Michael W. Krzyzewski Human Performance Laboratory, Division of Orthopaedic
Surgery and Duke Sports Medicine, Duke University Medical Center, Durham, NC
27710, USA. rescamil@duke.edu
PURPOSE: The specific aim of this project was to quantify knee forces and
muscle activity while performing squat and leg press exercises with
technique variations. METHODS: Ten experienced male lifters performed the
squat, a high foot placement leg press (LPH), and a low foot placement leg
press (LPL) employing a wide stance (WS), narrow stance (NS), and two foot
angle positions (feet straight and feet turned out 30 degrees ). RESULTS: No
differences were found in muscle activity or knee forces between foot angle
variations. The squat generated greater quadriceps and hamstrings activity
than the LPH and LPL, the WS-LPH generated greater hamstrings activity than
the NS-LPH, whereas the NS squat produced greater gastrocnemius activity
than the WS squat. No ACL forces were produced for any exercise variation.
Tibiofemoral (TF) compressive forces, PCL tensile forces, and patellofemoral
(PF) compressive forces were generally greater in the squat than the LPH and
LPL, and there were no differences in knee forces between the LPH and LPL.
For all exercises, the WS generated greater PCL tensile forces than the NS,
the NS produced greater TF and PF compressive forces than the WS during the
LPH and LPL, whereas the WS generated greater TF and PF compressive forces
than the NS during the squat. For all exercises, muscle activity and knee
forces were generally greater in the knee extending phase than the knee
flexing phase. CONCLUSIONS: The greater muscle activity and knee forces in
the squat compared with the LPL and LPH implies the squat may be more
effective in muscle development but should be used cautiously in those with
PCL and PF disorders, especially at greater knee flexion angles. Because all
forces increased with knee flexion, training within the functional 0-50
degrees range may be efficacious for those whose goal is to minimize knee
forces. The lack of ACL forces implies that all exercises may be effective
during ACL rehabilitation.
Michael W. Krzyzewski Human Performance Laboratory, Division of Orthopaedic
Surgery, Duke University Medical Center, Durham, NC 27710, USA. rescamil@duke.edu
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
rehabilitation. 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.
Michael W. Krzyzewski Human Performance Laboratory, Division of Orthopaedic
Surgery, Duke University Medical Center, Durham, NC 27710, USA. rescamil@duke.edu
PURPOSE: The purpose of this study was to quantify biomechanical parameters
employing two-dimensional (2-D) and three-dimensional (3-D) analyses while
performing the squat with varying stance widths. METHODS: Two 60-Hz cameras
recorded 39 lifters during a national powerlifting championship. Stance
width was normalized by shoulder width (SW), and three stance groups were
defined: 1) narrow stance squat (NS), 107 +/- 10% SW; 2) medium stance squat
(MS), 142 +/- 12% SW; and 3) wide stance squat (WS), 169 +/- 12% SW. RESULTS:
Most biomechanical differences among the three stance groups and between 2-D
and 3-D analyses occurred between the NS and WS. Compared with the NS at 45
degrees and 90 degrees knee flexion angle (KF), the hips flexed 6-11 degrees
more and the thighs were 7-12 degrees more horizontal during the MS and WS.
Compared with the NS at 90 degrees and maximum KF, the shanks were 5-9
degrees more vertical and the feet were turned out 6 degrees more during the
WS. No significant differences occurred in trunk positions. Hip and thigh
angles were 3-13 degrees less in 2-D compared with 3-D analyses. Ankle
plantar flexor (10-51 N.m), knee extensor (359-573 N.m), and hip extensor
(275-577 N.m) net muscle moments were generated for the NS, whereas ankle
dorsiflexor (34-284 N.m), knee extensor (447-756 N.m), and hip extensor
(382-628 N.m) net muscle moments were generated for the MS and WS.
Significant differences in ankle and knee moment arms between 2-D and 3-D
analyses were 7-9 cm during the NS, 12-14 cm during the MS, and 16-18 cm
during the WS. CONCLUSIONS: Ankle plantar flexor net muscle moments were
generated during the NS, ankle dorsiflexor net muscle moments were produced
during the MS and WS, and knee and hip moments were greater during the WS
compared with the NS. A 3-D biomechanical analysis of the squat is more
accurate than a 2-D biomechanical analysis, especially during the WS.
American Sports Medicine Institute, Birmingham, AL 35205, USA.
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).
METHODS: 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.
American Sports Medicine Institute, Birmingham, AL 35205, USA.
An analytical model of the knee joint was developed to estimate the forces
at the knee during exercise. Muscle forces were estimated based upon
electromyographic activities during exercise and during maximum voluntary
isometric contraction (MVIC), physiological cross-sectional area (PCSA),
muscle fiber length at contraction and the maximum force produced by an unit
PCSA under MVIC. Tibiofemoral compressive force and cruciate ligaments'
tension were determined by using resultant force and torque at the knee,
muscle forces, and orientations and moment arms of the muscles and ligaments.
