The location of the knee between the long bones of the lower
extremity, combined with its weight-bearing and locomotion functions,
makes it susceptible to injury, particularly during participation in
contact sports. A common injury mechanism involves the stretching
or tearing of soft tissues on one side of the joint when a blow
is sustained from the opposite side during weight bearing.
Injuries to the anterior cruciate ligament (ACL) are occurring
with increasing frequency: The number of all skiing-related injuries
reported to involve ACL rupture escalated from 6% in 1979
to 20% in 1996 (13). During 2000, there were an
estimated 80,000 ACL tears in the United States, with the highest risk
group being participants in the 15- to 25-year-old age range in
selected sports (39). ACL injuries are common in sports
such as basketball and team handball, which involve pivoting and
cutting, as well as in Alpine skiing, where a common mechanism involves
catching a ski tip in the snow, with the skier simultaneously twisting
and falling. Approximately 70% of ACL injuries are noncontact,
with most of these being sustained when the femur is rotated on
the planted leg with the knee close to full extension during cutting,
landing, or stopping (15, 57). These kinds of
activities involving sudden changes in direction combined with acceleration
or deceleration of the body produce large rotational moments and
varus/valgusforces at the knee, particularly when such
movements are inadequately planned (6, 7). The
ACL is loaded when the net shearforce at the knee is directed anteriorly
(98). So, for ACL rupture to occur, there must be excess
anterior translation or rotation of the femur on the tibia.
There is a striking gender disparity in the incidence of ACL
injuries, with women sustaining significantly more than do men.
A number of hypotheses related to anatomical or neuromuscular factors
have been advanced, but the reason for this disparity remains unknown.
Research has shown that during running, cutting, and landing, women,
as compared to men, tend to have less knee flexion, greater knee
valgus angles, more hip abduction, greater quadriceps activation,
less hamstring activation, and generally less variability in lower-extremity
coordination patterns (31, 32, 47, 66, 70, 85).
One study, however, showed less hip abduction in female, compared
to male, soccer players during cutting, with no differences in kinematics
or kinetics at the knee (84). Some have advocated strengthening
or stiffening of the hamstrings to protect the ACL, since female
athletes tend to have greater quadriceps-to-hamstrings strength
ratios than do male athletes (14, 71). Research
on the notion that tension in the hamstrings can reduce forces on
the ACL has produced conflicting results, however (61, 101).
Others have advocated strengthening the quadriceps to protect against
ACL injuries, since the quadriceps provide the primary muscular
restraint to anterior tibial translation during activities such
as running and jumping (16). Yet other research has shown
that women, as compared to men, have increased coactivation of the
quadriceps during knee flexion, which may increase anterior tibial
loads under dynamic conditions and increase the risk of ACL injury
(114). Still others have encouraged training programs for
female athletes focusing on landing and pivoting with increased
knee flexion and not allowing medial or lateral sagging of the knees
during cutting maneuvers to protect the ACL from excessive strain
(26, 71, 77). It has been demonstrated
that sagittal plane knee forces cannot rupture the ACL during sidestep cutting,
so during this maneuver, valgus loading is a more likely injury
Following unrepaired ACL rupture, most individuals have difficulty
with movements involving lateral or rotational loads at the knee,
and 70–80% experience knee instability or “giving
way” (99). There is also a notable lessening of
flexion–extension range of motion at the knee during walking,
which has been attributed to “quadriceps avoidance” (4).
This does not appear to be related to a deficit in quadriceps strength,
but instead may be attributed to the fact that quadriceps tension
produces an anteriorly directed force on the tibia when the knee is
near full extension. Individuals seem to adapt to the absence of
the ACL by minimizing activation of the quadriceps when the knee
is near full extension (4). There is also evidence of general
impairment of neuromuscular control of the quadriceps, with the
muscles remaining active during tasks not requiring quadriceps activation
(117). Although some ACL-deficient individuals are able
to stabilize their knees even during cutting and pivoting, for most,
the absence of the ACL results in local instability of the knee,
a change in the location of the center of rotation at the knee,
a change in the area of tibiofemoral contact during gait, and altered
joint kinetics, with subsequent onset of osteoarthritis (3, 23, 96, 103).
