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Relationship of kinetic demands of athletic training and knee joint laxity
Original Research
Relationship of kinetic demands of athletic training and knee joint laxity Gabriel Y.F. Ng and Murray E. Maitland Objectives: To compare the antero-posterior knee laxity and compliance in athletes and control subjects. Design: Cross-sectional comparison of different groups. Setting: Laboratory based study. Participants: sixty-four athletes and 25 control subjects aged 14±20. Athletes were trained for at least 2 years at a minimum of 6 h per week in their respective sports events, namely, basketball (n 27), running (n 20), swimming (n 17). Main outcome measure: Antero-posterior laxity, stiffness and rate of change in stiffness of the dominant knee at 100 N of anterior drawer force measured with a KT-2000 knee arthrometer. Results: Data were tested with multivariate analysis of variance with a 0.05. Results revealed signi®cant differences between groups in knee laxity (P 0.035) and the change in stiffness (P 0.034). There is a strong trend of difference between groups in stiffness (P 0.067). Post-hoc linear contrasts revealed that the swimming and basketball groups had signi®cantly less laxity and higher stiffness than the control group (P 5 0.05). Conclusions: These ®ndings suggest that sports training may improve the strength of soft tissues in the knee joint, and different kinetic demands of the sport events may result in unique response of c 2001 Harcourt Publishers Ltd the joint structures. *
Gabriel Y. F. Ng PhD, PT, Associate Professor, Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hong Kong Murray E. Maitland PhD, Associate Professor and Physical Therapist, Sports Medicine Centre, Faculty of Kinesiology, The University of Calgary, Canada Correspondence to: G. Ng, Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom, Hong Kong. Fax: 852 23308656; E-mail: rsgng@ polyu.edu.hk
66
Introduction The sagittal stability of the knee joint is largely dependent on the integrity of the anterior cruciate ligament (ACL) because it provides 85± 87% of restraint to anterior tibial translation (ATT) (Butler et al. 1980). Change in stiffness of this ligament could lead to increase or decrease in ATT (Cabaud, 1983; Grood et al. 1992). Injury to the ACL may render the knee joint unstable and develop early degenerative changes of the articular cartilages (Noesberger, 1992; Daniel, 1993). Rehabilitation for patients with ACL injury may not be adequate for them to return to their pre-injury level of sports performance because of functional instability. Also, many individuals experience residual laxity following ACL reconstruction and this is particularly worse in people who had associated meniscus injuries (Shelbourne & Gray, 2000). It is therefore important to determine the factors that enhance the mechanical properties of the
Physical Therapy in Sport
(2001) 2, 66±70
doi : 10.1054/ptsp.2001.0047, available online at http://www.idealibrary.com on
restraints to excessive knee joint laxity in order to prevent injury or improve the result of rehabilitation should injuries occur. There is evidence from animal studies that enforced weightbearing exercises improved the structural stiffness of the tissues in the knee joint (Cabaud et al. 1980; Oakes et al. 1982; Sakuma et al. 1993). In a 3-year study with an ACL hemitransection model in goat, Ng et al. (1996) reported no signi®cant difference in knee laxity from 12 weeks to 3 years after injury. In another study using the feline model, Maitland et al. (1998) found that force-displacement properties of the knee joint changed systematically over time following ACL transection. Even though similar studies in humans have not been reported, it is possible that sports training involving high intermittent loading to the knee can lead to adaptive changes in the structural and mechanical properties of the tissues, which would otherwise not occur with normal daily activities.
