The optimal program to rehabilitate the healing graft

Strain on the Anterior Cruciate Ligament during Closed Kinetic Chain Exercises ANNETTE HEIJNE 1, BRADEN C. FLEMING 2, PER A. RENSTROM 1, GLENN D. PEURA 3, BRUCE D. BEYNNON 3, and SUZANNE WERNER 1 1 Section of Sports Medicine, Department of Surgical Science, Karolinska Hospital, Stockholm, SWEDEN; 2 Bioengineering Laboratory, Brown Medical School/Rhode Island Hospital, Providence, RI; and 3 McClure Musculoskeletal Research Center, Department of Orthopaedics & Rehabilitation, University of Vermont, Burlington, VT ABSTRACT HEIJNE, A., B. C. FLEMING, P. A. RENSTROM, G. D. PEURA, B. D. BEYNNON, and S. WERNER. Strain on the Anterior Cruciate Ligament during Closed Kinetic Chain Exercises. Med. Sci. Sports Exerc., Vol. 36, No. 6, pp. 935 941, 2004. Purpose: The purpose of this investigation was to characterize the ACL strains produced during four commonly prescribed CKC exercises; the step-up, the step-down, the lunge, and the one-legged sit to stand. We hypothesized that the ACL strains produced during the lunge and one-legged sit to stand exercises (the exercises that challenge the leg musculature to a greater extent and utilize greater hip flexion) would be less than those produced during the step-up and step-down exercises. Methods: The strains in the anteromedial bundle of the ACL were measured while nine subjects, who had normal ligaments, performed the four exercises. Peak ACL strain values and the ACL strain patterns as a function of knee flexion angle were compared between exercises. Results: No significant differences were found between the peak ACL strain values (mean SEM) between exercises (step-up: 2.5 0.36; step-down: 2.6 0.34; lunge 1.9 0.50; one-legged sit to stand: 2.8 0.27). The mean ACL strain values as a function of knee flexion angle were not significantly different. On average, there was a significant increase in ACL strain as the knee was extended for each exercise. Conclusions: The ACL strain responses produced during these CKC exercises were equal and similar to those produced during other rehabilitation exercises (i.e., squatting, active extension of the knee) previously tested. Key Words: REHABILITATION, ACL, IN VIVO, LUNGE, STEP TRAINING The optimal program to rehabilitate the healing graft after anterior cruciate ligament reconstruction has been extensively studied (9,14,18,19,23,24). Closed kinetic chain (CKC) exercises rather than open kinetic chain (OKC) exercises play a primary role in ACL rehabilitation protocols. A CKC exercise is one where the foot is opposed by considerable resistance (e.g., squatting, bicycling). CKC activities are modeled as closed linkages where movement in one joint produces movements in all the other joints of an extremity (23). Conversely, an OKC exercise is one where the distal segment is free to move across one joint independently (e.g., kicking, active extension of the knee) (23). The rationale for prescribing CKC exercises after ACL reconstruction is based on the hypotheses that: 1) CKC exercises simulate the functional movements that are common in sports and the activities of daily life (23) and 2) CKC exercises increase tibiofemoral joint compression and emphasize co-contraction between hamstrings and quadriceps Address for correspondence: Annette Heijne, Section of Sports Medicine, Department of Surgical Science, Karolinska Hospital, 171 76 Stockholm, Sweden; E-mail: heijne@telia.com. Submitted for publication November 2003. Accepted for publication February 2004. 0195-9131/04/3606-0935 MEDICINE & SCIENCE IN SPORTS & EXERCISE Copyright 2004 by the American College of Sports Medicine DOI: 10.1249/01.MSS.0000128185.55587.A3 935 muscles (23). Thus, they are thought to stabilize the joint and minimize the strain on the healing ACL graft (9,22,29). However, there is evidence to suggest that CKC exercises may not strain shield the healing ACL graft (5). It has been determined that the peak strains produced on the ACL during active extension (an OKC exercise) and squatting (a CKC exercise) were equivalent (5). However, it has been shown that increasing the resistance torque across the knee joint (in an effort to increase the magnitude of the muscle forces) during an OKC exercise will further increase the peak ACL strains while a similar increase in resistance during a CKC exercise will not (13). This finding suggests that one of the advantages of CKC exercises is that the compressive load produced by body weight and muscle co-contraction may attenuate the peak strain values with an increase in resistance. There are numerous CKC exercises commonly used to rehabilitate the ACL graft. Several authors have analyzed the relationship between CKC exercises and the intersegmental loads at the knee (9,18,22,28,30). Stuart et al. (28) analyzed the muscle activity and the intersegmental forces about the tibiofemoral joint during two different squatting exercises and the lunge using EMG, video, and force plate measurements. Using an inverse dynamic model, they determined that the net shear forces of the tibiofemoral joint for all three CKC exercises remained posterior (tibia relative to the femur) throughout the flexion-extension cycle. This finding implies that the ACL, or ACL graft, would not be strained during these exercises. However, the distribution of

