Cruciate Ligament Loading During Common Rehabilitation Exercises

7/26/2019 Cruciate Ligament Loading During Common Rehabilitation Exercises

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Special Issue Article

Proc IMechE Part H:

J Engineering in Medicine

226(9) 670680IMechE 2012

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DOI: 10.1177/0954411912451839

pih.sagepub.com

Cruciate ligament loading duringcommon knee rehabilitation exercises

Rafael F Escamilla1, Toran D MacLeod2, Kevin E Wilk3,

Lonnie Paulos4 and James R Andrews5,6

Abstract

Cruciate ligament injuries are common and may lead to dysfunction if not rehabilitated. Understanding how to progressanterior cruciate ligament and posterior cruciate ligament loading, early after injury or reconstruction, helps cliniciansprescribe rehabilitation exercises in a safe manner to enhance recovery. Commonly prescribed therapeutic exercises

include both weight-bearing exercise and non-weight-bearing exercise. This review was written to summarize and pro-vide an update on the available literature on cruciate ligament loading during commonly used therapeutic exercises. Ingeneral, weight-bearing exercise produces smaller loads on the anterior cruciate ligament and posterior cruciate liga-ment compared with non-weight-bearing exercise. The anterior cruciate ligament is loaded less at higher knee angles(i.e. 50100). Squatting and lunging with a more forward trunk tilt and moving the resistance pad proximally on the legduring the seated knee extension unloads the anterior cruciate ligament. The posterior cruciate ligament is less loadedat lower knee angles (i.e. 050), and may be progressed from level ground walking to a one-leg squat, lunges, wall squat,leg press, and the two-leg squat (from smallest to greatest). Exercise type and technique variation affect cruciate ligamentloading, such that the clinician may prescribe therapeutic exercises to progress ligament loading safely, while ensuringoptimal recovery of the musculoskeletal system.

Keywords

Anterior cruciate ligament, anterior shear force, exercise therapy, reconstruction, strain

Date received: 3 May 2011; accepted: 23 March 2012

Introduction

Cruciate ligament injuries are common. After sustaining

an injury to the cruciate ligaments, or after anterior cruci-

ate ligament (ACL) or posterior cruciate ligament (PCL)

reconstruction, it is important to properly rehabilitate the

tibiofemoral joint to ensure optimal recovery of the heal-ing tissues, keep the joint healthy, and to prevent lower

extremity muscle atrophy. Understanding cruciate liga-

ment loading during commonly prescribed rehabilitation

exercises helps the clinician maximize treatment efficacy

and minimize the likelihood of injury.

The scientific literature on cruciate ligament loading

has not been recently reviewed and summarized to give

the clinician a current understanding of how ligament

loading is quantified, or to give an updated understand-

ing of loading across a wider range of exercises than pre-

viously described.113 Rehabilitation exercises commonly

used include both weight-bearing exercises (WBE), also

referred to as closed kinetic chain exercises, and non-weight-bearing exercises (NWBE), also referred to as

open kinetic chain exercises. The specific exercises

included in this review include squatting, lunging, step-

ping (e.g. stepping up and down stairs), leg press, seated

knee extension and knee flexion, stair climbing, station-

ary bicycling, drop landing, and walking (samples of

squatting and lunging exercises are shown in Figures 1

and 2). These exercises were chosen because using these

exercises as part of a cruciate ligament rehabilitation

1Department of Physical Therapy, California State University

Sacramento, USA2Department of Radiology and Biomedical Imaging, University of

California San Francisco, USA3Champion Sports Medicine, USA4Paulos Sports Injury and Joint Preservation Clinic, USA5American Sports Medicine Institute, USA6Andrews Institute, USA

This paper was submitted as part of the Lower Limb Musculoskeletal

Modelling Special Issue.

Corresponding author:Rafael F Escamilla, Department of Physical Therapy, California State

University Sacramento, 6000 J Street, Sacramento, CA 95819-6020, USA.

Email: [email protected]


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program after cruciate ligament injury or reconstruction

has been shown to significantly improve short- and long-

term knee function and enhance a successful return to

sport or activity.1416

Cruciate ligament loading will be investigated during

these exercises with varying resistance, speeds of

movement, and techniques. Understanding how the

cruciate ligaments are loaded during WBE and NWBE

rehabilitation can help clinicians better prescribe train-

ing and rehabilitation regimens in a safe manner, to

enhance recovery and the rehabilitation process.

