1. The Journal of Arthroplasty Vol. 18 No. 5 2003
Knee Strength After Total Knee Arthroplasty
Mauricio Silva, MD,* Eric F. Shepherd, MD,*† Walter O. Jackson, MD,*†
Jeffrey A. Pratt, MD, MPH,*† Christian D. McClung, MPhil (Cantab),* and
Thomas P. Schmalzried, MD*†
Abstract: Fifty-two knees in normal healthy subjects and 32 knees more than 2
years after total knee arthroplasty (TKA) were evaluated. Average isometric exten-
sion peak torque values in TKA patients were reduced by up to 30.7% (P .01).
Isometric flexion peak torque values in patients with TKA were, on average, 32.2%
lower than those from control subjects throughout the motion arc (P .004). Knee
Society Functional Scores were positively correlated to the average isometric exten-
sion peak torque (r 0.57; P .004) and negatively correlated to the average isomet-
ric hamstring to quadriceps (H/Q) ratio (r 0.78, P .0001). Relatively greater
quadriceps strength was associated with a better functional score. Older TKA patients
( 70 years) generated lower isometric extension peak torque values in terminal
extension than younger TKA patients ( 24.2%; P .05). Higher body mass index
(BMI) was associated with relative quadriceps weakness (r 0.44; P .007). These
results suggest that more thorough rehabilitation after TKA would improve func-
tional outcomes. Key words: knee, muscle strength, total knee arthroplasty, H/Q
ratio.
© 2003 Elsevier Inc. All rights reserved.
Improving knee function has become a premier Berman et al. [3] reported that after TKA, the
issue in total knee arthroplasty (TKA) [1]. Little quadriceps mechanism showed 83% of the strength
data exist, however, on knee strength after TKA of the contralateral knee at a minimum of 2 years
and its relationship to patient characteristics, out- after surgery. Patients with relatively better quad-
come measures, and appropriate controls. A dyna- riceps strength had a more normal gait. The authors
mometer can measure strength and provide objec- found no significant decrease in hamstring strength.
tive measures of knee function, and this instrument However, 6 of the contralateral “control” knees
is commonly used to assess knee strength in athletic underwent a TKA during the evaluation period [3].
training and the rehabilitation of knee injuries [2–10]. This suggests that the contralateral knee may not
necessarily be an appropriate control for knee
strength. Even if there is no arthritis, the strength of
From the *Joint Replacement Institute at Orthopaedic Hospital, Los the contralateral knee may be decreased because of
Angeles, California, and †the Harbor-UCLA Medical Center, Torrance, the functional limitation imposed by the other
California. knee. Huang et al. [2] measured knee strength 6 to
Submitted May 21, 2002; accepted March 6, 2003.
Funds were received from Peidmont Fund of the Los Angeles 13 years after TKA. There was no statistically signifi-
Orthopaedic Foundation in support of the research material cant difference between the ratio of hamstring
described in this article.
Reprint requests: Thomas P. Schmalzried, MD, the Joint Re-
strength to quadriceps strength in subjects with a TKA
placement Institute at Orthopaedic Hospital, 2400 S. Flower and the 9 control subjects [2]. Unfortunately, the
Street, Los Angeles, CA 90007. results were not stratified by subject characteristics.
© 2003 Elsevier Inc. All rights reserved.
0883-5403/03/1805-0011$30.00/0 Therefore, no comparisons of absolute strength could
doi:10.1016/S0883-5403(03)00191-8 be made between controls and TKA patients.
605

2. 606 The Journal of Arthroplasty Vol. 18 No. 5 August 2003
The goals of this study are to measure and com- patient performed 3 seconds of maximal knee ex-
pare knee strength in control subjects (no TKA) and tension (concentric quadriceps muscle contraction)
in subjects with a clinically well-functioning TKA immediately followed by 3 seconds of maximal
and correlate those measurements to categorical knee flexion (concentric hamstrings muscle con-
patient variables and clinical outcomes. traction). There was a 30-second rest period be-
tween testing at each position. During testing, a
computer monitor displayed a real-time column
Materials and Methods graph of the generated torque. The test subjects
were allowed to observe this graph as feedback in
Subjects an attempt to enhance effort.
