Chapter 5 Normal and pathologic gait
Each limb blends the patterns of motion, passive force, and muscular control into a sequence of activity (called a gait cycle or a stride), which is repeated endlessly until the desired destination is reached. The two limbs perform in a reciprocal manner, offset by 50% of the gait cycle. The head, neck, trunk, and pelvis are self-contained passengers riding on the limb’s locomotor system.15
The normal interactions of joint motion and muscle activity of walking serve four basic functions. Although each is described as a separate event, they occur in an overlapping fashion during the stride.
In a serial fashion, the extensor muscles maintain the limb’s ability to support body weight. This begins with the hamstrings and quadriceps preparing the swinging limb for stance. Responding to the rapid drop of body weight onto the foot, the hip extensors and quadriceps stabilize the flexed hip and knee, while the hip abductors support the pelvis. As body weight progresses over the foot, the ankle plantar flexors restrain the tibia and provide indirect extensor stability of the hip and knee.
This pattern of muscle control is dictated by the changing alignment of the body weight line (vector) with the individual joints. As the vector moves away from the joint center, a rotational moment develops that must be controlled by opposing muscles to preserve postural stability.
To advance the weight-bearing limb over the supporting foot (i.e., stance limb progression), three rocker actions are used. A fourth rocker initiates swing limb advancement. The sites of progression are the heel, ankle, forefoot, and toe.
The two forces stimulating progression are forward fall of body weight and momentum created by the swinging limb. From a quiet stance, forward fall is initiated by flexion of the swing limb and calf muscle relaxation, which allows the weight-bearing tibia to advance.
Selective relaxation of muscle action by substitution of momentum or passive positioning can conserve energy. Cocontraction of antagonists is rare. The normal occurrences are use of hamstrings and quadriceps during limb loading and anterior and posterior tibialis during medial foot control.
The simplest system for subdividing the continuum of activity that constitutes walking uses the timing of foot–floor contact as a frame of reference. The instant of initial floor contact has been designated as the start of the gait cycle. Within each cycle, the period of floor contact by any part of the foot is called stance. This is followed by an interval of midair limb advancement called swing. At the beginning and end of stance, there is an interval when both limbs are in contact with the floor for weight transfer. These intervals are called initial and terminal double stance. In between is a longer interval of single-limb stance, at which time the other foot is in the air. A more functional term is single-limb support. The limitation of this temporal classification system is that the divisions impart minimal indications of function.
To understand the purpose of individual joint motions and their modes of control, it is necessary to consider the action of the whole limb as the posture of each segment is influenced by the others. During a gait cycle, the limb moves through eight functionally distinct postural sequences, which are called phases of gait. Each has one or more events that are critical to accomplishing its purpose. These phases are combined into three primary tasks by the synergistic patterns of the muscles controlling the limb. Transitional actions between stance and swing create an overlap in the phase sequence. The actions in the final phase of swing (terminal swing) also prepare the limb for stance. Similarly, the final phase of stance (preswing) prepares the limb for swing before the toe is lifted.
During stance, loading the heel initiates motion at each joint. Prompt response by the muscles provides an eccentric force that limits the arc of joint motion and also serves as a shock-absorbing mechanism. Recent investigations of eccentric muscle function, using portable ultrasound sensors (taped over the target muscle) have differentiated muscle fascicles from tendon during walking and jumping. These findings have redefined eccentric muscle action, which commonly is defined as a lengthening contraction. However, an ultrasound display of the muscle fascicles showed no increase in length.9 The gain in length was stretch of the tendon while the muscle exerted an isometric contraction to stabilize the joint; muscles only shorten or maintain neutral length.12
During each stride, the ankle passes through four arcs of motion (Fig. 5-1). At the onset of stance, the ankle is in neutral dorsiflexion, and floor contact is by the heel. Rapid loading of the heel causes the ankle to quickly plantar flex and then return to neutral before forefoot contact. Ultrasound analysis of muscle function at this time identified that the motion was the result of tibialis anterior tendon stretch.6 Release of the stretch force occurred as the heel lever shortened with the advancement of the vector across the heel. Following forefoot contact with the ground, ankle motion reverses to 10 degrees dorsiflexion as the tibia advances over the stationary foot for stance limb progression. Then the ankle plantar flexes 20 degrees during the final phase of stance (preswing). As toe-off starts swing, the foot again dorsiflexes under control of the pretibial muscles. Full elevation of the foot to neutral, however, is not completed until midswing.
