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See how the muscles work to create ambulation

This is an excerpt from Rehabilitation of Musculoskeletal Injuries 5th Edition With HKPropel Online Video by Peggy A. Houglum,Kristine L. Boyle-Walker & Daniel E. Houglum.

Gait Kinetics

Movements that occur during ambulation result from forces acting on the body. These forces (kinetics) include primarily those produced by the muscles, ground reaction forces, gravity, and momentum. Before we look at ground reaction forces, we will see how the muscles work to create ambulation.

Muscle Actions: General Functions in Gait
In gait, the muscles perform one of three actions: acceleration, deceleration, or shock absorption. Muscles also work as stabilizers to secure the body or its segments during movement. Acceleration propels the body or segment forward. Acceleration is generally the result of concentric muscle activity. Deceleration slows down a segment’s or body’s movement to produce a smooth, controlled motion during ambulation. Deceleration occurs from eccentric activity. Like deceleration, shock absorption is primarily an eccentric activity. Shock absorption occurs primarily during early contact with the ground to reduce impact forces on the body. Since deceleration and shock absorption are both eccentric activities and occur either in preparation for stance or on impact with the ground, separation between these two activities is often not acknowledged, but keep in mind that they are separate tasks. Stabilizer muscles act as guy wires to hold a segment stable during movement. Isometric activity often produces stabilization. Some muscles may act as accelerators during gait and as decelerators at other times, while other muscles are primarily stabilizers throughout the gait cycle.

Obviously, not all muscles are active all the time during gait. The cyclic activity of a muscle in gait provides periods of rest for that muscle. Brief periods of peak muscle activity followed by less activity and rest periods give muscles enough recovery time so an activity like walking can continue for extended durations, if necessary. Because walking is the means by which we move our bodies from one location to another, it matters quite a bit that locomotion does not need any single muscle to perform continuously.

Muscles need the greatest amount of energy during the stance phase; less energy is needed during the swing phase. The periods of greatest muscle activity are the last 10% of the stance phase and the last 10% of the swing phase. In other words, the greatest muscle activity occurs during periods of acceleration (final stance phase) and deceleration (final swing phase).⁷⁸ Periods of relative inactivity occur during midstance and the swing phase. The swing phase is a relatively quiet time for muscle activity because the momentum produced during the final stages of stance propels the lower extremity forward.

Let’s take a brief look at the specific muscles that produce ambulation. Once you know what muscles are important for gait, it becomes easier to instruct patients in corrective gait training and to provide therapeutic exercises to correct gait deficiencies.

An easy way to look at gait muscles is to divide them into categories according to their functions. These categories include shock absorbers and decelerators, stabilizers, and accelerators. Some categories overlap because, for example, shock absorption requires deceleration. It is helpful to further divide categories according to the various body segments the muscles influence. Refer to figure 7.18 for a summary of the muscle activity described in the following sections.

Shock Absorbers and Decelerators Eccentric motion produces both shock absorption and deceleration. Not always, but sometimes, a muscle that absorbs shock is also decelerating the limb; it can be difficult at times to determine whether a muscle is acting as a shock absorber or a decelerator. The best way to determine the muscle’s action is to identify what the limb is doing when the muscle performs its task. During the first 15% of stance from initial contact to loading response, the quadriceps work as shock absorbers to reduce impact forces. These forces are based on the impact of the body contacting the ground and the ground pushing back in reaction to the body’s impact (ground reaction force, or GRF). This principle is based on Newton’s third law of motion regarding action–reaction. Depending on the speed of gait, the ground reaction force can be anywhere from 110% at normal walking speed¹⁰ to well over 700% of the body’s weight.⁸⁰ Muscles absorb these forces by eccentrically moving the lower-extremity joints. At initial contact, the ankle dorsiflexors work as decelerators to prevent the foot from slapping onto the ground. The quadriceps group decelerates knee flexion and controls the amount of knee flexion that occurs during the first 15% of the gait cycle. At the instant the heel makes contact with the floor, the ankle dorsiflexors are at their peak output as they work first isometrically to keep the forefoot off the floor, then immediately act both as decelerators to lower the forefoot and as shock absorbers to absorb impact forces so that the movement is smooth.^{10, 81}

During swing phase, the hamstrings work as decelerators of the knee to control the swing of the leg so initial contact occurs smoothly. The hamstrings also act as accelerators in the early portion of stance phase to bring the body’s center of mass forward onto the weight-bearing limb.