c 2001 Harcourt Publishers Ltd *
Relationship of kinetic demands of athletic training and knee joint laxity
It was reported in the literature that subjects with ACL reconstruction demonstrated increases in laxity of their knee joints immediately after endurance running (Sailor et al. 1995). This has been attributed to the transient lengthening of the ACL due to collagen creep (Woo et al. 1990). Relatively little has been published about the long-term cumulative effect of exercise on ATT in humans. A few studies that examined subjects with unilateral ACL injury have revealed that the uninjured knee of these subjects had signi®cantly greater anteroposterior laxity than the normal population (Bach et al. 1990; Daniel et al. 1985; Edixhoven et al. 1989). It was postulated that their injured knee might have also exhibited greater laxity than normal before the injury. These studies suggest that greater ATT could be associated with higher risk of knee injury. The long-term effects of exercise on the mechanical properties of soft tissues have not been well documented. Conteduca et al. (1991) performed subjective clinical laxity tests on 500 patients with ACL de®ciency. They found that patients with low levels of sports activities and those with sedentary lifestyles had higher laxity ratings than amateur and professional athletes. Wishart (1995) compared the antero-posterior laxity of the knee joint in long term endurance runners with relatively sedentary subjects and found that runners had less laxity than their sedentary counterparts at 308 of knee ¯exion and the same trend was shown at 908 of ¯exion. When the non-weightbearing joint laxity was taken as a variable, it did not affect the result, which suggested that difference was probably caused by the effect of running. This may be due to the frequent loading of the joint structures, in particular, the ACL of the runners, which had strengthened the ligament and rendered it to provide better restraint to the knee. Considering the kinetic demands of running, it is mostly repetitive extra-weightbearing in nature. It is not clear whether the kinetic nature of an exercise could affect the knee laxity. Furthermore, most previous studies only measured the total laxity but the stiffness was not known. In actual daily activities, the structural stiffness is an important parameter because it determines how complaint the knee joint would be at submaximal loading as in most daily activities.
c 2001 Harcourt Publishers Ltd *
Therefore, the present study aimed at comparing the antero-posterior laxity and structural stiffness of the knee joint in athletes engaged in different sports, namely basketball (high impact loading and cutting activity); endurance running (repetitive impact loading activity); swimming (non-weightbearing but high levels of muscular loading to the joint) and relatively sedentary subjects (control). If a signi®cant difference is found among these groups, it will contribute to the understanding of the relationship between kinetic demands of exercise and the constraints to knee joint motion. This will be very important in designing rehabilitation exercises for patients with knee injury. More importantly, incorporating the appropriate exercises may alter the force-displacement characteristics of the knee joint, so as to strengthen the knee amongst athletes.
Method A total of 64 athletes, aged 14 to 20 years, and 25 age-matched, control subjects were tested. The athletes consisted of 27 in the basketball group, 20 in the running group and 17 in the swimming group. All athletes had received at least 6 h of training per week in one of the above sporting events for more than 2 years. For the subjects in the control group, they must not have been engaged in any regular physical training program. None of the subjects had previous knee injuries that required medical treatment for more than 2 weeks. The Ethics Review Committee of the administering institute approved this study. All subjects gave their written consent prior to being tested. In order to control for the inter-tester variability, the same examiner, who is a quali®ed physical therapist, conducted all tests. Subjects were instructed to abstain from any strenuous physical activity at least 6 h prior to testing, so as to control for the immediate effect of exercise on joint laxity. All subjects were tested in a supine lying position with the knees rested on a testing block which maintained the knees at 308 of ¯exion. The dominant leg was tested, which was determined as the one that the subject used for kicking a ball.
