Issues involving action of the knee and exercises to strengthen or
rehabilitate knee action, such as the squat, knee extensions and leg curls,
arise so frequently in sport and strength training that I felt it useful to
compile a list of recent articles on this vast topic. The information
gathered here tends to depose to a large extent the still common view that
the squat is inherently a dangerous exercise and shows increasing support for
the use of the squat in training and rehabilitation, matched by strong
criticism of knee extensions and leg curls. Once considered a
contraindicated exercise for cruciate ligament rehabilitation, the squat
emerges as a useful rehabilitation tool in this regard, while questions are
raised about the effectiveness and safety of isokinetic devices and other
'open chain' movements like this.

Bear in mind that there are literally thousands of articles which focus on
the analysis, conditioning, rehabilitation and surgery of the knee, so that
this selection should be regarded as but a glimpse into the complexity of
this subject.

Mel Siff


Escamilla RF Knee biomechanics of the dynamic squat exercise Med Sci
Sports Exerc 2001 Jan; 33(1):127-41

PURPOSE: Because a strong and stable knee is paramount to an athlete's or
patient's success, an understanding of knee biomechanics while performing the
squat is helpful to therapists, trainers, sports medicine physicians,
researchers, coaches, and athletes who are interested in closed kinetic chain
exercises, knee rehabilitation, and training for sport. The purpose of this
review was to examine knee biomechanics during the dynamic squat exercise.

METHODS: Tibiofemoral shear and compressive forces, patellofemoral
compressive force, knee muscle activity, and knee stability were reviewed and
discussed relative to athletic performance, injury potential, and

RESULTS: Low to moderate posterior shear forces, restrained primarily by the
posterior cruciate ligament (PCL), were generated throughout the squat for
all knee flexion angles. Low anterior shear forces, restrained primarily by
the anterior cruciate ligament (ACL), were generated between 0 and 60 degrees
knee flexion. Patellofemoral compressive forces and tibiofemoral compressive
and shear forces progressively increased as the knees flexed and decreased as
the knees extended, reaching peak values near maximum knee flexion. Hence,
training the squat in the functional range between 0 and 50 degrees knee
flexion may be appropriate for many knee rehabilitation patients, because
knee forces were minimum in the functional range. Quadriceps, hamstrings, and
gastrocnemius activity generally increased as knee flexion increased, which
supports athletes with healthy knees performing the parallel squat (thighs
parallel to ground at maximum knee flexion) between 0 and 100 degrees knee
flexion. Furthermore, it was demonstrated that the parallel squat was not
injurious to the healthy knee.

CONCLUSIONS: The squat was shown to be an effective exercise to employ during
cruciate ligament or patellofemoral rehabilitation. For athletes with healthy
knees, performing the parallel squat is recommended over the deep squat,
because injury potential to the menisci and cruciate and collateral ligaments
may increase with the deep squat. The squat does not compromise knee
stability, and can enhance stability if performed correctly. Finally, the
squat can be effective in developing hip, knee, and ankle musculature,
because moderate to high quadriceps, hamstrings, and gastrocnemius activity
were produced during the squat.

My Note: Epidemiological studies comparing Weightlifting and Powerlifting
injury patterns do not corroborate the suggestion above that deep squats are
necessarily more risky than half squats. Some biomechanical studies even
state that half squats impose a greater patellofemoral force than full
squats, so that they may be inherently less safe. Some coaches and lifters
stress that it is relaxation of the muscles at the bottom of the squat which
makes the full squat more dangerous and that the full squat per se is not
morre dangeorus than the half squat. Almost heretically, other lifters remark
that ballistic recoil off tensed muscles out of the deep squat position is
safer than slow controlled squatting, but I have not come across any research
which substantiates this point of view.


Escamilla RF, Fleisig GS, Zheng N, Barrentine SW, Wilk K & Andrews JR
Biomechanics of the knee during closed kinetic chain and open kinetic chain
exercises. Med Sci Sports Exerc 1998 Apr; 30(4): 556-69

PURPOSE: Although closed (CKCE) and open (OKCE) kinetic chain exercises are
used in athletic training and clinical environments, few studies have
compared knee joint biomechanics while these exercises are performed
dynamically. The purpose of this study was to quantify knee forces and muscle
activity in CKCE (squat and leg press) and OKCE (knee extension). M

ETHODS: Ten male subjects performed three repetitions of each exercise at
their 12-repetition maximum. Kinematic, kinetic, and electromyographic data
were calculated using video cameras (60 Hz), force transducers (960 Hz), and
EMG (960 Hz). Mathematical muscle modeling and optimization techniques were
employed to estimate internal muscle forces.

