MS2 LE Test and Gait

List factors that could limit full (active or passive) knee extension.

· Paralysis or reflex inhibition of the quadriceps · Increased passive tension in the posterior capsule, collateral ligaments of the knee, or knee flexor muscles · Excessive scarring of the skin in the popliteal fossa · Excessive knee swelling · Dislodged menisci

Describe two possible consequences that may result from long-term weakness of the fibularis longus muscle.

(i) Prolonged weakness reduces the main source of active stability to the lateral side of the ankle. Furthermore, the more dominant (unopposed) invertor muscles allow the forefoot to follow the rearfoot into supination during mid to late stance phase. As a result, the person tends to walk on the lateral border of the foot, increasing the likelihood of lateral metatarsalgia and recurrent inversion ankle sprains. (ii) The fibularis longus stabilizes the first tarsometatarsal joint against the strong medial pull of the tibialis anterior. (Compare the two muscles' opposing distal attachments sites in Fig. 14.45.) Without this muscular stability, the first ray may migrate medially, possibly initiating pathomechanics-associated hallux valgus deformity.

As indicated in Figure 12-12, during the swing phase of walking the hip experiences (compression) forces of about 10-20% of body weight. What causes this force?

During the swing phase of walking, the forces from the contracting hip flexor muscles compress the head of the femur against the acetabulum.

Figure 12-22A shows a seated person performing a 30-degree anterior pelvic tilt. What structure(s) is/are most likely responsible for determining the end range of this motion?

-End range of extension of the lumbar apophyseal joints · Gluteus maximus · Deep fibers within the inferior and posterior parts of the hip capsule · Adipose tissue located between the front of the thigh and pelvis

A patient sustained a severe fracture of the femoral head and the acetabulum, with marked reduction in contact area between the articular surfaces of the joint. As part of the reconstructive surgery, the surgeon decides to slightly increase the internal moment arm of the hip abductor muscles. What is the likely rationale for this procedure?

A hip with markedly reduced contact area is subjected to higher than normal hip stress (pressure). Because the hip abductor muscles generate the largest of all muscular-based compression forces on the hip, a surgeon may decide to increase the leverage of this muscle group. In theory, increasing the internal moment arm of the hip abductor muscles will reduce the compressive stress produced on the joint during the single-limb support phase of walking. This protective measure may reduce the likelihood of the hip developing degenerative arthritis.

Explain why a patient with an inflamed capsule of the hip joint may be susceptible to a hip flexion contracture.

A person with an inflamed hip capsule often feels most comfortable with the hip partially flexed, a position that reduces intracapsular pressure. Over time, the capsule and hip flexor muscles may experience adaptive shortening, leading to a hip flexion contracture.

Describe the changes that typically occur in gait in aged persons. What natural protection may these changes provide?

A slower gait velocity is typically adopted as a person reaches advanced age. This is achieved by a combination of reduced step length and a slower cadence. These changes increase the amount of time spent in double-limb support, which leads to greater stability (Figure 15-9) and assumed reduced risk of falling.

At what points in the gait (walking) cycle is the potential energy (A) greatest and (B) least?

As illustrated in Figure 15-25, potential energy is greatest when the center of mass of the body is at its highest location, at 30% and 80% of the gait cycle. Conversely, potential energy is at its lowest during double-limb support when the center of mass is at its lowest location, at 5% and 55% of the gait cycle.

List muscles and ligaments capable of resisting external rotation of the knee. Why would this function be especially important from a femoral-on-tibial (weight-bearing) perspective?

All internal rotator muscles of the knee can resist external rotation. These internal rotators include the semitendinosus, sartorius, gracilis, semimembranosus, popliteus, and, perhaps slightly, the vastus medialis (oblique fibers). The medial collateral ligament (including the posterior-medial capsule) and oblique popliteal ligament also resist external rotation of the knee. The demands placed on these aforementioned muscles and ligaments can be high during the motion of planting one lower limb securely to the ground while vigorously "cutting" to the opposite side. For example, planting the right foot and lower leg securely to the ground and rotating the femur (and rest of the body) to the left require external rotation motion of the right knee. (This external rotation motion is the result of internal rotation of the femur relative to a fixed tibia.) This motion must be decelerated at its end range by eccentric activation of internal rotator muscles of the knee and by increased tension in the medial collateral ligament, posterior-medial capsule, and oblique popliteal ligament.