Techniques commonly used to measure

cruciate ligament biomechanics

Both in-vivo17 and experimental813 biomechanical

models have been developed to evaluate ACL strain or

tensile force during WBE and NWBE, and both these

approaches have advantages and limitations. The obvi-

ous advantage of in vivo studies is that they calculate

ACL strain directly by using strain sensors within the

ACL. The subjects in these in vivo studies were patients

that had strain sensors implanted within the anterome-

dial bundle of their healthy ACL during arthroscopic

surgery to repair damaged knee structures (partialmeniscectomy; capsule, and patellofemoral joint debri-

dement). Immediately after surgery, these patients were

asked to perform a variety of NWBE and WBE, includ-

ing lunging, squatting, leg press, step-up and step-down,

stair climbing, bicycling, and seated knee extension and

knee flexion. The strain within the anteromedial bundle

of the ACL was measured and referenced to an instru-

mented Lachman Test with 150 N of resistance.

There are several limitations to measuring ACL strain

in vivo, such as, the procedure is invasive, time consum-

ing, costly, performed in a patient population under sur-

gical conditions, and that the types of activities arelimited. Moreover, the exercise technique employed

while these patients performed selected WBE was gener-

ally not controlled. For example, there are many ways

to perform a squat that could affect muscle forces and

cruciate ligament loading, such as, using narrow stance

or a wide stance, turning the feet in or out, having a near

vertical trunk position or tilting the trunk forward 30

45 relative to vertical, and during the squat descent

moving the knees forward beyond the toes or keeping

the knees from moving forward beyond the toes.

Another limitation toin vivostudies is that both athletes

and non-athletes are employed in performing WBE, gen-erally only body weight or light external resistance is

employed during the exercises, and usually only selected

knee flexion angles are chosen for ACL strain data col-

lection. Therefore, the ability to generalize the results of

ACL strain in vivo from studies during WBE to the

active athletic population, which comprise the majority

of ACL injuries and who often trains with moderate to

heavy external resistance over a large knee range of

motion, is limited and should be interpreted cautiously.

Experimental biomechanical knee models, which

also have advantages and limitations, have been previ-

ously developed and described.8,9,1113,1721 The advan-

tage of using experimental models is that the estimatedloads are better generalized to the active athletic popu-

lation because variables are often better controlled. For

Figure 2. One leg squat.

Figure 1. Forward lunge.

Escamilla et al. 671


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example, moderate to heavy resistance could be used

during exercise using the experimental model, such is

more consistent to how athletes train, but only body

weight or light resistance could be used with thein vivo

model, because the subjects are all patients that are just

coming out of knee surgery. The obvious limitation of

experimental biomechanical knee models is that theydo not measure ACL loading directly, but only esti-

mate its value. However, if the same experimental

model is used for all the exercises, it still provides a

good relative comparison (assuming that the models

are physiologically realistic). Another limitation in

using experimental biomechanical knee models is that

these models were primarily limited to sagittal plane

motion because squatting, lunging, and similar exer-

cises are performed primarily in the sagittal plane with

only minimal transverse plane rotary motions and fron-

tal plane valgus/varus motions. However, performing

these types of exercises with excessive transverse planerotary motions and frontal plane valgus/varus motions

could affect cruciate ligament loading, and this should

be the focus of future studies. Further, most experimen-

tal models are constructed with the assumption that the

cruciate ligaments are the only restraints to tibiofe-

moral shear forces, and do not account for other soft

or hard tissues (meniscus, tibial slope, etc.) that likely

play a role.

Both in vivo and experimental models have draw-

backs, but there is evidence to suggest their validity

because severalin vivoexperiments found similar results

to studies using experimental models examining ACL

loading. Studies usingin vivomodeling17 reported peak

ACL loading for squatting and lunging of approxi-

mately 2.8% to 4% (about 100150 N) at knee flexion

angles between 0 and 30, corresponding to the peak

ACL forces calculated from experimental models8,9,11

13,1721 for the same exercises. This example demon-

strates that the magnitude of predicted forces from

experimental models are in general agreement with in

vivo direct measurement, thus providing some validity

to the measurements and suggesting that the two mea-

sures may be very cautiously compared between and

within techniques. There is another drawback related

to the two models, in that the ultimate tensile force isnot readily determined using live subjects, representing

a potential disagreement between at maximal force lev-

els between in vivo and experimental modeling studies.

Finally, because in vivo studies only include subjects

that have otherwise undergone surgery, the majority of

studies use experimental models and, therefore, the

majority of the work presented within this article is

based upon experimental models.

Commonly used graft mechanical

propertiesBoth autograft, which is tissue harvested from the

patient undergoing surgery, and allograft, which is

tissue harvested from a cadaver, are commonly used to

reconstruct the cruciate ligaments in the United

States.22,23 In healthy adults, the ultimate strength of

the native ACL is approximately 2000 N,24 and the

reconstructed ACL has similar ultimate strengths com-

pared with the healthy ACL, although these values can

change considerably depending on graft type, donorsage, and donor characteristics (e.g, autograft versus

allograft, patellar tendon versus hamstrings graft,

etc.).25 However, the healing graft and graft site may

be injured with considerably less force compared with

the ultimate strength of the graft, especially when it

involves soft tissue to bone fixation, such as the ham-

strings graft. The graft must mature, and as the

maturation process continues, the ultimate tensile prop-

erties of the graft increase in strength. Unfortunately it

is not known how much force to the graft site is too

much and how soon force can be applied to the healing

tissues after reconstruction. However, concerns regard-ing the loading properties of the cruciate ligaments

highlight the importance of biomechanical testing of

the cruciate ligaments.

