Dorsiflexion and Plantar Flexion

Dorsiflexion and plantar flexion occur in the sagittal plane about a medial-lateral axis of rotation (Figure 11-7, A). Dorsiflexion describes the motion of bringing the dorsal part (top) of any region of the foot toward the anterior aspect of the tibia. Plantar flexion, in contrast, most often describes the motion of pushing the foot downward or, more correctly, moving the dorsal part of any region of the foot away from the anterior aspect of the tibia. Plantar flexion of the ankle, for example, occurs as one pushes down on the accelerator of a car.

Inversion and Eversion

Inversion and eversion occur in the frontal plane about an anterior-posterior axis of rotation (Figure 11-7, B). Inversion turns a point anywhere on the plantar aspect of the foot toward the midline. Eversion turns a point on the plantar aspect of the foot laterally, or away from the midline.

Abduction and Adduction

These motions occur in the horizontal plane about a vertical axis of rotation (Figure 11-7, C). Adduction describes a horizontal plane rotation of the foot as a point on its anterior surface moves toward the midline. Abduction describes the opposite movement, in which a point on its anterior surface rotates away from the midline.

Pronation and Supination

These specialized applied motions are based on a combination of the fundamental movements described above. Pronation (Figure 11-8, A) is a combined movement that includes eversion, abduction, and dorsiflexion of any region of the ankle and foot. Supination, in contrast, is a combined movement of inversion, adduction, and plantar flexion of any region of the ankle and foot (Figure 11-8, B). As indicated in this figure, these specialized movements occur most regularly at the subtalar and transverse tarsal joints. This concept will be reinforced later in the chapter.

General Features

The talocrural joint, commonly called the ankle joint, is created by the articulation between the trochlea of the talus and the concavity formed by the distal tibia and fibula. This concave part of the joint is often referred to as the mortise because of its resemblance to a mortise joint used by carpenters (Figure 11-10).

Several factors contribute to the stability of this joint: the tight rectangular fit of the talus in the mortise, the support of numerous collateral ligaments and muscles, and the strength of the distal tibiofibular joint.

Supporting Structures

The following structures support the talocrural joint (Table 11-3):

• Interosseous membrane (see Figure 11-3, B)

• Anterior and posterior tibiofibular ligaments (Figure 11-11, A)

• Deltoid ligament (Figure 11-11, B)

• Lateral collateral ligaments (see Figure 11-11, A)

• Anterior talofibular ligament

• Calcaneofibular ligament

• Posterior talofibular ligament

Kinematics

The talocrural joint possesses 1 degree of freedom, permitting dorsiflexion and plantar flexion of the ankle. This sagittal plane motion is essential to the forward progression of movement while walking. Dorsiflexion and plantar flexion are also important in permitting squatting motions, such as when transitioning between sitting and standing. Note that in this type of motion, the tibia moves relative to the foot; consider, for example, the dorsiflexed position of the ankle when holding a deep squat.

Normal range of motion for dorsiflexion is about 0 to 20 degrees. The 0-degree or neutral position of the foot is determined by a 90-degree angle between the fifth metatarsal and the fibula. The normal range of motion for plantar flexion is 0 to 50 degrees (Figure 11-12), although these ranges vary considerably depending on the type and method of measurement.

Dorsiflexion and plantar flexion of the ankle occur about a near medial-lateral axis of rotation that travels through the tips of each malleolus (Figure 11-12, A). These easily identifiable bony markers allow visualization of the axis, enabling one to understand the function of the muscles that cross this joint. Muscles that course anterior to this medial-lateral axis of rotation perform dorsiflexion, whereas muscles that course posterior to this axis of rotation perform plantar flexion.

The arthrokinematics of the talocrural joint is traditionally based on the convex trochlea of the talus within the fixed concave mortise. This occurs when the foot is off the ground, as the convex trochlea rolls and slides in opposite directions within the mortise. Figure 11-13 highlights the arthrokinematics of dorsiflexion and plantar flexion of the talocrural joint. Realize, however, that most of the time, the foot is fixed to the ground during the stance phase of walking. In this case, the concavity formed by the mortise rolls and slides in the same direction over the convex articular surface of the talus.

Functional Considerations: Most- and Least-Stable Positions of the Talocrural Joint

While walking, maximal dorsiflexion occurs late in stance phase, just before the heel rises off the ground (at about 40% of the gait cycle; see Figure 11-2). Realize that while in the stance phase of walking, the term dorsiflexion describes the position of the leg relative to the foot. At this point in the gait cycle, the ankle is most stable because most of the collateral ligaments and all of the plantar flexor muscles are stretched (Figure 11-14, A). The dorsiflexed ankle is further stabilized as the wider, anterior part of the trochlea of the talus becomes wedged into the mortise (Figure 11-14, B). For these reasons, the close-packed position of the ankle is full dorsiflexion. Such stability is necessary in late stance to prepare for the action of the strongly activated plantar flexor muscles during jumping or the push-off phase of fast walking.