the net joint loads between the structures of the knee remains unknown. After ACL reconstruction, it is necessary to minimize muscle atrophy by prescribing exercises that challenge the knee muscles while protecting the healing ACL graft. Conventional physical therapy programs incorporate different CKC exercises that increase muscle activity, though the strain environment of the graft remains unknown. The objective of this investigation was to characterize the ACL strains produced during four commonly prescribed CKC exercises: 1) the step-up, 2) the step-down, 3) the lunge, and 4) the one-legged sit to stand. The research hypothesis was that the ACL strains produced during the lunge and one-legged sit to stand exercises (the exercises that would challenge the leg musculature to a greater extent) are less than those produced during the step-up and stepdown exercises. This experiment was performed using subjects with intact ligaments. It was assumed that the exercises that produced higher strains on the ACL would produce higher strains on a healing ACL graft. METHODS TABLE 1. The clinical data for the nine subjects. Age Sex Involved Side Surgical Procedure Pat 1 20 Male Left LM Pat 2 25 Female Left P Pat 3 49 Female Left LM Pat 4 34 Male Left PD, P Pat 5 32 Male Left LM Pat 6 30 Female Right MM Pat 7 37 Female Left FD, PD Pat 8 24 Male Left MM Pat 9 30 Male Right LM LM, lateral meniscectomy; MM, medial meniscectomy; P, plica debridement; FD, femoral chondral debridement; PD, patellar chondral debridement. FIGURE 1 The DVRT was arthroscopically inserted on the anteromedial bundle of the ACL. The body of the transducer is approximately 5 mm in length. The fixation barbs are 3-mm long and penetrate into the ligament (11). Test subjects. Nine subjects (five males and four females) who were candidates for arthroscopic partial meniscectomy (six subjects) or debridement (three subjects) under local anesthesia volunteered for this study (Table 1). Their ages ranged from 20 to 49 yr (mean age 31 yr). None of the subjects had a history of a knee ligament injury nor did they show any evidence of an injury via clinical and arthroscopic examinations. The study received approval from the Ethics Committee of the Karolinska Hospital and the Institutional Review Board at the University of Vermont. All subjects granted their informed consent before participating. Strain measurement technique. Displacement measurements of the anteromedial bundle of the ACL were measured using a differential variable reluctance transducer (DVRT; MicroStrain, Burlington VT) (6). The small displacement transducer was arthroscopically applied to the ACL through a lateral parapatellar portal as previously reported (6). Because the surgery was performed under local anesthesia, a tourniquet was not used. The measurement axis of the transducer was aligned with the anteromedial bundle and attached to the ligament midsubstance (Fig. 1). The arthroscopic portals were covered with a sterile dressing (Tegaderm; 3M, St. Paul, MN) for the duration of the test protocol. The electrical connection and removal sutures of the DVRT exited through the lateral portal to enable data acquisition and removal of the transducer after the experiment was completed. The ACL displacement measurements were converted to strain using the engineering strain formulation (6). Strain was equal to the change in length of the anteromedial bundle of the ACL relative to a reference length divided by the reference length and expressed as a percentage. The reference length was established by plotting DVRT displacement as a function of the anterior shear load applied to the tibia during an instrumented Lachman test (6,10). The DVRT length corresponding to the inflection point of the loaddisplacement curve served as the reference. This reference length has been shown to correspond to the slack-taut transition length of the ACL (10). Thus, a positive strain value indicates that the anteromedial bundle of the ACL was under strain, hence load bearing, whereas a negative strain indicates that it was slack, or nonload bearing. Experimental protocol. Immediately after the routine surgical procedure, the DVRT was implanted on the ACL. The portals were sealed, and an electrogoniometer (CA- 4000; Orthopaedic Systems Inc, Hayward, CA) was attached to the thigh and lower leg to record knee flexion angles. The patient was then escorted off the operating table to perform three repetitions of the four rehabilitation exercises under investigation: 1) the step-up (Fig. 2A), the step-down (Fig. 2B), the lunge (Fig. 2C), and the one-legged sit to stand (Fig. 2D). The exercise testing order was randomized. The outputs of the DVRT (the ACL displacement response) and the electrogoniometer (the knee flexion angle) were recorded as each subject performed the exercises. A physiotherapist was in the operating room to provide instruction and overlook the performance of each exercise. For the lunge, the subjects were instructed to position their upper body over the knee of the front leg (the instrumented 936 Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