ACL loading during selected rehabilitation

exercises

Both WBE and NWBE have been employed and shown

to be effective in enhancing ACL rehabilitation and

return to sport.26 However, it is believed by some clini-

cians that, compared with NWBE, individuals that per-

form predominately WBE in their rehabilitation tendto have less knee pain, more stable knees, are generally

more satisfied with the end result, and return to their

sport sooner than expected. Tables 13 present ACL

strain, ACL tensile force, and anterior shear force (the

force on the tibia in the anterior direction that loads

the ACL) data from selected articles in scientific litera-

ture, and each of these will now be summarized.

ACL strain

The ACL strains reported in Table 1 are from several

in vivo studies16 performed during a variety of NWBE

and WBE. Key points from Table 1 are as follows.

1. It should be emphasized that peak ACL strain

occurs at knee angles of less than 30. Therefore, if

the rehabilitation goal is to minimize ACL loading,

such as during the early phases after ACL recon-

struction surgery, training both NWBE and WBE

at higher knee angles (i.e. 50100) is recom-

mended, compared with training these exercises at

lower knee angles (i.e. 050). In addition, it

should be emphasized that ACL loading from both

NWBE and WBE at knee angles less than 60 are

of relatively small strain magnitudes (typically lessthan 3.7% from Table 1), which is similar to the

ACL strain observed with a 150 N Lachman test,

672 Proc IMechE Part H: J Engineering in Medicine 226(9)


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which produced 3.7% strain at a 30 knee flexion

angle.

2. Peak ACL strain is typically greatest at around1015 knee flexion, and gradually decreases

between 1550 knee angle, and between approxi-

mately 5090 knee angles there is minimal or no

ACL strain. For example, ACL strains during the

isometric seated leg extension using a 30 Nm tor-

que as resistance were 4.4% at 15 knee angle, 2%

at 30 knee angle, and no ACL strain at 60 and

90 knee angles.6 Moreover, when tested at 30,

50, and 70 knee angles, squatting, lunging and

step-up and step-down exercises had the greatest

ACL strain at 30 knee angle.6 ACL strain at full

knee extension (0) has not been reported during

exercise, but is assumed to be minimal owing to the

knee being in a very stable closed pack position.

3. Peak ACL strain was not significantly different

between squatting with or without 136 N of exter-

nal resistance, or between stair climbing at slower

versus faster rates.1,4,6,30 It can be concluded from

these WBE data that increasing resistance during

the squat, or increasing the rate of stepping during

stair climbing, may not increase ACL strain. This

may have occurred because adding resistance or

stepping faster may affect muscle recruitment pat-

terns, such as recruiting the hamstrings to a greater

extent (perhaps owing to changes in technique,such as a greater forward trunk tilt). Muscle force

from the hamstrings helps unload the ACL owing

to their posterior directed force on the leg. This

finding, during the WBE (such as squatting and

lunging), is different compared with the NWBEseated knee extension, in which ACL strain

increased from 2.8% without external resistance to

3.8% with adding only 45N (10 lbs) of external

resistance5 One possible explanation for this is that

technique variations typically do not occur during

the seated knee extension exercise (and the ham-

strings are not recruited to unload the ACL), but

techniques variations do occur during WBE, such

as the squat and lunge.

4. Peak ACL strain was generally greater in the

NWBE seated knee extension compared with most

WBE.5 For example, performing a leg press type

exercise with 40% bodyweight resistance, stair

climbing, and forward lunging, all produced less

ACL strain compared with performing a seated

knee extension with no external resistance.5

Interestingly, performing a NWBE seated knee

extension with no external resistance (quadriceps

activation only), produced the same amount of

ACL strain compared with performing a WBE one

leg sit-to-stand or stair climbing, with the WBE

being much more challenging in recruiting impor-

tant hip and thigh musculature (e.g. quadriceps,

hamstrings, and gluteals) that help stabilize the

knee and protect the ACL.6

Therefore, WBEsminimize ACL strain to a greater extent compared

with the NWBE seated knee extension, and WBEs

Table 1. Peak ACL strain and knee angle for commonly performed rehabilitation exercises.