RESULTS: Overall, the squat generated approximately twice as much hamstring
activity as the leg press and knee extensions. Quadriceps muscle activity
was greatest in CKCE when the knee was near full flexion and in OKCE when the
knee was near full extension. OKCE produced more rectus femoris activity
while CKCE produced more vasti muscle activity. Tibiofemoral compressive
force was greatest in CKCE near full flexion and in OKCE near full
extension. Peak tension in the posterior cruciate ligament was approximately
twice as great in CKCE, and increased with knee flexion. Tension in the
anterior cruciate ligament was present only in OKCE, and occurred near full
extension. Patellofemoral compressive force was greatest in CKCE near full
flexion and in the mid-range of the knee extending phase in OKCE.

CONCLUSION: An understanding of these results can help in choosing
appropriate exercises for rehabilitation and training.


Stuart MJ, Meglan D, Lutz G, Growney E & An K Comparison of intersegmental
tibiofemoral joint forces and muscle activity during various closed kinetic
chain exercises. Am J Sports Med 1996 Nov-Dec; 24(6): 792-9

The purpose of this study was to analyze intersegmental forces at the
tibiofemoral joint and muscle activity during three commonly prescribed
closed kinetic chain exercises: the power squat, the front squat, and the

Subjects with anterior cruciate ligament-intact knees performed repetitions
of each of the three exercises using a 223-N (50-pound) barbell. The results
showed that the mean tibiofemoral shear force was posterior (tibial force on
femur) throughout the cycle of all three exercises. The magnitude of the
posterior shear forces increased with knee flexion during the descent phase
of each exercise. Joint compression forces remained constant throughout the
descent and ascent phases of the power squat and the front squat. A net
offset in extension for the moment about the knee was present for all three
exercises. Increased quadriceps muscle activity and the decreased hamstring
muscle activity are required to perform the lunge as compared with the power
squat and the front squat.

A posterior tibiofemoral shear force throughout the entire cycle of all three
exercises in these subjects with anterior cruciate ligament-intact knees
indicates that the potential loading on the injured or reconstructed anterior
cruciate ligament is not significant. The magnitude of the posterior
tibiofemoral shear force is not likely to be detrimental to the injured or
reconstructed posterior cruciate ligament. These conclusions assume that the
resultant anteroposterior shear force corresponds to the anterior and
posterior cruciate ligament forces.


Wilk KE, Escamilla R, Fleisig G, Barrentine S, Andrews J & Boyd M A
comparison of tibiofemoral joint forces and electromyographic activity during
open and closed kinetic chain exercises. Am J Sports Med 1996 Jul-Aug;
24(4): 518-27

We chose to investigate tibiofemoral joint kinetics (compressive force,
anteroposterior shear force, and extension torque) and electromyographic
activity of the quadriceps, hamstring, and gastrocnemius muscles during open
kinetic chain knee extension and closed kinetic chain leg press and squat.

Ten uninjured male subjects performed 4 isotonic repetitions with a 12
repetition maximal weight for each exercise. Tibiofemoral forces were
calculated using electromyographic, kinematic, and kinetic data. During the
squat, the maximal compressive force was 6139 1708 N, occurring at 91
degrees of knee flexion; whereas the maximal compressive force for the knee
extension exercise was 4598 2546 N (at 90 degrees knee flexion). During the
closed kinetic chain exercises, a posterior shear force (posterior cruciate
ligament stress) occurred throughout the range of motion, with the peak
occurring from 85 degrees to 105 degrees of knee flexion. An anterior shear
force (anterior cruciate ligament stress) was noted during open kinetic chain
knee extension from 40 degrees to full extension; a peak force of 248 259 N
was noted at 14 degrees of knee flexion. Electromyographic data indicated
greater hamstring and quadriceps muscle co-contraction during the squat
compared with the other two exercises.

During the leg press, the quadriceps muscle electromyographic activity was
approximately 39% to 52% of maximal velocity isometric contraction; whereas
hamstring muscle activity was minimal (12% maximal velocity isometric
contraction). This study demonstrated significant differences in tibiofemoral
forces and muscle activity between the two closed kinetic chain exercises,
and between the open and closed kinetic chain exercises.


Pandy MG & Shelburne K Dependence of cruciate-ligament loading on muscle
forces and external load. J Biomech 1997 Oct; 30(10): 1015-24

A sagittal-plane model of the knee is used to predict and explain the
relationships between the forces developed by the muscles, the external loads
applied to the leg, and the forces induced in the cruciate ligaments during
isometric exercises.