Statistical analyses. The peak ACL strains produced during the four exercises for the flexion and extension portion of the cycle were compared using a two-way repeated measures analysis of variance. The two within subject factors were exercise type and cycle direction. The ACL strain patterns (i.e., ACL strain as a function of knee flexion angle) were compared using a three-way repeated measures analysis of variance. The three within-subject factors were exercise type (step-up, step-down, lunge, and one-legged sit to stand), knee flexion angle (30, 50, and 70 ) and cycle direction (flexion vs extension). Fisher s least significant difference procedure was used to make pairwise comparisons. The strain reference values that were obtained from the repeated normal Lachman tests were compared using a paired t-test to ensure that the DVRT measurements did not change (6). The interclass correlation coefficient was calculated to ensure reproducibility of the sensor. Statistical significance for all analyses were determined at 0.05. FIGURE 2 Four closed kinetic chain exercises that are commonly prescribed after ACL reconstruction were compared: A. the step-up exercise, B. the step-down exercise, C. the lunge exercise, and D. the one-legged sit to stand exercise. knee) to align the torso center of gravity with the knee joint. The back leg served as a stabilizer to maintain balance. For the step-up, step-down, and the one-legged sit to stand, the subjects were instructed to move normally. All subjects practiced each exercise two to three times before data were collected. An instrumented Lachman test was performed both before and immediately after the exercise testing protocol. Anterior-posterior directed shear loads, between the limits of 90 (posterior) and 130 (anterior) Newtons, were applied to the tibia while the knee was supported at 30 of flexion while the femur was aligned in the horizontal plane (6). The shear loads were applied perpendicular to the long axis of the tibia at the level of the tibial tuberosity. The subjects were instructed to relax their leg musculature during the test. The data that were obtained during the repeated Lachman test served two purposes: 1) to determine the reference for the strain calculation (10), and 2) to serve as a repeated normal to ensure that the DVRT measurements were reproducible (6). After completion of the repeated Lachman test, the lateral arthroscopic portal was reopened and the DVRT was removed by pulling on the removal sutures. RESULTS Peak ACL strain. No significant differences were found between the peak ACL strain values produced due to exercise type (P 0.25) or by cycle direction (P 0.34) (Table 2). The average peak ACL strains produced during the step-up exercise were 2.5% (SEM 0.36) during the extension portion of the exercise cycle and 2.5% (SEM 0.31) during the flexion portion of the exercise cycle. The average peak ACL strains produced during the step-down exercise were 2.5% (SEM 0.34) during the extension portion of the exercise cycle and 2.6% (SEM 0.34) during the flexion portion of the exercise cycle. The average peak ACL strains produced during the lunge was 1.8% (SEM 0.62) during the extension portion of the exercise cycle and 2.0% (SEM 0.50) during the flexion portion of the exercise cycle. The average peak ACL strains produced during the one-legged sit to stand exercise were 2.8% (SEM 0.23) during the extension portion of the exercise cycle and 2.8% (SEM 0.27) during the flexion portion of the exercise cycle. On average, the peak strains occurred when the knee was nearing extension (Table 2). ACL strain patterns. No significant differences were found between the ACL strains produced during the four exercises when the knee was at 30, 50, and 70 of flexion (P 0.15) (Fig. 3, A D). No differences were found between the extension and flexion directions of the cycle (P 0.18). However, the strains produced when the knees were at 30 (1.7 0.22%; pooled mean 1 SEM) were significantly greater than those produced at 50 (0.7 0.19%) and 70 (0.1 0.24%) of knee flexion. Repeated normal testing. For the instrumented Lachman tests performed before and after the exercises, the mean difference in the reference strains across subjects was equal to 0.0 0.04 mm. Because the mean change in reference length before and after the exercise bout was not significant (P 0.73) and the interclass correlation coefficient of the reference lengths was high (ICC 0.98), the ACL STRAIN AND CLOSED KINETIC CHAIN EXERCISES Medicine & Science in Sports & Exercise 937

