Low Versus High Intensity Plyometric Exercise During Rehabilitation After Anterior Cruciate

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The American Journal of Sports
 http://ajs.sagepub.com/content/44/3/609
The online version of this article can be found at:
 
DOI: 10.1177/0363546515620583
 2016 44: 609 originally published online January 21, 2016Am J Sports Med
Troy N. Trumble, Jonathan J. Shuster, Flavia M. Cicuttini and Christiaan Leeuwenburgh
Terese L. Chmielewski, Steven Z. George, Susan M. Tillman, Michael W. Moser, Trevor A. Lentz, Peter A. Indelicato,
Ligament Reconstruction
Low- Versus High-Intensity Plyometric Exercise During Rehabilitation After Anterior Cruciate
 
 
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Low- Versus High-Intensity Plyometric
Exercise During Rehabilitation After
Anterior Cruciate Ligament Reconstruction
Terese L. Chmielewski,*yz PT, PhD, Steven Z. George,y PT, PhD, Susan M. Tillman,§ PT,
Michael W. Moser,|| MD, Trevor A. Lentz,§ PT, Peter A. Indelicato,|| MD, Troy N. Trumble,{ DVM, PhD,
Jonathan J. Shuster,# PhD, Flavia M. Cicuttini,** PhD, and Christiaan Leeuwenburgh,yy PhD
Investigation performed at the University of Florida, Gainesville, Florida, USA
Background: Plyometric exercise is used during rehabilitation after anterior cruciate ligament (ACL) reconstruction to facilitate
the return to sports participation. However, clinical outcomes have not been examined, and high loads on the lower extremity
could be detrimental to knee articular cartilage.
Purpose: To compare the immediate effect of low- and high-intensity plyometric exercise during rehabilitation after ACL recon-
struction on knee function, articular cartilage metabolism, and other clinically relevant measures.
Study Design: Randomized controlled trial; Level of evidence, 2.
Methods: Twenty-four patients who underwent unilateral ACL reconstruction (mean, 14.3 weeks after surgery; range, 12.1-17.7
weeks) were assigned to 8 weeks (16 visits) of low- or high-intensity plyometric exercise consisting of running, jumping, and agility
activities. Groups were distinguished by the expected magnitude of vertical ground-reaction forces. Testing was conducted
before and after the intervention. Primary outcomes were self-reported knee function (International Knee Documentation Commit-
tee [IKDC] subjective knee form) and a biomarker of articular cartilage degradation (urine concentrations of crosslinked
C-telopeptide fragments of type II collagen [uCTX-II]). Secondary outcomes included additional biomarkers of articular cartilage
metabolism (urinary concentrations of the neoepitope of type II collagen cleavage at the C-terminal three-quarter–length fragment
[uC2C], serum concentrations of the C-terminal propeptide of newly formed type II collagen [sCPII]) and inflammation (tumor
necrosis factor–a), functional performance (maximal vertical jump and single-legged hop), knee impairments (anterior knee laxity,
average knee pain intensity, normalized quadriceps strength, quadriceps symmetry index), and psychosocial status (kinesiopho-
bia, knee activity self-efficacy, pain catastrophizing). The change in each measure was compared between groups. Values before
and after the intervention were compared with the groups combined.
Results: The groups did not significantly differ in the change of any primary or secondary outcome measure. Of interest, sCPII
concentrations tended to change in opposite directions (mean 6 SD: low-intensity group, 28.7 6 185.5 ng/mL; high-intensity
group, –200.66 255.0 ng/mL; P = .097; Cohen d = 1.03). Across groups, significant changes after the intervention were increased
the IKDC score, vertical jump height, normalized quadriceps strength, quadriceps symmetry index, and knee activity self-efficacy
and decreased average knee pain intensity.
Conclusion: No significant differences were detected between the low- and high-intensity plyometric exercise groups. Across
both groups, plyometric exercise induced positive changes in knee function, knee impairments, and psychosocial status that
would support the return to sports participation after ACL reconstruction. The effect of plyometric exercise intensity on articular
cartilage requires further evaluation.
Registration Number: Clinicaltrials.gov NCT01851655
Keywords: ACL; knee; articular cartilage; loading; psychosocial; outcomes
An anterior cruciate ligament (ACL) rupture is a common
injury in sports that involve cutting, jumping, or pivot-
ing.18 Most people with an ACL injury require ACL recon-
struction surgery to regain the knee stability necessary for
resuming sports participation.24 ACL reconstruction is
then followed by several months of supervised rehabilita-
tion.1,60 Despite surgical advances and extensive rehabili-
tation, recent literature has revealed less than optimal
short- and long-term outcomes after ACL reconstruction.