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(2001) 2, 66±70
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A KT-2000 knee ligament arthrometer (MEDmetric Corporation, San Diego, CA) was used to measure the antero-posterior knee laxity of the subjects. The KT-2000 arthrometer was applied to the lower leg of the subject according to the instruction manual of the manufacturer. The analogue signal of the KT2000 arthrometer was output to a data acquisition unit (DataQ Instruments, Akron) which performed A-D conversion at a sampling frequency of 200 Hz. The digital signals were then input to a personal computer installed with a custom designed program to calculate the total sagittal displacement (laxity in mm), instantaneous rate of change of displacement (stiffness in Nmm ÿ1), and the rate of change of stiffness (Nmm ÿ2). The output was displayed real-time on the computer. The examiner applied a cyclic antero-posterior loading of 67 N, as suggested by the manufacturer, through the handle of the arthrometer so as to acquaint the subject with the test. After setting the reference position, the cyclic anteroposterior force was applied until the KT-2000 arthrometer output consistently registered 0/ ÿ0.5 mm at rest. The examiner would perform a 67 N of posterior push followed by 134 N of anterior drawer test for each subject. Knee laxity tests may be confounded by the size of the subject because of the weight of the leg. In order to obtain measurements at the equivalent anterior force levels, the weight of the subjects' legs were estimated from kinanthropometic regression equations as calculated by Chandler (1975). Leg segment lengths and weights were used to normalize the knee laxity test force for each individual. The weight-corrected data were then analysed at the point of 100 N of anterior drawer force according to the method of Maitland et al. (1995). Three parameters were obtained with the analysis, namely, total sagittal displacement (laxity), rate of change of displacement (stiffness), and rate of change of stiffness.
Data analysis Statistical analysis of the data was performed with a SPSS 9.0 computer program (SPSS Inc., Chicago). Normality of the data was tested and the results showed that most of the data were normally distributed. The data were analysed
68
Physical Therapy in Sport
(2001) 2, 66±70
with multivariate analysis of covariance. The covariates were gender, body weight and height. Results revealed that none of the covariates was signi®cant with P 0.757, 0.902 and 0.823 for gender, body weight and height, respectively. Therefore, the data were analysed with multivariate analysis of variance. The level of signi®cance was set at 5% and signi®cant results were further analysed with post-hoc linear contrasts to identify the data pairs that were signi®cantly different.
Results Table 1 presents the means, standard deviations (in brackets), and P values of each parameter tested for the four groups. There were statistically signi®cant differences between groups in laxity and rate of change in stiffness. Post-hoc linear contrasts revealed that the athletic groups were not signi®cantly different from each other in any of the parameters tested. However, the athletic groups were signi®cantly different to the sedentary control group. Given that the P value of the stiffness test was quite low (P 0.067), therefore post-hoc contrast was also performed for this parameter. Results reveal that subjects in the swimming group had less laxity but higher stiffness and rate of change of stiffness than the control group (P 0.013, 0.008 and 0.007, respectively). The basketball group had less laxity and higher rate of change of stiffness than the control group (P 0.025 and 0.023, respectively).
Discussion This study shows that there is an association between long-term sports training and measurements of joint laxity. Stresses or strains associated with sports activity may have contributed to differences in knee joint constraints, such that there was decreased in anterior translation of the tibia on the femur, and an increase in the rate of change of stiffness during the passive drawer manoeuvre. The differences between mean values were not statistically signi®cant but revealed a general trend in tissue stiffness with different sports events. The purpose of measuring stiffness and rate of change in stiffness in addition to joint
c 2001 Harcourt Publishers Ltd *
Relationship of kinetic demands of athletic training and knee joint laxity
Table 1 Means (SD) of antero-posterior knee laxity, stiffness and rate of change of stiffness of the different groups. * denotes the P values of 50.05 Basketball (Group 1, n 27)
Running (Group 2, n 20)
Swimming (Group 3, n 17)
Control (Group 4, n 25)
Laxity (mm)
6.349 (2.292)
7.330 (2.078)
5.961 (2.303)
7.789 (2.404)
0.035*
1 1 1 2 2 3
vs. vs. vs. vs. vs. vs.
2: 3: 4: 3: 4: 4:
0.149 0.584 0.025* 0.072 0.504 0.013*
Stiffness (N/mm)
47.542 (23.088)
45.206 (23.089)
57.769 (32.368)
37.856 (15.903)
0.067
1 1 1 2 2 3
vs. vs. vs. vs. vs. vs.
2: 3: 4: 3: 4: 4:
0.737 0.163 0.141 0.108 0.300 0.008*
Rate of change of stiffness (N/mm2)
25.742 (33.669)
18.226 (27.084)
33.086 (43.260)
4.685 (26.811)
0.034*
1 1 1 2 2 3
vs. vs. vs. vs. vs. vs.
