limb has minimal intrinsic momenturn. Active knee extension in terminal swing completes the contribution of the swing limb to propulsion.

Due ta the small weight of the shank and Figure 3. Four factors contribute: 1 ankle is dorsiflexed to allow the vector to He over the metatarsophalangeal joint MPJ ; 2 the limb is rapidly unloaded by the transfer of body weight to the other limb shaded areas ; 3 residual gastrosoleus muscle action plantar flexes the foot about the forefoot rocker; 4 the resulting tibial advancement flexes the ee; and 5 adductor longus action flexes the hip as it controls coronal plane balance.

By the end of these actions the total advancement of the thigh and tibia results in a limb posture that is appropriate to catch the falling body weight at the onset of the next stance period. A new progressional cycle is begun. Shock Absorption Transfer of body weight from the rear to forward foot is a fairly abrupt exchange, even though it occurs during a double stance interval. At the end of the single support period, body weight is significantly ahead of the are a of forefoot support.

This creates an unbalanced situation with the body faUing forward toward the floor. At the same time, the foot of the forward limb, while positioned for stance, still is about 1cm above the floor's surface Figure 3. The full intensity of this floor impact is reduced by shock-absorbing reactions at the ankIe, knee and hip. An three motion patterns occur during the loading response phase of gait. Ankle plantar flexion is an irnrnediate reaction to initial floor contact by the heel. The rate at which fa11ing body weight is transferred onto to the floor is correspondingly reduced. Knee flexion is the second and greater shock-absorbing mechanism.

This motion also is a reaction to the heel rocker initiated by floor contact. As the pretibial museles act to restrain foot fall, their bony attachrnents to the tibia and fibula create a tie that causes the leg to foUow the falling foot. Forward ro11of the tibia reduces the support available to the femur and thus allows the body to drop.

This also causes knee flexion, as the joint center is anterior to the body vector. Action by the quadriceps to decelerate the rate of knee flexion transfers some of the loading force to the thigh musele mass Figure 3. Hence, the intensity of the joint loading force is reduced floor impact is less. This is reflected by the foreeplate as an absence of an initial high impact verticalload Figure 3. Abrupt loading of the weight acceptance limb similarly unloads the other limb.

This removes the support from that side of the pelvis, introducing a contralateral pelvic drop. The HAT resting on the middle of the pelvis also fa11s. The rate of pelvic drop is restrained by the stance limb's abductor museles. Again, the impact of limb loading is absorbed by muscular action. As a result the totalload experienced by the stance hip joint is reduced Figure 3. This is the distance between the heel and the flaar at the end of terminal swing. Shaded area - loading response.

During walking, preservation of stance stability by selectively restraining falling body weight and advancing the swinging limb as the body progresses along the desired distance constitutes the work being perforrned. The ama unt of muscular effort reguired in these actions determines the energy cost. In the terms of physics, the word work indicates controlled motion, that is, kilogram-meters of displacement.

Physiologically there are two other concerns. The intensity of the muscular effort as a percent of its capacity indicates the person's capability of performing the task. The amount of energy reguired by the muscular action indicates the person's endurance. Unlimited endurance reguires the energy cost of walking be below the cardiopulmonary midpoint in a person's maximum energy production capacity. Thus, walking is not sa effortless as assumed. Ta maintain the "Iow" effort level, the normal stride inc1udes two mechanisms to conserve energy.

These are CjG alignment modulation and selective muscular control. Both serve ta reduce the intensity and duration of the muscular action involved. CjG Control. Minimizing the amount that the body's center of gravity is displaced from the line of progression is the major mechanism for reducing the muscular effort of walking and, conseguently, saving energy. The least energy would be used if the weight being carried remained at a constant height and followed a single central path.

Then no additional lifting effort would be needed ta recover from the intermittent falls downward ar laterally. This is equally true for moving the body during walking; displacement should be minimized. Dependence on reciprocal bipedal locomotion, however, presents two potentially costly situations during each stride.

As the right and left limbs alternate their support roles, the body must shift from one side ta the other. The limbs also change their vertical alignrnent between double and single support, causing a change in the height of the pelvis, leading ta the body mass mov ing up and down. The body is at its lowest point when the limbs become obliguely aligned during the two double support periods initial and terminal. Then in mid stance the body is raised to its highest position right ar left when the supporting limb is vertical Figure 3.

The potential difference in hip height is approximately 9. Repeatedly lifting the body this amount would guickly be exhaustive. Potential transverse displacement of the body from side ta side could egual the average stride width of 8cm. In addition, abrupt changes in direction are avoided, which is another energy-saving maneuver.?