An optimization program was used to minimize the errors caused by the
estimation of the muscle forces. The model was used in a ten-subject study
of open kinetic chain exercise (seated knee extension) and closed kinetic
chain exercises (leg press and squat). Results calculated with this model
were compared to those from a previous study which did not consider muscle
length and optimization. Peak tibiofemoral compressive forces were 3134 +/-
1040 N during squat, 3155 +/- 755 N during leg press and 3285 +/- 1927 N
during knee extension. Peak posterior cruciate ligament tensions were 1868
+/- 878 N during squat, 1866 +/- 383 N during leg press and 959 +/- 300 N
for seated knee extension. No significant anterior cruciate ligament (ACL)
tension was found during leg press and squat. Peak ACL tension was 142 +/-
257 N during seated knee extension. It is demonstrated that the current
model provided better estimation of knee forces during exercises, by
preventing significant overestimates of tibiofemoral compressive forces and
cruciate ligament tensions.
American Sports Medicine Institute, Biomechanical Laboratory, Birmingham,
Alabama, USA.
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.
Biomechanics Laboratory, Mayo Clinic, Rochester, Minnesota, USA.
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
lunge. 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.
Department of Engineering Science, University of Oxford, England.
A computer-based model of the knee was used to study forces in the cruciate
ligaments induced by co-contraction of the extensor and flexor muscles, in
the absence of external loads. Ligament forces are required whenever the
components of the muscle forces parallel to the tibial plateau do not
balance. When the extending effect of quadriceps exactly balances the
flexing effect of hamstrings, the horizontal components of the two muscle
forces also balance only at the critical flexion angle of 22 degrees. The
calculations show that co-contraction of the quadriceps and hamstring
muscles loads the anterior cruciate ligament from full extension to 22
degrees of flexion and loads the posterior cruciate at higher flexion angles.
In these two regions of flexion, the forward pull of the patellar tendon on
the tibia is, respectively, greater than or less than the backward pull of
hamstrings. Simultaneous quadriceps and gastrocnemius contraction loads the
anterior cruciate over the entire flexion range. Simultaneous contraction of
all three muscle groups can unload the cruciate ligaments entirely at
flexion angles above 22 degrees. These results may help the design of
rational regimes of rehabilitation after ligament injury or repair.
Sports Medicine Section, Department of Orthopaedic Surgery, The David Geffen
UCLA School of Medicine, Los Angeles, California 90095, USA.
Knee injuries are common in sports activities. Understanding the mechanisms
of injury allows for better treatment of these injuries and for the
development of effective prevention programmes. Tibial torque and knee
flexion angle have been associated with several mechanisms of injury in the
knee. This article focuses on the injury to the anterior cruciate ligament (ACL),
the posterior cruciate ligament (PCL) and the meniscus of the knee as they
relate to knee flexion angle and tibial torque. Hyperflexion and
hyperextension with the application of tibial torque have both been
implicated in the mechanism of ACL injury. A combination of anterior tibial
force and internal tibial torque near full extension puts the ACL at high
risk for injury. Hyperflexion also increases ACL force; however, in this
position, internal and external tibial torque only minimally increase ACL
force. Several successful prevention programmes have been based on these
biomechanical factors. Injury to the PCL typically occurs in a flexed or
hyperflexed knee position. The effects of application of a tibial torque,
both internally and externally, remains controversial. Biomechanical studies
have shown an increase in PCL force with knee flexion and the application of
internal tibial torque, while others have shown that PCL-deficient knees
have greater external tibial rotation. The meniscus must endure greater
compressive loads at higher flexion angles of the knee and, as a result, are
more prone to injury in these positions. In addition, ACL deficiency puts
the meniscus at greater risk for injury. Reducing the forces on the ACL, PCL
and meniscus during athletic activity through training, the use of
appropriate equipment and safe surfaces will help to reduce injury to these
structures.
The load moment of force about the knee joint during machine milking and
when lifting a 12.8 kg box was quantified using a computerized static
sagittal plane body model. Surface electromyography of quadriceps and
hamstrings muscles was normalized and expressed as a percentage of an
isometric maximum voluntary test contraction. Working with straight knees
and the trunk flexed forwards induced extending knee load moments of maximum
55 Nm. Lifting the box with flexed knees gave flexing moments of 50 Nm at
the beginning of the lift, irrespective of whether the burden was between or
in front of the feet. During machine milking, a level difference between
operator and cow of 0.70 m - 1.0 m significantly lowered the knee extending
moments. To quantify the force magnitudes acting in the tibio-femoral and
patello-femoral joints, a local biomechanical model of the knee was
developed using a combination of cadaver knee dissections and lateral knee
radiographs of healthy subjects. The moment arm of the knee extensor was
significantly shorter for women than for men, which resulted in higher knee
joint forces in women if the same moment was produced. A diagram for
quantifying patellar forces was worked out. The force magnitudes given by
the knee joint biomechanical model correlated well with experimentally
forces measured by others. During the parallel squat in powerlifting, the
maximum flexing knee load moment was estimated to 335-550 Nm when carrying a
382.5 kg burden and the in vivo force of a complete quadriceps tendon-muscle
rupture to between 10,900 and 18,300 N. During isokinetic knee extension,
the tibio-femoral compressive force reached peak magnitudes of 9 times body
weight and the anteroposterior shear force was close to 1 body weight at
knee angles straighter than 60 degrees, indicating that high forces stress
the anterior cruciate ligament. A proximal resistance pad position decreased
the shear force considerably, and this position is recommended in early
rehabilitation after anterior cruciate ligament repairs or reconstructions.