As described in this chapter, the maximum-effort torques produced by the internal and external rotator muscles of the knee (when tested at 90 degrees of flexion) are of about equal magnitudes. How can this fact be justified given the disparity in the number of internal and external rotator muscles?

Although the number (and total cross-sectional area) of the internal rotator muscles of the knee exceeds that of the sole external rotator (i.e., the biceps femoris), the moment arm of the biceps femoris for external rotation torque is three times greater than the average moment arm of the internal rotator muscles for internal rotation torque. This fact likely justifies why, at least with the knee flexed to about 90 degrees, maximal-effort torques of the two muscle groups are about the same.

Explain why a person with a weak calf muscle may complain of "buckling" of the knee before the push off phase of walking.

An important function of the gastrocnemius-soleus muscle group during the stance phase of walking is to decelerate the rate and control the extent of dorsiflexion of the talocrural joint (i.e., forward rotation of the tibia relative to the talus). Without a sufficiently strong force from the gastrocnemius-soleus, the tibia may rotate too far or too fast over the talus just before the push off phase. In this event, the forward tibia displaces the line of gravity (due to body weight) posterior to the medial-lateral axis of rotation at the knee. Without a sufficient and relatively strong contraction of the quadriceps, this sudden (external) flexion torque at the knee can create a buckling (uncontrolled flexion) of the knee.

Describe how contraction of the quadriceps muscle could elongate (strain) the anterior cruciate ligament. How is the strain on the ligament affected by (a) the knee joint angle and (b) the magnitude of quadriceps and hamstring muscle coactivation?

An isolated contraction of the quadriceps can create an anterior force on the proximal tibia that elongates and therefore increases the tension and length in the ACL. In general, the tension in the ACL is proportional to the contractile force in the quadriceps. The magnitude of the muscular-based tension in the ACL increases as the knee moves closer to full extension because the quadriceps produces a greater anterior translation force on the tibia (based on the increased angle of insertion of the patellar tendon onto the tibia). Because of the unloading effect of hamstring activation on the ACL, coactivation of the quadriceps and hamstring muscles reduces the tension on the ACL, bringing it close to zero at knee angles greater than 30 degrees of flexion.

Between about 30% and 50% of the gait cycle, a person with a tight or short heel cord (Achilles tendon) often makes kinematic compensations within the ankle and foot as a way to allow continued forward rotation of the leg relative to the ground. Describe a kinematic compensation that may allow this, including the specific joint(s) where it may occur.

As discussed in the chapter, several strategies may be used to compensate for a lack of ankle dorsiflexion in the later part of stance phase. The individual may demonstrate an early heel off, the heel coming off the ground before 30-40% of the gait cycle. This results in a "bouncing" type gait pattern with exaggerated vertical fluctuation of the center of mass. Another possible compensation is to externally rotate the lower extremity at the hip and use what is referred to as a "toeing-out" gait pattern. While this lower extremity alignment reduces the need for ankle dorsiflexion, it may force the foot into excessive pronation.

At 10% into the gait cycle, describe the position and direction of rotation of the hip, knee, and ankle with respect to the sagittal plane.

As illustrated in Figure 15-13, at 10% of the gait cycle, the hip is in approximately 30 degrees of flexion and is starting to move toward extension as the body is moving forward. At this time, the knee is in approximately 15 degrees of flexion and is still flexing as the lower extremity is accepting the weight of the body. The ankle is in a small amount of plantar flexion but is progressively moving toward dorsiflexion as the lower leg (tibia) is moving forward over the fixed foot.

(A) Describe the rotation at the ankle between 5% and 40% of the gait cycle with respect to the sagittal plane. (B) Describe the type of muscle activation (eccentric, isometric, concentric) of the ankle plantar flexor and dorsiflexor muscles within the context of the kinematics described in part A.