Stabilizers The hip and torso muscles act as stabilizers to keep the trunk erect as the weight transfers from one leg to the other, preventing excessive side tilting of the pelvis or trunk. The hamstrings stabilize the pelvis to prevent the trunk from leaning forward during weight bearing and during weight transfer from side to side;^{35, 82, 83} the gluteus medius, gluteus minimus, and adductor magnus (ischiocondylar adductor) stabilize the pelvis on the femur in the frontal plane;^{82, 84} and the internal obliques, external obliques, serratus anterior, upper trapezius, and lower trapezius balance the head, arms, and trunk (HAT) on the pelvis.^{29, 46, 79, 85, 86} These groups reach their peak levels of activity in normal gait during the beginning and late stages of stance when weight is transferred from one leg to the other.^{29, 48, 79} The tensor fasciae latae also works during initial swing phase to stabilize the pelvis.⁸⁷

Accelerators Accelerators in the leg and thigh have peak outputs at various times during gait. The posterior calf accelerators exhibit peak activity during the end of stance phase as they propel the leg forward, providing a push-off to produce an accelerated passive momentum of the extremity forward during swing phase.⁸⁸ The posterior calf muscles begin to act during the middle portion of the weight-bearing phase as they provide control and balance during weight bearing. This is especially true of the lateral leg muscles, the ankle inverters and evertors.¹⁰ These lateral leg muscles assist with foot stability and balance.⁸⁹

During swing, the foot and toe dorsiflexors lift the foot and toes to clear the floor and position the foot as the limb prepares for heel strike. The thigh accelerators work primarily in the early and middle stages of swing to increase hip flexion so the foot clears the ground.¹⁰ The psoas activity peaks during swing phase, providing motions of hip flexion, femoral lateral rotation, and contralateral lumbar rotation to help keep the body’s center of mass over the stance leg.⁹⁰

Muscle Actions Specific to Phases of Gait
Now that we have an idea of how muscles work in gait, let us take a look at each phase of gait to identify how muscles work together during walking. Each aspect of the gait cycle is presented in this section. To view how each body segment functions, refer to the images in figure 7.17 as you read through the muscle activities within each phase of gait. Following are the gait cycles using the Rancho Los Amigos terminology with the clinical terminology in parentheses.

Initial Contact (Heel Strike) The limb’s rate of speed during the swing phase is rapid, especially in the tibia where normal walking speed creates tibial swing at 350°/s.¹⁰ In moving from this rate to essentially stopping when the limb contacts the ground, stabilization is vital, and it is the primary activity that occurs during initial contact. The erect position of the trunk and the hip-flexed position are stabilized through efforts of the biceps femoris⁸³ as the limb makes sudden contact with the ground.

The hamstrings complete their task at the knee that began in terminal swing, decelerating the knee in preparation for ground contact. Knee stabilization occurs through passive forces; with the knee anterior to the body’s center of mass and posterior to the heel’s contact with the ground, the downward vector force stabilizes knee extension.¹⁰

In the thorax, the posterior deltoid, trapezius, and latissimus dorsi are active in humeral extension and thorax rotation to the same side.^{47, 79} These muscles stabilize the humerus and thorax during weight acceptance of the ipsilateral lower extremity. The ipsilateral internal obliques and transversus abdominis and the contralateral external obliques also help stabilize our center of mass over the stance leg.²⁹