TABLE 2. The peak ACL strains (and the corresponding knee flexion angle that the peak strains occurred) for the four exercises. Exercise 1 2 3 4 Step-up Extension cycle (% strain) 2.51 2.65 4.62 1.9 2.4 2.18 1.79 3.45 0.75 Knee angle at peak ( ) 30 25 40 25 25 40 15 20 50 Flexion cycle (% strain) 2.03 3.13 3.8 2.28 2.9 2.53 1.85 3.1 0.51 Knee angle at peak ( ) 25 25 30 25 25 55 20 25 65 Knee angle range 75 25 75 25 70 15 70 20 80 25 80 40 65 15 55 20 75 50 Step-down Extension cycle (% strain) 1.98 3.51 3.76 3.42 1.91 3.27 2.38 2.95 2.18 Knee angle at peak ( ) 30 15 70 20 15 30 15 25 40 Toward flexion (% strain) 1.13 3.26 3.72 3.26 2.7 3.65 2.6 3.15 2.17 Knee angle at peak ( ) 65 20 70 20 15 50 15 25 40 Knee angle range 80 20 80 15 80 25 75 20 75 15 90 25 80 15 90 25 85 40 Lunge Extension cycle (% strain) 1.19 4 2.02 2.87 1.93 3.6 2.01 2.38 0.03 Knee angle at peak ( ) 30 10 75 35 20 25 20 30 45 Toward flexion (% strain) 0.57 3.95 0.38 2.77 1.94 4 2.37 2.44 0.2 Knee angle at peak ( ) 30 10 75 35 20 25 20 30 45 Knee angle range 105 25 80 10 105 20 40 25 80 20 95 25 95 10 80 30 100 45 Chair Extension cycle (% strain) 2.84 3.33 2.27 1.74 2.28 4.6 1.32 2.26 1.2 Knee angle ( ) 60 15 40 25 20 30 30 90 45 Toward flexion (% strain) 2.64 3.69 2.23 2.15 2.9 4.66 1.58 2.12 1.4 Knee angle ( ) 70 15 80 30 30 30 35 90 75 Knee angle range 70 15 75 15 80 10 70 15 70 20 90 30 70 15 65 25 80 40 5 6 7 8 9 output of the DVRT was considered reproducible over the exercise bout. DISCUSSION This study shows that the step-up, step-down, lunge, and one-legged sit to stand exercises did not produce greater strains on the ACL than the traditional two-legged squat (6). The best exercises for rehabilitating the knee after ACL reconstruction are those that can maximize the patients ability to achieve full range of joint motion while minimizing muscle atrophy and risk to further injury. Over the last decade rehabilitation protocols are generally based on the hypotheses that CKC exercises do not strain the anterior cruciate ligament graft in a harmful way (24 26). Thus, exercises like mini-squats, weight shifts and balance training are frequently prescribed in the early phase of rehabilitation. OKC exercises and the demanding CKC exercises such as lunges, parallel squats, stair climbing and deep leg press are normally introduced around 6 8 wk postoperatively (8,20,23,28). No previous studies that directly measuring the strain or force in the ligament have been performed for these four CKC exercises. However, the ACL strains produced in this study follow similar patterns and magnitudes to those of other rehabilitation exercises (Table 3), such as squatting (4,5,11,12). Using inverse dynamics, Ohkoshi et al. (22) found that the shear forces at the knee were posteriorly directed and increased with increased hip flexion during knee extension during a static squat. These findings support the indications of the present study that CKC exercises may strain the ACL less when the hip is in a more flexed position as compared with those with less hip flexion such as the step-up and step-down exercises (Fig. 3). This study suggests that all four exercises will strain a healing graft a similar amount. The anteromedial part of the ACL was strained as the knee neared extension for the four exercises. However, the strain values were not greater than those produced during a two-legged squat (5). Thus, the high demand exercises such as the lunge could be prescribed at the same time as a squat if and when one feels that the squat is a safe activity to perform after ACL reconstruction. The step-up, step-down, and the one-legged sit to stand exercises are performed standing on one leg. With these exercises, higher muscle forces are produced in the lower extremity as compared with a normal squat. Thus, the use of these exercises would allow the patient to increase the muscle forces about the knee without significantly increasing the graft forces. Finally, the strain values were similar to those produced during active extension of the knee against gravity, an open kinetic chain exercise that was previously tested using the same technique (5). Therefore, these data suggest that the OKC exercise could be initiated at the same time during the postoperative rehabilitation program as the more aggressive closed kinetic chain exercises. There are secondary effects that also need to be considered when evaluating and introducing different rehabilitation exercises after ACL reconstruction, including pain, swelling, reduced range of joint motion and proprioception, and muscle atrophy, factors not evaluated in this laboratory controlled study. Mikkelsen et al. (21) demonstrated that there was no difference in anterior knee joint laxity or function after graft healing when comparing the early (6 wk) and late (12 wk) introduction of OKC exercises in postoperative rehabilitation programs in a prospective randomized study. Therefore, open kinetic chain exercises are not detrimental to healing and could play a role in postoperative care. In contrast, Bynum et al. (8) found that the reconstructed knee of a subject who had undergone a CKC exercise rehabilitation program had less increased anterior knee joint laxity over time compared with those who had undergone an OKC 938 Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

