anterior cruciate ligament ( acl ) injuries are one of the most common and devastating knee 
 injuries associated with sport participation1 . surgical repair of complete rupture of the acl is recommended to 
 prevent knee joint instability . while surgery corrects the structural defect , post - operative 
 impairments in knee strength , function and endurance are inevitable corollary problems 
 associated to the repair . many patients experience some degree of muscle weakness post - acl 
 reconstruction ( aclr ) , especially weakness of the quadriceps muscles , which can limit their 
 functional ability2 . palmieri - smith et 
 al.3 assessed the deficit in quadriceps 
 muscle strength between the reconstructed and non - injured knees in patients six months after 
 aclr . several studies have suggested use of an accelerated progression of knee strength , 
 function and endurance following aclr to improve outcomes4 , 5.while most recent studies 
 have focused on the effects of postoperative rehabilitation protocols , only a few studies 
 exist that have studied about effects of preoperative exercise on post - aclr outcomes6,7,8 . keays et al.7 reported on the beneficial effects of a 5-week home - based exercise 
 program for improving quadriceps strength and knee function early post - aclr injury . based on 
 available research evidence , if a preoperative program of exercise can restore muscle 
 strength and function before surgery , it could be of benefit to improve outcomes 
 post - surgery . based on the study eitzen et al.9 the preoperative quadriceps strengthening may be a significant 
 predictor of knee function two year after aclr9 . preoperative deficits in quadriceps strength of greater than 20% 
 have been associated with significant persistent muscle weakness two years after aclr . 
 however , there is still little evidence regarding the benefits of preoperative exercise on 
 improving postoperative knee weakness . we hypothesized that a preoperative program of exercise would improve functional outcomes 
 post - aclr . therefore , the aim of the present study was to examine the effects of a 4 week , 
 hospital - based preoperative program of exercise , focusing on quadriceps strengthening and 
 single - leg hop function , on after aclr outcomes . a prospective , randomized , controlled study was initiated to evaluate the effectiveness of 
 a 4 weeks preoperative program of exercises on after aclr knee strength and function . patients with acl rupture were recruited from samsung medical orthopedics centers from april 
 2012 to august 2014 . the inclusion criteria were men between the ages of 20 and 35  years , 
 presenting with an isolated acl rupture in the knee . the exclusion criteria were previous 
 aclr or meniscus repair , injury to other ligaments in the same knee , and any associated 
 fractures . the 80 patients were randomly assigned into two groups , the preoperative exercise 
 group ( peg , n=40 ) and no preoperative exercise group ( npeg , n=40 ) . the mean height and body weight peg were 173.85.6  cm and 75.28.9  kg ; for the npeg , they were 172.84.4  cm and 
 74.319.77  kg . there were no significant between - group differences in height and weight 
 between groups . the peg participated in a 4-week exercise program preoperatively and in a 12 week 
 postoperative program , while the npeg only participated in the 12 weeks postoperative 
 program . knee muscle strength and function were measured at 4 weeks before surgery and 3 
 months after surgery . all patients were supervised at least three times a week in a sports 
 medicine center . the preoperative exercise program was based on known impairments associated with acl 
 injuries , focused mainly on strengthening , functional balance , muscle control , and 
 co - contraction7 . the exercise program mainly focused 
 on lower limb strengthening , with particular attention paid to strengthening of the 
 quadriceps muscles . prior to performing the main strength training program , after completion of the warm - up , subjects 
 proceeded to a series of lower - body strengthening exercises , short arc extension , wall 
 squat , leg press , isometric leg extension , and leg curls as well as other exercises ( table 1table 1.a summary of the preop exercise programexercise categoryexerciserepetitions and setscardio exercisestationary bike 20 minrange of motion exerciseseat flexion / extension10 minwall slidesstrengthening exerciseopen - chain strengtheningshort arc extension 102straight leg raises102leg curl ( isometric)102close - chain strengtheningleg extension ( rom 3080)102leg press152half squat152balance / proprioceptionsingle - leg standing30 sec3balance board30 sec3 ) . selected exercises reduced anterior shear force across the tibio - femoral 
 joint and , therefore , helped strengthen the muscles without risk of damaging intra - articular 
 structures of the knee . the postoperative exercise program was based on a combination of 
 evidence - based exercises identified from the scientific literature . immediately after aclr , the limb was immobilized in a postoperative functional brace , and 
 patients were instructed to perform straight leg raising and quadriceps setting exercises . patients were allowed to undergo partially weight bearing at 2 weeks after their surgery and 
