Abstract
Background and Purpose: The authors previously reported that focused eccentric resistance training during the first 15 weeks following anterior cruciate ligament reconstruction (ACL-R) induced greater short-term increases in muscle volume, strength, and measures of function relative to standard rehabilitation. The purpose of this study was to evaluate the effects of early progressive eccentric exercise on muscle volume and function at 1 year after ACL-R.
Participants and Methods: Forty patients who had undergone an ACL-R were randomly assigned to 1 of 2 groups: a group that received early progressive eccentric exercise (n=20) and a group that received standard rehabilitation (n=20). Seventeen participants in the eccentric exercise group and 15 participants in the standard rehabilitation group completed a 1-year follow-up. Magnetic resonance images of the thighs were acquired 1 year after ACL-R and compared with images acquired 3 weeks after surgery. Likewise, routine knee examinations, self-report assessments, and strength and functional testing were completed 1 year after surgery and compared with previous evaluations. A 2-factor analysis of variance for repeated measures (group × time) was used to analyze the data.
Results: Compared with the standard rehabilitation group, improvements in quadriceps femoris and gluteus maximus muscle volume in the involved lower extremity from 3 weeks to 1 year following ACL-R were significantly greater in the eccentric exercise group. Improvements in quadriceps femoris and gluteus maximus muscle volume were 23.3% (SD=14.1%) and 20.6% (SD=12.9%), respectively, in the eccentric exercise group and 13.4% (SD=10.3%) and 11.6% (SD=10.4%), respectively, in the standard rehabilitation group. Improvements in quadriceps femoris muscle strength and hopping distance also were significantly greater in the eccentric exercise group 1 year postsurgery.
Discussion and Conclusions: A 12-week focused eccentric resistance training program, implemented 3 weeks after ACL-R, resulted in greater increases in quadriceps femoris and gluteus maximus muscle volume and function compared with standard rehabilitation at 1 year following ACL-R.
The restoration of muscle volume and strength (force-generating capacity) following anterior cruciate ligament reconstruction (ACL-R) continues to be a rehabilitation challenge. Interventions that can safely and effectively overload muscle early are needed to minimize the residual atrophy and weakness that often are recalcitrant to standard management approaches. The application of progressive, high-force eccentric resistance is one such intervention that has been shown to safely increase muscle volume and strength in various populations, including individuals who have had an ACL-R.1–8 We previously demonstrated that a 12-week focused eccentric resistance training program, implemented 3 weeks after ACL-R, could safely induce statistically significant and clinically meaningful short-term structural and functional changes in key muscle groups.1,2 Compared with a standard rehabilitation program, improvements in quadriceps femoris and gluteus maximus muscle volume were more than 2-fold greater with the addition of eccentric exercise training during the first 15 weeks following surgery. Likewise, significantly greater results in quadriceps femoris muscle strength and hopping distance were observed with eccentric exercise training compared with standard rehabilitation. Although these initial short-term results were positive and encouraging, the typical recovery period following ACL-R often approaches or exceeds 1 year in duration; thus, follow-up at that time is essential.
The purpose of this study was to evaluate the effects of early progressive eccentric exercise on muscle volume and function at 1 year after ACL-R. We hypothesized that, compared with standard rehabilitation, an eccentrically biased rehabilitation program would result in significantly greater improvements in quadriceps femoris and gluteus maximus muscle volume of the involved thigh assessed 1 year after surgery. Furthermore, we hypothesized that these muscle volume improvements would lead to superior results in quadriceps femoris muscle strength and performance on functional tests.
Materials and Method
Participants
The study sample consisted of the same patients who participated in our previous short-term study.1 Figure 1 illustrates the randomization process via a Consolidated Standards of Reporting Trials (CONSORT) diagram.9 Patients were included in the study if they were between 18 and 50 years of age, moderately active prior to injury (a score of ≥4 on the Tegner Activity Scale10), and willing to adhere to the 12-week training program (starting 3 weeks after surgery). Patients were excluded if they had had a previous fracture or reconstructive surgery in either lower extremity; an abnormal knee radiograph; or a concurrent injury to the posterior cruciate ligament or lateral collateral ligament, a grade III tear of the medial collateral ligament, or a significant articular cartilage lesion. Patients with large, vertical, longitudinal meniscus tears also were excluded. Those who had had a partial meniscectomy or a small meniscus repair were allowed to participate. Two surgeons performed all of the ligament reconstructions in the patients for this study, and each surgeon used an arthroscopically assisted technique with a semitendinosus-gracilis tendon or bone-patellar tendon-bone autograft. The graft selection was based on the patient's desire or request after he or she had been educated about graft-type choice. The surgeons had a bias toward using bone-patellar tendon-bone grafts in younger patients and hamstring muscle grafts in older patients. All patients provided informed consent before participating.