Author Rehabilitation exercise Peak ACLstrain (%)

Knee flexionangle ()

Beynnon et al. (1997)1 Squatting with or without 136 N (30 lb) resistance 3.6-4.0 10Beynnon et al. (1995)4 Dynamic seated leg extension using with a 45N (10 lb) force as resistance 3.8 10

Dynamic seated leg extension without external resistance 2.8 10Isometric seated leg extension using a 30Nm torque as resistance 4.4 15Isometric seated leg extension using a 30Nm torque as resistance 2.0 30Isometric seated leg extension using a 30 N m torque as resistance 0 60Isometric seated leg extension using a 30 N m torque as resistance 0 90

Beynnon et al. (1992)3 Isometric seated leg extension using a 27Nm torque as resistance 3.2 30Isometric seated leg extension using a 27 N m torque as resistance 0 90150 N (33 lbs) Lachman test 3.7 30Anterior drawer test 150 N (33 lbs) 1.8 90

Heijne et al. (2004)6 One-legged sit to stand (without external resistance) - tested at 30, 50,and 70knee angle

2.8 30

Step-up (without external resistance) tested at 30, 50, and 70knee angle 2.5 30Step-down (without external resistance) tested at 30, 50, and 70kneeangle

2.52.6 30

Leg press using 40% bodyweight resistance 2.1 20

Forward lunge (without external resistance) tested at 30, 50, and 70knee angle 1.82.0 30

Stationary bicycling 1.7 30Fleming et al. (1999)5 Stair climbing (112 steps per minute) (without external resistance) 2.8 20

Stair climbing (80 steps per minute) (without external resistance) 2.7 11Fleming et al. (1998)7 Stationary bicycling (175 W, 60 r/min) 0 Near full

extension150 N (33 lbs) Lachman test 3.0 30

ACL: anterior cruciate ligament.



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Table 2. Peak ACL tensile forces and knee angles for commonly performed rehabilitation exercises.

Author Rehabilitation exercise ACL

Peakforce (N)


Escamilla et al. (1998)9 Barbell squat using 12 repetition maximum resistance** 0

Leg press using 12 repetition maximum resistance** 0Dynamic seated knee extension using 12 repetition maximum resistance** 158 15

Escamilla et al. (2001)27 Barbell squat with narrow stance using 12 repetition maximum resistance** 0Barbell squat with wide stance using 12 repetition maximum resistance** 0Leg press with narrow stance with high foot placement using 12 repetitionmaximum resistance**

0

Leg press with wide stance with high foot placement using 12 repetitionmaximum resistance**

0

Leg press with narrow stance with low foot placement using 12 repetitionmaximum resistance**

0

Leg press with wide stance with low foot placement using 12 repetitionmaximum resistance**

0

Escamilla et al. (2009)8 Wall squat with heels position far from wall using 12 repetition maximumdumbbell resistance**

0

Wall squat with heels positioned close to wall using 12 repetition maximumdumbbell resistance**

0

One-leg squat using 12 repetition maximum dumbbell resistance** 59 30Escamilla et al. (2010)11 Forward lunge while taking a long step forward using 12 repetition maximum

dumbbell resistance**0

Forward lunge while taking a short step forward using 12 repetition maximumdumbbell resistance**

0

Escamilla et al. (2010)11 Forward lunge while taking a normal length step forward using 12 repetitionmaximum dumbbell resistance**

0

Side lunge while taking a normal length step sideways using 12 repetitionmaximum dumbbell resistance**

0

Lunging forward and sideways while taking a normal length step using 12repetition maximum dumbbell resistance**

0

Lunging forward and sideways while keeping both feet stationary using 12repetition maximum dumbbell resistance**

0

Toutoungi et al. (2000)

18

Isokinetic seated knee extension at 60

/s 349 3540Isokinetic seated knee extension at 120/s 325 3540Isokinetic seated knee extension at 180/s 254 3540Isokinetic seated knee flexion at 60/s 0Isokinetic seated knee flexion at 120/s 0Isokinetic seated knee flexion at 180/s 0Isometric seated knee extension 396 3540Isometric seated knee flexion 0Squat with heel-off-ground without external resistance 95 \ 50Squat with heel-on-ground without external resistance 28 \ 50Squat one-legged without external resistance 142 \ 50

Shelburne et al. (2005)19 Level ground walking 303 1520Shelburne et al. (2002)20 Dynamic squat-to-stand 20 25Pflum et al. (2004)13 Two-feet drop landing stepping off of 60 c m height platform 253 3348Shin et al. (2007)21 Single leg landing from running to a stop 1294 2530

**Used the heaviest resistance possible that allowed the performance of 12 consecutive repetitions with proper form and technique.ACL: anterior cruciate ligament.

Table 3. Peak anterior shear force (N) (ACL loading) and knee angle () for commonly performed rehabilitation exercises.