Loading Response (Foot Flat) The gluteus medius, adductor magnus (ischiocondylar adductor), and hamstring muscles continue to provide trunk stabilization to maintain an erect position as muscles of the more distal limb absorb forces from the impact.^{82-84, 91} As the limb begins to accept the body’s weight, the gluteus medius limits the contralateral hip’s drop in the frontal plane. The tensor fasciae latae also controls the hip and helps stabilize the knee in the frontal plane.³⁶ With the hip in flexion and the foot now anchored to the ground, an additional forward torque tends to move the body’s center of mass forward; however, the ipsilateral internal obliques assist the low traps and hamstrings in maintaining the trunk’s upright position.^{29, 48, 83, 92} As the knee starts to flex to absorb impact forces, the quadriceps eccentrically controls the rate and amount of knee flexion. Because the tibia is moving forward from its anchor site at the ankle as the ankle dorsiflexors contract and the knee flexes slightly via eccentric quadriceps contraction, the hamstrings contract to counteract these cumulative stresses that are being placed on the anterior cruciate ligament.¹⁰ The ankle continues to absorb impact stresses via the eccentric activity of the anterior muscles through the first half of the loading response.¹⁰ The subtalar joint moves into supination via concentric effort of the tibialis posterior. Peroneal muscles assist in stabilizing the ankle along with the tibialis posterior.⁹³

The ankle plantar flexors begin to help stabilize the ankle as the body’s center of mass continues to move forward. Ankle frontal plane motion involves moving the subtalar joint into eversion, primarily through the eccentric efforts of the tibialis anterior with some assistance from the tibialis posterior. Subtalar eversion with subsequent pronation causes medial rotation of the tibia and subsequently also the femur; this rotation is limited by the efforts of the biceps femoris to counteract the semimembranosus and subtalar forces.¹⁰

Midstance (Midstance) During this phase, this limb is the only weight-bearing extremity, so as you may guess, medial–lateral stability and continued progression forward are most important. Midstance is also when the body’s center of mass moves from behind the weight-bearing ankle to directly over it and then ahead of the ankle in the last moments of midstance. Gluteus medius and minimus muscles, along with the tensor fasciae latae, provide lateral hip stability and control the amount of contralateral hip drop during this time of single-leg support. Until the center of mass moves over the foot, the quadriceps are controlling knee flexion so the hamstring can perform hip extension to pull the body forward over the anchored limb.^{35, 83} Once the center of mass moves over the foot and forward of it, the quadriceps are no longer needed to control knee extension; gravity’s vector force between the body’s center of mass and the foot on the ground provide passive extension of the knee. A gradual increase in activity of the posterior calf muscles occurs from late loading response and into the remaining aspects of stance.

As the body’s center of mass moves over the anchored foot, the posterior muscles control the body’s forward progression and then, when the center of mass is forward of the foot, they are responsible for moving the limb forward, controlling knee flexion and managing ankle plantar flexion. The soleus provides stability for the ankle. The gastrocnemius controls knee motion eccentrically to make for a smooth transition from knee flexion to extension. The foot’s intrinsic muscles also activate during this single-leg stance phase to help convert the foot to a progressively more rigid structure to prepare for the end of the stance phase.⁹⁴

During this phase of gait, there is significant upper extremity muscle activity. The anterior and posterior deltoids, triceps, trapezius, and latissimus dorsi are all active; of these muscles, the posterior deltoid and trapezius demonstrate the most activity.⁷⁹ Some of these muscles are working as accelerators, while others are decelerating.

To date, evidence for the upper body’s role during the gait cycle is inconclusive. This may be, at least in part, because arm swing is different at different walking speeds,⁴⁵ and some studies had specific walking speeds while others used subject-selected speeds. Also, the relative amount of muscle activity in the upper body is minimal; Kuhtz-Buschbeck and Jing⁴⁷ found that the average muscle activity throughout the gait cycle for these muscles was well below 5% of their maximum voluntary isometric contraction (MVIC). Some investigators concluded that the purpose of upper-body activity is to assist the lower body,^{47, 48} while others asserted that the upper body’s role is to reduce head and neck overactivity,⁴⁶ and still others determined that upper-body activity reduces joint reaction forces in the spine.⁴³ Additional investigations with more consistent methods are needed to enable us to understand the full importance of upper-body contributions to gait.