For example, up to two-thirds of those who undergo ACL
reconstruction do not return to preinjury sport activities.3
Additionally, by 10 years after surgery, up to 80% of the
ACL reconstruction population show radiographic signs
The American Journal of Sports Medicine, Vol. 44, No. 3
DOI: 10.1177/0363546515620583
� 2016 The Author(s)
609
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of posttraumatic knee osteoarthritis (OA),40,48 which can lead
to pain and limited ability to perform weightbearing activi-
ties. Rehabilitation interventions that facilitate a return to
sports participation and/or reduce the risk of posttraumatic
knee OA after ACL reconstruction are highly desirable.
Rehabilitation after ACL reconstruction is broadly divided
into early and late phases. The early phase focuses on resolv-
ing knee impairments (eg, pain, effusion, range of motion def-
icit, quadriceps muscle weakness, and antalgic gait), and the
late phase focuses on preparing the patient to return to sport
activities.1,60 Running, jumping, and agility drills are typical
interventions in the late phase of rehabilitation after ACL
reconstruction.1,60 These interventions involve lower extrem-
ity landing, followed by propulsion, which invokes the
stretch-shortening cycle in extensor muscles (eg, quadriceps).
The stretch-shortening cycle is an identifying feature of plyo-
metric exercise.6 Therefore, running, jumping, and agility
drills are all forms of plyometric exercise. In uninjured peo-
ple, lower extremity plyometric exercise improves motor
recruitment,5 increases muscle strength,5,47 and enhances
sports-related performance.5,6,22,43 Plyometric exercise might
assist the return to sports participation after ACL recon-
struction by improving quadriceps muscle strength and
knee function, but the intervention has not been examined
in this population.1
Plyometric exercise produces vertical ground-reaction
forces that range from 2 to over 6 times the body
weight.12,29,57 The magnitude of vertical ground-reaction
force indicates the intensity of plyometric exercise.13,28
Higher intensity plyometric exercise includes activities per-
formed on a single leg, with greater effort, or from a higher
box height.12,28,29,57 Posttraumatic knee OA is characterized
by the loss of articular cartilage, which results when the
metabolism of articular cartilagematrix molecules (eg, type
II collagen) is imbalanced such that degradation outpaces
synthesis.17 Excessive loads on articular cartilage increase
degradation,51 but it is unclear how this translates to loads
applied during rehabilitation after ACL reconstruction. It is
of interest to know if plyometric exercise, especially high-
intensity plyometric exercise, has negative effects on articu-
lar cartilage after ACL reconstruction.
Articular cartilage status may be monitored through
biomarkers of articular cartilage metabolism in blood,
urine, or synovial fluid.11 A widely studied articular carti-
lage degradation biomarker is crosslinked C-telopeptide
fragments of type II collagen in urine (uCTX-II).27 uCTX-
II concentrations are elevated in patients with knee OA,
those with focal articular cartilage lesions, and those who
have undergone ACL reconstruction.7,8,46 Chronically high
uCTX-II concentrations or increases over about a year are
associated with the progression of knee OA on radio-
graphs.45,49 While uCTX-II is generally considered an artic-
ular cartilage biomarker, some studies suggest that
concentrations also reflect changes at the bone-cartilage
interface.37,56 Another biomarker of articular cartilage deg-
radation is the neoepitope of type II collagen cleavage at the
C-terminal three-quarter–length fragment in urine (uC2C).
uC2C concentrations are elevated in patients with knee
OA,8,19 and uC2C concentrations are elevated in synovial
fluid after ACL injuries.31,61 Some research suggests that
a ratio of articular cartilage degradation to synthesis (eg,
serum concentrations of the C-terminal propeptide of newly
formed type II collagen [sCPII]) better discriminates knee
OA than individual biomarkers.8,55
The purpose of this study was to compare the immediate
effects of low- and high-intensity plyometric exercise on
knee function and articular cartilage metabolism in
patients after ACL reconstruction. We hypothesized that
self-reported knee function and articular cartilage degra-
dation would increase based on the intensity of plyometric
exercise. Because relatively little is known about plyomet-
ric exercise during rehabilitation after ACL reconstruction,
other clinically relevant measures were included as sec-
ondary outcomes.
METHODS
Study Design
This was a double-blind (participant and tester), random-
ized controlled clinical trial comparing low- and high-
intensity plyometric exercise during rehabilitation after
ACL reconstruction. Testing was conducted before ran-
domization and within 1 week after the intervention at
the University of Florida and UF Health Rehab Center at
the Orthopaedic and Sports Medicine Institute. Interven-
tions were conducted at the UF Health Rehab Center at
the Orthopaedic and Sports Medicine Institute.
Setting and Participants
Patients who underwent ACL reconstruction were recruited
from the clinical practices of 2 board-certified orthopaedic
*Address correspondence to Terese L. Chmielewski, PT, PhD, TRIA Orthopaedic Center, UF Health Orthopaedic and Sports Medicine Institute, 8100
Northland Drive, Bloomington, MN 55431, USA (email: terese.chmielewski@tria.com).
yDepartment of Physical Therapy, University of Florida, Gainesville, Florida, USA.
zTRIA Orthopaedic Center, Bloomington, Minnesota, USA.