Author Rehabilitation exercise Anterior shearforce (N)


Wilk et al. (1996)28 Barbell squat using 12 repetition maximum resistance** 0Leg press using 12 repetition maximum resistance** 0Dynamic seated knee extension using 12 repetitionmaximum resistance**

248 14

Nagura et al. (2006)29 Full squat using no external resistance 66 10.9

Rising from kneeling 111 40.9Level ground walking 355 16.8Stair climbing 146 50.8

Pflum et al. (2004)13 Drop landing 220 3348

**Used the heaviest resistance possible that allowed the performance of 12 consecutive repetitions with proper form and technique.



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are more functional multi-joint, multi-muscle exer-

cises that are effective in developing important hip

and thigh musculature, such as the gluteals, ham-

strings, quadriceps, and adductors and abductors.

Peak ACL tensile force

ACL tensile force levels that are injurious to the recon-

structed ACL are unknown, although it likely depends,

in part, on the number of weeks post reconstruction.

ACL tensile forces are generally lower during WBE

compared with NWBE,6,9,28 and are typically absent in

both WBE and NWBE between 50100 knee

angles.6,12,28 Therefore, employing higher knee angles

of between 50100 during WBE and NWBE mini-

mizes the risk of injury to the healing graft site.

Table 2 presents ACL tensile force during a variety

of NWBE and WBE. Key points of emphasis from

Table 2 are as follows.

First, like the ACL strain data from Table 1, peak

ACL tensile force is of relatively low magnitude (typi-

cally under 150 N for WBE, between approximately

150350 N for the NWBE seated knee extension) com-

pared with loading of the posterior cruciate ligament,9

and occurred at lower knee angles, typically between

1535. The highest ACL tensile forces between

NWBE and WBE occurred during maximal-effort iso-

kinetic seated knee extension exercises, in which ACL

tensile force was approximately 40% greater at a

slower 60/s speed, compared with faster 180/s speed.

Rapid deceleration activities, such as one-leg landing

from a jump, or running and cutting movements, havebeen shown to generate very high ACL loading and are

often implicated in ACL injuries.31 For example, dur-

ing a running plyometric-type exercise involving a

single-leg landing and rapidly coming to a stop, high

deceleration forces are produced that result in approxi-

mately 1300 N of ACL tensile force.31 This high ACL

loading demonstrates that high explosive deceleration-

type plyometric exercises should not be performed until

the later stages of ACL rehabilitation, after the ACL

graft has healed, revascularized, and strengthened ade-

quately. In contrast, a two-leg drop jump from a 60 cm

platform only resulted in approximately 250 N of ACLtensile force,31 which is similar to the ACL loading that

occurred during the NWBE seated knee extension.

Therefore, lower-intensity plyometric exercises, such as

the two-leg drop jump, should precede higher intensity

plyometric exercises, such as the single-leg drop jump.

The rate of deceleration should also be considered

when performing plyometric exercises, as a higher rate

of deceleration will result in greater ACL loading.

Second, squatting typically resulted in minimal or no

ACL tensile force, and one-leg squatting producing

slightly greater ACL loading compared with two-leg

squatting. The minimal or absence of ACL loading dur-

ing the squat is, in part, owing to the increased ham-strings activity and force generated during squatting.

For example, peak hamstring activity during the barbell

squat was approximately 50% of a maximum voluntary

isometric contraction, which helps unload the ACL.9

Moreover, peak hamstring force reported during the

one-leg squat has been reported to be approximately

200 N.8 The increased hamstring activity and force

from one- and two-leg squatting was, in part, owing to

a forward trunk tilt of approximately 3040 at maxi-mum knee flexion.8,9 Progressively increasing the for-

ward trunk tilt during the squat tends to increase

hamstring activity and decrease quadricep activity,

both which result in ACL unloading at knee angles of

less than 60.32 Also, squatting with the heels off the

ground, which typically results in increased forward

knee movement beyond the toes, resulted in over three

times the ACL loading compared with squatting with

the heels on the ground. Like the squat, the absence of

ACL loading during the forward and side lunge is, in

part, owing to the relatively high hamstring force, peak-

ing at approximately 150 N at knee angles of less than

30.11,12

Wall squat exercises may be a better choice com-

pared with the one-leg squat early after ACL recon-

struction because of greater ACL forces generated

during the one-leg squat compared with the wall squat.

However, because peak ACL force during the one leg

squat was only approximately 60 N, it is not likely that

the one leg squat will produce forces that would be

injurious to the healing ACL graft, and mild strain to

the graft may enhance the healing process.33 During

both the wall squat and one-leg squat, as well as other

WBE such as the leg press and lunge, employing larger

knee angles (i.e. 50100) before progressing to smallerknee angles (i.e. 050) may be desirable during the

early stages of ACL rehabilitation because ACL forces

primarily occur at smaller knee angles of less than 50.