2: 3: 4: 3: 4: 4:
0.437 0.470 0.023* 0.171 0.170 0.007*
displacement was because these parameters had been shown to have higher sensitivity than laxity when assessing subjects with ligamentous injuries (Maitland et al. 1995). The loaddisplacement characteristics of an articulation can be expressed with different mathematical functions. Laxity is a parameter that measures the magnitude of displacement under a given level of force. The limitation of this measure is that it does not provide information on the quality of displacement throughout the loading phase. Stiffness ( ®rst derivative of loaddisplacement curve) is a functional parameter that signi®es the biomechanical response of the joint to external load during a prescribed level of force. It is more indicative of the mechanical joint function at a submaximal loading condition. The rate of change in stiffness (second derivative of the load-displacement curve) signi®es the onset of constraints and the sharpness with which constraints come into play. The rate of change in stiffness has also been reported to contain important information regarding the end-feel assessment of a joint (Maitland & Kawchuk, 1997). We did not ®nd a signi®cant difference between the running group and control group in contrast to the previous ®ndings of Wishart (1995), in which the runners were found to have signi®cantly less laxity than the control subjects.
c 2001 Harcourt Publishers Ltd *
MANOVA P values
Post-hoc linear contrasts (P values)
We recruited subjects with at least 2 years of training in running. Most of our subjects were trained between 2 and 4 years. For the previous study, the training intensity was decidedly higher than our study, as most of the subjects had more than 5 years of training. The relatively lower training intensity of our subjects could explain the insigni®cant difference between the runners and control subjects. Considering the other two sports groups, they had signi®cantly lower laxity and higher stiffness than the control (Table 1). From the point of mechanical consideration, the nature of basketball would impose very high intermittent compressive and shear loading to the joint structures due to the high speed running, jumping and cutting actions. In swimming, the compressive loading to the joint would not be very high, but the action of the quadriceps muscles could create high anterior shearing loading to the joint thus stretching the ACL (O'Connor et al. 1990). Actually, the shear force that crosses the knee is more important than the joint compression in ligament loading, as the shear force to the joint due to muscle contraction requires the restraining action of the ligaments to maintain joint integrity. When considering the swimming action, it is an open kinetic chain activity, in which the shear force to the joint puts direct tensile loading to the
Physical Therapy In Sport
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Physical Therapy in Sport
ACL that provides the sagittal restraining forces. It is possible that the ACL adapted by collagen remodelling (Cabaud et al. 1980; Oakes et al. 1982; Sakuma et al. 1993) and therefore this group of subjects presented with the least laxity and highest stiffness. The stiffness in the soft tissues is important for injury prevention and functional restraint to excessive movements of the joint. Our ®nding provides insight to coaches in their training of athletes in order to improve knee joint stability. However, our study was conducted with ablebodied subjects, it is not known whether similar effects would also exhibit in patients. Further studies need to be performed in subjects with ACL insuf®ciency so as to examine the effects of exercises in the patient population.
Acknowledgements The authors gratefully acknowledge the Sports Development Board Research Fund for ®nancial support of this study, Mr Simon Yeung of Hong Kong Polytechnic University, Mr Kunloi Tang of Hong Kong Amateur Athletic Association and Mr Martin Lam of Hong Kong Sports Institute for administrative support.
