Training Affects Knee Kinematics and Kinetics in Cutting Maneuvers in Sport

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Training Affects Knee Kinematics and Kinetics
in Cutting Maneuvers in Sport
JODIE L.COCHRANE
1,2
,DAVID G.LLOYD
1
,THOR F.BESIER
1,3
,BRUCE C.ELLIOTT
1
,
TIM L.A.DOYLE
1
,and TIMOTHY R.ACKLAND
1
1
School of Sport Science,Exercise and Health,University of Western Australia,Perth,AUSTRALIA;
2
School of Exercise,
Biomedical and Health Sciences,Edith Cowan University,Perth,AUSTRALIA;and
3
Department of Orthopaedics,
Stanford University,Palo Alto,CA
ABSTRACT
COCHRANE,J.L.,D.G.LLOYD,T.F.BESIER,B.C.ELLIOTT,T.L.A.DOYLE,and T.R.ACKLAND.Training Affects Knee
Kinematics and Kinetics in Cutting Maneuvers in Sport.
Med.Sci.Sports Exerc.
,Vol.42,No.8,pp.1535–1544,2010.
Purpose
:The
current study examined how different training affects the kinematics and applied moments at the knee during sporting maneuvers and
the potential to reduce loading of the anterior cruciate ligament (ACL).The training programs were 1) machine weights,2) free weights,
3) balance training,and 4) machine weights + balance training.
Methods
:Fifty healthy male subjects were allocated either to a control
group or to one of four 12-wk training programs.Subjects were tested before and after training,performing running and cutting
maneuvers from which knee angle and applied knee moments were assessed.Data analyzed were peak applied flexion/extension,varus/
valgus,and internal/external rotation moments,as well as knee flexion angles during specific phases of stance during the maneuvers.
Results
:The balance training group decreased their peak valgus and peak internal rotation moments during weight acceptance in all
maneuvers.This group also lowered their flexion moments during the sidestep to 60
-
.Free weights training induced increases in the
internal rotation moment and decreases in knee flexion angle in the peak push-off phase of stance.Machine weights training elicited
increases in the flexion moment and reduced peak valgus moments in weight acceptance.Machine weights + balance training resulted
in no changes to the variables assessed.
Conclusions
:Balance training produced reductions in peak valgus and internal rotation
moments,which could lower ACL injury risk during sporting maneuvers.Strength training tended to increase the applied knee loading
known to place strain on the ACL,with the free weights group also decreasing the amount of knee flexion.It is recommended that
balance training be implemented because it may reduce the risk of ACL injury.
Key Words:
MOMENTS,SIDESTEPPING,
BALANCE,STRENGTH,PREVENTION,ACL INJURY
T
he knee is one of the most commonly injured joints
in sport.Clinically,the knee accounts for nearly
50% of all sporting injuries (39).Within these,rup-
ture of the anterior cruciate ligament (ACL) is common
and devastating,occurring often during noncontact cutting
and landing maneuvers (8,12).Previous studies have shown
that large loads are exerted on the knee during these sport-
ing maneuvers,placing the ACL at risk of injury,especially
in unanticipated circumstances (3–5).
The ACL primarily supports anterior draw of the tibia
with respect to the femur,a movement produced during active
leg extension,and it also acts as a major restraint to varus,
valgus,and internal rotation moments (30,31).Combined
loading or joint loading in this article refers to the combined
externally applied moments at the knee.Combined loading
fromanterior drawand varus,valgus,or internal rotation mo-
ments place greater load on the ACL compared with anterior
draw alone,and these loads are exacerbated when the knee
is extended (30,31).These findings have been gathered from
non–weight-bearing situations.However,the experiments
have been replicated during static
in vivo
conditions (18),
which also revealed the ACL strain increased when valgus
and internal rotation moments were applied to the extended
knee in weight bearing.
Previous laboratory-based research demonstrated that,
during sidestepping tasks,large valgus and internal rotation
moments were applied to the knee while the quadriceps
were producing a knee extension moment (5).This work
also showed that these moments were further magnified
when maneuvers were performed in unanticipated condi-
tions (4) or when a defensive player was present (35).Such
knee loading scenarios are likely mechanisms for ACL
rupture (3–5,26,34,35);therefore,reducing the magnitude
APPLIED SCIENCES
Address for correspondence:David G.Lloyd,Ph.D.,School of Sport
Science,Exercise and Health,University of Western Australia,35 Stirling
Hwy.Crawley,WA 6009,Australia;E-mail:dlloyd@cyllene.uwa.edu.au.
Submitted for publication June 2009.
Accepted for publication December 2009.