The knees moving forward beyond the toes during

squatting and lunging may also increase ACL loading,

especially if excessive (approximately 810cm or

more).8,11,12 ACL loading was significantly greater in

the one-leg squat, in which the knees moved forward

beyond the toes 106 2cm, and in the lunge using a

short step, in which the knees moved forward beyond

the toes 96 2cm, compared with a lunge with a long

step and a wall squat exercise in which the knees didnot move forward beyond the toes.8,11,12 Moreover,

squatting with a more erect trunk position at the lowest

position of the squat compared with squatting with a

3040forward trunk tilt position tends to cause more

forward movement of the knees beyond the toes, as

well as greater quadricep activation (which increases

ACL tensile force at lower knee angles) and less ham-

string activation (which results in less unloading of the

ACL).8,9 Furthermore, as the knee goes forward

beyond the toes, the tibia plateau slopes anteriorly,

resulting in an increase in ACL loading.29,34

Forward trunk tilt may also affect ACL loading dur-

ing the squat and forward lunge exercises. Squattingand lunging with increased forward trunk tilt, com-

pared with a more erect trunk position, has been shown



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to increase hamstrings, which may decrease ACL load-

ing.9,28,32,35 For example, Ohkoshi et al.32 reported no

ACL loading at all knee angles tested (15, 30, 60,

and 90) while maintaining a squat position with trunk

tilted forward from 090, with 30 or more forward

trunk tilt optimal for recruiting relatively high ham-

strings activity and minimizing ACL loading.Technique variations during the NWBE seated knee

extension also can affect ACL tensile force. For exam-

ple, given a constant external knee torque applied to the

leg, ACL force decreases when the resistance pad is

moved up the leg more proximal to the knee compared

with being more distal to the knee closer to the ankle

(Figure 3).17 From Figure 3, when a constant external

knee torque is applied to the leg at 30 knee angle, the

ACL tensile force is approximately twice as great when

the resistance pad is positioned near the ankle (approxi-

mately 400 N) compared with when it is placed near the

middle of the leg (approximately 200 N). Also, Figure 3demonstrates that ACL loading decreases progressively

from 15 knee angle (approximately 500 N when the

resistance pad is near the ankle and approximately

325 N when the resistance pad is placed near the middle

of the leg) to 60knee angle (approximately 100 N when

the resistance pad is near the ankle and approximately

0 N when the resistance pad is positioned near the mid-

dle of the leg), with no ACL loading at knee angles

greater than 60. Nisell et al.34 reported a similar find-

ing of less ACL loading with a more proximally posi-

tion resistance pad on the leg during isokinetic seated

knee extension at 30/s and 180/s. It can be concluded

from these data that, when the goal is to minimize ACL

loading while using the seated knee extension exercise,

this exercise should be performed at higher knee angles

(50100) and with the resistance pad position more

proximal on the leg compared with a more distal posi-

tion. Moreover, it should be emphasized that if the

ACL is torn, there is no ligament to restrain anterior

tibial translation on the femur. Therefore, performing

exercises that would normally load the ACL may causeanterior tibial translation, which may result in altered

and possibly injurious tibiofemoral joint loading.36,37

Wilk and Andrews38 have reported that ACL-deficient

knees during isokinetic exercises tibial translation can

be reduced by utilizing a proximal pad and performing

higher angular velocities (e.g. 180 /s and 300 /s) com-

pared with slower speeds (e.g. 60 /s).

Peak anterior shear force

Peak anterior shear force during WBE and NWBE are

shown in Table 3. From Table 3, it is interesting thatlevel ground walking resulted in greater anterior shear

force (ACL loading) compared with both NWBE and

WBE. Compared with WBE, anterior shear forces were

greater in the NWBE seated knee extension. Peak ante-

rior shear forces occurred at knee angles of 50 or less.

Moreover, from Table 2 peak ACL tensile force during

level walking was approximately 300 N and occurred

near opposite foot toe-off (approximately 1520knee

flexion). Therefore, peak ACL loading during level

walking is similar to peak ACL loading during NWBE

seated isokinetic and isometric knee extension exercises,

and these ACL loading magnitudes are several times

greater than the ACL tensile forces reported for the

WBE.

Figure 3. Changes in ACL loading during the seated knee extension exercise with proximal or distal resistance applied on the leg.

The location of the restraining force is given relative to the distance from the knee joint. Given a constant external knee torqueapplied to the leg, moving the restraining force closer to the knee joint axis decreases ACL force. Adapted from Pandy and

Shelburne17 with kind permission of Elsevier.ACL: anterior cruciate ligament.



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PCL loading during selected rehabilitation

exercises

Table 4 presents PCL tensile force data from selected

articles in the scientific literature during a variety of

NWBE and WBE commonly used in cruciate ligament

rehabilitation. Key points of emphasis from Table 4

are as follows.