A Consolidated Standards of Reporting Trials (CONSORT) diagram showing the flow of participants through each stage of the randomized trial, including the 1-year follow-up.
Eccentric and Standard Rehabilitation Programs
A randomized matched design was used after the surgery to randomly assign patients to either an eccentric exercise group or a standard rehabilitation group. Participants were matched by graft type, sex, and age. The standard rehabilitation protocol that all participants followed was a criterion- and time-based, 3-phase rehabilitation program at our institution that emphasized weight-bearing exercises, functional and resistance training, and early gains in knee range of motion.1 The exercise prescription was determined by the individual response to exercise. Specifically, if exercises were completed without an increase in knee pain or effusion, the participant was considered ready to progress. Other exercises subsequently were added or current exercises were continued at a higher intensity, frequency, or duration.
After ACL-R, all participants completed 2 to 3 weeks of phase I exercises that focused on controlling pain and effusion, gaining full range of motion of the knee, and attaining basic quadriceps femoris muscle function. Beginning 3 weeks following surgery, participants in the eccentric exercise group continued with standard rehabilitation and began a 12-week, progressive, eccentrically induced, negative work exercise program using 1 of 2 recumbent eccentric exercise ergometers as described previously.1–3,8,11 During each exercise session, the negative work rate was visible on the computer monitor, and the total amount of negative work (measured in kilojoules) was recorded. The pedal speed was self-selected and ranged from 20 to 40 rpm. Participants were positioned on the ergometer so that the negative work would occur from approximately 20 to 60 degrees of knee flexion, effectively minimizing the possibility of a knee hyperextension injury. The intensity of exercise was based on the Borg Rating of Perceived Exertion Scale.12 The first session was 5 minutes in duration at a “very, very light” intensity. If a participant had a favorable individual response to exercise (ie, absence of increased knee pain, effusion, excessive fatigue, and so on), he or she was allowed to gradually progress to a “hard” intensity and a maximum duration of 30 minutes. Participants had to complete a minimum of 80% of the training sessions for their data to be included in the data analysis. Beginning 3 weeks postoperatively, the participants in the standard rehabilitation group continued with the standard rehabilitation protocol. In an attempt to equalize the total exercise time between the groups, the standard rehabilitation group was instructed to follow an exercise regimen similar to that used by the eccentric exercise group except that the standard rehabilitation group used a concentric ergometer (ie, gradually progressing to a “hard” intensity and a duration of 30 minutes).
After the 12-week training program was complete (approximately 15 weeks following surgery), supervised rehabilitation was discontinued for both groups. Participants were instructed and encouraged to perform traditional progressive resistance exercises 2 to 3 times per week as a home exercise program and to gradually increase activity as tolerated until at least 1 year following ACL-R. Periodic routine physical evaluations were continued during this time.