0195-9131/10/4208-1535/0
MEDICINE & SCIENCE IN SPORTS & EXERCISE
￿
Copyright
￿
2010 by the American College of Sports Medicine
DOI:10.1249/MSS.0b013e3181d03ba0
1535
Copyright © 2010 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

of these loads may help to reduce the risk of ACL injury.
Decreasing this loading has been achieved using sidestep
technique training (14).In addition,more general forms of
neuromuscular and physiological training,such as strength
and/or balance training,may be effective (10,29,37,44);
however,the mechanisms by which these work are yet to
be fully determined.This information is important to assist
efforts to reduce the incidence of ACL injury in sport and
understand its long-term detrimental effects on knee joint
health (13).Specifically,it is unknown what impact the
aforementioned neuromuscular training may have on knee
loading,thereby altering the risk of ACL injury.
Balance training has been implemented in sports with
the objective of preventing knee ligament injuries (10,29,
37,44).In addition to providing stability,the knee ligaments
have an important neurosensory function (41).Mechano-
receptors within the ligaments function to detect tissue
strains providing feedback to the central nervous system,
which may be used to alter feed-forward muscle activation
strategies and movement patterns to reduce knee loading,
thereby protecting ligaments from injury (9,16,41).Training
programs that load and stimulate the knee ligaments,such
as balance or perturbation training,may facilitate this pro-
tective mechanism (2,26).Indeed,there is evidence to sug-
gest that balance training can reduce the incidence of ACL
injury (10,29,37,44).However,the mechanisms underlying
these changes are still to be clearly defined.In particular,it
is not known what effect balance training has on the knee
loading and kinematics during the execution of running and
cutting maneuvers in sport.
Strength training is commonly adopted by most sport-
ing teams for improving performance.It is known that re-
sistance training can decrease muscular cocontraction about
the knee and increase coordination of synergist muscles
(11).For example,studies on leg flexion and extension
strength training (11,40) found that cocontraction of the
hamstrings and quadriceps muscles decreased and there was
greater coordination of agonist muscles.Therefore,training
optimized muscle activation patterns needed to perform the
movement and usually decreased cocontraction (11,17,36,40).
The reduction of cocontraction may diminish the activation
patterns needed to protect the ligaments of the knee (3,27),
so there may be a negative outcome from strength training.
However,it is not yet known if the reduced levels of co-
contraction map over into performance of maneuvers that
challenge knee stability.There are studies that support the
notion that neuromuscular changes from strength training
carry over into dynamic maneuvers (36,45).If this does oc-
cur,the effect on stabilization of the knee joint may be un-
favorable with decreased cocontraction levels,providing less
support for the knee during the sporting maneuvers.It has
been speculated that free weights are better than machine
weights for training stability because of greater balance and
joint stability requirements and because of the more func-
tional nature of free weight exercises (2,19).However,there
has been no research on the effect that free weights or ma-
chine weights training may have on knee joint loading and
kinematics during the performance of sporting maneuvers.
The purpose of this study was to investigate the effect
of strength training and balance training on knee joint load-
ing and lower limb kinematics during sporting maneuvers
of running and cutting.An intervention study was per-
formed to assess whether the loading at the knee joint could
be altered by training.Four hypotheses were proposed:1)
balance training reduces the applied loading on the knee dur-
ing sporting maneuvers,2) strength training using machine-
based resistance increases loading on the knee joint during
sporting maneuvers,3) strength training using free weights
increases loading of the knee joint to the same level as
machine-based resistance training,and 4) combined balance
and machine-based resistance training results in no significant
change in knee joint loading during sporting maneuvers be-
cause their potential effects would counter one another.
METHODS
Participants recruited were 50 healthy male subjects with
no history of lower limb pain or injury and had limited
previous exposure to endurance,strength,or balance train-
ing (age = 23.0
T
5.5 yr,height = 1.82
T
0.05 m,mass =
78.2
T
9.7 kg).The study was approved by the human
research ethics committee at the University of Western
Australia,and informed written consent was obtained from
all subjects before commencement of the study.Subjects
from Australian Rules Football teams were randomly al-
located either to a control group or to one of four training
groups (10 players per group).
This study comprised three stages:pretesting,training,
and posttesting.The pretesting and posttesting followed the
same methodology as developed previously by Besier et al.
(3–5).Pretesting was conducted within 1 wk before com-
mencement of the training programs,and the posttesting
was conducted within 1 wk after the 12-wk training sched-
ule.Kinematic and kinetic variables were obtained for each
subject while performing sidestepping to 30
-
and 60
-
(S30
and S60),crossover cutting to 30
-
(XOV),and straight line
running (RUN) (3–5).All maneuvers were performed as
both preplanned and unanticipated.Subjects executed these
maneuvers at a speed of 4.0–4.5 m
I
s
j
1
,stepping from their
preferred leg.Subjects performed three acceptable trials of
each maneuver in the preplanned condition and a further
three trials in the unanticipated condition.For the unantic-
ipated trials,individual time delays were determined before
each testing session.The delay time was adjusted to suit
each individual’s reactions so that the appropriate LED in-
dicating the trial type illuminated at a point where the sub-
ject was just able to react in time to perform the maneuver
successfully.The order of trials was randomized to prevent
subject anticipation,and to reduce the effects of learning,
subjects were given a 3-min rest between each trial.