As illustrated in Figure 15-13D, from 5% to 40% of the gait cycle, the ankle progressively moves toward greater dorsiflexion as the result of the lower leg moving forward over the fixed foot. Through concentric activation, the ankle dorsiflexors may have a small initial role in advancing the lower leg over the foot between 8% and 20% of the gait cycle (Figure 15-42D). Thereafter the movement of dorsiflexion is primarily controlled through an eccentric action of the ankle plantar flexors as illustrated in Figure 15-42D.

(A) For 0% to about 50% of the gait cycle, describe the kinematics of the hip joint in the horizontal plane. (B) Using Figure 15-29A as a guide, discuss a possible role of the gluteus minimus and the gluteus medius muscles during these kinematics.

As illustrated in Figure 15-21, the hip rotates toward internal rotation from 0% to 50% of the gait cycle. This reflects the forward movement of the contralateral pelvis and advancing lower extremity. As the gluteus minimus and medius of the stance limb are active at this time (Figure 15-29A) and are capable of internally rotating the hip, these muscles likely assist with the forward advancement of the contralateral pelvis and lower extremity during its swing phase.

Referring to Figure 13-38, how does increasing forward trunk lean while landing from a jump alter the magnitude of the muscular response of the quadriceps and hamstring muscles? What are the clinical implications of this alteration?

As indicated in Figure 13-38B, increasing trunk flexion (or forward lean) typically decreases the (external) flexion moment arm at the knee, and increases the (external) flexion moment arm at the hip. As a consequence (and in contrast with Figure 13-38A), the hip extensors (including the hamstrings) respond strongly while the quadriceps respond only moderately. The higher hamstring-to-quadriceps activation ratio in Fig 13-38B reduces the anterior stain on the ACL. Also, in general, reducing the activation of the quadriceps reduces the stress on the patellofemoral joint—a point that may be relevant in persons with patellofemoral joint pathology. Figure13-38 nicely demonstratives a global principle of "protecting" joints from damage because of potentially large muscle-based joint forces. The figure specifically applies to the hips and knees. In general, the joint reaction forces can be reduced by orienting the line of gravity (body weight) closer to the joint that requires protection (i.e., relative "unloading"). The posture depicted in Figure 13-38A favors reduced muscular and joint loading at the hip at the expense of increasing these forces at the knee. In contrast, Figure 13-38B favors reduced muscular and joint loading at the knee at the expense of increasing these forces at the hip.

During about 5% to 20% of the gait cycle, correlate the functional association between the frontal plane kinematics at the stance hip with the type of muscle activation of the gluteus medius.

As shown in Figure 15-36, during about 5% to 20% of the gait cycle, the hip of the stance limb is moving toward adduction while a strong action of the gluteus muscle is taking place. Therefore, the gluteus medius is active eccentrically to control the downward movement of the pelvis of the side of the swing limb.

Using Figure 14-43 as a reference, contrast the inversion torque potential of the tibialis anterior and the extensor hallucis longus (at the subtalar joint).

Assuming equivalent cross-sectional areas, the tibialis anterior has a greater inversion torque potential than the extensor hallucis longus, at least from the anatomic position. This assessment is based on the different lengths of the two muscles' moment arms, relative to the subtalar joint's axis of rotation.

Weakness of the ankle dorsiflexor muscles is associated with some very typical gait deviations. Contrast gait deviations at the ankle and foot that may likely occur in persons with the three following levels of dorsiflexion weakness: (a) mild (30% loss of strength or 4/5 based on the standard manual muscle testing scale), (b) moderate (greater than 50% loss of strength or 3-/5 strength), or (c) severe (80-90% loss of strength or 2/5 strength).