The geometry of the model bones is adapted from cadaver data. Eleven elastic
elements describe the geometric and mechanical properties of the cruciate
ligaments, the collateral ligaments, and the posterior capsule. The model is
actuated by 11 musculotendinous units, each unit represented as a
three-element muscle in series with tendon. For isolated contractions of the
quadriceps, ACL force increases as quadriceps force increases for all flexion
angles between 0 and 80 degrees; the ACL is unloaded at flexion angles
greater than 80 degrees. When quadriceps force is held constant, ACL force
decreases monotonically as knee-flexion angle increases. The relationship
between ACL force, quadriceps force, and knee-flexion angle is explained by
the geometry of the knee-extensor mechanism and by the changing orientation
of the ACL in the sagittal plane.

For isolated contractions of the hamstrings, PCL force increases as
hamstrings force increases for all flexion angles greater than 10 degrees;
the PCL is unloaded at flexion angles less than 10 degrees. When hamstrings
force is held constant, PCL force increases monotonically with increasing
knee flexion. The relationship between PCL force, hamstrings force, and
knee-flexion angle is explained by the geometry of the hamstrings and by the
changing orientation of the PCL in the sagittal plane.

At nearly all knee-flexion angles, hamstrings co-contraction is an effective
means of reducing ACL force. Hamstrings co-contraction cannot protect the ACL
near full extension of the knee because these muscles meet the tibia at small
angles near full extension, and so cannot apply a sufficiently large
posterior shear force to the leg. Moving the restraining force closer to the
knee-flexion axis decreases ACL force; varying the orientation of the
restraining force has only a small effect on cruciate-ligament loading.


Note what this next reference says about squats versus knee extension

Yack HJ, Collins C & Whieldon T Comparison of closed and open kinetic
chain exercise in the anterior cruciateligament-deficient knee. Am J Sports
Med 1993 Jan-Feb; 21(1): 49-54

The purpose of this study was to quantify the amount of anterior tibial
displacement occurring in anterior cruciate ligament-deficient knees during
two types of rehabilitation exercises: 1) resisted knee extension, an open
kinetic chain exercise; and 2) the parallel squat, a closed kinetic chain
exercise. An electrogoniometer system was applied to the anterior cruciate
ligament-deficient knee of 11 volunteers and to the uninvolved normal knee in
9 of these volunteers. Anterior tibial displacement and the knee flexion
angle were measured during each exercise using matched quadriceps loads and
during the Lachman test.

The anterior cruciate ligament-deficient knee had significantly greater
anterior tibial displacement during extension from 64 degrees to 10 degrees
in the knee extension exercise as compared to the parallel squat exercise. In
addition, the amount of displacement during the Lachman test was
significantly less than in the knee extension exercise, but significantly
more than in the parallel squat exercise. No significant differences were
found between measurements in the normal knee.

We concluded that the stress to the anterior cruciate ligament, as indicated
by anterior tibial displacement, is minimized by using the parallel squat, a
closed kinetic chain exercise, when compared to the relative anterior tibial
displacement during knee extension exercise.


Note what this reference says about exercises, such as supine leg curls,
which significantly recruit gastrocnemius during rehabilitation after knee
injury. This information should be carefully considered by any therapists
who still insist on treating cruciate ligament injuries with leg curls.

Durselen L, Claes L & Kiefer H The influence of muscle forces and external
loads on cruciate ligament strain. Am J Sports Med 1995 Jan-Feb; 23(1):

We know it is important to avoid excessive strain on reconstructed ligaments,
but we do not know how individual muscles affect cruciate ligament strain. To
answer this, we studied the effect of muscle forces and external loads on
cruciate ligament strain.

Nine cadaveric knee joints were tested in an apparatus that allowed
unconstrained knee joint motion. Quadriceps, hamstring, and gastrocnemius
muscle forces were simulated. Additionally, external loads were applied such
as varus-internal or valgus-external rotation forces. Cruciate ligament
strain was recorded at different knee flexion angles. Activation of the
gastrocnemius muscle significantly strained the posterior cruciate ligament
at flexion angles larger than 40 degrees. Quadriceps muscle activation
significantly strained the anterior cruciate ligament when the knee was
flexed 20 degrees to 60 degrees and reduced the strain on the posterior
cruciate ligament in the same flexion range. Activation of the hamstring
muscles strained the posterior cruciate ligament when the knee was flexed 70
degrees to 110 degrees. Combined varus and internal rotation forces
significantly increased anterior cruciate ligament strain throughout the
flexion range.

The results suggest that to minimize strain on the ligament after posterior
cruciate ligament surgery, strong gastrocnemius muscle contractions should be
avoided beyond 30 degrees of knee flexion. The study also calls into question
the use of vigorous quadriceps exercises in the range of 20 degrees to 60
degrees of knee flexion after anterior cruciate ligament r

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