Motion analysis of the subjects was performed using
a six-camera Vicon 370 motion analysis system (Oxford
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Metrics,Oxford,UK) with a 1200
￿
600-mm
2
force plate
(AMTI,Watertown,MA) sampling at 2000 Hz.The in-
frared cameras sampled at 50 Hz tracking retroreflective
markers were placed externally on each subject’s skin at
selected landmarks.A Fourier analysis was performed on
the ground reaction forces recorded during pilot testing of
the maneuvers to investigate the movement frequency con-
tent,and it was found that 98% of the frequency content
was below 19 Hz,with 95% below 10 Hz.These results
confirmed that the motion analysis frame rate was sufficient
to represent the underlying movement frequencies without
aliasing.However,to ensure that there were enough points
in the stance phase on which to carry out the analyses,we
up-sampled the marker motion data from 50 to 200 Hz
using an interpolating cubic spline.To view the effectiveness
of the resulting sampling rate,typical extension/flexion,
valgus/varus,and internal/external rotation moment curves
are provided for the weight acceptance (WA) phase (defined
below) of sidestepping (Fig.1).Figure 4 in the study of
Dempsey et al.(15) also displays typical mean
T
SD curves
collected in our laboratory.
Motion data were collected using the ‘‘UWA marker set’’
with definitions of segment and joint coordinate systems and
joint axis as per the ‘‘UWAkinematic and kinetic model’’ (6).
This lower body marker set consisted of markers placed on
anatomical landmarks and clusters of three markers on each
segment.Before testing,subject calibration trials were per-
formed (6),which were used to locate anatomical landmarks
and define joint coordinates systems.In this,functional hip
and knee tasks were performed to locate hip joint centers
by fitting a sphere to the motion of the thigh markers,with
the knee joint flexion/extension axes defined using a mean
helical axis–based method.The subject also stood on a foot
calibration rig,which was used to establish the position of
the foot markers and to measure foot abduction/adduction
and rear foot inversion/eversion angles.These protocols have
been shown to improve repeatability of joint kinematic and
kinetic data (6).
Filtering of the marker trajectory and force plate data was
performed using a low-pass fourth-order,zero-lag Butter-
worth filter with a cutoff frequency of 18 Hz.Applying the
same filter to the motion and force plate data follows the
recommendations of van den Bogert and de Koning (42)
and studies by McLean et al.(33–35).Selection of the best
cutoff frequency was conducted using a residual analysis
and visual inspection of the final kinematic and kinetic data.
The three-dimensional kinetics were calculated using in-
verse dynamics (6) for the knee joint.Knee joint moments
were expressed as those externally applied to the joint in the
distal segment’s anatomical coordinate system.It is intuitive
to express moments as those applied externally because
knee injury most likely occurs when these moments exceed
the limits of joint strength.The definitions of the moments
were as follows:flexion moment is one that acts to flex
the knee and
vice versa
for an extension moment,a valgus
moment is one that acts to cause abduction of the knee (i.e.,
places the knee into a knock-kneed position) and
vice versa
for a varus moment,and an internal rotation moment is
one that acts to internally rotate the tibia on the femur and
vice versa
for an external rotation moment.
The knee flexion angle and three-dimensional knee mo-
ments were analyzed in different phases.Stance was di-
vided into two phases;the WA and peak push-off (PPO)
phases.WA was defined from heel strike to the start of the
main knee power absorption during stance (which corre-
sponded to the trough after the heel strike transient in the
ground reaction force).PPO phase was the active phase,
i.e.,10% of stance on either side of peak ground reaction
force during stance phase (4,5).Variables were calculated
in each of these gait phases to enable comparisons across
testing sessions and training groups.
Knee joint moments were normalized (divided by) to
height
￿
weight to account for between-subject variation
in body proportions.Visual inspection of graphs for each
moment type across the maneuvers revealed particular mo-
ment peaks or areas of importance for subsequent analysis.
Similar to other studies (14,15,34),the valgus peak was
FIGURE 1—Typical extension/flexion,valgus/varus,and internal/
external rotation moment curves in the WA phase of sidestepping.