§UF Health Rehab Center at the Orthopaedics and Sports Medicine Institute, Gainesville, Florida, USA.
||Department of Orthopaedics & Rehabilitation, University of Florida, Gainesville, Florida, USA.
{Veterinary Population Medicine, University of Minnesota, Minneapolis, Minnesota, USA.
#Department of Health Outcomes and Policy, University of Florida, Gainesville, Florida, USA.
**School of Public Health and Preventive Medicine, Monash University, Melbourne, Australia.
yyInstitute on Aging, University of Florida, Gainesville, Florida, USA.
One or more of the authors has declared the following potential conflict of interest or source of funding: This study was funded by NFL Charities. T.L.C.’s
time and effort were supported by a grant from the National Institutes of Health (K01-HD052713). Support for the study was also provided by the Claude D.
Pepper Older Americans Independence Center (OAIC) Metabolism and Translational Science Core. The OAIC is funded by a grant from the National Insti-
tutes of Health/National Institute on Aging (1P30AG028740).
610 Chmielewski et al The American Journal of Sports Medicine
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surgeons (M.W.M. and P.A.I.) at the University of Florida.
Eligible participants were between 15 and 30 years of age,
had undergone ACL reconstruction surgery no more than
6 months after injury, participated at least 50 hours per
year in level 1 or 2 activities before injury (ie, sports that
include cutting, jumping, or pivoting),10 and met clinical
requirements for initiating advanced rehabilitation (at least
12 weeks after surgery, full active knee extension, active
knee flexion within 5� of the nonsurgical side, pain rating
no greater than 2 of 10 during activities of daily living,
and quadriceps index of �60%). Exclusion criteria included
a bilateral knee injury, prior knee ligament injury and/or
surgery, concomitant ligamentous injury .grade 1, menis-
cal repair, cartilage repair procedure, surgical complications
requiring rehabilitation modification, and renal disease.
The inclusion and exclusion criteria were meant to create
a population of active participants with an acute, unilateral,
and relatively isolated ACL injury. Patients gave written
consent or assent (minor participants) to participate in
this study on a form approved by the University of Florida
Institutional Review Board.
Randomization and Interventions
After preintervention testing, participants were random-
ized to the low- or high-intensity plyometric exercise
group. The randomization scheme was computer gener-
ated, balanced to ensure equal allocation to each treatment
group, and further stratified by sex. The randomization
scheme was maintained by the study coordinator and com-
municated to the treating physical therapist when a partic-
ipant entered the study. The mean time from surgery to
the start of the intervention was 14.3 weeks (range, 12.1-
17.7 weeks). Participants were asked to refrain from par-
ticipating in plyometric exercise outside of the study.
Study interventions were administered 2 times per week
for 8 weeks (16 visits) by a physical therapist with board-cer-
tified credentialing in sports physical therapy (S.M.T.) or
sports physical therapy residency training (T.A.L.). A brief
warm-up on a stationary bicycle was performed at the start
of the treatment session. Plyometric exercise consisted of
running, jumping, and agility activities, and groups were dis-
tinguished by the expected magnitude of vertical ground-
reaction forces (Appendixes 1 and 2, available online at
http://ajsm.sagepub.com/supplemental). Compared with the
low-intensity group, the high-intensity group increased per-
ceived effort at a faster rate and performed higher intensity
activities such as sprinting, jump landing from boxes, single-
legged drop land, and single-legged line jump (Table 1).
Exercise volume was matched between groups, and the
intensity, volume, and neuromuscular demands were grad-
ually increased to minimize delayed-onset muscle soreness
and knee joint inflammation. Verbal and visual cues were
given during exercise performance to ensure the proper
technique. All participants performed a standardized pro-
gram of lower extremity strengthening (leg presses,
machine squats, and knee extensions; 3 sets 3 10 repeti-
tions each), flexibility (standing gastrocnemius and quadri-
ceps stretches and long-sitting hamstring stretches; 2 3
30 seconds each), and proprioception (standing on foam or
a tilt board; 3 3 30 seconds each). The starting resistancefor the plyometric leg press and lower extremity strengthen-
ing was considered moderate by participants’ self-report.
Cryotherapy was used after treatment as needed for knee
symptoms.
Before each treatment, participants reported any thigh
muscle soreness or knee pain, and knee girth was mea-
sured with a standard tape measure. The treatment ses-
sion was rescheduled if muscle or joint soreness did not
resolve after a warm-up on a stationary bicycle or knee
TABLE 1
Differences in Plyometric Exercise Between the Low- and High-Intensity Groups
Exercises Performed by Both Groups: Variable Manipulated
Running
� 10-minute walk:jog (greater jog time in high-intensity group)
� 10-minute jog, 10-minute jog:run (initiated earlier in high-intensity group)
Jumping
� Leg press jump (high-intensity group progressed faster to alternating legs and surgical leg only)
� Wall jump (initiated earlier in high-intensity group)
� 2-legged forward hop (high-intensity group progressed to more demanding jumps)
� 2-legged line jump (high-intensity group progressed to more demanding jumps)
� Squat jump (initiated earlier in high-intensity group)
Agility
� Side shuffle, shuttle run, carioca, 45� cut, 90� cut (low-intensity group progressed to 50% effort, and high-intensity group progressed to
75% effort)
Exercises Performed Only in High-Intensity Group
Running
� Run:sprint
Jumping
� 2-legged drop land, single-legged drop land, drop vertical jump, cone jump, tuck jump, single-legged line jump
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girth increased more than 3 cm from the previous session.