The least stable position of the talocrural joint is full plantar flexion. Full plantar flexion—the loose-packed position of the joint—slackens most of the collateral ligaments and all of the plantar flexor muscles. The position of full plantar flexion also causes the mortise (distal tibia and fibula) to “loosen its grip” on the talus; this places the narrower width of the top of the talus between the malleoli, thereby releasing tension within the mortise. Weight bearing over a fully plantar flexed ankle, therefore, places the talocrural joint at a relatively unstable position. Wearing high heels or landing from a jump in a plantar flexed (and usually inverted) position increases the likelihood of spraining the ankle and injuring the lateral ligaments.

General Features

The subtalar joint is located within the rearfoot (see Figure 11-1). This joint consists of the articulation between the facets on the inferior surface of the talus and the matching facets on the superior surface of the calcaneus. The shape of this joint is specifically designed to allow frontal and horizontal plane motions between the foot and the lower leg. These motions are essential for adapting to uneven ground surfaces or cutting laterally or medially while walking or running.

Kinematics

The kinematics of the subtalar joint allows the combined motions of inversion/adduction and eversion/abduction of the rearfoot (Figure 11-15). (Recall that these motions are components of supination and pronation, respectively.) To appreciate the components that make up these motions, firmly grasp the calcaneus (heel) and twist it in a side-to-side and rotary fashion. The side-to-side motions—inversion and eversion—are most often used clinically to evaluate the strength and range of motion of the subtalar joint. The rotary (horizontal plane) motions are adduction and abduction. During subtalar motions, the trochlea of the talus is usually well stabilized within the mortise shape of the talocrural joint.

The motions just described for the subtalar joint involve the calcaneus moving underneath the fixed talus, which occurs when the foot is off the ground. More realistically, however, the subtalar joint usually operates in a weight-bearing position when the calcaneus is fixed against the ground during the stance phase of walking. Because the talus is firmly stabilized within the mortise, subtalar joint motion is most often expressed as a combined movement of both the talus and the lower leg, relative to a stationary calcaneus.

Functional Considerations: Subtalar Joint—Critical Kinematic Link Between the Leg and Foot

As has been described, motion at the subtalar joint is commonly expressed in one of two ways: when the calcaneus is free, such as during the swing phase, or when it is in firm contact with the ground during the stance phase of walking. While in the stance phase, the leg and talus move as one mechanical unit over the fixed calcaneus. Although the motion at the subtalar joint is small, it is nevertheless important. The subtalar joint allows a dissipation of the relatively slight horizontal and frontal plane rotations of the leg and talus that naturally occur when the lower extremity is in contact with the ground during the stance phase. To understand the importance of these motions, consider the consequences of a fused subtalar joint. In this scenario, the leg, talus, and calcaneus would all be forced to move together—following the rotating lower extremity. This would significantly alter an individual’s balance and ability to ambulate over uneven ground.

The normal subtalar joint is well utilized during walking and running, especially on unlevel terrain. To illustrate, consider the following example: When standing on level ground, the leg and talus are in relative alignment with the calcaneus, as is indicated by the red dots across the subtalar joint in Figure 11-16, A. Consider, however, what happens when the foot encounters uneven ground. Figure 11-16, B, illustrates the response of the subtalar joint as the medial side of the foot steps on a rock. In this scenario, the calcaneus rotates, resulting in inversion of the subtalar joint. This “righting” mechanism of the foot allows the leg to remain vertical, even while standing or walking on uneven surfaces. If this motion is excessive, however, it may result in a sprain of the lateral ligaments.

In other circumstances, it may be necessary that the calcaneus remain firmly planted on the ground, while the leg and body cut in medial or lateral directions. As illustrated in Figure 11-16, C, with the calcaneus well fixed, a medially directed movement of the talus and leg can occur as subtalar joint inversion. Realize that, although different bones are moving in Figure 11-16, B and C, the final position of the subtalar joint in both scenarios is the same—inversion. Without the available motion provided by the subtalar joint, walking on uneven surfaces would be extremely difficult and would likely result in loss of balance or injury to the ankle and foot.

General Features

The transverse tarsal joint separates the rearfoot from the midfoot (see Figure 11-1). This extensive joint consists of two separate articulations: the talonavicular joint and the calcaneocuboid joint. This pair of joints allows the midfoot to move independently of the rearfoot (i.e., the calcaneus and talus). The most important feature of this articulation, however, is its ability to perform the most pure form of pronation and supination. (Recall that pronation has nearly equal elements of eversion, abduction, and dorsiflexion; supination has nearly equal elements of inversion, adduction, and plantar flexion.) Figure 11-17 shows a physical therapist assistant moving a foot through an arc of pronation (Figure 11-17, A) and supination (Figure 11-17, B).

The oblique motions of pronation and supination at the transverse tarsal joint provide tremendous kinematic versatility to the foot. The midfoot (and attached forefoot) can mold into many positions, allowing the entire foot to conform to many different terrains while walking or running. Often the transverse tarsal joint functions in conjunction with the subtalar joint to control the final components of pronation and supination across the entire foot.