TRAINING AND KNEE KINETICS DURING CUTTING Medicine & Science in Sports & Exercise
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identified as occurring during the WA phase of the side-
stepping and XOV maneuvers.Both varus and valgus peaks
were common in WA and PPO during sidestepping,so peak
varus moments were also calculated for both phases.As in
our previous studies (14,15,34),peak flexion moment was
not found in WA,but it was present in PPO.Mean flexion
moment in WA was very small and had large variance,so
only the peak flexion moment in PPO was analyzed.Peak
internal rotation moment was present and determined for
both phases,and mean knee flexion angle was also calcu-
lated for each phase (4,5).
The results presented throughout the article refer to the
percentage change in knee joint moments and knee flexion
angle.Actual values are presented to demonstrate the mag-
nitude of moments.
Training.
The training programs were as follows:1)
Machine weights strength training only—using pin-loaded
isotonic resistance machines for leg curl and leg press
exercises.2) Free weights strength training only—using leg
curl and squat exercises with free weights.3) Balance
training only—balance exercises using equipment such as
wobble boards,tilt boards,mini trampolines,dura discs,
and Swiss balls.The exercises included progressions from
double-legged to single-legged static balancing,double-
legged to single-legged squats,and additional stability-
challenging exercises while maintaining balance on the
various equipment.The athletes were progressed in exercise
difficulty when they could competently complete exercises
at the current level of difficulty.4) Machine weights +
balance training—using the machine-based exercises as in
the machine weights group and balance exercises as in the
balance training only group.All training groups exercised
for 30 min,three times per week for 12 wk.Strength
training groups followed a progressive overload plan start-
ing at 80% of one-repetition maximum
￿
eight repeti-
tions
￿
three sets
￿
three times per week.The machine
weights + balance training performed 15 min of resistance
training exercises and 15 min of balance training exercises.
The balance training components became progressively
more difficult as the players became more proficient in
the tasks.At the end of 12 wk of balance training,all sub-
jects were able to perform full squats on the Swiss balls.
The control group carried out their normal team training
programs.
Data treatment.
The influence of training on knee
loading during each maneuver performed under preplanned
and unanticipated conditions was determined using a four-
factor repeated-measures ANOVA (training group
￿
ma-
neuver
￿
anticipation
￿
before–after training).ANOVAwas
collapsed to a two-factor model when no interactions for
anticipation or maneuvers were found.The maneuvers were
combined in the collapsed model to demonstrate the over-
all effect of training on these tasks.LSD
post hoc
analysis
was also performed,and the significance level was set at
P
G
0.05.A three-factor repeated-measures ANOVA (train-
ing group
￿
maneuver
￿
anticipation) was also conducted
on the data at baseline to identify if any differences existed
between the training groups before testing.The ANOVA
was collapsed to a two-factor model when no significant
interactions were identified between the groups at baseline.
Data analyses were conducted in DataDesk statistical soft-
ware Version 6.1 (Data Description,Inc.,Ithaca,NY).
If significant differences were found in any measures
between the groups at baseline,they were not included in
the results or they have been discussed in the results where
necessary.No differences at baseline would indicate that
any changes after posttesting could be attributed to the
training.In most cases,the values presented are those that
occurred in each phase for all sporting maneuvers com-
bined.However,if an interaction did occur among the train-
ing group,session,and trial type,the changes for each trial
type are presented.
TABLE 1.Summary of changes in the knee moments and flexion angle after the different training methods that have the potential to change the risk of ACL i
njury.
Training Type
a
Peak Valgus
Moment
Peak Varus
Moment
Peak Internal
Rotation Moment
Peak Flexion
Moment
Mean Knee
Flexion Angle
Control WA
j
(
j
)
PPO
,
(
j
)
Balance training WA
,
(+) WA
,
(+) WA
,
(+)
PPO
,
(+) PPO
,
(+) PPO
j
RUN (
j
)
,
S60 (+)
Free weights
(
j
)
PPO
j
(
j
) PPO
,
S60 (+) PPO
,
(
j
)
Machine weights WA
,
(+) WA
,
(+)
PPO
,
(+) PPO
,
(+) PPO
j
(
j
)
Machine weights + balance training
PPO
,
S30 (+)
j
indicates an increase in load or knee flexion.
,
indicates a decrease in load or knee flexion.
(+) indicates a possible decrease in injury risk,i.e.,favorable.
(
j
) indicates a possible increase in injury risk,i.e.,less favorable.
a
If the phase has no maneuver named,it indicates the change was significant across all maneuvers.However,if a maneuver is named,this indicates that s
ignificant differences were
evident for that maneuver only.All changes in the table are significant (
P
G
0.05).