References Bach B R, Warren R F, Flynn W M, Kroll M, Wickiewiecz T L 1990 Arthrometric evaluation of knees that have a torn anterior cruciate ligament. Journal of Bone and Joint Surgery 72-A: 1299±1306 Butler D L, Noyes F R, Grood E S 1980 Ligamentous restraints to anterior-posterior drawer in the human knee. A biomechanical study. Journal of Bone and Joint Surgery 62A: 259±270 Cabaud H E, Chatty A, Gildengorin V, Feltman R J 1980 Exercise effects on the strength of the rat anterior cruciate ligament. American Journal of Sports Medicine 8: 79±86 Cabaud H E 1983 Biomechanics of the anterior cruciate ligament. Clinical Orthopaedics and Related Research 172: 26±31 Chandler R F 1975 . In: Enoka R (ed). Neuromechanical basis of kinesiology, 2nd edn. Champaign, Human Kinetics (1994), p 44±48 Conteduca F, Ferretti A, Mariani P P, Puddu G, Perugia L 1991 Chondromalacia and chronic anterior instabilities of the knee. American Journal of Sports Medicine 19: 119±123 Daniel D M, Malcom L L, Losse G, Stone M L, Sachs R, Burks R 1985 Instrumented measurement of anterior laxity of the knee. Journal of Bone and Joint Surgery 67A: 720±726 Daniel D M 1993 Selecting patients for ACL surgery. In: Jackson D, Arnoczky S, Woo S L-Y, Frank C, Simon T
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(eds). The anterior cruciate ligament: current and future concepts. Raven Press, New York, p 251±258 Edixhoven P, Huiskes R, de Graaf R 1989 Anteroposterior drawer measurements in the knee using an instrumented test device. Clinical Orthopaedics and Related Research 247: 232±242 Grood E S, Walz-Hasselfeld K A, Holden J P et al. 1992 The correlation between anterior-posterior translation and cross-sectional area of anterior cruciate ligament reconstructions. Journal of Orthopaedic Research 10: 878±885 Maitland M E, Bell G D, Mohtadi N G H, Herzog W 1995 Quantitative analysis of anterior cruciate ligament instability. Clinical Biomechanics 10: 93±97 Maitland M E, Kawchuk G N 1997 Towards the quanti®cation of end-feel for the assessment of passive joint motion. Physical Therapy Review 2: 217±226 Maitland M E, Leonard T, Frank C B, Shrive N G, Herzog W 1998 Longitudinal measurement of tibial motion relative to the femur during passive displacements in the cat before and after ACL transection. Journal of Orthopaedic Research 16: 448±454 Ng G Y F, Oakes B W, McLean I D, Deacon O W, Lampard D 1996 The long-term biomechanical and viscoelastic performance of repairing anterior cruciate ligament after hemitransection injury in a goat model. American Journal of Sports Medicine 24: 109±117 Noesberger B 1992 Diagnosis of acute tears of the anterior cruciate ligament, and the clinical features of chronic anterior instability. In: Jakob R, Staubli H (eds). The Knee and the cruciate ligaments. Springer-Verlag, Berlin 143±156 Oakes B W, Parker A W, Norman J 1982 Changes in collagen ®ber populations in young rat cruciate ligaments in response to a one month intensive exercise program. Connective Tissue Research 9: 212 O'Connor J, Biden E, Bradley J et al. 1990 The musclestabilized knee. In: Daniel D, Akeson W, O'Connor J (eds). Knee ligaments: structure, function, injury and repair. Raven Press, New York, p 239±277 Sailor M E, Keskula D R, Perrin D H 1995 Effect of running on anterior knee laxity in collegiate-level female athletes after anterior cruciate ligament reconstruction. Journal of Orthopaedic and Sports Physical Therapy 21: 233±239 Sakuma K, Mizuta H, Kai K, Takagi K, Iyama K 1993 Ultrastructural changes of collagen ®bers in the anterior cruciate ligament of bipedal rats after enforced running. Journal of the Japanese Orthopaedic Association 67: 655±661 Shelbourne K D, Gray T 2000 Results of anterior cruciate ligament reconstruction based on meniscus and articular cartilage status at the time of surgery: ®ve to ®fteen-year evaluations. American Journal of Sports Medicine 28: 446±452 Wishart G R 1995 The long term effect of running on anterior tibial translation. Thesis, School of Physiotherapy, La Trobe University, Australia Woo S L Y, Young E P, Kwan M K 1990 Fundamental studies in knee ligament mechanics. In: Daniel D, Akeson W, O'Connor J (eds). Knee ligaments: structure, function, injury and repair. Raven Press, New York, p 115±134