Fig. 5-1 Typical motion pattern of the limb during a gait cycle: Hip (top), knee (middle), and ankle (bottom). DF, Dorsiflexion; PF, plantar flexion; LR, loading response; MSt, midstance; TSt, terminal stance; PSw, preswing; ISw, initial swing; MSw, midswing; TSw, terminal swing. 0, Onset of gait cycle; 60, end of stance; 100, end of gait cycle.
The subtalar joint moves into eversion following initial floor contact by the heel. This unlocks the midtarsal joint, allowing it to dorsiflex slightly (arch flattens) following forefoot impact with the floor. Then the subtalar joint progressively inverts and locks the midtarsal joint through late midstance and terminal stance.
Dorsiflexion control is provided by the pretibial muscles (tibialis anterior, extensor hallucis longus, and extensor digitorum) during the loading response in addition to swing. The soleus and gastrocnemius control the tibia during stance limb progression. During terminal stance the EMG intensity of the gastrocnemius and soleus muscle mass increases rapidly in response to the dorsiflexion moment generated by the advancement of the body mass over the forefoot rocker. This same moment also stretches the tendon and gains 5 degrees of dorsiflexion at the ankle.9 In preswing, the tension of the Achilles tendon is abruptly released by the rapid transfer of body weight to the other limb. This creates a large power burst of plantar flexion by elastic recoil. The muscle mass is inactive (no EMG).9 The “push off” power generated is sufficient to initiate swing.12
Subtalar inversion control is created by the anterior tibialis, posterior tibialis, and soleus. The peroneus brevis and peroneus longus muscles restrain inversion as they produce an eversion force on the lateral side of the foot. Midtarsal restraint of the dorsiflexing forces created by body weight advancement is provided by the intrinsic flexor muscles as well as the subtalar muscles and the long toe flexors. The primary role of the flexor hallucis longus and flexor digitorum longus is to stabilize the MP joint during heel rise.
Within each gait cycle, the knee alternately flexes and extends both in stance and in swing (see Fig. 5-1). From a position of full extension at initial contact, the knee rapidly flexes 18 degrees during weight acceptance. Ultrasound analysis shows this motion is the result of patellar tendon stretch while the quadriceps muscle is undergoing an isometric contraction.6 This is followed by progressive extension throughout the period of single stance, reaching a final position of 5 degrees flexion. The knee then rapidly flexes to 40 degrees during preswing and continues to 60 degrees in initial swing. From this position, the knee then extends to neutral.
The quadriceps restrains knee flexion in stance and assists extension. All the vasti respond simultaneously. The gluteus maximus through its iliotibial band insertion also contributes to knee extensor stability. Brief and occasional action of the rectus femoris (and less frequently the vastus intermedius) restrains excessive preswing flexion. Knee flexion in swing is aided by the short head of the biceps femoris. Terminal swing knee extension is limited by the hamstring muscle group.
The major hip motions occur in the plane of progression (see Fig. 5-1). This consists of an arc of extension through stance, reaching 10 degrees hyperextension in terminal stance. A similar arc of flexion occurs from preswing through midswing. The resulting 35-degree flexed posture is maintained in terminal swing and loading response. In the other planes, there are small (4 to 5 degree) arcs of postural accommodation, which are described as pelvic motions.
Hip extensor muscle action begins with the hamstrings in terminal swing and proceeds to the gluteus maximus and adductor magnus during the loading response. Lateral stability of the hip in stance is provided by the gluteus medius–gluteus minimus complex and the tensor fascia lata.
The pelvis moves through small (5 degree) arcs in each plane as it yields to body weight in stance and follows the advancing limb in swing. Stability is provided by the muscles of the weight-bearing hip.
The basic function of the head and trunk is to maintain an upright posture. The small (5 degree) arcs of motion that occur reflect the uneven support provided by the reciprocal actions of the two limbs. Motion is greatest in the lumbar area and decreases at each higher segment. The spinal muscles act to preserve balance, absorb shock, and minimize head displacement.
Weight acceptance is the first determinant of the ability to walk. Two objectives determine the events that occur during this task: establishment of a stable limb for weight bearing and minimization of the shock of floor impact. The last phase of swing and the first two stance phases are dedicated to optimum weight acceptance.