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RESULTS
All resistance training groups significantly increased their
three-repetition maximum strength after the intervention
(
P
G
0.001).The free weights group increased strength by
60% on the squat exercises and by 74% on the leg curl
exercises.The machine weights group increased strength by
53% on the leg press exercises and by 66% on the leg curl
exercises,whereas the machine weights + balance training
group increased by 58%on the leg press and by 43%on the
leg curl exercises.In the performance of the maneuvers,no
significant differences were found for approach velocity
between the groups when compared within and between the
sessions for each maneuver type.Generally,in the WA
phase of all maneuvers,the knee was close to full exten-
sion,with a mean
T
SD flexion angle of 11.3
-T
4.8
-
,
whereas the knee was more flexed in PPO (45.1
-T
7.2
-
).
In the following sections,the changes that occurred in the
two phases will be presented.Within each section,the order
of changes presented is peak valgus (in WA) or peak varus
(PPO),peak internal rotation (in both WA and PPO) and
peak flexion moments (in both WA and PPO),and then the
knee flexion angle.Further,because there were no differ-
ences between results for the preplanned and unanticipated
conditions,the data were collapsed,and these results were
combined.A summary of all the results is presented in
Table 1.
WA phase.
The balance training group decreased peak
valgus moments during WA by an average of 62%reducing
from 0.09 N
I
m
I
kg
j
1
before testing to 0.03 N
I
m
I
kg
j
1
after
testing for all maneuvers (
P
G
0.001;Fig.2).The machine
weights group also experienced a reduction in the peak
valgus moments (27%,
P
G
0.05) from 0.13 N
I
m
I
kg
j
1
be-
fore testing to 0.09 N
I
m
I
kg
j
1
after testing (Fig.2).The
control group increased their peak valgus moment by 26%
(from 0.09 to 0.11 N
I
m
I
kg
j
1
,
P
G
0.05) during the 12-wk
period.The peak valgus moments for the balance training
group were similar to those for the control group before
training but were significantly less than those for the control
group after training (
P
G
0.001).None of the other training
groups exhibited differences from the control group before
or after training.
In the WA,peak varus moment decreased from before
to after training in the balance training and the machine
weights groups across all sporting maneuvers (Fig.3).In
the balance training group,this moment decreased by
24% from 0.32 N
I
m
I
kg
j
1
before testing to 0.24 N
I
m
I
kg
j
1
after training (
P
G
0.001),and the machine weights group
experienced a 21% reduction from 0.28 to 0.22 N
I
m
I
kg
j
1
(
P
G
0.001).
Balance training also altered the peak internal rotation
moments in all maneuvers (Fig.4).The internal rotation
moment at the knee was reduced with balance training
during the WA phase by 32% from 0.08 to 0.06 N
I
m
I
kg
j
1
(
P
G
0.001).
PPO phase.
In the PPO,the peak varus moment de-
creased from before to after training in the balance train-
ing and the machine weights groups across all sporting
FIGURE 2—Percentage pre- to posttesting change in the peak valgus
moment during the WA phase for each training group from all
maneuvers.
*
P
G
0.05.
FIGURE 3—Percentage pre- to posttesting change in the peak varus
moment in the WA and PPO phases for each training group from all
maneuvers.
*
P
G
0.05.
FIGURE 4—Percentage pre- to posttesting change in the peak applied
internal rotation moment in the WA and PPO phases for each training
group from all maneuvers.
*
P
G
0.05.
TRAINING AND KNEE KINETICS DURING CUTTING Medicine & Science in Sports & Exercise
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maneuvers (Fig.3).In the balance training group,this
moment decreased by 8% from 0.55 N
I
m
I
kg
j
1
before test-
ing to 0.50 N
I
m
I
kg
j
1
after training (
P
= 0.018),and the
machine weights group experienced an 18% reduction from
0.65 to 0.53 N
I
m
I
kg
j
1
(
P
G
0.001).
In the PPO phase,the peak internal rotation moment
increased by 28%after free weights training,increasing from
0.13 to 0.16 N
I
m
I
kg
j
1
after the training (
P
G
0.001;Fig.4).
The balance training group decreased the peak internal
rotation moments by 15% from 0.19 to 0.16 N
I
m
I
kg
j
1
(
P
G
0.001),and the machine weights group decreased by
11% from 0.18 to 0.16 N
I
m
I
kg
j
1
(
P
= 0.018) after train-
ing (Fig.4).However,the balance training and machine
weights groups were significantly greater than the control
group in this measure before testing and were decreased to
values not significantly different to the control group after
testing.
In PPO,there was a significant increase in the peak
flexion moment after machine weights training,with a
13% increase from 0.86 to 0.97 N
I
m
I
kg
j
1
after training
(
P
G
0.001;Fig.5).Balance training,machine weights +
balance training,and free weights tended to decrease the
peak knee flexion moment,but these changes were not
significant.A deeper examination of the results revealed
significant changes when considering the maneuver type.