Terminal Stance (Heel-Off) Now that the body’s center of mass is ahead of the foot on the ground, forces between the center of mass and the foot allow passive extension of the hip and knee, so little muscle effort is needed for these segments. The heel rises passively as the limb moves forward of the foot; during this time, the soleus provides stability for the more proximal joints, and the gastrocnemius controls ankle stability.¹⁰ As the heel continues to rise, the peroneals and tibialis posterior continue their work, stabilizing the ankle in the frontal plane and placing the subtalar joint into supination. The intrinsic foot muscles contract isometrically in this phase to add to the foot’s stability as the body’s forward momentum moves the foot from foot flat to heel-off.⁹⁵

Preswing (Toe-Off or Push-Off) It is during this phase that the contralateral limb makes contact with the ground, so the contralateral pelvis begins to elevate as the limb begins to accept body weight. The erector spinae help the hamstrings to maintain an erect trunk,⁹⁶ while the hip abductors and adductors stabilize lateral motion during initiation of double-limb support.¹⁰ Hip movement toward flexion and pelvic motion begins with concentric effort from the psoas, iliacus, gluteus maximus, piriformis, and adductor magnus, and continues with activity from the rectus femoris.^{36, 97} Knee flexion occurs passively from the combined motions of hip flexion and ankle plantar flexion. The ankle achieves its maximum plantar flexion as the foot leaves the ground.³⁶ The posterior calf muscles are responsible for propulsion during preswing,⁹⁸ but their activity ceases before the end of this phase when the anterior ankle muscles contract concentrically to dorsiflex the ankle in preparation for swing.⁹⁹

The arm is now in shoulder flexion and helps transfer the body’s center of mass toward the opposite side as the body prepares to move its weight onto the other lower extremity.⁴⁸ The posterior deltoid, trapezius, and latissimus dorsi demonstrate the most activity of the upper extremity muscles working at this time.⁷⁹

Initial Swing (Early Swing) During this short phase, the stance leg becomes the swing leg and begins its advance forward. The hip continues to flex by concentric contraction of the psoas and iliacus. Ankle and toe extensor muscles continue to work concentrically to maintain ankle dorsiflexion so the foot clears the ground during swing. Most other lower-extremity muscles are relatively inactive during this phase since the limb’s momentum, which accumulated from force production during weight bearing, is released as the limb is propelled forward. This is an efficient mechanism that is effective during bipedal motion.¹⁰⁰

Midswing (Midswing) Toward the end of this phase, the tibia becomes perpendicular with the ground. The most important actions in this phase are toe and foot clearance from the ground and continued forward progression of the limb. Hip flexor muscle activity begins to diminish. Hamstrings begin to activate as decelerators at the very end of midswing, while knee motion continues to be passively produced. Toe and ankle dorsiflexors continue their isometric contraction to maintain foot clearance from the floor during swing.

While the leg is in midswing, the ipsilateral shoulder moves toward shoulder extension. This upper extremity movement promotes upper trunk rotation in the opposite direction from pelvic rotation (as the upper extremity and trunk rotate in a posterior direction, the ipsilateral hip and pelvis rotate in a forward direction), allowing the head to maintain a forward-looking position and creating normal walking mechanics.⁴⁵

The upper body muscle most active in this phase is the trapezius.⁷⁹ Although the relative motions are small, researchers suggest that arm swing and thorax rotation during this phase of gait are important because they add to overall stability and enhance lower-extremity function.^{43, 47, 48, 85}