Fig. 5-2 Terminal swing pattern of muscle control. The limb is positioned for stance by synergistic action of the hamstrings (posterior thigh), quadriceps (anterior thigh), and tibialis anterior (anterior leg).
Rapid, intense action by the hamstring muscles (semimembranosus, semitendinosus, biceps femoris long head) stops hip flexion and terminates swing. These muscles then reduce their intensity and allow the quadriceps to extend the knee. The continuation of mild hamstring action prevents knee hyperextension from the residual tibial momentum. Pretibial muscle action supports the dorsiflexed foot.
Fig. 5-3 Initial contact by the heel with pretibial muscle control (tibialis anterior shown) establishes the heel rocker. Vertical line represents the body weight vector. Both ground impact (large arrow) and base of the body weight vector (small arrow) are at the heel.
Fig. 5-4 Loading response vector (vertical line) is anterior to the hip (flexor moment is restrained by the gluteus maximus), posterior to the knee (quadriceps restraint of the flexor moment), and posterior to the ankle (plantar flexor moment is restrained by the tibialis anterior).
For both shock absorption and progression, the heel rocker drives the foot toward the floor as the limb is loaded. Response of the pretibial muscles to decelerate the dropping foot pulls the tibia forward. This places the vector behind the knee, leading to rapid knee flexion for shock absorption. Prompt quadriceps response opposes the vector’s flexor moment to preserve knee stability and absorb the shock of the initial floor impact. Knee extensor stability is aided by the femoral stability gained from the adductor magnus and gluteus maximus. Prompt relaxation of the hamstring muscles avoids an unnecessary flexor force.
The basic function is advancement of the limb (and body) over the supporting foot. This is the second determinant of the ability to walk. Two phases of single-limb support are involved as the means of progression differ.
Fig. 5-5 Midstance progression of the limb over the stationary foot generates two patterns of muscle action. In early midstance (left), the vector is behind the hip (no muscle action required), closer to the knee (less quadriceps) and anterior to the ankle (this dorsiflexor moment is retrained by the soleus). By late midstance (right), the vector is anterior to the knee, and no quadriceps action is needed. Ankle dorsiflexor moment has increased.
Fig. 5-7 Terminal stance progression advances the vector across the forefoot, and the heel rises. The vector remains behind the hip and knee joints (knee hyperextension moment is restrained by the gastrocnemius). Vector alignment at the ankle creates a maximal dorsiflexion moment, which is restrained by the soleus and gastrocnemius.
The ability to lift the foot is the third determinant of walking ability. Flexing the limb for floor clearance and swing advancement begins in the terminal double-support period of stance. Because the purpose is limb advancement rather than weight bearing, the phase has been titled preswing. The other actions occur throughout swing.
Fig. 5-8 Preswing transfer of body weight to the other limb reduces the vector. The base of the vector now is at the metatarsophalangeal joint. The unloaded foot falls forward with the tibia as it follows the dorsiflexion moment. Gastrosoleus tension induces ankle plantar flexion. The knee flexes in response to the posterior moment, with rectus femoris restraint if needed. Posterior hip moment is opposed by the flexor component of the adductor longus and rectus femoris.
Following floor contact by the other foot, body weight is rapidly transferred to that limb to catch the forward fall. The equally abrupt unloading of the trailing limb initiates a series of actions commonly called push-off. A rapid arc of ankle plantar flexion to 20 degrees is accompanied by passive knee flexion to 40 degrees, increased toe dorsiflexion, and release of the extended hip. The initial force is a large burst of plantar flexion power. Because there is no corresponding EMG, the source of the power is attributed to elastic energy generated by the abrupt release of the previously tense soleus and gastrocnemius muscles: push-off positions the limb for swing and initiates the action, allowing several small forces to be effective. As the limb’s trailing posture reduces the foot’s floor contact to the anterior margins of the metatarsal heads and the toes (fourth rocker), there is no stabilizing force, so the foot and the leg are free to roll forward. This is accelerated by the rapid ankle plantar flexion stimulated by the release of the tension stored in the eccentrically stretched soleus and gastrocnemius. Passive knee flexion is initiated. Unloading the limb also releases the tension in the hip flexors. This force combined with adductor longus action initiates early hip flexion and assists knee flexion.