The peak flexion moment in the PPO phase increased in
the RUN (
P
G
0.001),S30 (
P
= 0.04),and XOV (
P
G
0.001)
after the machine weights training by 26% (from 0.67 to
0.84 N
I
m
I
kg
j
1
),10% (from 0.94 to 1.04 N
I
m
I
kg
j
1
),and
24% (from 0.78 to 0.97 N
I
m
I
kg
j
1
),respectively.The
machine weights + balance training group experienced
a decreased moment during S30 of 10% from 1.25 to
1.13 N
I
m
I
kg
j
1
(
P
G
0.001).The balance training group ex-
perienced an increased moment in the RUN of 10% from
0.95 to 1.05 N
I
m
I
kg
j
1
(
P
= 0.04) but decreased during
the S60 by 11%,from 1.27 to 1.48 N
I
m
I
kg
j
1
(
P
G
0.001).
The free weights group also decreased the peak flexion
moment in the S60 by 11%,from 0.75 to 0.65 N
I
m
I
kg
j
1
(
P
= 0.001) after training.
The free weights group experienced a small but signifi-
cant 5% (
P
G
0.001) decrease in mean knee flexion angle
in the PPO phase (Fig.6).There was also a small (3.5%,
P
G
0.05) but significant decrease in the control group knee
flexion angles.
DISCUSSION
The objective of this study was to investigate the effect
of different types of training on knee joint loading and ki-
nematics during sporting maneuvers to highlight potential
training methods that may prevent ACL injury.It was hy-
pothesized that the balance training group would experience
a reduction of applied loading at the knee during sporting
maneuvers and that the machine weights and free weights
groups would experience increased loading,whereas the
machine weights + balance training group would experience
no changes.These hypotheses were generally supported ex-
cept in regard to the machine weights + balance training be-
cause this group also experienced a general decrease in knee
loading.For easy reference,the results of the current study
are summarized in Table 1.
In a previous research,differences were found between
preplanned and unanticipated trials (4).However,in this
study,no significant differences were observed,which may
be because of learning in the training that occurred or be-
cause of the faster running speeds used in the current study.
Therefore,the study analyzed all trials of the same type
together,and significant differences were observed,which
may have implications for knee joint loading and risk of
ACL injury in both preplanned and unanticipated cutting
maneuvers.
Several studies have shown that training can reduce the
incidence rate of ACL injuries;however,few have attempted
FIGURE 5—Percentage pre- to posttesting change in the peak applied
flexion moment loading in the PPOphase for each training group from
all maneuvers.
*
P
G
0.05.
FIGURE 6—Percentage pre- to posttesting change in the mean knee
flexion angle in the PPO phases for each training group from all
maneuvers.
*
P
G
0.05.
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to investigate the underlying mechanisms,and no studies
have investigated changes in these mechanisms in running
and cutting maneuvers.One of the initial studies was that of
Caraffa et al.(10),who reported that players who underwent
balance training had about 10 times fewer ACL injuries than
a no-intervention,but active,control group.Holm et al.(23)
demonstrated that neuromuscular training improved dynamic
balance,and this effect was maintained a year after training.
Hewett et al.(20) demonstrated that plyometric training re-
duced the incidence of ACL injuries.The research demon-
strated decreased peak landing force,lowered knee adduction
and abduction moments,increased hamstring-to-quadriceps
muscle peak torque ratios,and increased hamstring muscle
power after training (22).Although these results looked at
landing after a jump,and not sidestepping,parallels can be
drawn with the current study.
It is unknown when ACL rupture actually occurs during
sporting maneuvers.Most research to date has shown that
the applied loads are lower during the WA phase and
greater in PPO (4,5,34,35).Flexion/extension moments are
small during WA,which may mean less support of the
varus/valgus and internal/external rotation moments by the
knee flexor and extensor musculature (27,28).However,it is
in this phase that peak valgus moments occur (15,34).Some
evidence suggests that ACL rupture occurs immediately
after contact (8,24,38),with the knee giving way in valgus
and internal rotation after injury (8,12,24,38).Furthermore,
in the WA phase,the knee is at more extended angles
(
È
10
-
),when higher strains are placed on the ACL (30,31).
ACL rupture could also occur in midstance (PPO) where
the loads are large,although knee flexion angle is around
45
-
.Many studies have shown that the ACL is under
greatest strain when the knee is close to full extension,but
the ACL can also be strained in the 20
-
–50
-
knee flexion
range (7).Markolf et al.(30) showed that,when a valgus
moment is added to anterior tibial translation force,the
ACL loading is greatest around 30
-
–40
-
knee flexion and
similar to that in 0
-
–10
-
of extension from anterior tibial
translation force alone.This suggests that the ACL may
be loaded in the PPO phases of sporting maneuvers,es-
pecially sidestepping,which can subject the knee to large
flexion,valgus,and internal rotation moments (4,5).How-
ever,it remains unclear in which phase ACL rupture actu-
ally occurs,and therefore,changes in both phases were
considered.Further,if the applied knee loading can be
decreased in any or both phases,then the risk of strain and
injury on the ACL should be lessened.