Otherwise, the protocol was implemented, and resistance
was increased by 10% in the lower extremity strengthening
program as long as a good technique was demonstrated.
Demographic Information
Demographic information was collected at testing before
the intervention and included age, sex, body mass index,
mechanism of injury, preinjury Tegner Activity Scale52
score, time from injury to surgery, time from surgery to
pretesting, graft type, and meniscectomy procedures. A
Tegner score of �5 indicates participation in sports.52
Outcome Measures
Primary Outcomes. Knee function was assessed with the
2000 International Knee Documentation Committee
(IKDC) subjective knee form, which includes items related
to knee symptoms and functional activities. Scores range
from 0 to 100, and a higher score indicates better knee
function. The IKDC has good test-retest reliability (intra-
class correlation coefficient [ICC] = 0.94).25
Articular cartilage metabolism was determined from bio-
markers in urine and blood. Fasting, early morning (within
2 hours of waking), second-void urine and blood samples
were collected and stored at –20�C until analysis. Biomarker
concentrations were analyzed in duplicate with commercially
available enzyme-linked immunosorbent assay (ELISA) kits.
The primary biomarker of interest was uCTX-II (Urine Car-
tiLaps; Nordic Bioscience). Urine samples were diluted as
needed (range, 1:1 to 1:90). Urinary concentrations of creati-
nine (Cayman Chemical Co) were determined and used to
correct uCTX-II values according to the following formula:
[corrected concentration (ng/mmol) = 1000 3 uncorrected
concentration (ng/L)/creatinine (mmol/L)]. Intra- and inter-
assay coefficients of variation were \6% and \12%,
respectively.
Secondary Outcomes. Functional performance was
assessed with maximal vertical jump and single-legged for-
ward hop test. A knee brace was not worn during testing.
Participants performed 3 practice trials, followed by 3
maximal-effort test trials. The average of the test trials
was analyzed. For the maximal vertical jump test, reach dis-
tance was recorded with the arms raised overhead (Vertec;
Power Systems). Next, participants performed a squat coun-
termovement, jumped vertically as high as possible, touched
the measuring arm, and landed solidly on both feet. The ver-
tical jump height was calculated as the jump distance minus
the reach distance (cm). The single-legged forward hop test
was only performed after the intervention because of safety
concerns. Participants stood on the test limb and hopped for-
ward maximally, and the distance was recorded (cm). The
nonsurgical limb was tested first. The single-legged forward
hop test index was computed with the following formula:
(surgical side distance/nonsurgical side distance) 3 100.
Knee impairments included anterior knee laxity, average
knee pain intensity, and quadriceps strength. Anterior knee
laxity was measured with a knee arthrometer (KT-1000
arthrometer; MEDmetric Corp) using a manual maximum
pull.44 Laxity was recorded on both sides (mm), and the dif-
ference between sides (surgical – nonsurgical) was calcu-
lated. For average knee pain intensity, participants
verbally rated their worst and least knee pain intensity in
the past week as well as their current knee pain intensity
on the 11-point Numeric Pain Rating Scale (NPRS; 0 = no
pain, 10 = worst pain imaginable),58 and the 3 ratings
were averaged. The NPRS has been shown to be a reliable
and valid method of measuring pain.23,58 Knee extensor tor-
que (quadriceps strength) was measured with an isokinetic
dynamometer (Biodex System 3; Biodex Medical Systems)
and a test speed of 60 deg/s. Participants were seated and
stabilized with their hips in 90� of flexion, and the dyna-
mometer moved through a range of 100� to 10� of knee flex-
ion. Testing was performed on the nonsurgical side first,
followed by the surgical side. Participants performed 5 prac-
tice trials and 5 maximal-effort test trials. The peak knee
extensor torque (N�m) was determined from the test trials.
Quadriceps strength variables included normalized peak
knee extensor torque (peak knee extensor torque/body
mass [kg]) and the quadriceps index: (peak knee extensor
torque on surgical side/peak knee extensor torque on non-
surgical side) 3 100. A secondary biomarker of articular
cartilage degradation was uC2C (IB-C2C-HUSA; IBEX
Pharmaceuticals Inc). Urine samples were diluted as
needed (range, 1:2 to 1:15). uC2C values were corrected
for urinary concentrations of creatinine. sCPII (Procollagen
II C-Propeptide; IBEX Pharmaceuticals Inc) concentrations
were used to assess articular cartilage synthesis. Ratios of
articular cartilage degradation to synthesis (uCTX-II/sCPII
and uC2C/sCPII) were created. Intra- and interassay coeffi-
cients of variation across biomarkers were\6% and\12%,
respectively.