Determination of Muscle Volume by Magnetic Resonance Imaging
A 1.5-T Signa LX magnetic resonance imaging instrument and body coil* was used to acquire a coronal scout scan and axial spin-echo T1-weighted images. Both thighs were scanned from the superior border of the femoral head to the tibiofemoral joint line while the participant lay supine in the scanner. The scans were acquired with an image matrix of 256 × 256; a field-of-view of 40 to 44 cm, depending on the size of the participant; a slice thickness of 8 mm; and an interslice distance of 15 mm. After electronic data transfer of magnetic resonance images, cross-sectional areas were measured with use of MATLAB custom-written image-analysis software† on a desktop personal computer. Muscle volumes were determined by measuring muscle cross-sectional area in sequential axial sections across the length of the muscle.13 On each image, the entire muscle of interest (independent of skin, bone, and fat) was identified and captured. The cross-sectional area of each slice was automatically computed with use of the averaged gray-scale density of the captured muscle. The muscle volume was calculated by multiplying the average of 2 consecutive measurements of cross-sectional area by the slice thickness plus the interslice distance (23 mm) and then summing those values across the length of the muscle. The validity of the volume measurement was determined by analysis of images obtained from a cadaveric thigh phantom that approximated the size of the quadriceps femoris muscle group. The volume of the phantom, measured by water displacement 5 hours after magnetic resonance imaging, was 100.7% of the magnetic resonance-determined value. There was a 0.012% difference between repeated volume displacement measurements of the phantom by the same investigator.14
For this study, magnetic resonance image scans taken 1 year after surgery were compared with scans taken 3 weeks after surgery (prior to the training program). Because of the high correlation between muscle volume (in cubic centimeters) and peak cross-sectional area (in square centimeters) of the thigh musculature reported in the previous study (r2=.95), only muscle volume is reported for the current study. The same investigator (JPG) performed all structural measurements in a highly reproducible manner (intraclass correlation coefficients=>.99).
Knee Laxity Assessment and Functional Status
In a similar manner as previously conducted,1 routine clinical examinations, which included an assessment of knee laxity with use of the KT-1000 device,‡ were completed 1 year following ACL-R. These examinations also included isokinetic strength testing and the single-leg hop-for-distance test. Quadriceps femoris and hamstring muscle strength (peak torque) were assessed with use of a Kin Com isokinetic dynamometer.§ Participants were tested concentrically at 60°/s in a seated position with the hips and knees in 90 degrees of flexion and the thighs, pelvis, and upper body firmly strapped to the seat of the dynamometer. Prior to testing, a warm-up consisting of 3 repetitions (at 50%, 75%, and 100% intensity) was completed. After a 1-minute rest period, the participants completed 3 separate trials at 100% intensity. The peak torques of the 3 trials were averaged, and the average was recorded. For the hop-for-distance test, the participants were instructed to hop as far as possible, always landing on the same leg. Hopping for maximal distance with each leg was tested 3 times, and the average of the 2 farthest hops was recorded. Participants also completed the Activities of Daily Living Scale of the Knee Outcome Survey,15 the Lysholm Knee Rating Scale, and the Tegner Activity Scale.
Data Analysis
Data were analyzed with SPSS software (version 13.0).‖ Descriptive statistics for categorical variables and measures of central tendency for continuous variables were calculated to summarize the data. Tests for outliers and assumptions of the parametric statistical tests were performed. Assumptions of parametric testing were met, and all data were included for analysis. Separate 2-factor analyses of variance for repeated measures (group × time) were used to analyze the effects of time and group assignment and the group × time interaction for each of the dependent variables. Significance levels for all statistical analyses were set at α<.05. Post hoc examination of mean values was performed.
Results
Thirty-two of the 40 enrolled participants completed all aspects of the study (Fig. 1). Of those not returning for the 1-year follow-up, 2 participants cited being too busy and 2 participants had moved out of the area. Four participants traumatically reinjured their involved knee between 4 and 12 months following initial ACL-R, causing graft disruption. These injuries were sustained by 2 male participants with patellar tendon autografts (1 in each group) and 2 female participants with semitendinosus-gracilis tendon autografts (1 in each group). Due to the number of participants who could not complete the 1-year follow-up, an intention-to-treat analysis was performed. Thus, all 40 participants were included for statistical analyses. Twenty individuals (10 in each intervention group) had ACL-R with the semitendinosus-gracillis tendon autograft, and that same number (10 individuals per group) had ACL-R with the bone-patellar tendon-bone autograft. Presurgical demographic and physical characteristics were similar between intervention groups (Tab. 1).