FIGURE 3 The mean ACL strains as a function of knee flexion angle for the four exercises: A. the step-up exercise, B. the step-down exercise, C. the lunge, and D. the one-legged sit to stand exercise. exercise program. Although the strain magnitudes produced during the active knee extension exercise were similar to the closed kinetic chain exercises tested in our study, there is recent evidence to suggest that increasing the resistance to the lower leg during the active extension exercise (i.e., quadriceps sets performed on a weight bench) will increase the peak strain values (13). Increasing the level of resistance during closed kinetic chain exercises does not produce this increase (13). The DVRT allowed for precise strain measurements of the anteromedial bundle of the ACL. Application of multiple DVRT could potentially provide a detailed mapping of the strain distribution across the different bundles of the ACL. Due to the size of the DVRT and the location of the ACL with respect to the posterior cruciate ligament and the osseous walls of the intercondylar notch, the technique is currently limited to the anteromedial bundle using one transducer. We recognize that the ACL has a strain distribution about its cross-section area (7). However, the measurements presented here may be sufficient since surgeons generally attempt to reconstruct the function of the anteromedial band when performing an ACL reconstructive procedure (15). This investigation was performed on subjects with normal ACL to gain insight into the strains produced on an ACL graft during exercise after reconstructive surgery. It is currently impossible to evaluate the strain in an ACL substitute, in vivo, during dynamic activities that involve the leg musculature because the surgical reconstruction should not be performed under local anesthesia. Furthermore, additional variables that are associated with the reconstructive procedure would also increase the variability of the strain response. Therefore, it is advantageous to perform these measurements on the normal ACL. It seems reasonable, however, to extend these data to the knee with a properly positioned ACL graft. The elongation pattern of the bonepatellar ligament-bone graft during passive extension of the knee joint has been previously measured, in vivo, and was found to be similar to the normal ACL (3). Thus, a loading condition that causes a decrease in normal ACL strain should cause a similar decrease in a properly positioned ACL STRAIN AND CLOSED KINETIC CHAIN EXERCISES Medicine & Science in Sports & Exercise 939

TABLE 3. Rank comparison of peak ACL strain values during commonly prescribed rehabilitation exercises (mean SE). Peak Rehabilitation Activity Strain Isometric quads contraction @ 15 (30 N m of 4.4 (0.6)% 8 Squatting w/sport cord 4.0 (0.6)% 8 Active flexion-extension of the knee with 45-N 3.8 (0.5)% 9 weight boot Lachman test (150 N of anterior shear load; 30 3.7 (0.8)% 10 flexion) Squatting 3.6 (0.5)% 8 Active flexion-extension (no weight boot) of the 2.8 (0.8)% 18 knee Simultaneous quads and hams contraction @ 15 2.8 (0.9)% 8 Isometric quads contraction @ 30 (30 N m of 2.7 (0.5)% 18 Stair climbing 2.7 (1.2)% 5 Weightbearing @ 20 of knee flexion 2.1 (0.5)% 11 Anterior drawer (150 N of anterior shear load; 90 1.8 (0.9)% 10 flexion) Stationary bicycling 1.7 (0.7)% 8 Isometric hams contraction @ 15 (to 10 N m of 0.6 (0.9)% 8 flexion torque) Simultaneous quads and hams contraction @ 30 0.4 (0.5)% 8 Passive flexion-extension of the knee 0.1 (0.9)% 10 Isometric quads contraction @ 60 (30 N m of 0.0% 8 Isometric quads contraction @ 90 (30 N m of 0.0% 18 Simultaneous quads and hams contraction @ 60 0.0% 8 Simultaneous quads and hams contraction @ 90 0.0% 8 0.0% 8 Isometric hams contraction @ 30, 60, and 90 ( 10 N m of flexion torque) No. of Subjects ACL graft. Most likely, the strain patterns for other graft types would be similar; however, this has not been verified. The results of this study were based on subjects with normal ACL undergoing arthroscopic surgery for a partial meniscectomy, chondral debridement, or plica excision under local anesthesia. Although the subjects presented