An inflammation biomarker was analyzed because high
loads on articular cartilage can cause joint inflammation
that contributes to articular cartilage degradation.14 Blood
samples were collected (see Primary Outcomes section),
and serum concentrations of tumor necrosis factor–a
(TNF-a) were analyzed with a high-sensitivity ELISA
(R&D Systems).
Psychosocial status can influence the return to sports
participation.9,15 Kinesiophobia, or fear of movement/
reinjury, impedes a return to sports participation4 and
was measured with the shortened version of the Tampa
Scale for Kinesiophobia (TSK-11).59 Items are scored from
1 (strongly disagree) to 4 (strongly agree) and summed to
create a total score ranging from 11 to 44. Higher scores
indicate higher kinesiophobia. The TSK-11 has shown
good test-retest reliability (ICCs = 0.8159 and 0.9316) in
patients with low back pain. Self-efficacy, or confidence,
related to the knee can facilitate a return to sports partici-
pation4 and was measured with a 10-item Knee Activity
Self-efficacy questionnaire (KASE) (Appendix 3, available
online) developed by us after considering a published ques-
tionnaire53 and our clinical experience. Items are scored
from 0 (strongly disagree) to 10 (strongly agree) and
summed to createa total score ranging from 0 to 100.
Higher scores indicate greater self-efficacy in knee-related
activities. The test-retest reliability of the KASE question-
naire was analyzed in 53 patients who had undergone
612 Chmielewski et al The American Journal of Sports Medicine
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ACL reconstruction (29 male patients) and who completed
the questionnaire at 8 and 9 weeks after surgery. The ICC
(model 2,1) was 0.852. Pain catastrophizing, or negative
thoughts about pain, can contribute to chronic pain develop-
ment32 that might impede a return to sports participation34
and was measured with the Pain Catastrophizing Scale
(PCS).50 Items on the PCS are scored from 0 (not at all) to
4 (all the time) and summed to create a total score ranging
from 0 to 52. Higher scores indicate higher pain catastroph-
izing. The PCS has good test-retest reliability (ICC = 0.93)
in patients with low back pain.16
Statistical Analysis
Power calculations were based on detecting group differen-
ces in the 2 primary outcome measures (IKDC score and
uCTX-II concentrations) with a 2-sided test, a level of
.05, and power of 0.80. Group differences in the IKDC score
were set at the minimal clinically important difference of
11 points,25,26 and the standard deviation was conserva-
tively estimated at 11 points. Data on uCTX-II concentra-
tions in an ACL reconstruction population were not
available at the time of study planning. Therefore, uCTX-
II concentrations in uninjured participants who perform
running or swimming exercise (different loading intensi-
ties) were used.41 A sample size of 13 participants per
group was deemed necessary to satisfy power calculations.
Statistical analysis was performed with SPSS version 21.0
(IBM Corp) and SAS version 9.3 (SAS Institute Inc). uCTX-II
and uC2C values were log transformed before analysis
because of a nonnormal distribution. uCTX-II/sCPII and
uC2C/sCPII ratios were computed with raw values, and the
result was log transformed. Descriptive statistics were gener-
ated for demographic variables and the primary and second-
ary outcome measures. To examine group differences,
a univariate general linear model was created for each pri-
mary and secondary outcome measure. Group assignment
was the independent variable, and the change in the outcome
measure (value after the intervention – value before the inter-
vention) was the dependent variable. Values before the inter-
vention were included as covariates, and age was also an
additional covariate in models with uCTX-II and uC2C
because past research37 and the present study show that
these measures are negatively associated with age. For the
single-legged forward hop test index, the value after the
intervention was compared between groups with an indepen-
dent-samples t test. Effect sizes were calculated for all out-
come measures with Cohen d. The effect of plyometric
exercise was examined by combining groups and comparing
values before and after the intervention with paired-samples
t tests. An a level of .05 was used for qualifying significance.
RESULTS
Study enrollment is shown in Figure 1. A total of 25 partic-
ipants were enrolled; however, 1 participant was withdrawn
after preintervention testing because of a quadriceps index
\60%. Thus, 24 patients participated (12 participants in
each treatment group). Demographic information can be
found in Table 2. A participant in the high-intensity group
injured her surgical knee in an accident outside of the study.
The postinjury physical examination indicated that the
graft was intact (ie, firm endpoint), and magnetic resonance
imaging showed no further injury to the knee structures.
The patient could not participate in further treatment ses-
sions because of knee pain and swelling, but testing after
the intervention was performed was consistent with an
intent-to-treat analysis.
