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.

MP joint dorsiflexion is an essential component of heel rise. The foot rolls up over the base of the toes, particularly the great toe, as the trailing limb advances.¹⁴

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.

Knee

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.

Hip

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.

Hip flexion results from serial activation of several muscles: adductor longus (plus rectus femoris), iliacus, sartorius, and gracilis.

Pelvis

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.

Head and trunk

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.

Integrated function of the limb

Task I: Weight acceptance

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.

Phase 8—Terminal swing: To prepare the swinging limb for stance, hip flexion is interrupted, the knee extends, and the ankle remains dorsiflexed (Fig. 5-2).

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.

Phase 1—Initial contact: Floor contact by the heel is the critical event (Fig. 5-3). Its purpose is to initiate the heel rocker for progression and shock absorption. The significant postures are ankle dorsiflexion and full knee extension. Initial floor contact by the heel is a forceful event, which begins with 1 cm of free fall between the foot and the ground. The impact registers 50% to 125% body weight during the first 10 to 20 ms of stance (1% to 2% of the gait cycle).^17,¹⁹ The heel responds to the impact by initiating small arcs of ankle plantar flexion and subtalar inversion. Anterior tibialis control of the foot determines heel rocker effectiveness.

Phase 2—Loading response (initial double stance): This is a highly demanding phase. At the ankle, the pretibial muscles (especially the anterior tibialis) preserve the heel rocker by intense isometric activity, while stretch of the anterior tibialis tendon allows a small arc of plantar flexion for shock absorption. This reverses the early plantar flexion arc to neutral (6° at 6% of the gait cycle) by the end of the loading response.¹⁸ The limb is destabilized by the heel rocker and then supported by strong extensor muscular response. There are three critical events (Fig. 5-4).

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.

At the hip, there is a rapid response by the abductor muscle group to stabilize the pelvis, which lost its contralateral support with the transfer of body weight to the forward limb.

Task II: Stance limb progression

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.

Phase 3—Midstance: The critical event is ankle dorsiflexion for progression of the stance limb over a stationary, flat foot (Fig. 5-5). As momentum from the contralateral swing limb moves the vector along the foot, the soleus (quickly assisted by the gastrocnemius) modulates the tibial advancement so the lower leg proceeds less rapidly than the femur. This provides passive extension of the hip and knee for weight-bearing stability. As a result, the hip extensor and quadriceps muscles rapidly relax and stability of the hip and knee become dependent on the strength of the plantar flexor muscles.

At the hip, there also is a major adducting moment as lifting the other limb for swing removes the support for that side of the pelvis (Fig. 5-6). This creates a large medial vector, which is restrained by the gluteus medius.

Phase 4—Terminal stance (late single stance): Heel rise is the critical event that continues progression (Fig. 5-7). Free dorsiflexion mobility of the MP joints also is essential. Both the foot and the limb roll forward over the forefoot rocker. The soleus and gastrocnemius muscles virtually lock the slightly dorsiflexed ankle, thus making the forefoot the site of limb rotation.

This creates a lever (ankle to metatarsal heads), which enlarges the arc of limb rotation. Heel height is preserved while greater advancement of the center of mass adds to step length. Heel rise is 3.5 cm at contralateral initial contact.¹⁴

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. 5-6 Single-limb stance creates an adductor moment at the hip as the pelvis, with midline vector medial to hip, is unsupported. Gluteus medius action restrains adductor moment.

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.

Task III: Limb advancement

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.

Phase 5—Preswing: Passive knee flexion to 40 degrees is the critical event because this is the primary contributor to foot clearance of the floor in swing (Fig. 5-8).

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.

Phase 6—Initial swing: The critical event is knee flexion sufficient for the toe to clear the floor as the thigh advances. This involves total limb flexion (Fig. 5-9). Hip flexion may be a passive continuation of the preswing events or result from direct action by the iliacus, sartorius, and gracilis. Attainment of full knee flexion largely depends on the imbalance between the forward momentum of the femur generated by hip flexion and inertia of the tibia. Active assistance also is provided by the biceps femoris, short head. Brisk activation of the pretibial muscles initiates ankle dorsiflexion, but the arc is incomplete in initial swing.

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