At initial heel contact, the ground reaction forces applied to the calcaneus require a strong eccentric action of the ankle dorsiflexor muscles to bring the rest of the foot to the ground in a controlled plantar flexed manner. A person with mild weakness of the dorsiflexors often shows a clinical sign of "foot slap"—the characteristic sound made by the foot uncontrollably and rapidly hitting the ground immediately after initial heel contact. In this scenario, the ankle dorsiflexors have sufficient strength to hold the ankle near its normal neutral position during the swing phase but are not sufficiently strong to control the descent of the foot to the ground after heel contact. In cases of moderate weakness of the dorsiflexors, the lower limb often strikes the ground with a relative "foot flat "appearance. This occurs because during the swing phase the weakened muscles are not able to sufficiently dorsiflex the foot to allow the heel to make initial contact with the ground at the beginning of stance. Instead, initial contact is made by the entire surface of the foot—hence the term "foot flat." In a yet more severe scenario of dorsiflexor weakness the ankle is held plantar flexed throughout swing phase (often referred to as "drop foot"). As a result, initial contact with the ground at early stance is made by the forefoot region, with the ankle moving into dorsiflexion immediately after initial contact. A plantar flexed position of the ankle during swing may also be associated with a fixed plantar flexion contracture (i.e., pes equinus deformity). But, if the person does indeed have a pes equinus deformity, the plantar flexed foot posturing would in some manner persist throughout the stance phase.

Explain why the patellofemoral joint is least mechanically stable in the last 20 to 30 degrees of knee extension.

At the last 20-30 degrees of full knee extension, the patella is less stable and more susceptible to lateral dislocation because (a) it is less physically engaged within the trochlear groove of the femur, (b) the Q-angle is greatest owing to the external rotation component of the screw-home mechanism, and (c) the compression forces due to quadriceps contraction are relatively low at the patellofemoral joint.

Justify how bilateral tightness in the adductor longus and brevis could contribute to excessive lumbar lordosis while standing.

Because the adductor longus and adductor brevis are also hip flexors, their bilateral tightness may be expressed as an exaggerated anterior pelvic tilt, at least while standing. An increased anterior pelvic tilt is associated with an increased lumbar lordosis.

Using Figure 15-35A,B,D, explain the exchange of mechanical energy for sagittal plane motion of the hip (Figure 15-35C) for the transition from stance to swing phase (35-60% of the gait cycle).

Between 35% and 50% of the gait cycle, the hip is going toward extension while a flexion torque partially created by eccentric activation of the hip flexors is present. The flexion internal torque during this time is also due, in part, to the elongation (and subsequent passive tension) in the anterior structures at the hip (e.g., capsular ligaments). The combination of movement toward hip extension and a flexion internal torque results in energy absorption at the hip. At 50% through 60% of the gait cycle, hip flexion movement is initiated through an internal flexion torque at the hip, which is due to concentric activation of the hip flexors. This combination of movement and torque results in energy generation. In summary, during the transition from stance to swing phase, the eccentric activation of the hip flexor muscles first decelerate hip extension, corresponding to energy absorption, followed by a concentric activation that accelerates the hip toward flexion, corresponding to energy generation.

Which part of the gait cycle requires greater dorsiflexion at the talocrural joint: the stance phase or the swing phase?

Dorsiflexion of the ankle is greatest during the stance phase, at about 40% of the gait cycle (Figure 14-19). This maximal range of dorsiflexion occurs just before the initiation of the push off phase of the gait cycle.

Describe the timing and type of muscular activity of the quadriceps muscle during the early part of the stance phase of gait.

During the early part of the stance phase of gait, the quadriceps are active eccentrically to control the slight flexion of the knee. This muscular action helps absorb the impact of the lower limb striking the ground.

At what point in the gait (walking) cycle are the (A) semitendinosus and (B) gastrocnemius most likely at their greatest length?

Estimating the length of these muscles requires considering the angular position of the two joints each muscle crosses. The semitendinosus is elongated by hip flexion and knee extension. Based on the sagittal plane kinematics illustrated in Figure 15-13B,C, the combination of hip and knee positions leading to maximal elongation of this muscle is just before heel contact (90-95% of the gait cycle), when the knee is near full extension and the hip is nearly maximally flexed. For the gastrocnemius, elongation occurs through a combination of ankle dorsiflexion and knee extension. Figures 15-13C,D illustrate that maximum elongation of the gastrocnemius is likely near 40% to 50% of the gait cycle, when maximum ankle dorsiflexion is reached (just before heel off) and when the knee is in nearly full extension.

A patient has excessive anteversion of the femur and acetabulum. Which extreme hip motion (in the horizontal plane) would most likely be associated with a spontaneous anterior dislocation?

External rotation

Describe kinematic strategies typically used to optimize the vertical and medial-lateral displacements of the center of mass of the body during walking.