Treatment logs were reviewed for compliance with the
16 treatment sessions. In the high-intensity group, the
participant who sustained a knee injury completed 9 treat-
ment sessions; additionally, 1 participant completed 14
treatment sessions, and 2 participants completed 15 treat-
ment sessions, all because of missed appointments that
were not rescheduled. In the low-intensity group, 1 partic-
ipant completed 10 treatment sessions because of missed
appointments that were not rescheduled. Therefore, the
number of treatment sessions completed according to pro-
tocol was 182 of 192 (95%) in the high-intensity group
and 186 of 192 (97%) in the low-intensity group.
Data on the articular cartilage metabolism biomarker
were not analyzable at the preintervention time point for 1
participant in the high-intensity group. The creatinine con-
centration was more than 75 times lower than the next low-
est value in the sample, and the sCPII concentration was 10
times lower than the next lowest value in the sample. Thus,
articular cartilage metabolism biomarkers were analyzed for
11 participants in the high-intensity group. The serum con-
centrations of TNF-a after the intervention were below the
threshold of detection in 6 participants (low-intensity group:
n = 4; high-intensity group: n = 2). For these participants, the
value of 0.203 pg/mL was substituted, which is half of the
minimum value in the remaining sample.
High-intensity group (n = 12)
• Received allocated
intervention (n =12)
Low-intensity group (n = 12)
• Received allocated
intervention (n = 12)
Allocation
Excluded (n =1) 
• Did not meet inclusion criteria
Enrolled in study
(N = 25)
Randomized (n = 24)
Follow-up
Lost to follow-up (n = 0)
Missed appointments
(n = 1)
Lost to follow-up (n = 0)
Discontinued intervention
due to injury (n = 1)
Missed appointments
(n = 3)
AnalysisAnalyzed (n = 12)
• •
•
Analyzed (n = 12)
Figure 1. Flow diagram of study participants.
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Group differences were not found in the change of any
outcome measure (Tables 3 and 4). However, sCPII concen-
trations seemed to change in opposite directions between
groups, with a positive mean value in the low-intensity
group and a negative mean value in the high-intensity
group (P = .097) (Table 4). Effect sizes were below 0.50
for all outcome measures, except for an effect size of 1.03
for sCPII concentrations (Table 4).
Several clinical measures significantly changed in both
groups after the intervention (Table 3). Measures that
increased were the IKDC score (P \ .001), vertical jump
height (P = .001), normalized knee extensor torque (P =
.018), quadriceps index (P = .004), and KASE score (P \
.001); and the mean NPRS score decreased (P\ .001).
DISCUSSION
This study examined the effect of plyometric exercise inten-
sity on knee function, articular cartilage metabolism, and
other clinically relevant measures in patients who had
undergone ACL reconstruction. We hypothesized that knee
function and articular cartilage degradation would increase
based on plyometric exercise intensity, but differences were
not found between the low- and high-intensity groups. Signif-
icant changes after the intervention were increased self-
reported knee function, vertical jump height, normalized
quadriceps strength, quadriceps symmetry index, and knee
activity self-efficacy; and decreased average knee pain inten-
sity. Thus, plyometric exercise had positive effects on knee
function, knee impairments, and psychosocial status in
patients who had undergone ACL reconstruction, regardless
of intensity.
Group differences were not found in the primary and sec-
ondary outcomes possibly because of overlap in intensity
between groups. Many plyometric activities form a common
progressionduring rehabilitation after ACL reconstruction
and were included in both groups. For these activities,
perceived effort was increased more quickly in the high-
TABLE 2
Demographic Information for Low- and High-Intensity Plyometric Exercise Groups
Low-Intensity Group (n = 12) High-Intensity Group (n = 12)
Male sex, n 7 8
Age, mean 6 SD, y 20.7 6 4.9 19.3 6 3.8
Body mass index, mean 6 SD, kg/m2 24.2 6 3.2 24.5 6 2.2
Mechanism of injury, n
Contact 2 4
Noncontact 10 8
Time from injury to surgery, mean 6 SD (range), wk 5.8 6 3.9 (2-14) 11.7 6 9.5 (3-36)
Time from surgery to preintervention testing, mean 6 SD (range), wk 14.6 6 1.6 (12-17) 14.0 6 0.9 (12-15)
Preinjury Tegner activity rating, mean 6 SD 7.7 6 0.8 7.8 6 1.3
Graft type, n
Allograft 5 3
Autograft 7 9
Meniscectomy, n
None 4 11
Medial 3 0
Lateral 3 1
Medial and lateral 2 0
TABLE 3
Clinical Outcomes for Low- and High-Intensity Plyometric Exercise Groupsa
Low-Intensity Group High-Intensity Group
Before
Intervention
After
Intervention Change
Before
Intervention
After
Intervention Change P Value
Effect
Size
IKDC scoreb 70.0 6 13.1 82.1 6 12.9 12.1 6 7.5 72.2 6 10.9 87.7 6 8.4 15.5 6 6.8 .147 0.47
Single-legged hop test index, % — 88.7 6 9.1 — — 92.2 6 5.2 — .257 0.47
NPRS scoreb 1.0 6 0.9 0.6 6 0.6 –0.4 6 0.5 1.0 6 1.1 0.5 6 0.7 –0.5 6 0.6 .639 0.18