c 2001 Harcourt Publishers Ltd *
Relationship of kinetic demands of athletic training and knee joint laxity Gabriel Y.F. Ng and Murray E. Maitland Objectives: To compare the antero-posterior knee laxity and compliance in athletes and control subjects. Design: Cross-sectional comparison of different groups. Setting: Laboratory based study. Participants: sixty-four athletes and 25 control subjects aged 14±20. Athletes were trained for at least 2 years at a minimum of 6 h per week in their respective sports events, namely, basketball (n 27), running (n 20), swimming (n 17). Main outcome measure: Antero-posterior laxity, stiffness and rate of change in stiffness of the dominant knee at 100 N of anterior drawer force measured with a KT-2000 knee arthrometer. Results: Data were tested with multivariate analysis of variance with a 0.05. Results revealed signi®cant differences between groups in knee laxity (P 0.035) and the change in stiffness (P 0.034). There is a strong trend of difference between groups in stiffness (P 0.067). Post-hoc linear contrasts revealed that the swimming and basketball groups had signi®cantly less laxity and higher stiffness than the control group (P 5 0.05). Conclusions: These ®ndings suggest that sports training may improve the strength of soft tissues in the knee joint, and different kinetic demands of the sport events may result in unique response of c 2001 Harcourt Publishers Ltd the joint structures. *
Gabriel Y. F. Ng PhD, PT, Associate Professor, Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hong Kong Murray E. Maitland PhD, Associate Professor and Physical Therapist, Sports Medicine Centre, Faculty of Kinesiology, The University of Calgary, Canada Correspondence to: G. Ng, Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom, Hong Kong. Fax: 852 23308656; E-mail: rsgng@ polyu.edu.hk
66
Introduction The sagittal stability of the knee joint is largely dependent on the integrity of the anterior cruciate ligament (ACL) because it provides 85± 87% of restraint to anterior tibial translation (ATT) (Butler et al. 1980). Change in stiffness of this ligament could lead to increase or decrease in ATT (Cabaud, 1983; Grood et al. 1992). Injury to the ACL may render the knee joint unstable and develop early degenerative changes of the articular cartilages (Noesberger, 1992; Daniel, 1993). Rehabilitation for patients with ACL injury may not be adequate for them to return to their pre-injury level of sports performance because of functional instability. Also, many individuals experience residual laxity following ACL reconstruction and this is particularly worse in people who had associated meniscus injuries (Shelbourne & Gray, 2000). It is therefore important to determine the factors that enhance the mechanical properties of the
Physical Therapy in Sport
(2001) 2, 66±70
doi : 10.1054/ptsp.2001.0047, available online at http://www.idealibrary.com on
restraints to excessive knee joint laxity in order to prevent injury or improve the result of rehabilitation should injuries occur. There is evidence from animal studies that enforced weightbearing exercises improved the structural stiffness of the tissues in the knee joint (Cabaud et al. 1980; Oakes et al. 1982; Sakuma et al. 1993). In a 3-year study with an ACL hemitransection model in goat, Ng et al. (1996) reported no signi®cant difference in knee laxity from 12 weeks to 3 years after injury. In another study using the feline model, Maitland et al. (1998) found that force-displacement properties of the knee joint changed systematically over time following ACL transection. Even though similar studies in humans have not been reported, it is possible that sports training involving high intermittent loading to the knee can lead to adaptive changes in the structural and mechanical properties of the tissues, which would otherwise not occur with normal daily activities.