Figure 15-27 provides a visual summary of the four strategies used to optimize the vertical displacement of the center of mass while walking. Two strategies help limit the downward displacement of the center of mass at initial heel contact and later in the stance phase at toe off. The amount of hip flexion (angle of the lower extremity away from vertical) is reduced by the horizontal forward movement of the opposite pelvis during swing phase and functional lengthening of the lower extremity is achieved by the relatively large size of the calcaneus at heel contact and the length of the foot segment at toe off. The small amount of knee flexion that is maintained during mid stance and the small amount of frontal plane adjustment of the pelvis help reduce the elevation of the center of mass at mid stance. Medial-lateral displacement of the center of mass is controlled by placement of the foot.

At what points in the gait (walking) cycle are the vertical ground reaction forces (A) greatest and (B) least?

Figures 15-30 and 15-31C provide a visual representation of the vertical ground reaction forces while walking. Note that the maximum vertical ground reaction forces are at approximately 10% and 45% of the gait cycle (Figure 15-31C) for the right lower extremity, and 60% and 95% of the gait cycle for the left lower extremity. These peak forces correspond to the time of weight acceptance and push off for each lower extremity. The lowest vertical ground reaction forces are at 30% of the gait cycle for the right lower extremity and 80% of the gait cycle for the left side. This corresponds to mid stance on each lower extremity when there is reversal in the direction of movement of the center of mass of the body, which has just reached its maximum height.

Contrast the arthrokinematics of (femoral-on-pelvic) hip flexion and extension with those of internal and external rotation.

Flexion and extension involve a spin between the femoral head and the lunate surface of the acetabulum. From the anatomic position, internal and external rotation involves a roll and opposite-directed slide of the femoral head relative to the acetabulum.

(A) From about 30% to 50% of the gait cycle, describe the likely position and direction of movement of the subtalar joint in the frontal plane. Use frontal plane movements of inversion and eversion (of the calcaneus) as a reference for your description. (B) Using Figure 15-29B as a guide, explain the most likely role of the tibialis posterior muscle in controlling these kinematics.

From 30% to 50% of the gait cycle, the subtalar joint progressively moves from an everted to an inverted position (Figure 15-19). Functionally this transforms the foot from a relatively supple (pliable) structure to a more rigid structure. As illustrated in Figure 15-29B, the tibialis posterior is active during this part of the gait cycle as it decelerates and limits eversion, and then initiates inversion. At the talocrural joint, the muscle also decelerates dorsiflexion followed by an initiation of plantar flexion.

What factors contribute to the stability of the talocrural joint in full dorsiflexion?

Full dorsiflexion of the ankle elongates several of its collateral ligaments as well as the tendons of the many plantar flexor muscles, most notably the Achilles tendon. Full dorsiflexion also wedges the wider anterior part of the talus within the mortise. These factors help stabilize the fully dorsiflexed talocrural joint.

Using a ruler and Figure 12-30 as a reference, which muscle appears to have the greatest moment arm for hip abduction?

Gluteus medius.

Which of the following activities create greater compression stress (pressure) on the articular surfaces of the patellofemoral joint: (a) maintain holding a partial squat with knees flexed to 10-20 degrees or (b) holding a deeper squat with knees flexed to 60-90 degrees? Why?

Holding a deeper squat creates greater joint compression stress on the patellofemoral joint because of the increased force demands placed on the quadriceps in conjunction with the reduced knee joint angle. As shown in Figure 13-28B, the greater knee flexion increases the sum of the quadriceps and patellar tendon forces that oppose the patellofemoral joint.

A standard way for persons to stretch their own rectus femoris is to simultaneously flex the knee while extending the hip. When performing this stretch, some persons with a tightened rectus femoris also hold their extended hip slightly abducted. Why would this be?

In the anatomic positon, the rectus femoris has a slight overall moment arm to abduct the hip. Holding the extended hip slightly abducted therefore slackens the muscle slightly, thereby reducing some of the discomfort caused by the relatively extensive stretch placed on the muscle by the flexed knee and extended hip.

At about what arc of knee motion does the quadriceps muscle produce its largest internal torque? What factor(s) most likely accounts for this?