Knee laxity difference, mm 3.4 6 1.5 3.0 6 0.9 –0.4 6 1.0 2.0 6 0.9 1.9 6 0.9 –0.1 6 0.6 .219 0.09
Knee extensor torque,b N�m/kg 2.3 6 0.5 3.0 6 0.7 0.7 6 0.5 2.2 6 0.5 2.8 6 0.5 0.6 6 0.5 .751 0.20
Quadriceps index,b % 79.7 6 14.4 87.1 6 14.3 7.4 6 14.0 82.4 6 17.2 92.7 6 9.4 10.3 6 13.9 .311 0.21
TSK-11 score 17.8 6 6.9 17.6 6 5.2 –0.2 6 3.8 17.3 6 3.9 17.4 6 4.5 0.1 6 4.1 .964 0.08
KASE scoreb 67.2 6 23.5 87.9 6 15.1 20.7 6 20.1 80.3 6 12.2 93.3 6 6.1 13.0 6 10.0 .801 0.49
PCS score 3.7 6 4.5 2.5 6 4.2 –1.2 6 2.8 3.2 6 5.1 2.8 6 4.8 –0.4 6 2.0 .297 0.33
aData are reported as mean 6 SD. Group differences were not found in the magnitude of change from before intervention to after intervention. IKDC, Inter-
national Knee Documentation Committee; KASE, Knee Activity Self-efficacy questionnaire; NPRS, Numeric Pain Rating Scale; PCS, Pain Catastrophizing
Scale; TSK-11, shortened version of Tampa Scale for Kinesiophobia.
bSignificant difference between scores before the intervention and after the intervention with the groups combined (P\ .05).
614 Chmielewski et al The American Journal of Sports Medicine
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intensity group, but perception of effort was not measured.
Also, individual movement patterns can affect the magni-
tude of vertical ground-reaction force (eg, greater hip and
knee flexion are associated with lower vertical ground-
reaction force).20,35 Finally, high-intensity activities that
were unique to the high-intensity group did not start until
later in the protocol (eg, box jumps in week 3 or single-
legged jumps in week 6) to allow time to progressively
increase loads on the knee. Published ACL reconstruction
protocols have not specified plyometric exercise prescrip-
tion,1,21,38,62 so this study can inform protocol development
for future research. Caution should be taken when general-
izing the results of this study to other protocols that are sub-
stantially lower or higher in intensity.
Despite the possible overlap in plyometric exercise inten-
sity between groups, it was of interest that sCPII concentra-
tions changed in opposite directions (increase in low-
intensity group and decrease in high-intensity group),
which might be attributed to subtle differences in loading.
Basic science research indicates that articular cartilage syn-
thesis increases with moderate loading and shifts toward
degradation with excessive loading.51 However, sCPII con-
centrations were elevated in the high-intensity group com-
pared with the low-intensity group at the preintervention
time point; therefore, differences in the change in sCPII con-
centrations could reflect a regression toward the mean in
the high-intensity group. Exploratory post hoc analyses
did not show that meniscectomy status or time from injury
to surgery explained differences in sCPII concentrations.
Subchondral bone injuries were not measured in this study
but have the potential to influence articular cartilage sta-
tus.30 Protecting knee joint health after ACL reconstruction
is important but often overlooked in rehabilitation because
of insufficient evidence to guide clinical decision making.
Further research is warranted to better understand the
effect of loading intensity on articular cartilage in patients
who have undergone ACL reconstruction.
Several measures changed favorably from before the
intervention to after the intervention across groups. The
IKDC score improved more than the minimal clinically
important difference,26 and the value after the interven-
tion approximated that of patients who return to sports
at 1 year after surgery.33 Vertical jump height and quadri-
ceps strength increased, mirroring the positive effects of
plyometric exercise in uninjured participants.42 Moreover,
mean single-legged hop test index and quadriceps index
after intervention were 90%, which is a benchmark often
used for clearance to return to sports.54 KASE scores
increased, and while there is no previous research with
which to compare, increased confidence bodes well for
a return to sports participation.2 Finally, average knee
pain intensity decreased after the intervention. Overall,
changes after plyometric exercise would facilitate a return
to sports participation after ACL reconstruction.
Interestingly, the mean TSK-11 score, which indicates
kinesiophobia or fear of reinjury, did not change after the
intervention. A closer inspection of the data showed that
TSK-11 scores after the intervention increased in about
one-third of the sample, which agrees with reports of
increased kinesiophobia at the time of return to sports after
an injury.4,36 Plyometric exercise would seem appropriate to
decrease kinesiophobia because it exposes patients to
sports-related activities in a controlled setting,1 and graded
exposure treatment is effective in patients with low back
pain.39 However, graded exposure treatment focuses on activ-
ities that cause fear, and plyometric activities in this study
were selected to gradually increase loads on the lower
extremity.