c 2001 Harcourt Publishers Ltd *
Relationship of kinetic demands of athletic training and knee joint laxity
It was reported in the literature that subjects with ACL reconstruction demonstrated increases in laxity of their knee joints immediately after endurance running (Sailor et al. 1995). This has been attributed to the transient lengthening of the ACL due to collagen creep (Woo et al. 1990). Relatively little has been published about the long-term cumulative effect of exercise on ATT in humans. A few studies that examined subjects with unilateral ACL injury have revealed that the uninjured knee of these subjects had signi®cantly greater anteroposterior laxity than the normal population (Bach et al. 1990; Daniel et al. 1985; Edixhoven et al. 1989). It was postulated that their injured knee might have also exhibited greater laxity than normal before the injury. These studies suggest that greater ATT could be associated with higher risk of knee injury. The long-term effects of exercise on the mechanical properties of soft tissues have not been well documented. Conteduca et al. (1991) performed subjective clinical laxity tests on 500 patients with ACL de®ciency. They found that patients with low levels of sports activities and those with sedentary lifestyles had higher laxity ratings than amateur and professional athletes. Wishart (1995) compared the antero-posterior laxity of the knee joint in long term endurance runners with relatively sedentary subjects and found that runners had less laxity than their sedentary counterparts at 308 of knee ¯exion and the same trend was shown at 908 of ¯exion. When the non-weightbearing joint laxity was taken as a variable, it did not affect the result, which suggested that difference was probably caused by the effect of running. This may be due to the frequent loading of the joint structures, in particular, the ACL of the runners, which had strengthened the ligament and rendered it to provide better restraint to the knee. Considering the kinetic demands of running, it is mostly repetitive extra-weightbearing in nature. It is not clear whether the kinetic nature of an exercise could affect the knee laxity. Furthermore, most previous studies only measured the total laxity but the stiffness was not known. In actual daily activities, the structural stiffness is an important parameter because it determines how complaint the knee joint would be at submaximal loading as in most daily activities.
c 2001 Harcourt Publishers Ltd *
Therefore, the present study aimed at comparing the antero-posterior laxity and structural stiffness of the knee joint in athletes engaged in different sports, namely basketball (high impact loading and cutting activity); endurance running (repetitive impact loading activity); swimming (non-weightbearing but high levels of muscular loading to the joint) and relatively sedentary subjects (control). If a signi®cant difference is found among these groups, it will contribute to the understanding of the relationship between kinetic demands of exercise and the constraints to knee joint motion. This will be very important in designing rehabilitation exercises for patients with knee injury. More importantly, incorporating the appropriate exercises may alter the force-displacement characteristics of the knee joint, so as to strengthen the knee amongst athletes.
Method A total of 64 athletes, aged 14 to 20 years, and 25 age-matched, control subjects were tested. The athletes consisted of 27 in the basketball group, 20 in the running group and 17 in the swimming group. All athletes had received at least 6 h of training per week in one of the above sporting events for more than 2 years. For the subjects in the control group, they must not have been engaged in any regular physical training program. None of the subjects had previous knee injuries that required medical treatment for more than 2 weeks. The Ethics Review Committee of the administering institute approved this study. All subjects gave their written consent prior to being tested. In order to control for the inter-tester variability, the same examiner, who is a quali®ed physical therapist, conducted all tests. Subjects were instructed to abstain from any strenuous physical activity at least 6 h prior to testing, so as to control for the immediate effect of exercise on joint laxity. All subjects were tested in a supine lying position with the knees rested on a testing block which maintained the knees at 308 of ¯exion. The dominant leg was tested, which was determined as the one that the subject used for kicking a ball.