Maximal-effort knee extensor torque typically occurs between 45 and 70 degrees of knee flexion. The relatively high torque can be explained primarily by the extensor muscle's internal moment arm length, which is greatest throughout most of this range of motion

Explain how excessive tibial torsion could mask the functional expression of excessive femoral anteversion.

Persons with excessive femoral anteversion may walk with the hip in excessive internal rotation ("in-toeing") as a way to optimize the fit of the hip joint (Chapter 12). The presence of excessive tibial torsion (excessive external rotation of the distal tibia relative to the proximal tibia) may mask this internal rotation posturing of the hip.

Referring to Figure 15-42, explain the virtual lack of energy (power) generation or absorption during 10% to 30% of the gait cycle despite the presence of an increasing plantar flexion torque. As part of the explanation, contrast this virtual lack of energy exchange between 10% and 30% of the cycle to (a) the small energy absorption observed from 0% to 8% of the gait cycle and (b) the significant energy generation present from 45% to 60% of the gait cycle.

Power, or rate of work, is the product of angular velocity and internal torque. Therefore, despite the presence of an internal (plantar flexion) torque observed between 10% and 30% of the gait cycle, the relatively low (near zero) ankle joint angular velocity at the same time results in near absent power (energy) generation or absorption. The small amount of power absorption observed between 0% and 8% of the gait cycle is the result of a small amount of internal torque occurring simultaneously with a relatively higher velocity of ankle plantar flexion movement. Finally, between 45% and 60% of the gait cycle, just prior to toe-off, a large plantar flexion internal torque and rapid plantar flexion movement combine to create significant power generation.

How can severe hyperextension of the knee cause injury to both the ACL and the PCL?

Severe hyperextension of the knee while in a weight-bearing position may cause injury to the ACL because of the excessive posterior femoral slide that could accompany this (femoral-on-tibial) movement. Severe hyperextension may also injure the PCL if the injury also involves excessive posterior "gapping" of the knee.

Based on Figure 12-35, which muscle has (a) the least leverage and (b) the greatest leverage for producing internal rotation torque?

The adductor brevis has the least leverage and the anterior fibers of the gluteus medius the greatest leverage for internal rotation torque

Considering the first tarsometatarsal joint, which muscle is considered the most direct antagonist to the fibularis longus?

The anterior tibialis.

List the bones that make up (a) the ankle and (b) the rearfoot. Which bone is common to both regions?

The bones of the ankle consist of the distal tibia, the distal fibula, and the talus. The bones of the rearfoot consist of the talus and the calcaneus. The talus is common to both regions.

What is one likely role of the adductor longus muscle at 60% to 75% of the gait cycle?

The burst of activity of the adductor longus illustrated in Figure 15-29A likely reflects the potential role of the adductors as hip flexors when the toes come off the ground and the swing limb is brought forward.

Explain how a reduced center-edge angle of the acetabulum could favor a dislocation of the hip.

The center-edge (CE) angle describes the extent to which the acetabulum covers the top of the femoral head. As indicated by Figure 12-13A, a smaller CE angle indicates less coverage of the femoral head, which increases the likelihood of a dislocation

What characteristics define the close-packed position of the hip? How do these characteristics differ from most other synovial joints of the body?

The close-packed position of the hip is defined by the position that creates the greatest (stretch) tension in the capsular ligaments. (In the hip this position is extension, slight internal rotation, and abduction.) In many other joints of the body, the position that stretches most of the ligaments is also the position where the joint is most congruent. This is not the case with the hip; the hip is most congruent in flexion, external rotation, and abduction.

Why do the medial collateral ligament and the medial meniscus often become traumatized by a similar mechanism of injury?

The deeper fibers of the medial collateral ligament attach partially to the medial meniscus. Excessive tension applied to this ligament during an excessive and combined valgus and axial rotation stress to the knee, for example, may be transferred to the medial meniscus, possibly creating injury.

Describe how the first tarsometatarsal joint is frequently involved with the development of hallux valgus (bunion).

The development of hallux valgus is often mechanically associated with a deviation of the first ray towards the midline of the body. The abnormal alignment of the first tarsometatarsal joint is often referred to as an adduction (or varus) deformity. Based on the pathomechanics of a "zig-zag" deformity (Chapter 7), the medial deviation of the first metatarsal can exaggerate the lateral deviation of the first phalanx of the great toe.