Surgical repair of ACL rupture involves reconstruction of the
ligament using either the middle third of the patellar tendon, the
semitendinosus, or the semitendinosus and gracilis (33).
Problems that follow trauma to the knee, whether the trauma is injury-
or surgery-induced, include notable weakness and loss of mass in
the knee extensor muscles, dramatic reduction of joint range of
motion, and impaired joint proprioception (105). The reasons
for these changes are not understood, and may be neural or mechanical
in origin, and/or a product of deconditioning (43).
One factor hypothesized to play a major role in precipitating these
changes is muscle inhibition, or the
inability to activate all motor units of a muscle during maximal
voluntary contraction (104). It has been shown that muscle
inhibition can persist for an extended time and may be responsible for
long-term strength deficits that alter joint kinetics and lead to
osteoarthritis (48, 102, 104).
Posterior cruciate ligament (PCL) injuries most commonly result
from sport participation or motor vehicle accidents (21, 51).
When the PCL is ruptured in isolation, with no damage to the other
ligaments or to the menisci, the mechanism is usually hyperflexion
of the knee with the foot plantar flexed (51). Impact with
the dashboard during motor vehicle accidents, on the other hand,
with direct force on the proximal anterior tibia, results in combined
ligamentous damage in 95% of cases (51). Isolated
PCL injuries are usually treated nonoperatively.
Blows to the lateral side of the knee are much more common than
blows to the medial side, because the opposite leg commonly protects
the medial side of the joint. When the foot is planted and a lateral
blow of sufficient force is sustained, the result is sprain or rupture
of the medial collateral ligament (MCL). Modeling studies suggest
that the muscles crossing the knee are able to resist approximately
17% of external medial and lateral loads on the knee, with
the remaining 83% sustained by the ligaments and other
soft tissues (63). However, direct measurements of strain
to the knee ligaments indicate that external tibial torque is more
dangerous than medially directed tibial force for the MCL (46).
In contact sports such as football, the MCL is more frequently injured,
while both medial and lateral collateral ligament sprains occur
To prevent knee ligament injuries, especially during contact
sports, some athletes wear prophylactic knee braces. The wearing
of such braces by healthy individuals has been a contentious issue since
the American Academy of Orthopaedics issued a position statement
against their use in 1987. Research on this topic has shown that
knee braces can protect the ACL against anterior and torsional loads
on the tibia by significantly reducing the strain present in the
ligament under these conditions (12). Knee braces can also
contribute 20–30% added resistance against lateral
blows to the knee, with custom-fitted braces providing the best
protection (1). A possible concern, however, is that knee
braces act to change the pattern of lower-extremity muscle activity
during gait, with less work performed at the knee and more at the
hip (24). Other documented problems that appear to affect
some athletes more than others and may be brace-specific include
reduced sprinting speed and earlier onset of fatigue (1).
Because the medial collateral ligament attaches to the medial
meniscus, stretching or tearing of the ligament can also result
in damage to the meniscus. A torn meniscus is the most common knee
injury, with damage to the medial meniscus occurring approximately
10 times as frequently as damage to the lateral meniscus. This is
the case partly because the medial meniscus is more securely attached
to the tibia, and therefore less mobile than the lateral meniscus.
In knees that have undergone ACL rupture, the normal stress distribution
is disrupted such that the force on the medial meniscus is doubled
(82). In the absence of ACL reconstruction, this results
in an increased incidence of medial meniscal tears, although loading
of the meniscus returns to normal if the ACL is reconstructed (2, 45).
A torn meniscus is problematic in that the unattached cartilage
often slips from its normal position, interfering with normal joint
mechanics. Symptoms include pain, which is sometimes accompanied
by intermittent bouts of locking or buckling of the joint.
The tensor fascia lata develops tension to assist with stabilization
of the pelvis when the knee is in flexion during weight bearing.
This can produce friction of the posterior edge of the iliotibial
band (ITB) against the lateral condyle of the femur around the time
of footstrike, primarily during foot contact with the ground (80).