After obtaining Institutional Review Board ap- At each position, peak torque values (foot-
proval and informed consent, 52 control knees (no pounds) of flexion (hamstrings) and extension
TKA) in 31 volunteer subjects (16 women and 15 (quadriceps) were recorded and then used to calcu-
men) were evaluated. All control knees were clin- late the hamstring to quadriceps (H/Q) ratios. The
ically normal: no pain or other limitation. For this ratio of knee flexion strength to knee extension
reason, not all knees in control subjects were in- strength, the so-called H/Q ratio (hamstrings/quad-
cluded. Demographics of control subjects are pro- riceps), is an established method to assess relative
vided in Table 1. strength of the muscle groups [3].
Nineteen patient volunteers with a total of 32
knee arthroplasties were recruited because the ar- Statistical Analysis
throplasties were clinically well-functioning, and
the patient had no physical or mental condition that The statistical analysis was performed using the
would prohibit or inhibit participation. The out- Stata 5.0 software (Stata, College Station, TX). Dif-
come of the TKA was evaluated using the Knee ferences between groups were compared using a
Society Clinical Rating System [11]. All TKAs were 2-sample Student’s t-test. The outcome measures
cemented and posterior-stabilized, with a cemented (isometric flexion and extension torques and H/Q
all-polyethylene patellar component. All patients ratios) were adjusted for patient characteristics
were at least 2 years after surgery (average, 2.8 (age, gender, weight, height, and BMI) using a
years; maximum, 6 years). Thirteen subjects had step-wise multivariate regression analysis. The con-
bilateral TKAs. Demographics of TKA subjects are trol subjects were younger (P .0001), taller
provided in Table 1. (P .09), lighter (P .1), and had lower BMI
(P .008) than the subjects with a TKA.
Test Protocol In addition to the step-wise multivariate analysis,
we also compared subsets of matched patients. Ten
Using a LIDO Active Dynamometer (LIDO 2.1 control subjects (7 women, 3 men) and 16 subjects
model 200 300 A; Loredan Biomedical, Davis, CA), with TKAs (12 women, 4 men) were selected based
isometric peak extension and flexion torques were on similarities in age, height, weight, and BMI. For
measured from 0° to 90° of knee flexion. the 10 control subjects (15 knees), the average age
To warm-up for testing, subjects walked on a was of 62.0 years (range, 51.4 –72.2 years; SD, 7.3
treadmill at a moderately vigorous rate (2.5 to 3.5 years), the average height was 168.8 cm (range,
miles per hour) for 5 minutes. Subjects were then 153.7–188.0 cm; SD, 11.6 cm), the average weight
seated on the LIDO test apparatus and stabilized was 82.4 kg (range, 56.4 –106.4 kg; SD, 18.3 kg),
around the pelvis and mid-thigh (Fig. 1). With the and the average BMI was 28.9 (range, 21.9 –38.2;
knee flexed to 90°, the center of rotation of the SD, 5,9). For the 16 subjects with TKAs (25 knees),
LIDO lever arm was aligned in parallel with the the average age was of 65.1 years (range, 50.4 –78.9
femoral condyles. The lower extremity was at- years; SD, 8.1 years), the average height was 168.0
tached to the LIDO lever arm by way of a padded cm (range, 147.3–198.1 cm; SD, 12.6 cm), the av-
cuff with a fastener just above the ankle. Subjects erage weight was 87.6 kg (range, 55.9 –101.8 kg;
were instructed on how to perform the tests, em- SD, 12.9 kg), and the average BMI was 31.1 (range,
phasizing the importance of maximum effort dur- 23.4 –36.9; SD, 4.4). There were no significant dif-
ing the test and encouraged during the test to push ferences, in age, height, weight, or BMI between
as hard as they could. these 2 subgroups.