with minor cartilage or meniscal problems, the overall function of their knee joints were assumed to be normal. Cadaver investigations have shown that knee kinematics are not altered in the ACL intact knee after medial or lateral meniscectomy (1,16,17). Thus, it seemed reasonable to assume that a partial meniscectomy would have a negligible effect on knee kinematics, and hence, ACL strain behavior. It is also possible that the local anesthesia could influence the strain response because it eliminates sensory perception within the joint capsule. Local anesthesia, however, has not been shown to alter knee joint proprioception (2). The power of the analysis of variance used to compare the peak strain values between exercises was relatively low (approximately 30%) if we assume a minimal detectable difference of 1% strain and alpha equals 0.05. The power could be increased to 80% if we were to triple our sample size. The mean peak strains that were recorded for these exercises were less than those measured previously for the simple squat and the passive Lachman test (5,6). Furthermore, the sample size was similar to those of our previous studies of ACL rehabilitation where significant differences were found. Considering the invasiveness of the experiment, the clinical relevance of the small differences observed between the exercises evaluated in this study is at least questionable. Nonetheless, the low power of the analysis is a limitation to the study. The variability inherent to the strain measurements is most likely related in part to the variation in exercise performance. The repeated normal test (i.e., the instrumented Lachman test) ensures us that the variability due to the measurement technique is minimal. It is important to note that the subjects were instructed by a physical therapist on how to perform each exercise as they would in the clinic. The subjects were allowed several trials of each exercise before data was collected to eliminate any potential learning effects. However, the position of the torso relative to the knee was not formally controlled or documented. The location of the center of gravity relative to the knee joint has been shown to influence the net shear loads across the knee (22). It is possible that location of the center of gravity was different across subjects and this may account for some of the variability. At this time, it is not possible to identify which exercises are safe or harmful to a healing graft because the strain thresholds that are beneficial and/or detrimental to graft healing remain unknown. Physical therapists introduce different types of CKC exercises postoperatively in an attempt to replicate the functional movements utilized in daily life and sports as soon as possible (20,24 26,28). The peak strains produced during the quadriceps-dominated exercises could conceivably produce damaging effects if they are introduced too early during rehabilitation. This is particularly true if they are performed with the knee joint near full extension, or if they are advanced to more challenging levels of muscle contraction. We know that hamstrings dominant exercises produce little or no strain on the ACL. Step-up, step-down, and mini-squats are usually performed with the knee closed to full extension. Lunges and parallel squats are performed with the hip in a more flexed position, which make the hamstrings more activated (22,23,30). Thus, these activities should be relatively safe for a healing ACL graft. It is known from the literature that the co-contraction of the quadriceps and hamstrings muscles is frequently altered in ACL-deficient knee subjects. For example, the hamstring activity has been shown to increase in many ACL-deficient patients. An increase in hamstrings activity would theoretically serve to decrease the shear forces on the tibia and thereby minimize the strains on the ACL graft (27). It is possible that the changes in co-contraction patterns seen in ACL reconstructed patients may be lower than the subjects with normal ACL who participated in the present study. Whether the differences in hamstrings activity contribute significantly to reduce ACL-loading during strenuous activities remains unknown. Knowledge about the strain response of the healing graft is necessary to determine which exercise should be included in an exercise program to optimize rehabilitation of the ACL graft. The peak strain produced during these closed kinetic chain exercises are less than those produced during a passive Lachman test. The strain data obtained from the normal ACL may enable us to design prospective, randomized, longitudinal studies to optimize rehabilitation and shed the light on the strain thresholds that may be beneficial and/or detrimental to the healing of different types of grafts. 940 Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

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