Control group.
Except for an increase in the peak
valgus moment in WA,the control group experienced no
changes in the applied loading on the knee joint from before
to after testing (Table 1).The increase in control group’s
valgus moment may have been due to random variation or
typical in-season training.Nevertheless,the general lack of
changes in the controls indicates that any changes experi-
enced in the other groups were probably because of their
specific training programs.Significant changes were seen in
the knee flexion angle in PPO for the control group,but
these were very small,being only a 3.5% decrease.
Balance training.
Previous researchers have shown a
decrease in the incidence of ACL or lower limb injuries
after a balance training intervention (10,29,37,43,44).The
current study demonstrates the mechanisms that possibly
underlie these changes.It is suggested that the 62% re-
duction in the peak valgus moment across all maneuvers in
WA after balance training (Fig.2 and Table 1) is a critical
factor for the reduction in these injury rates.During the early
stance phase of sidestepping,McLean et al.(35) observed
knee moments that fluctuate between varus and valgus with
a clearly obvious valgus peak,whereas others have noticed
valgus moments (4,5) again with distinct valgus peaks
(14,15).Valgus moments are a likely contributor to ACL
injury because analysis of videos of these injuries has shown
that the knee gives way in the valgus direction after ACL
rupture (8,12,24,38).Hewett et al.(21) also showed that
compared with noninjured athletes,those who had ACL in-
juries also had greater valgus knee loading during landing
tasks in preseason testing.Furthermore,valgus moments
combined with other anterior draw loading increases the
force on the ACL,particularly at knee flexion angles be-
tween 10
-
and 40
-
(1,30),such as those occurring in the WA
phase (11.3
-T
4.84
-
).Taken together,these studies suggest
that valgus moments contribute to ACL injuries and that the
decrease in the peak valgus moments after balance training
may lower the risk of ACL injury.
After balance training,there was also a general reduction in
peak internal rotation moments in both WA and PPO across
all maneuvers.In WA during sidestepping,the applied inter-
nal rotation moments changed to external rotation,with sig-
nificant changes occurring in the S30.These changes have the
potential to reduce ACL injury risk because an external rota-
tion moment applied to the knee places less force on the ACL
compared with a similar internal rotation moment magni-
tude (30,31).Moreover,Markolf et al.(30) demonstrated that
internal rotation moments cause the greatest loading of the
ACL between full extension and 20
-
flexion and that ACL
strain further increased when valgus or varus loading was
added to internal rotation moments.Hence,this reduction
to peak internal rotation moments in WA,along with the
decrease in the peak valgus moment after balance training,
should result in a decreased risk of ACL injury.Although not
explicitly examined,it could be speculated that the bal-
ance training group had better control of their upper body
than the other groups.Intuitively,the decreased moments
around the knee may have been because of good control of
the very large mass of the upper body.Indeed,our previous
work has shown that changes to the upper body posture can
increase knee valgus and internal rotation moments in side-
stepping (14,15),and Zazulak et al.(46) have demonstrated
that deficits in trunk stability are predictive of ACL injury.
Although there was a general reduction,no significant de-
crease in the applied flexion moment occurred after balance
training when all maneuvers were considered.However,a
TRAINING AND KNEE KINETICS DURING CUTTING Medicine & Science in Sports & Exercise
d
1541
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significant 10% decrease did occur in the more challenging
S60 in PPO,where the greatest applied flexion moments oc-
cur (4,5).These applied flexion moments can also be viewed
as internally generated extension moments by the quadriceps.
The quadriceps cause anterior tibial translation (7,32) with
the ACL being the main restraint in knee postures from 30
-
flexion to full extension (25,30,31).As a consequence,these
decreased knee flexion moments may lower the risk of ACL
injury,again possibly because of improved control of the
upper body.
Free weights training.
Free weights training elicited
mixed results with respect to risk of ACL injury.After the
free weights training,the peak internal rotation moments
significantly increased in PPO (Table 1).However,in PPO,
there were significant decreases in the applied peak flexion
moment.As already stated,the higher internal rotation
moments would place the ACL under high strain (1,30,31).
However,in PPO,this may be moderated by the lower
flexion moment and knee flexion angles of around 45
-
,
although flexion angle decreased slightly (4%) after train-
ing.This is because,at these knee postures,the internal rota-
tion moments do not transfer to high ACL loading (1,30,31),
and therefore,free weights training may not increase the risk
of ACL injury.