Isometric testing was performed at 7 positions, Correlations between patient characteristics and
beginning with 90° of flexion and moving to full outcome measures were obtained using univariate
extension in 15° increments. At each position, the and multivariate regression analyses. A Pearson

3. Knee Strength After Total Knee Arthroplasty • Silva et al. 607
product-moment coefficient of correlation (r)
181.0 (167.6–198.1) [8.3] 166.5 (147.3–198.1) [12.6] 161.3 (147.3–170.2) [7.0] 186.1 (177.8–198.1) [9.1]
93.5 (74.1–100.0) [13.0]
67.1 (50.4–78.9) [12.0]
27.0 (23.4–30.7) [3.2]
greater than 0.75 indicated a very good to excellent
4)
correlation; 0.51 to 0.75 indicated a moderate to
good correlation; 0.25 to 0.50 indicated a fair degree
Men (n
of correlation; and equal or less than 0.25 was
considered as little or no correlation. A P value of
.05 was considered statistically significant.
19)
85.9 (55.9–101.8) [12.3]
Results
Patients Undergoing TKA (n
67.3 (53.0–83.2) [8.6]
33.1 (25.8–45.9) [5.0]
15)
Isometric Extension Torque
Women (n
Isometric extension peak torque values decreased
as the knee came into extension (Table 2). There
was a high degree of variability in isometric exten-
sion peak toque at all positions tested. On average,
women control subjects generated 40.4% lower
87.5 (55.9–101.8) [12.5]
isometric extension peak torque values than men
67.3 (50.4–83.2) [9.1]
31.8 (23.4–45.9) [5.3]
controls (P .0001). Regression analysis indicates a
19)
correlation between average isometric extension
peak torque values and height (r 0.67, P .0001)
All (n
and age (r 0.82; P .0001) in control subjects.
Table 1. Subject Demographics
On average, women TKA patients generated 52.4%
lower isometric extension peak torque values than
men TKA patients (P .0001). Height and weight
were positively correlated to isometric extension
85.9 (66.4–106.4) [13.1]
38.1 (20.1–72.2) [17.3]
26.2 (21.6–34.0) [3.3]
peak torque values in subjects with a TKA (r 0.82;
15)
P .0001 and r 0.47; P .007, respectively). In ter-
minal extension (30°, 15°, and 0° of flexion), older
Men (n
TKA patients ( 70 years) generated lower isomet-
ric extension peak torque values than younger TKA
patients (24.2%, P .05; 26.5%, P .05; 29.0%,
P .05, respectively).
After adjustments in patient characteristics, iso-
164.4 (152.4–177.8) [18.2]
Abbreviations: BMI, body mass index; TKA, total knee arthroplasty.
31)
74.3 (53.6–133.6) [22.6]
metric extension peak torque values in control sub-
41.7 (15.9–71.0) [18.2]
27.6 (20.4–52.2) [8.7]
16)
jects were, on average, 9.7 ft-lb (95% CI, 0.7 to
NOTE: Values are given as Mean (range) [standard deviation].
Control Subjects (n
19.4; P .05) higher than those in TKA patients. A
Women (n
difference in adjusted isometric extension peak
torque values between control subjects and TKA
patients was evident at all positions tested (Table 2).
Isometric Flexion Torque
172.4 (152.4–198.1) [11.4]
79.8 (53.6–133.6) [19.1]
Isometric flexion peak torque values increased with
40.0 (15.9–72.2) [17.6]
26.9 (20.4–52.2) [6.6]
knee extension (Table 2). There was a high degree
31)
of variability in isometric flexion peak torque at all
positions tested. On average, women control sub-
All (n
jects generated 43.6% lower isometric flexion peak
torque values than men controls (P .0001). Iso-
metric flexion peak torques were correlated to
height (r 0.71, P .0001), age (r 0.51, P .0001)
and weight (r 0.38, P .005). On average, women
Weight (kg)
Height (cm)
TKA patients generated 44% lower isometric flex-
Age (y)
ion peak torque values than men (P .0001). In
BMI
TKA patients, age was not correlated to the average

4. 608 The Journal of Arthroplasty Vol. 18 No. 5 August 2003
Fig. 1. Subjects were seated on the LIDO test apparatus and stabilized around the pelvis and mid-thigh.