Propose a mechanism that could explain why active plantar flexion torque at the ankle is about 20% to 30% greater with the knee extended than when flexed.

The extended knee increases the length of the gastrocnemius. The increased length may augment the muscle's force output (based on its length-tension relationship), thereby increasing the overall plantar flexion torque at the ankle.

Which structures (joints and connective tissues) bind the fibula to the tibia?

The fibula and tibia are bounded by the interosseous membrane and ligament, the anterior tibiofibular ligament, and the posterior tibiofibular ligament.

Compare the distal attachments of the fibularis brevis and fibularis tertius. Justify how these muscles have different actions within the sagittal plane but similar actions in the frontal plane.

The fibularis brevis attaches distally to the styloid process of the fifth metatarsal. The fibularis tertius attaches distally to the dorsal base of the fifth metatarsal. Even though the muscles have a relatively similar distal attachment, each has an opposite action at the talocrural joint. The fibularis tertius is a dorsiflexor because it passes anterior to the axis of rotation at the talocrural joint. Because the tendon of the fibularis brevis passes posterior to the lateral malleolus, it exerts a force that is posterior to the axis of rotation at the talocrural joint, and therefore performs plantar flexion. Both the fibularis brevis and the fibularis tertius produce eversion at the subtalar joint because they exert a force that passes lateral to the axis of rotation at this joint.

Describe the path of the tendon of the flexor hallucis longus, from its belly to its insertion on the great toe.

The flexor hallucis longus originates from the distal two-thirds of the fibula. The tendon of the muscle courses within a connective tissue sleeve located between the medial and lateral tubercles of the talus and the inferior edge of the sustentaculum talus. The tendon continues to its distal attachment at the base of the distal phalanx of the great toe

Describe how the ischiofemoral ligament becomes taut in full internal rotation and extension of the hip. Include both femoral-on-pelvic and pelvic-on-femoral perspectives in your description.

The ischiofemoral ligament arises from the pelvis near the posterior and inferior rim of the acetabulum. The ligament attaches distally to the apex of the greater trochanter. From a femoral-on-pelvic perspective, internal rotation and extension (of the femur) move the apex of the greater trochanter away from the ligament's pelvic attachments. From a pelvic-on-femoral perspective, internal rotation and extension of the hip move the posterior--inferior rim of the acetabulum farther away from the ligament's femoral attachments. (A skeletal model and Figure 12-15 may help visualize these movements.) In both femoral-on-pelvic and pelvic-on-femoral movements, the ischiofemoral ligament is stretched and becomes taut.

Which muscle depicted in the dynamic bilateral hip adduction event of Figure 12-33 is active eccentrically? Please justify your answer.

The left gluteus medius is active eccentrically. Assuming this muscle is indeed active, the rotation of the left hip into (pelvic-on-femoral) adduction would elongate the gluteus medius. Any muscle that is active while being pulled into a longer length is experiencing eccentric activation.

What is the primary mechanism by which the menisci reduce pressure across the articular surfaces of the knee?

The menisci reduce pressure across the articular surfaces of the knee by increasing the fit and contact area between the tibia and femur. This protective function requires that the menisci are securely attached to the intercondylar area of the tibia.

Justify (a) why the popliteus is called the "key to the knee" and (b) how the popliteus can provide both medial and lateral stability to the knee.

The popliteus is called the "key to the knee" because of its relatively strong biomechanical potential to produce internal rotation torque. Internal rotation "unlocks" the extended knee whereas external rotation "locks" the knee in extension. While in a weight-bearing position, the popliteus indirectly stabilizes the medial side of the knee as follows. As a primary internal rotator muscle of the knee, the popliteus (along with the "pes" muscles) can decelerate external rotation of the knee. By actively decelerating external rotation of the knee (resisting internal femoral rotation relative to a firmly planted lower leg), the popliteus can limit the tension placed on structures such as the medial collateral ligament and posterior-medial capsule. Along with the lateral collateral ligament, the thick intracapsular tendon of the popliteus directly stabilizes the lateral side of the knee, resisting a varus movement of the knee.