Machine weights training.
As with the free weights,
machine weights training elicited changes to the knee load-
ing,which may both increase and decrease risk of ACL
injury,although most changes would act to decrease injury
risk.Nevertheless,the most striking change after machine
weights training was an increase in the applied flexion mo-
ment in PPO (Table 1):more than 20% in the RUN and
XOV and approximately 10% in the S30 trials.These in-
creased flexion moments can potentially increase anterior
tibial translation.However,given the large knee flexion
angle in PPO,this increase may not be that important for
risk of ACL injury.Importantly,the machine weights train-
ing reduced the peak valgus and peak varus moments in
WA,although not as large as that seen in balance training
group.These changes have the potential to lower ACL
loading and risk of injury during WA.
Machine + balance training.
The balance training
group tended to have numerous favorable changes that may
decrease the potential risk of ACL injury,whereas the ma-
chine weights group experienced a mixture of changes.The
machine weights + balance training group,however,tended
to be neutral with only a few changes that may reduce the
risk of injury (Table 1).These results support the research
of Wedderkopp et al.(43),who showed that,compared
with strength training alone,a combination of strength and
balance training had significantly fewer traumatic lower
limb injuries.The current results show the possible reasons
for this difference,that is,machine weights + balance training
seems better in preventing ACL injury than machine weights
alone.However,on the basis of results from the current re-
search,machine weights + balance training may not be as
beneficial as balance training alone.
In reading these results,there are several limitations that
are important to note.First,the testing and training took
place in a laboratory,and results in the real world may differ
from what are found in the current study.Future research
should look to evaluate a balance-based intervention in the
field to assess its efficacy with respect to decreasing ACL
injuries.Second,the weight training programs consisted of
only two exercises,and it may not represent a complete weight
training program as part of an athlete’s complete physical
preparation.The balance training was matched for training
time but not volume on the basis of repetitions.In addition,
the strength training was limited to sagittal plane exercise,
whereas the balance training exercises involved coordination
in all three planes.It is not known if similar results would
have been found if the strength training was performed in
all planes.
An additional limitation is that this study investigated the
peak loadings on the knee joint,and although these were
analyzed in phases,it did not distinguish the exact temporal
aligning of the peak valgus and peak internal rotation
moments.If the peak moments occurred at the same time,it
is likely that the ACL loading would increase.For example,
in WA,typically the peak valgus moment occurred slightly
before peak internal rotation moments,although there was
variability (Fig.1).In the instance that the peak valgus
moment occurs at a similar time as the peak internal rota-
tion moment,as demonstrated in Figure 4 in the study of
Dempsey et al.(15),ACL load would be raised.Further-
more,the subjects in the resistance training groups ex-
perienced increases in strength,and it is unknown if the
ligaments are being strained to a greater degree with the
increase in applied loading after these strength increases.
EMG-driven knee models such as that developed by Lloyd
et al.(28) could be used with the data from this study to
estimate ligament and soft tissue loading at the knee and to
monitor how this loading changes after the various training
programs.Finally,reported in this article are the moments
at the knee;although high knee moments will certainly put
a person at an increased risk of ACL injury,these knee
moments are likely not the risk factor
per se
because these
moments are a measure of the outcome of many other ac-
tions before initiating and performing a sidestep.Further
research is required to elucidate what other measurable in-
dicators may be risk factors for ACL injury,for example,
poor upper body stability or one-legged balance.
CONCLUSIONS
The results indicate that balance training tended to induce
positive changes in joint loading that serve to reduce the risk
of ACL injury,whereas strength training elicited changes that
could reduce or increase injury potential,depending on the
variable and phase of stance evaluated.The balance training
group reduced the loading at the knee with decreases in the
applied varus/valgus,flexion,and internal rotation moments
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1542
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during the performance of sporting maneuvers.Alternatively,
the strength training tended to induce changes,which could
predispose the subject to a greater risk of ACL injury.The
free weights group experienced reduced knee flexion and in-
creased internal rotation load.The machine weights group
experienced increases in the applied flexion moment,but
these may have been moderated by decreased peak valgus
and peak varus loading in WA.
The take home message for athletes and coaches is that
balance training may reduce the risk of ACL injury.
Strength training,needed to maximize other aspects of per-
formance,is recommended to be done in combination with
balance training because it may reduce the potential for
ACL injury.
The authors thank the Australian Football Research and Devel-
opment Board for their financial support.The authors also thank the
help of Donna-Lee Ferguson for her assistance in data collection,
training,and processing of data and John Wilkie,Catherine Hill,and
students at the University of Western Australia who assisted with
the processing of data.
Results of the present study do not constitute endorsement by
the American College of Sports Medicine.
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