Table 2. Isometric Extension Torque, Isometric Flexion Torque, and Hamstring to Quadriceps Ratio
Control Raw Difference 95% CI for the P value for the
All Knees Knees TKAs Between Difference Between Adjusted Adjusted
(n 84)* (n 52)* (n 32)* Groups Groups† Difference Difference
Isometric extension torque (ft-lb)
90° 109.3 (59.5) 135.2 (59.0) 67.2 (28.6) 68.0 67.9 45.7 to 90.1 .0001
75° 115.2 (57.2) 142.8 (51.3) 70.5 (33.2) 72.3 23.7 8.0 to 39.4 .004
60° 106.6 (50.2) 129.9 (44.6) 68.7 (32.9) 61.2 18.5 5.4 to 31.6 .006
45° 89.8 (38.1) 105.9 (35.2) 63.6 (26.7) 42.3 13.4 2.3 to 24.5 .02
30° 69.8 (29.6) 81.5 (27.7) 50.8 (22.0) 30.7 30.7 19.2 to 42.1 .0001
15° 59.2 (23.2) 59.3 (22.1) 37.9 (18.5) 21.4 21.3 12.0 to 30.7 .0001
0° 35.1 (18.7) 41.1 (18.8) 25.5 (14.3) 15.6 15.6 7.9 to 23.3 .0001
Isometric flexion torque (ft-lb)
90° 46.5 (29.0) 61.1 (27.0) 22.1 (8.6) 39.0 11.6 3.4 to 19.3 .003
75° 54.8 (31.2) 70.8 (28.7) 28.8 (11.4) 42.0 15.0 7.0 to 22.9 .0001
60° 59.7 (32.5) 75.6 (31.0) 33.9 (12.0) 41.7 12.1 3.5 to 20.7 .006
45° 63.9 (32.8) 79.0 (30.8) 39.2 (15.3) 39.8 12.2 3.0 to 21.4 .01
30° 68.5 (33.2) 83.9 (31.2) 43.6 (17.5) 40.3 13.1 3.5 to 22.6 .008
15° 72.4 (36.9) 88.6 (35.4) 46.0 (20.6) 42.6 9.6 0.5 to 19.7 .06
0° 69.2 (34.0) 84.2 (32.3) 44.8 (19.7) 39.4 9.1 1.7 to 19.9 .09
H/Q ratio
90° 0.42 (0.12) 0.46 (0.99) 0.35 (0.12) 0.11 0.11 0.06 to 0.16 .0001
75° 0.47 (0.12) 0.49 (0.11) 0.43 (0.13) 0.06 0.06 0.01 to 0.11 .03
60° 0.56 (0.15) 0.57 (0.10) 0.54 (0.22) 0.03 0.03 0.04 to 0.10 .44
45° 0.70 (0.17) 0.74 (0.13) 0.65 (0.20) 0.09 0.09 0.02 to 0.16 .02
30° 1.01 (0.42) 1.08 (0.46) 0.92 (0.32) 0.16 0.16 0.03 to 0.34 .1
15° 1.42 (0.39) 1.49 (0.29) 1.32 (0.50) 0.17 0.24 0.06 to 0.43 .01
0° 2.20 (0.97) 2.18 (0.64) 2.22 (1.36) 0.04 0.04 0.48 to 0.40 .86
*Mean (SD).
†Adjusted by patient characteristics.
‡Degrees of flexion.
Abbreviations: TKAs, total knee arthroplasties; CI; confidence interval; H/Q, hamstring to quadriceps ratio; SD, standard deviation.