Participant Characteristics of the Eccentric Exercise and Standard Rehabilitation Groups Before Anterior Cruciate Ligament Reconstructiona
The presurgical evaluation was conducted the week before surgery (at a mean of 4.0 days [SD=2.6] in the eccentric exercise group and at a mean of 4.4 days [SD=2.4] in the standard rehabilitation group). The pretraining magnetic resonance image was obtained for each participant 3 weeks after ACL-R (eccentric exercise group: X̄=23.1 days [SD=4.0]; standard rehabilitation group: X̄=22.7 days [SD=3.9]). The follow-up magnetic resonance image and evaluation were conducted 1 year after ACL-R (eccentric exercise group: X̄=367.5 days [SD=19.2]; standard rehabilitation group: X̄=369.6 days [SD=17.2]). During the 1-year evaluation, 16 participants (9 in the eccentric exercise group and 7 in the standard rehabilitation group) reported that they had lifted weights for the lower extremity an average of 2 or more times per week over the past 6 to 9 months.
Quadriceps Femoris Muscle Volume
From 3 weeks after surgery (pretraining) to 1 year after surgery, quadriceps femoris muscle volume of the involved thigh increased significantly in both groups (time effect, P<.001). However, these structural increases were significantly greater, by more than 50%, in the eccentric exercise group compared with the standard rehabilitation group (group × time interaction, P<.01). Quadriceps femoris muscle volume improved 23.3% (SD=14.1%) in the eccentric exercise group and 13.4% (SD=10.3%) in the standard rehabilitation group (Tab. 2, Fig. 2). There was no significant group effect.
Quadriceps femoris and gluteus maximus muscle volume improvement of the involved lower extremity that occurred from 3 weeks after surgery (pretraining) to 1 year after surgery. Asterisk (*) indicates that statistical differences in muscle volume improvement were observed between the eccentric exercise (black bars) and standard rehabilitation (blue bars) groups (P≤.05).
Muscle Volume (in Cubic Centimeters) Measured 3 Weeks (Pretraining) and 1 Year After Anterior Cruciate Ligament Reconstruction for the Eccentric Exercise Group (n=20) and the Standard Rehabilitation Group (n=20)a
Gluteus Maximus Muscle Volume
The distal portion of the gluteus maximus muscle beginning from the superior border of the femoral head was available for analysis. From 3 weeks after surgery (pretraining) to 1 year after surgery, gluteus maximus muscle volume of the involved lower extremity increased significantly in both groups (time effect, P<.001). These structural increases were significantly greater, by more than 50%, in the eccentric exercise group (group × time interaction, P<.05). Volume improved 20.6% (SD=12.9%) in the eccentric exercise group and 11.6% (SD=10.4%) in the standard rehabilitation group (Tab. 2, Fig. 2). There was no significant group effect.
Hamstring Muscle Volume
Hamstring muscle volume of the involved thigh increased significantly in both groups (time effect, P<.001) from pretraining to the 1-year follow-up. There were no significant group or interaction effects in hamstring muscle volume of the involved thigh between groups (P=.70) (Tab. 2).
Gracilis Muscle Volume
Gracilis muscle volume of the involved thigh decreased significantly in both groups (time effect, P<.01) from pretraining to the 1-year follow-up. There were no significant group or interaction effects in gracilis muscle volume of the involved thigh between groups (P=.62) (Tab. 2).
Knee Stability Assessment and Functional Status
Functional status measurements are shown in Table 3. There were no significant differences in knee laxity, as measured with the KT-1000 device (with manual maximum force), between the eccentric exercise group (X̄=1.7 mm, SD=1.9) and the standard rehabilitation group (X̄=1.9 mm, SD=1.5) 1 year after ACL-R (P=.56).
Functional Status Measurements Taken Preoperatively and 1 Year After Anterior Cruciate Ligament Reconstruction for the Eccentric Exercise Group (n=20) and the Standard Rehabilitation Group (n=20)a
From presurgery to 1 year after surgery, quadriceps femoris muscle strength (peak torque) gains were significantly greater in the eccentric exercise group than in the standard rehabilitation group (time and group × time interaction effects, P<.01). The magnitude of improvement was approximately 33% in the eccentric exercise group and 9% in the standard rehabilitation group (Tab. 3). There was no significant group effect.
From presurgery to 1 year after surgery, hopping distance increased by a significantly greater amount in the eccentric exercise group compared with the standard rehabilitation group (time and group × time interaction effects, P<.01). The magnitude of improvement was approximately 50% in the eccentric exercise group and 21% in the standard rehabilitation group (Tab. 3). There was no significant group effect.