Three of the energy-conserving motions relate ta changes in alignment of the pelvis. These are contralateral drop, horizontal rotation and lateral displace- Basic Functions t9.

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This occurs as weight is rapidly dropped onto Ihe loading limb Iarge arrow as Ihe olher limb is being lifted smaJl arrow. Lateral displacement of the pelvis relates to the transfer of body weight onto the limb. During loading response and early mid stance both vertical and lateral realignment of the body cle occurs. Contralateral drop of the pelvis lowers the base of the HAT.

Shifting weight to the stance limb at the onset of stance removes the support from the swing side of the pelvis, causing a contralateral pelvic drop. Half of this drop is experienced by the body center as it lies at the midpoint of the pelvic width between the two hips. Lateral displacement of the pelvis with limb loading involves two factors.

First is the natural valgus angle between the femur and the tibia. This places the knees and supporting feet closer to each other than a vertical line down from the hip joints would provide. Anatomical width between the hip joints approximates cm. Normal step width is 8cm.

The third pelvic motion is forward rotation, which occurs as the swing limb advances. This action moves the hip joint of the swing limb anterior to that of the stance limb, placing the width of the pelvis into a relatively sagittal position. Initial Contact L Figure 3. Each deviation is approximately 2cm up and to each side. The resulting horizontal segment between the two hips functionally lengthens the limbs by increasing the distance between the points of the fIoor contact and the base of the trunk. Pelvic rotation also moves the hip joints and thus the supporting feet closer to the midline.

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Both effects reduce the amount of limb obliquity needed to accomplish the desired step length. The effect, greatest in terminal stance, is a decrease in the amount that the body center is lowered during double support. Limb motions also contribute to smoothing the path of the body's vertical travel. The mechanics vary with the phase of gait. During the double stance intervals, ankle control is critical.

Heel rise in terminal stance relatively lengthens the trailing limb by lifting the ankle. Initial contact by the heel similarly adds length to the forward limb Figure 3. The interchange between ankle and knee motion is a second means of reducing body mass displacement. As the limb is loaded the combination of increasing ankle plantar fIexion and knee fIexion decreases the rate of body elevation, part of the loading response becomes more vertical by rolling BasicFunctions 43 4 o Pelvic Drop a re 3.

Leit: No accommodation. Right: Terminal stance heel rise lengthens the trailing limb. Initiai contact heel strike increases the length of the forward limb. Following forefoot contact with the fIoor, limb advancement continues as ankle dorsiflexion while the knee increases its fIexion. Subsequent blending of progressive dorsiflexion that lowers the tibia and knee extension that elevates the femur continues the avoidance of an abrupt change in the level of the body center during the weight-bearing period Figure 3.

In addition to the anatomical narrowing of step width, the body does not fully align itself over the supporting foot as would occur in quiet standing. The potential imbalance is controlled by inertia. By the time body weight loses its lateral momentum and would falI to the unsupported side, the swing limb has completed its advancement and is prepared to accept the load.

Transverse rotation also narrows the distance between the hip joints. Thus, in summary, vertical lift of the passenger unit during single limb support is lessened by lateral and anterior tilt of the pelvis combined with stance limb ankle plantar flexion and knee fIexion.

Lowering of the body center by double limb support is reduced by terminal stance heel rise, initial heel contact combined with full knee extension, and horizontal rotation of the pelvis. Lateral displacement is similarly minimized by the pelvic rotations, medial femoral angulation, and the substitution of inertia for complete coronal balance. As a resuIt, the body's center of gravity follows a smooth three-dimensional sinusoidal path that intermingles vertical and horizontal deviations. Selective Control. By substituting passive posturing and available momentum Flgure 3.

Basic Functions 45 for muscle action whenever possible, less energy is expended as the desired progression and the necessary joint stability are accomplished. Both the timing and intensity of muscular activity are selectively modulated. Throughout stance the musc1es contract on1y when body alignment creates a torque antagonistic to weight-bearing stability of the limb and trunk.

That is, the body vector is aligned to create instability. The demand torques that must be controlled in the sagittal plane are those that induce hip flexion, knee flexion and ankle dorsiflexion Figure 3. In the coronal plane, the threatening alignments are hip adduction and abduction and subtalar joint inversion and eversion.

There also are transverse rotational demands at each joint that must be controlled. The intensity of the muscular responses is proportional to the magnitude of the demand torque that must be restrained.