5. Knee Strength After Total Knee Arthroplasty • Silva et al. 609
Table 3. Knee Strength Data Summary by Matched Subgroup
90° 75° 60° 45° 30° 15° 0°
Isometric extension torque (ft-lb)
Control knees (n 15) 83.6 (30.5) 100.8 (36.7) 92.6 (32.4) 81.1 (29.3) 59.7 (24.0) 44.6 (16.4) 30.1 (13.6)
TKAs (n 25) 67.9 (32.2) 69.8 (37.1) 68.9 (36.9) 63.9 (30.0) 51.6 (24.3) 39.2 (20.2) 26.4 (15.5)
Isometric flexion torque (ft-lb)
Control knees (n 15) 37.1 (16.0) 47.4 (21.4) 50.3 (21.6) 56.6 (23.2) 62.1 (22.5) 64.4 (27.8) 62.2 (24.7)
TKAs (n 25) 22.6 (8.8) 28.4 (12.2) 33.3 (12.6) 38.8 (16.4) 44.0 (18.7) 46.5 (22.5) 44.4 (21.2)
H/Q Ratio
Control knees (n 15) 0.45 (0.11) 0.47 (0.11) 0.54 (0.09) 0.69 (0.11) 1.21 (0.81) 1.42 (0.34) 2.25 (0.94)
TKAs (n 25) 0.35 (0.12) 0.44 (0.14) 0.55 (0.23) 0.64 (0.20) 0.92 (0.30) 1.28 (0.46) 2.17 (1.42)
NOTE: Values are given as mean (standard deviation). All groups are matched subgroups.
° Degrees of flexion.
Abbreviation: H/Q, hamstring to quadriceps.
isometric flexion peak torque (r 0.16, P .4) but and BMI was found (r 0.44, P .007); more obese
height (r 0.62, P 0.0001) and weight (r 0.44, patients have relatively lower quadriceps strength.
P .01) were. Multivariate regression analysis indi- After adjustments in patient characteristics, H/Q
cates that the average isometric flexion peak torque ratios in control subjects were, on average, 0.8
is strongly correlated to height (r 0.72, P .009). (95% CI, 0.03 to 0.2; P .2) higher than those in
Isometric knee flexion and extension strength were TKA patients. A difference in adjusted H/Q ratios
highly correlated in all subjects (r 0.95, P .0001). between control subjects and TKA patients was
After adjustments in patient’s characteristics, iso- evident at all but 2 of the position tested (60° and
metric flexion peak torque values in control sub- 0°) (Table 2).
jects were, on average, 12.1 ft-lb (95% CI, 4.2 to
20.0; P .003) higher than those in TKA patients. A Matched Subgroups
difference in adjusted isometric flexion peak torque
values between control subjects and TKA patients Isometric extension peak torque values in TKA pa-
was evident at all positions tested (Table 2). tients were highly variable and, on average, 21.2%
lower than those from control subjects, throughout
the motion arc (P .09) (Table 3). A reduction in
H/Q Ratios average isometric extension peak torque of 18.8%
(P .1), 30.7% (P .01), 25.6% (P .05), and 21.2%
For all subjects, isometric H/Q ratios increased with
(P .08) was observed at 90°, 75°, 60°, and 45° of
knee extension (Table 2). There was a high degree
flexion, respectively, in the TKA group (Fig. 2).
of variability in isometric H/Q ratios at all positions
tested. Univariate and multivariate regression anal-
ysis showed no correlation between average iso-
metric H/Q ratios and other variables such as age,
gender, weight, height, or BMI. No significant dif-
ferences in isometric H/Q ratios were found be-
tween men and women or between younger and
older subjects.
There was a trend for the isometric H/Q ratio to
increase near terminal extension as patient age
increased. Older TKA subjects ( 70 years old) had
isometric H/Q ratios that were 18.3% (P .15),
22.9% (P .1), and 46.3% (P .07) higher than
younger TKA subjects at 30°, 15°, and 0° of flexion,
respectively. Univariate regression analysis indi-
cates that BMI and height are correlated to isomet-
ric H/Q ratios in TKA patients (r 0.35, P .05, and Fig. 2. Isometric extension. Knee extension strength was
r 0.42, P .02, respectively). At 90° of flexion, a generally lower in subjects with a TKA. Error bars indi-
stronger correlation between isometric H/Q ratio cate standard deviation.

6. 610 The Journal of Arthroplasty Vol. 18 No. 5 August 2003
a function of gender, age, height, and degree of
obesity. Although knee strength can be restored to
normal levels after a TKA, it is uncommon. In the
present study, average isometric knee extension
and flexion strength of TKA subjects was more than
30% lower than matched control subjects (P .01).