First, in contrast to the peak, ACL loading at lower

knee angles between approximately 050 shown in

Tables 1 and 2, peak PCL loading occurs at higherknee angles between approximately 50100 (typically

around 8090 knee angles). Thus, to control PCL

loading, the rehabilitation process is the complete

opposite of that of the ACL program. Therefore, if the

rehabilitation goal is to minimize PCL loading, such as

during the early phases after PCL reconstruction sur-

gery, training both NWBE and WBE at lower knee

angles (e.g. 050) would be recommended compared

with training these exercises as higher knee angles (e.g.

50100).

Second, peak PCL tensile force was generally greater

in the NWBE seated knee flexion compared with WBE.For example, performing the NWBE seated isometric

knee flexion at 90 knee angle produced greater PCL

Table 4. Peak PCL tensile force and knee angle for commonly performed rehabilitation exercises.

Author Rehabilitation exercise Peak PCLforce (N)


Escamilla et al. (2009)8 Wall squat with heels position far from wall using 12 repetitionmaximum dumbbell resistance**

757 80

Wall squat with heels positioned close to wall using 12 repetitionmaximum dumbbell resistance** 786 90

One-leg squat using 12 repetition maximum dumbbellresistance**

414 90

Escamilla et al. (2010)11 Forward lunge while taking a long step forward using 12repetition maximum dumbbell resistance**

765 70

Forward lunge while taking a short step forward using 12repetition maximum dumbbell resistance**

612 90

Escamilla et al. (2010)11 Forward lunge while taking a normal length step forward using12 repetition maximum dumbbell resistance**

765 70

Side lunge while taking a normal length step sideways using 12repetition maximum dumbbell resistance**

641 50

Lunging forward and sideways while taking a normal length stepusing 12 repetition maximum dumbbell resistance**

733 60

Lunging forward and sideways while keeping both feet stationary

using 12 repetition maximum dumbbell resistance**

652 80

Escamilla et al. (1998)9 Barbell squat using 12 repetition maximum resistance** 1868 63Leg press using 12 repetition maximum resistance** 1866 95Dynamic seated knee extension using 12 repetition maximumresistance**

959 79

Escamilla et al. (2001)39 Barbell squat with narrow stance using 12 repetition maximumresistance**

2066 77

Barbell squat with wide stance using 12 repetition maximumresistance**

2212 76

Leg press with narrow stance with high foot placement using 12repetition maximum resistance**

1703 94

Leg press with wide stance with high foot placement using 12repetition maximum resistance**

1726 88

Leg press with narrow stance with low foot placement using 12repetition maximum resistance**

1690 95

Leg press with wide stance with low foot placement using 12repetition maximum resistance**

1726 95

Toutoungi et al. (2000)18 Isokinetic seated knee extension at 60 /s 74 90Isokinetic seated knee extension at 120 /s 59 90Isokinetic seated knee extension at 180 /s 55 90Isokinetic seated knee flexion at 60 /s 2701 90Isokinetic seated knee flexion at 120 /s 2394 90Isokinetic seated knee flexion at 180 /s 1952 90Isometric seated knee extension 0Isometric seated knee flexion 3330 90Squat with heel-off-ground without external resistance 2222 90100Squat with heel-on-ground without external resistance 2704 90100

Shelburne et al. (2005)19 Level ground walking Approximately 160 1520

**Used the heaviest resistance possible that allowed the performance of 12 consecutive repetitions with proper form and technique.

PCL: posterior cruciate ligament.



9/11

tensile force (3330 N) compared with all WBE. The

NWBE seated isokinetic knee flexion at 60 /s, 120 /s,

and 180 /s, and the two-leg squat, produced the next

highest PCL tensile force, ranging between approxi-

mately 19002700 N. PCL tensile forces were approxi-

mately 17001900 N during the leg press, approximately

750800 N during the wall squat, approximately650750 N during the forward and side lunges, approxi-

mately 400N during the one-leg squat, and approxi-

mately 160 N during level ground walking.

Comparing technique variations within exercises,

performing the forward lunge while taking a long step

forward, produces a significantly greater PCL tensile

force compared with performing the forward lunge

while taking a short step forward (the short step lunge

causes the knees to move forward over the toes approx-

imately 8 cm). Moreover, performing the forward lunge

produced a significantly greater PCL tensile force com-

pared with performing the side lunge, and performing

the forward and side lunge while taking a normal

length step (and then pushing back to the upright start-

ing position) produced a significantly greater PCL ten-

sile force compared with performing the forward and

side lunge while keeping both feet stationary (and sim-

ply lunging up and down). Therefore, there are differ-

ent progressions both within an exercise and between

exercises that can be employed during PCL rehabilita-

tion, and as a general rule NWBE and WBE should

first begin with no external resistance and progress to

increasing amounts of external resistance. Given the

PCL loading shown in Table 4, one example of exercise

progression for lower to higher PCL loading mayinclude performing NWBE and WBE initially between

050 knee flexion and progressing to 50100 knee

flexion. Exercises that may be appropriate early in

rehabilitation might include seated knee extensions

between 050 knee flexion, level ground walking,

one-leg squats with no resistance, and forward and side

lunges with no resistance. Resistance can slowly be

added to these exercises, and the leg press, squat, and

seated knee flexion exercises can be added later, ini-

tially without resistance and progressing to resistance.