Terminal Swing (Late Swing) During this phase, the limb is preparing to come once again into contact with the ground. Hamstrings, especially the medial hamstrings, slow the forward swing of the hip and prepare for weight acceptance.⁷⁸ The hamstrings are simultaneously slowing the forward swing of the tibia, preventing hyperextension of the knee, and performing posterior pelvic rotation in late swing, all in preparation for initial contact.³⁵ Toward the end of this phase, the quadriceps activate to stabilize the knee for initial contact.¹² The ankle and foot dorsiflexors prepare the foot for contact as well; the subtalar joint inverts, and the foot and ankle are in neutral dorsiflexion.¹⁰¹

Immediately before initial contact, the shoulder concludes its movement into maximum extension with scapular retraction to help the body’s center of mass move toward the new stance limb.^{85, 86} While the ipsilateral trapezius and the deltoids are most active,⁷⁹ the ipsilateral internal obliques and transversus abdominis are also active to help shift the center of mass to prepare for heel strike.²⁹

Ground Reaction Forces
Ground reaction forces (GRF) are the forces exerted between the body and the ground during ambulation. Since we move and live in three dimensions, the ground produces reaction forces in three planes. Two are shearing forces that are parallel to the ground, and the third is an impact force that is perpendicular to the ground (y-axis). Shear forces occur in a fore–aft direction (x-axis) and a lateral–medial direction (z-axis). At initial contact, the fore–aft shear force is a forward force between the ground and the foot as the forward-moving foot contacts the ground. During preswing, a backward GRF is produced when the foot pushes into the ground as it moves off the ground and into swing. If you step on ice and lose your footing at initial contact, the forward force causes your extremity to slip forward, so you may land on your backside if you fall. The reverse is true if you lose your footing during preswing; your foot slips backward, causing your body to move forward, so you may land on your outstretched arms, protecting your face and head as you fall.

Shortening stride length reduces fore–aft shear forces but increases vertical forces (figure 7.19). This is why it is safer to walk on ice with a shortened stride length: There is less forward force to slip the foot forward at initial contact and less backward force to slip the foot backward during preswing. Also, with a shortened stride, more of the foot surface is in contact at both the start and the end of stance, so forces are distributed over a larger area.

Fore–aft, or anteroposterior, forces are indicative of the deceleration forces that slow the body during initial contact and the acceleration forces that speed up the body’s forward motion before preswing. The medial–lateral shear force is predominantly a medial shear force during initial contact; since the foot hits the ground on the lateral heel, the force produced moves from lateral to medial, or is directed medially. As the entire foot contacts the ground and the subtalar joint pronates, the force becomes a lateral shear force. Although there is slight wavering of the medial–lateral force as the foot moves from a heel-off to a toe-off position, the force remains slightly lateral through the completion of the stance phase.

Vertical forces applied during stance are the effects of several factors, including the body’s weight, stride length, cadence, impact style, footwear, and ground surface.¹⁰² The amount of vertical force varies through the gait cycle and reflects the changing forces from shock absorption and deceleration during heel strike to acceleration as the extremity propels forward and moves into the swing phase. The greatest vertical force occurs during push-off as acceleration for forward propulsion occurs.

As you would expect, vertical forces are at minimal levels when the body weight is shared with the other lower extremity during double-limb support (figure 7.20).

It is important to be aware of ground reaction forces while examining for gait and musculoskeletal injuries. Some of the greatest forces applied to the foot occur during acceleration.¹⁰³ This can be crucial information when you are treating a patient who is a runner or participates in any activity in which ground reaction forces affect performance. For example, a pitcher who has first metatarsophalangeal joint pain will have difficulty at ball release and will need treatment of the great toe, since ground reaction forces applied to the painful joint significantly affect the pitcher’s follow-through.

Clinical Tips

Ground reaction forces occur because of gravity’s effect on all bodies. As we walk, our bodies respond to the earth, complying with Newton’s third law of motion regarding action and reaction. The more force that is applied downward when hitting the ground with the foot, the more force the earth applies against the foot. Some of those forces are absorbed by body segments and some are transmitted to other body parts. Although the greatest force is usually applied on the y-axis, vertically, we must not forget that fore–aft forces and medial–lateral forces also affect us when we walk.

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