Regardless of statistical analyses, such reductions in
strength have practical significance [12]. The reduc-
tion in muscle strength seen in TKA subjects is
probably the result of muscle atrophy caused by
disuse before the TKA that has not been recovered
after the TKA [13].
Fig. 3. Isometric flexion. Knee flexion strength was con- Knee strength is an important factor in the clin-
sistently lower in subjects with a TKA. Error bars indicate ical outcome after TKA. In the current study, we
standard deviation. found that isometric extension peak torque and the
H/Q ratio had a strong correlation with the Knee
Society Functional Score (r 0.57, P .004 and
r 0.78, P .0001, respectively). The need for ad-
equate extensor mechanism function is a prerequi-
Isometric flexion peak torque values in patients site for common activities of daily living such as
with a TKA were highly variable and, on average, climbing stairs, so it is logical that quadriceps
32.2% lower than those from control subjects strength is associated with the functional score.
throughout the motion arc (P .004) (Table 3). Re- Caution should be taken in assigning any cause and
duction of 39.5% (P .001), 40.0% (P .001), 33.9% effect relationship. It could be argued that better
(P .003), 31.4% (P .007), 29.2% (P .009), functioning knees allow more vigorous activity, and
27.8% (P .03), and 28.6 (P .02) was found at 90°, greater quadriceps strength is a result of higher
75°, 60°, 45°, 30°, 15°, and 0°, respectively, in the activity.
TKA group (Fig. 3). Isometric H/Q ratios in subjects Compared with normal controls, a significant re-
with TKA were, on average, 9.5% lower than those duction in flexion strength was observed at every
from control subjects, throughout the motion arc point on the arc of motion tested. This may be the
(P .3). result of surgical technique, the design and result-
ant biomechanics of total knee prostheses, the
Knee Society Scores quadriceps-focused rehabilitation of our TKA pa-
The average Knee Society (KS) Clinical Score was tients, the postoperative activities of the patients, or
92 (range, 76 –100) and the average KS Functional a combination of these or other factors.
Score was 92 (range, 70 –100). Average isometric As detected by the KSS, relative hamstring weak-
extension or flexion strength did not show a corre- ness had a lower level of functional significance
lation with the clinical score (r 0.09, P .66 and (r 0.33, P .1). The absence of a stronger correla-
r 0.15, P .46, respectively). The functional tion between hamstring weakness after TKA and
scores were, however, positively correlated to the the KSS is a reflection of the relatively low-level
average isometric extension peak torque (r 0.57, activities assessed by the KSS. Hamstring weakness
P .004) and to the average isometric flexion peak would become apparent in more vigorous activities
torque (r 0.33, P .1). The clinical score was not such as fast walking, uphill walking, and running.
correlated to the average isometric H/Q ratio In a study of patients with a torn anterior cruciate
(r 0.2, P .3). Functional scores were negatively ligament, it was found that subjects whose ham-
correlated to the average isometric H/Q ratio string strength was equal to or greater than the
(r .78, P .0001); in other words, relatively quadriceps strength in the involved limb returned
greater quadriceps strength was associated with a to higher levels of participation in sports than did
better functional score. subjects whose hamstring strength was less than
their quadriceps strength [12].
In the present study, nearly 70% of the patients
Discussion were women. Although this is biased toward
women, the female to male ratio for TKA is approx-
As would be expected in a study of human perfor- imately 3 to 1 [14]. Because each subject is their
mance, there is great variability in knee strength as own control, H/Q ratios are less affected by patient

7. Knee Strength After Total Knee Arthroplasty • Silva et al. 611
characteristics than the absolute values of extension 2. Huang CH, Cheng CK, Lee YT, Lee KS: Muscle
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and relatively obese subjects tended to have higher total knee arthroplasty using isokinetic testing. Clin
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Compared with rehabilitation protocols after ath- stituting knee arthroplasties. J Arthroplasty 13:906,
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After anterior cruciate reconstruction, 52 weeks of of muscle strength of posterior cruciate-retained ver-
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