The strengths of this study include a randomized con-
trolled study design, a focus on a common rehabilitation
intervention for which little is known,1 and a comprehensive
testing protocol. The potential overlap in plyometric exer-
cise intensity between groups is a study limitation that
could be addressed by extending the protocol duration as
tolerance to high-impact activity increases over time. It is
unknown if plyometric exercise intensity has a differential
effect in the long term because the study only had a short-
term follow-up (immediately after the intervention). A
short-term follow-up was appropriate for this study because
exercise intensity and frequency would vary across patients
after the return to sports. The study included a standardized
program of strengthening, stretching, and proprioception
exercises and did not have a control group (no plyometric
exercise) for comparison. Therefore, the standardized
TABLE 4
Biomarker Outcomes for Low- and High-Intensity Plyometric Exercise Groupsa
Low-Intensity Group High-Intensity Group
Before
Intervention
After
Intervention Change
Before
Intervention
After
Intervention Change P Value
Effect
Size
uCTX-II (log), ng/mmol 3.34 6 0.47 3.29 6 0.54 –0.05 6 0.13 3.45 6 0.48 3.36 6 0.46 –0.09 6 0.20 .856 0.24
uC2C (log), ng/mmol 2.62 6 0.29 2.65 6 0.44 0.03 6 0.43 2.78 6 0.44 2.72 6 0.45 –0.06 6 0.22 .694 0.26
sCPII, ng/mL 764.3 6 226.3 793.1 6 317.6 28.86 185.5 1007.2 6 317.4 806.6 6 279.0 –200.6 6 255.0 .097 1.03
Log uCTX-II/sCPII 0.48 6 0.45 0.42 6 0.52 –0.06 6 0.18 0.43 6 0.50 0.47 6 0.50 0.04 6 0.19 .237 0.49
Log uC2C/sCPII –0.25 6 0.29 –0.22 6 0.42 0.03 6 0.45 –0.20 6 0.42 –0.16 6 0.46 0.04 6 0.21 .859 0.03
TNF-a, pg/mL 2.3 6 3.2 1.7 6 3.9 –0.6 6 2.0 1.8 6 1.5 1.5 6 1.5 –0.3 6 1.6 .731 0.17
aData are reported as mean 6 SD. After accounting for baseline values, the magnitude of change in the biomarkers was not significantly different between
groups. However, the effect size for change in sCPII was high. sCPII, serum concentrations of the C-terminal propeptide of newly formed type II collagen (a
biomarker of articular cartilage synthesis); TNF-a, tumor necrosis factor–a (a biomarker of inflammation); uC2C, urine concentrations of the neoepitope of
type II collagen cleavage at the C-terminal three-quarter–length fragment (a biomarker of articular cartilage degradation); uCTX-II, urine concentrations of
crosslinked C-telopeptide fragments of type II collagen (a biomarker of articular cartilage degradation).
AJSM Vol. 44, No. 3, 2016 Plyometric Exercise Intensity After ACL Reconstruction 615
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program contributed to changes after the intervention, and
it cannot be determined if the same outcomes would be
achieved without plyometric exercise. The study had a mod-
est sample of 12 participants per group. The sample size
was estimated at 13 participants per group but was based
on biomarker data reported for uninjured participants. We
are cautious to interpret that plyometric exercise did not
influence articular cartilage metabolism. Future research
can be strengthened by a larger sample size and potentially
using articular cartilage measures that are local to the
knee. Preoperative differences were found between groups
including the time from injury to surgery and the preva-
lence of meniscectomy procedures, which may have con-
founded the outcomes. Finally, participants in this study
had an isolated, acute, and unilateral ACL injury, and
reconstruction was performed with allograft or autograft
tissue. Different results might be obtained with a multiple
ligament injury, a chronic injury, revision ACL reconstruc-
tion, or a sample with a homogeneous graft type.
In summary, regardless of intensity, 8 weeks of plyo-
metric exercise implemented during rehabilitation after
ACL reconstruction had positive effects on knee function,
knee impairments, and psychosocial status. These
improvements should facilitate a return to sports partici-
pation and possibly increase rates of return to sports after
ACL reconstruction. Further research is needed to deter-
mine if high-intensity plyometric exercise has negative
effects on articular cartilage so that long-term knee joint
health is not sacrificed for short-term functional gain.
ACKNOWLEDGMENT
The authors acknowledge Matt Walser, ATC, PA-C, and
Chris Koenig, ATC, for assisting with patient recruitment;
Caroline Davis, Brandon Lally, and Brittney Herder for
assisting with clinical testing and data entry; and Brian
Bouverat, MS, for assisting with biomarker analysis.
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