The methods presented quantify muscle activity, sagittal knee joint moments
and forces, enabling assessments to be made of different work postures,
training exercises and joint derangements.
Bioemchanics of the entire knee joint including tibiofemoral and
patellofemoral joints were investigated at different flexion angles (0
degrees to 90 degrees ) and quadriceps forces (3, 137, and 411 N). In
particular, the effect of changes in location and magnitude of restraining
force that counterbalances the isometric extensor moment on predictions was
investigated. The model consisted of three bony structures and their
articular cartilage layers, menisci, principal ligaments, patellar tendon,
and quadriceps muscle. Quadriceps forces significantly increased the
anterior cruciate ligament, patellar tendon, and contact forces/areas as
well as the joint resistant moment. Joint flexion, however, substantially
diminished them all with the exception of the patellofemoral contact force/area
that markedly increased in flexion. When resisting extensor moment by a
force applied on the tibia, the force in cruciate ligaments and tibial
translation significantly altered as a function of magnitude and location of
the restraining force. Quadriceps activation generated large ACL forces at
full extension suggesting that post ACL reconstruction exercises should
avoid large quadriceps exertions at near full extension angles. In isometric
extension exercises against a force on the tibia, larger restraining force
and its more proximal location to the joint substantially decreased forces
in the anterior cruciate ligament at small flexion angles whereas they
significantly increased forces in the posterior cruciate ligament at larger
flexion angles.
Department of Biokinesiology and Physical Therapy, University of Southern
California, Los Angeles 90089-9006, USA.
STUDY DESIGN: Single-group repeated measures design. OBJECTIVE: To quantify
patellofemoral joint reaction forces and stress while squatting with and
without an external load. BACKGROUND: Although squatting exercises in the
rehabilitation setting are often executed to a relatively shallow depth in
order to avoid the higher joint forces associated with increased knee
flexion, objective criteria for ranges of motion have not been established.
Methods and Measures: Fifteen healthy adults performed single-repetition
squats to 90 degrees of knee flexion without an external load and with an
external load (35% of the subject's body weight [BW]). Anthropometric data,
three-dimensional kinematics, and ground reaction forces were used to
calculate knee extensor moments (inverse dynamics approach), while a
biomechanical model of the patellofemoral joint was used to quantify the
patellofemoral joint reaction forces and patellofemoral joint stress. Data
were analyzed during the eccentric (0-90 degrees) and concentric (90-0
degrees phases of the squat maneuver. RESULTS: In both conditions, knee
extensor moments, patellofemoral joint reaction forces, and patellofemoral
joint stress increased significantly with greater knee flexion angles (P <
0.05). Peak patellofemoral joint force and stress was observed at 90 degrees
of knee flexion. Patellofemoral joint stress at 45 degrees, 60 degrees, 75
degrees, and 90 degrees of knee flexion during the eccentric phase, and at
75 degrees and 90 degrees during the concentric phase, was significantly
greater in the loaded trials versus the unloaded trials. CONCLUSION: The
data indicate that during squatting, patellofemoral joint stress increases
as the knee flexion angle increases, and that the addition of external
resistance further increases patellofemoral joint stress. These findings
suggest that in order to limit patellofemoral joint stress during squatting
activities, clinicians should consider limiting terminal joint flexion
angles and resistance loads.
Human Performance Laboratories, The University of Memphis, Memphis,
Tennessee 38152, USA. afry@memphis.edu
Some recommendations suggest keeping the shank as vertical as possible
during the barbell squat, thus keeping the knees from moving past the toes.
This study examined joint kinetics occurring when forward displacement of
the knees is restricted vs. when such movement is not restricted. Seven
weight-trained men (mean +/- SD; age = 27.9 +/- 5.2 years) were videotaped
while performing 2 variations of parallel barbell squats (barbell load =
body weight). Either the knees were permitted to move anteriorly past the
toes (unrestricted) or a wooden barrier prevented the knees from moving
anteriorly past the toes (restricted). Differences resulted between static
knee and hip torques for both types of squat as well as when both squat
variations were compared with each other (p < 0.05). For the unrestricted
squat, knee torque (N.m; mean +/- SD) = 150.1 +/- 50.8 and hip torque = 28.2
+/- 65.0. For the restricted squat, knee torque = 117.3 +/- 34.2 and hip
torque = 302.7 +/- 71.2. Restricted squats also produced more anterior lean
of the trunk and shank and a greater internal angle at the knees and ankles.
The squat technique used can affect the distribution of forces between the
knees and hips and on the kinematic properties of the exercise. PRACTICAL
APPLICATIONS: Although restricting forward movement of the knees may
minimize stress on the knees, it is likely that forces are inappropriately
transferred to the hips and low-back region. Thus, appropriate joint loading
during this exercise may require the knees to move slightly past the toes.