It is not well understood what PCL force magnitudes

become injurious to the healthy or reconstructed PCL.In healthy adults, the ultimate strength of the PCL is

approximately 4000 N,40 although these values depend

on age and anatomical factors. Therefore, the PCL

loads generated during both NWBE and WBE appear

to be well within a safe limit for the healthy PCL. The

reconstructed PCL has similar ultimate strengths com-

pared with the healthy PCL. However, the healing graft

site may be injured with considerably less force com-

pared with the ultimate strength of the graft, although

it is not well understood how much force to the healing

graft site is too much and how soon force can be

applied after reconstruction. Therefore, the peak PCL

forces that occur during NWBE and WBE may be pro-blematic early after PCL reconstruction when the graft

site is still healing, especially between 50100 of knee

angles. It may be prudent to employ smaller knee angles

(e.g. 050) before progressing to larger knee angles

(e.g. 50100) during NWBE and WBE, because PCL

forces increase as knee angle increases.

SummaryThis review allows the clinician to select specific thera-

peutic exercises, broken down by weight bearing status,

knee range of motion, and technique variations, to

progress cruciate ligament loading over the course of

rehabilitation safely while ensuring optimal recovery of

the musculoskeletal system. In general, WBE produces

smaller loads on the ACL and PCL compared with

NWBE. For the ACL, performing seated knee exten-

sions with resistance produces significantly greater

ACL loading compared with most WBE. Further, peak

ACL loading during level walking is similar to peak

ACL loading during NWBE seated isokinetic and iso-metric knee extension exercises, and these ACL loading

magnitudes are several times greater than the ACL ten-

sile forces reported for the WBE. For the PCL, the

highest loading occurred in the two-leg squat, followed

by the leg press, wall squat, forward and side lunges,

one-leg squat, and level ground walking. WBE has the

benefit of being much more functional and challenging

in terms of hip and thigh muscle recruitment compared

with NWBE. Therefore, early after injury or recon-

struction of the cruciate ligament the clinician should

prescribe WBE rather than NWBE, and progress to

NWBE as tolerated and to facilitate isolated musclefunctional groups such as the quadriceps.

Peak ACL loading occurs at knee angles of between

1015, and progressively decreases between 1560

knee angles. Beyond 5060 knee angles there is mini-

mal or no ACL loading. In contrast, PCL loading

occurs at higher knee angles (i.e. 50100), with peak

PCL loading typically occurring around 8090 knee

angles. These arc ranges of motion should serve as

guidelines to limit the therapeutic exercise technique

early on in the rehabilitation protocol.

Exercise technique variation should also be considered

when prescribing a rehabilitation protocol. For ACL

rehabilitation, anterior knee movement of 8 cm or more

beyond the toes may also increase ACL loading during

squatting, lunging, leg press, and other WBE. Moreover,

squatting with the heels off the ground, which typically

results in increased anterior knee movement beyond the

toes, resulted in over three times the ACL loading com-

pared with squatting with the heels on the ground.

Squatting and lunging with a more forward trunk tilt

tends to unload the ACL to a greater extent compared

with squatting and lunging with a more erect trunk posi-

tion. Moving the resistance pad up the leg proximally

towards the knee when performing the seated knee

extension, rather than being positioned closer to theankle, decreases ACL loading. Rapid deceleration activi-

ties, such as one-leg landing from a jump, or running



10/11

and cutting movements, have been shown to generate

very high ACL loading and are often implicated in ACL

injuries, so cautious progression is required. These higher

intensity plyometric type exercises should be performed

only during the later stages of ACL rehabilitation. When

prescribing therapeutic exercise for the PCL, the forward

lunge with a long step forward produced greater PCLloading compared with forward lunge with a short step

forward. The forward lunge produced greater PCL load-

ing compared with the side lunge, and lunging by taking

a forward or sideways step and pushing back to the

starting position produced greater PCL loading com-

pared with lunging up and down with both feet station-

ary. The exercise guidelines provided in this review may

be used by the clinician to progress a cruciate ligament

injured individual to maximize the potential benefits and

minimize the chance for injury.

Funding

This research received no specific grant from any fund-

ing agency in the public, commercial, or not-for-profit

sectors.

This article was submitted as part of the Lower Limb

Musculoskeletal Modelling Special Issue.

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Cruciate Ligament Loading During Common Rehabilitation Exercises