Compared with preoperative values, scores on the Activities of Daily Living Scale of the Knee Outcome Survey, the Lysholm Knee Rating Scale, and the Tegner Activity Scale improved significantly 1 year after surgery in both groups (time effect, P<.01), but no significant differences between groups were observed (group and group × time interaction effects).
Discussion
In support of our primary hypothesis, this investigation demonstrated that the addition of progressive eccentric exercise, implemented 3 weeks after ACL-R, resulted in muscle volume and strength gains in key muscle groups 1 year after surgery that exceeded those changes following a standard rehabilitation program. The overall magnitude of improvement in quadriceps femoris and gluteus maximus muscle volume at 1 year was more than 50% greater in the eccentric exercise group. Overall functional improvement (quadriceps femoris muscle strength and hopping distance) also was significantly greater in the group that performed early focused eccentric exercise training. These findings reinforce the potential benefits that can be achieved from safely overloading muscle via high force-inducing resistance exercise during the early rehabilitation stages following ACL-R.
In our previous publication,1 we reported that the magnitude of quadriceps femoris muscle atrophy of the involved thigh approached 25% to 30% just 3 weeks after ACL-R.1 From that point, those participants who performed standard rehabilitation exercises during the early 12-week training period achieved a quadriceps femoris muscle volume increase of 9%, whereas those participants who added early progressive resistance training achieved a quadriceps femoris muscle volume increase of 23%. Improvements in quadriceps femoris muscle volume 1 year after ACL-R in the current study were similar to the short-term improvements found in our previous study (approximately 13% improvement in the standard rehabilitation group and 23% improvement in the eccentric exercise group). Although the 1-year findings reinforce the importance of resistance training during the early rehabilitation stages following ACL-R, it is unknown whether implementing a similar intervention that utilizes high loads and induces high muscle forces during other time frames after ACL-R would lead to similar 1-year results.
Optimizing muscle volume and strength gains after ACL-R is best accomplished by using an intervention designed to overload muscle. Despite persistent muscle volume and strength deficits often observed years after ACL-R, some rehabilitation programs do not emphasize early resistance exercises if patients are content and improving functionally. Other programs may underdose the resistance exercises due to the reasonable concern for graft and joint safety. These factors may contribute to the 20% to 30% side-to-side quadriceps femoris muscle volume and strength deficits reported during the first 3 months following ACL-R and the approximate 10% deficits common at 1 year.16–31 The eccentric resistance intervention in the current study, which utilized high loads and induced high muscle forces, was specifically designed to produce muscle overload.1,2,7 Through the gradual, progressive, and individualized nature of the resistance training, we observed the positive combination of effectiveness and safety as quadriceps femoris muscle volume and function improved while graft stability was maintained. We believe that an intervention specifically designed to safely overload muscle can be an ideal addition to rehabilitation programs early after ACL-R.
The considerable clinical attention and effort toward mitigating quadriceps femoris muscle atrophy and weakness during the first 3 months following surgery suggest this is a critical time period for restoring muscle volume and function. The results of this study certainly support this notion. However, perhaps intervening with eccentric resistance training even sooner or prior to reconstruction is a reasonable clinical question to be explored. In this study, the majority of participants had surgery 4 to 6 weeks following rupture of the anterior cruciate ligament (ACL). Unfortunately, by that time (3 weeks after surgery), the quadriceps femoris muscle volume of the involved side was already more than 25% smaller than that of the uninvolved side.1 Considering that the uninvolved quadriceps femoris muscle in all likelihood also atrophied because of decreased activity, total quadriceps femoris muscle atrophy of the involved side prior to the intervention was dramatic. Further research is necessary to determine whether an eccentric exercise training program prior to ACL-R could prevent the obligatory atrophy and strength loss often associated with ACL injuries.