 to move through full - range of knee joint motion and perform closed chain exercise at 4 weeks 
 after surgery . all patients underwent assessment of knee extensor strength and distance on 
 single - leg hop 4 weeks prior to surgery and again at 3 months after surgery . isokinetic 
 strength and endurance were measured using a csmi isokinetic dynamometer ( csmi medical 
 solution , stoughton , ma , usa ) . knee strength was measured through the range of 0 to 90 at 
 an angular speed of 60/sec , with four repetitions completed at an angular speed of 
 180/sec , with 20 repetitions completed to calculate average power10 . the highest peak torque value for each velocity was 
 compared with the uninjured side and described as percent of strength deficit . for the 
 single - leg hop for test , the mean average distance was quantified by limb symmetry index 
 ( lsi ) using the following formula : ( distance for injured leg / distance for uninjured leg )  
 10011 . to investigate the change in knee extensor strength and single - leg hop distance between the 
 peg and npeg , a repeated measures analysis using the independent sample t - test . the knee extensor strength deficit was calculated as the percentage difference between the 
 injured and uninjured side . the knee extensor strength deficits was significantly different 
 between the groups , both at an angular velocity of 60/s ( p=0.018 ) and 180/s ( p=0.033 ) . patients in the peg showed a significantly greater improvement in post - operative strength 
 than patient in the npeg at 60/s , and180/s ( table 
 2table 2.change in knee extensor muscle strength and single - leg hop functionmain outcome measurementpeg ( n=40)npeg ( n=40)knee extensor 60/sec deficit ( % ) preop22.813.723.515.8post - op28.59.0 * 36.510.7knee extensor 180/sec deficit ( % ) preop16.610.617.511.9post - op23.39.0 * 27.912.6single - leg hop lsi ( % ) preop75.110.376.58.9post - op85.37.4 * peg : preop exercise group ; npeg : no pre - op exercise 
 group . * p<0.05).the peg also showed significant improvement in the single hop distance test 
 ( p=0.029 ) . peg : preop exercise group ; npeg : no pre - op exercise 
 group . * p<0.05 the purpose of this study was to examine the effects of a 4 weeks preoperative program of 
 the knee extensor muscle strengthening on post - operative quadriceps muscle strength and knee 
 function . outcomes confirmed the benefits of the peg in significantly lowering 
 post - operative deficits in quadriceps strength and improving single - leg hop distance 
 compared to patients who did not participate in a preoperative exercise program . knee extensor strength deficit following aclr is a common problem which has been reported 
 in previous studies12 , 13 . in a recent review of the literature , 11 studies 
 including research by palmier - smith et al.3 , reported knee extensor muscle strength deficit , ranging between 24% 
 and 40.5% , 6 months after aclr , while another 10 studies reported a residual , deficit in 
 quadriceps strength of 10% to 27% , 12 months after aclr3 . furthermore , long - term negative outcomes of strength deficits on 
 knee function for sports activities and even during daily activities . thus , the recovery of 
 quadriceps muscles strength is one of the most important factors after aclr14 . recent studies have investigated the relationship between pre and postoperative quadriceps 
 muscle strength deficits and function . according eitzen et al.9 reported patients with greater than 20% deficit in quadriceps 
 strength to have a significantly greater impairment on functional knee scores and strength . further , shelbourne et al.8 reported that 
 the postoperative quadriceps muscle strength to be significantly higher in patients with 
 greater preoperative strength than in patients with poor preoperative strength . these 
 studies provide strong evidence that preoperative quadriceps muscle strength positively 
 affects postoperative muscle strength and function . therefore , adequate and suitable program 
 of exercise before surgery would have an important role of improving post - operative muscle 
 strength . in our study , there was a significant difference in postoperative knee extensor 
 strength deficit between the npeg and peg groups , with a mean strength deficit of 36.5% for 
 the npeg compared to 28.5% . restoration of muscle strength , which was measured by 
 performance on the single leg - hop distance test , is considered to be an important 
 determinant for a successful return to sports after aclr9 , 17 . the limb symmetry index 
 ( lsi ) has been the most frequently reported criterion for comparing muscle strength and hop 
 performance with the uninjured side18 . our study demonstrated that patients in the showed peg significantly improved in the single 
 leg - hop distance compared to the npeg . the results indicate that the greater improvements in 
 quadriceps strength in the peg had a positive effect on knee function test , such as the 
 single - leg hop , compared to the npeg . the result indicated that preoperative exercises not 
 only prevented quadriceps weakness , but also accelerated the recovery of muscle strength , 
 which helped patients to quickly adapt to the rehabilitation environment . it is also 
 anticipated that possible re - injury could be prevented by having better strength and 
 function .