The result is inflammation of the distal portion of the ITB, as
well as the knee joint capsule under the ITB, with accompanying
symptoms of pain and tenderness over the lateral aspect of the knee
(56). This condition is an overuse syndrome that affects
approximately 2–12% of runners and can also affect
cyclists (90). Both training errors and anatomical malalignments
within the lower extremity increase the risk of ITB syndrome. Training
factors in running include excessive running in the same direction
on a track, greater-than-normal weekly mileage, and downhill running
(34). Improper seat height, as well as greater-than-normal
weekly mileage, predisposes cyclists to the syndrome (29).
Predisposing malalignments include excessive femoral anteversion,
increased Q-angle (see below), lateral tibial torsion, tibial genu
varum or valgum, subtalar varus, and excessive pronation (58).
This syndrome has been attributed to a number of possible causes,
including anatomical malalignment(s), imbalance between the vastus
medialis oblique (VMO) and the vastus lateralis (VL) in strength
or activation timing, weakness in the VMO and vastus medialis longus
(VML), and overactivity (22, 65, 78, 86, 87, 93).
A condition of pain and tenderness localized on the medial aspect
of the knee is often associated with performance of the whip kick,
the kick used with the breaststroke. The forceful whipping together
of the lower legs that provides the propulsive thrust of the kick
often forces the lower leg into slight abduction at the knee, with
subsequent irritation to the medial collateral ligament and the
medial border of the patellar tract. A survey of 391 competitive
swimmers revealed incidences of knee pain among 73% of
the breaststroke specialists and 48% of nonbreaststrokers
(113). In a study of breaststroke kinematics, it was found
that angles of hip abduction of less than 37˚ or greater
than 42˚ at the initiation of the kick resulted in a dramatically
increased incidence of knee pain (113) (Figure 8-16).
The likelihood of acquiring breaststroker’s
knee is much lower if the angle of hip abduction is between 37˚ and
42˚ at the beginning of the propulsive phase of the kick.
Painful patellofemoral joint motion involves anterior knee pain
during and after physical activity, particularly activities requiring
repeated flexion at the knee, such as running, ascending and descending
stairs, and squatting. It is more common among females than among
males. This syndrome has been attributed to a number of possible
causes, including anatomical malalignment(s), imbalance between
the VMO and the VL in strength or activation timing, and overactivity
(22, 65, 78, 86, 87, 93).
While the causes of this disorder are unknown, most research
attention has focused on the relationship between the VMO and the
VL. Weakness of the VMO relative to the VL has been shown to be
associated with a lateral shift of the patella, particularly early
in the range of knee flexion (86, 93). Research
has also documented that individuals with patellofemoral pain display
reduced patellofemoral joint loads with strengthening of the VMO
(78). Such individuals also demonstrate onset of VL activation
prior to VMO activation during stair stepping (22). An
experiment in which the VMO and VL were electrically stimulated
showed a respective increase and decrease in patellofemoral joint
loading when VMO activation was delayed (78).
An anatomical factor hypothesized to contribute to lower-extremity
malalignment that could trigger patellofemoral pain is an excessively
large Q-angle, the angle formed between
the anterior superior iliac spine, the center of the patella, and
the tibial tuberosity (42). The Q-angle provides an approximation
of the angle of pull of the quadriceps on the patella, and it has
been hypothesized that a large Q-angle could lead to lateral patellar
dislocation or increased lateral patellofemoral contact pressures
(74). To date, however, research has not documented a relationship
between Q-angle and incidence of patellofemoral pain. The one anatomical
factor that has been found to be related to pateller maltracking
is a shallow intercondylar groove (87).
Generalized pain along the anterolateral or posteromedial aspect
of the lower leg is commonly known as shin
splints. This is a loosely defined overuse injury, often associated
with running or dancing, that may involve microdamage to muscle
attachments on the tibia and/or inflammation of the periosteum. Common
causes of the condition include running or dancing on a hard surface
and running uphill. A change in workout conditions or rest usually
alleviates shin splints.