Timing and type of intervention (one specifically designed to overload muscle) are 2 important factors in preventing atrophy or restoring muscle volume and strength following ACL injury. Other neuromuscular factors also are important to consider. During weight-bearing activity (ie, gait, squats, leg press, eccentric ergometry), both the knee and hip extensors are active, but a shift toward a hip extensor strategy rather than a knee extensor strategy could develop.32,33 Although we intended for the eccentric ergometry intervention to be quadriceps femoris muscle specific, we made an early observation that gluteal muscles also were being loaded, as participants frequently reported soreness in the gluteal region as a result of training. Perhaps resistance exercises biased toward developing a pure knee extensor strategy (by isolating the quadriceps femoris muscle from the hip extensors via non–weight-bearing resistance training) may be helpful in restoring the proper knee-to-hip extensor relationship. Further research in this area is warranted.
It is worth noting that we observed a significant decrease in gracilis muscle volume from pretraining to the 1-year follow-up. In our previous study,1 we noted that the decrease in gracilis muscle volume appeared to be graft-dependent. Gracilis muscle volume was relatively unchanged in those individuals who had ACL-R with the bone-patellar tendon graft but was substantially decreased in those individuals who had ACL-R with the semitendinosus-gracilis tendon graft.1 Although it was not the purpose of this study to compare graft types, it appears that our previous statement would still apply at 1 year following ACL-R. Comparing gracilis muscle volume longitudinally between graft types after ACL-R would be an interesting topic for a future study.
Limitations
Several limitations characterize the current study. Only 80% of the original sample completed all aspects of the study up to the 1-year evaluation. However, because the directional short-term results of this smaller cohort were statistically supported at 1 year, we believe the current sample is an adequate representation of the original group. We also used an intention-to-treat analysis to be more conservative in our statistical approach.
Another potential limitation in this study was the lack of a control group or a more-detailed description of resistance training-specific activities during the home exercise program that occurred from the posttraining evaluation to the 1-year evaluation. Considering that one group could have participated in a different training regimen than the other group during this period is a confounding variable, especially because measures of muscle volume and function were the primary outcome variables. The bottom line is that the differences observed at the 1-year follow-up cannot be solely attributed to the eccentric exercise training that was performed during weeks 3 to 15.
Conclusions
This study demonstrated that the addition of progressive eccentric exercise, implemented 3 weeks after ACL-R, resulted in muscle volume and strength gains in key muscle groups 1 year after surgery that exceeded those changes following a standard rehabilitation program. The overall magnitude of improvement in quadriceps femoris and gluteus maximus muscle volume at 1 year was more than 50% greater in the eccentric exercise group compared with the standard rehabilitation group. Overall functional improvement (quadriceps femoris muscle strength and hopping distance) also was significantly greater in the group that performed early focused eccentric exercise training. These findings clearly emphasize the importance of progressive resistance training during the early rehabilitation stages following ACL-R and further suggest that adding a high-force eccentric exercise intervention is a viable option as part of a comprehensive rehabilitation program.
Footnotes
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All authors provided concept/idea/research design. Dr Gerber, Dr Marcus, Dr Dibble, and Dr LaStayo provided writing. Dr Gerber and Dr Marcus provided data collection. Dr Gerber, Dr Marcus, and Dr Dibble provided data analysis. Dr Gerber and Dr LaStayo provided project management and fund procurement. Dr Greis and Dr Burks provided participants. Dr Marcus, Dr Dibble, and Dr LaStayo provided facilities/equipment. Dr LaStayo provided institutional liaisons. Dr Marcus, Dr Dibble, Dr Greis, Dr Burks, and Dr LaStayo provided consultation (including review of manuscript before submission).
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This study received approval from the Institutional Review Board at the University of Utah.
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This research, in part, was presented at the Combined Sections Meeting of the American Physical Therapy Association; February 6–9, 2008; Nashville, Tennessee.
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This study was funded, in part, by the American Physical Therapy Association Orthopedic Section Clinical Research Grant to Dr Gerber and Dr LaStayo.
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The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Departments of the Army or Defense.
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↵* General Electric Medical Systems, 4855 W Electric Ave, Milwaukee, WI 53219-1628.
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↵† The MathWorks, 3 Apple Hill Dr, Natick, MA 01760-2098.
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↵‡ MEDmetric Corp, 7542 Trade St, San Diego, CA 92121.
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↵§ Isokinetic International, 6426 Morning Glory Dr, Harrison, TN 37341-9764.
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↵‖ SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.
- Received January 29, 2007.
- Accepted September 8, 2008.
- American Physical Therapy Association
References
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