While standing, a person performs a full posterior pelvic tilt while keeping the trunk essentially stationary. Describe how this movement could alter the tension in the anterior longitudinal ligament and the ligamentum flavum in the lumbar region.

The posterior pelvic tilt decreases the lumbar lordosis. The associated increased lumbar flexion slackens the anterior longitudinal ligament and increases tension in the ligamentum flavum.

Describe the primary arthrokinematics of inversion and eversion at the talonavicular joint.

The primary arthrokinematics at the talonavicular joint involve a spinning motion between the concave, posterior side of the navicular bone and the head of the talus

Polio affecting the L2-L4 spinal nerve roots would theoretically cause paralysis of what muscle group of the knee? (Hint: Consult Appendix IV, Part A.)

The quadriceps would theoretically be completely paralyzed

What structures convert the greater sciatic notch to a foramen? List three structures (nerves or muscles) that pass through this foramen.

The sacrotuberous and sacrospinous ligaments convert the greater sciatic notch to a greater sciatic foramen. The piriformis muscle, sciatic nerve, and inferior gluteal nerve exit through the greater sciatic foramen.

Figure 15-40 shows the primary mechanics associated with the production of a varus torque at the knee through most of stance phase. Which tissues at the knee are capable of limiting this torque?

The varus torque at the knee is resisted by structures on the lateral aspect of the knee, which include the vastus lateralis, biceps femoris, lateral head of the gastrocnemius, the tensor fascia lata, and the posterolateral capsule and ligaments.

A person sustained an injury of the cauda equina resulting in reduced function of spinal nerve roots L3 and below. What pattern of muscular tightness may develop without adequate physical therapy intervention? (Consult Appendix IV, Part A, for assistance in responding to this question.)

This injury would spare the spinal nerve roots associated with L1 and L2 of the lower extremity. As noted in the chart in Appendix IV, Part A, many of the hip flexor and adductor muscles would remain at least partially innervated, with paralysis of all other muscles. This situation would increase the likelihood of developing a hip flexion and adduction contracture.

Why do most persons have slightly greater active knee flexion range of motion with the hip fully flexed as compared to fully extended?

Two factors can account for the fact that active knee flexion range of motion is usually less when performed with the hip in full extension (as compared with full flexion). First, performing active (or passive) knee flexion from a position of full hip extension creates increased passive tension in the elongated rectus femoris. This increased passive tension naturally opposes knee flexion. Second, performing active knee flexion with the hip in full extension requires that the hamstrings function at an overly shortened length. The shortened length reduces the muscle's ability to actively flex the knee, especially against the increased passive tension generated by the stretched rectus femoris.

What are the two basic kinematic mechanisms used to increase walking speed?

Walking speed is increased by simultaneously increasing step length and step rate until a maximum step length is achieved. Thereafter, step rate increases until periods of double-limb support are no longer possible, at which point running is initiated.

Describe the type of quadriceps and hamstring muscle activation (i.e., eccentric, concentric, etc.) that occurs across the hip and knee while one slowly sits into a chair.

While slowly sitting into a chair, the quadriceps are active eccentrically to control the extent and speed of knee flexion. The hamstring muscles are active eccentrically to control the extent and speed of hip flexion.

Describe the roll-and-slide arthrokinematics of dorsiflexion at the talocrural joint with the foot free (Figure 14-18A) and with the foot fixed (Figure 14-20A).

With the foot free, dorsiflexion occurs by an anterior roll and posterior slide of the talus. (To help visualize the rolling of the talus, it may be helpful to follow the rotation of an imaginary point on the inferior aspect of the bone rather than on its superior [trochlear] surface.) With the foot fixed, dorsiflexion occurs by an anterior roll and anterior slide of the tibia and fibula (concave segment of the mortise) relative to the talus.

Which deformity would most likely develop after weakness of the invertor muscles? Which muscles would you stretch? Which muscles would you attempt to strengthen?

Without appropriate therapeutic intervention, weakness of the invertor muscles (especially if severe) could cause tightness in the evertor muscles, possibly leading to their adaptive shortening. A pes valgus deformity is therefore likely in the rearfoot and midfoot.