Physical Therapy In Sport
(2001) 2, 66±70
67
Physical Therapy in Sport
A KT-2000 knee ligament arthrometer (MEDmetric Corporation, San Diego, CA) was used to measure the antero-posterior knee laxity of the subjects. The KT-2000 arthrometer was applied to the lower leg of the subject according to the instruction manual of the manufacturer. The analogue signal of the KT2000 arthrometer was output to a data acquisition unit (DataQ Instruments, Akron) which performed A-D conversion at a sampling frequency of 200 Hz. The digital signals were then input to a personal computer installed with a custom designed program to calculate the total sagittal displacement (laxity in mm), instantaneous rate of change of displacement (stiffness in Nmm ÿ1), and the rate of change of stiffness (Nmm ÿ2). The output was displayed real-time on the computer. The examiner applied a cyclic antero-posterior loading of 67 N, as suggested by the manufacturer, through the handle of the arthrometer so as to acquaint the subject with the test. After setting the reference position, the cyclic anteroposterior force was applied until the KT-2000 arthrometer output consistently registered 0/ ÿ0.5 mm at rest. The examiner would perform a 67 N of posterior push followed by 134 N of anterior drawer test for each subject. Knee laxity tests may be confounded by the size of the subject because of the weight of the leg. In order to obtain measurements at the equivalent anterior force levels, the weight of the subjects' legs were estimated from kinanthropometic regression equations as calculated by Chandler (1975). Leg segment lengths and weights were used to normalize the knee laxity test force for each individual. The weight-corrected data were then analysed at the point of 100 N of anterior drawer force according to the method of Maitland et al. (1995). Three parameters were obtained with the analysis, namely, total sagittal displacement (laxity), rate of change of displacement (stiffness), and rate of change of stiffness.
Data analysis Statistical analysis of the data was performed with a SPSS 9.0 computer program (SPSS Inc., Chicago). Normality of the data was tested and the results showed that most of the data were normally distributed. The data were analysed
68
Physical Therapy in Sport
(2001) 2, 66±70
with multivariate analysis of covariance. The covariates were gender, body weight and height. Results revealed that none of the covariates was signi®cant with P 0.757, 0.902 and 0.823 for gender, body weight and height, respectively. Therefore, the data were analysed with multivariate analysis of variance. The level of signi®cance was set at 5% and signi®cant results were further analysed with post-hoc linear contrasts to identify the data pairs that were signi®cantly different.
Results Table 1 presents the means, standard deviations (in brackets), and P values of each parameter tested for the four groups. There were statistically signi®cant differences between groups in laxity and rate of change in stiffness. Post-hoc linear contrasts revealed that the athletic groups were not signi®cantly different from each other in any of the parameters tested. However, the athletic groups were signi®cantly different to the sedentary control group. Given that the P value of the stiffness test was quite low (P 0.067), therefore post-hoc contrast was also performed for this parameter. Results reveal that subjects in the swimming group had less laxity but higher stiffness and rate of change of stiffness than the control group (P 0.013, 0.008 and 0.007, respectively). The basketball group had less laxity and higher rate of change of stiffness than the control group (P 0.025 and 0.023, respectively).
Discussion This study shows that there is an association between long-term sports training and measurements of joint laxity. Stresses or strains associated with sports activity may have contributed to differences in knee joint constraints, such that there was decreased in anterior translation of the tibia on the femur, and an increase in the rate of change of stiffness during the passive drawer manoeuvre. The differences between mean values were not statistically signi®cant but revealed a general trend in tissue stiffness with different sports events. The purpose of measuring stiffness and rate of change in stiffness in addition to joint
c 2001 Harcourt Publishers Ltd *
Relationship of kinetic demands of athletic training and knee joint laxity
Table 1 Means (SD) of antero-posterior knee laxity, stiffness and rate of change of stiffness of the different groups. * denotes the P values of 50.05 Basketball (Group 1, n 27)
Running (Group 2, n 20)
Swimming (Group 3, n 17)
Control (Group 4, n 25)
Laxity (mm)
6.349 (2.292)
7.330 (2.078)
5.961 (2.303)
7.789 (2.404)
0.035*
1 1 1 2 2 3
vs. vs. vs. vs. vs. vs.
2: 3: 4: 3: 4: 4:
0.149 0.584 0.025* 0.072 0.504 0.013*
Stiffness (N/mm)
47.542 (23.088)
45.206 (23.089)
57.769 (32.368)
37.856 (15.903)
0.067
1 1 1 2 2 3
vs. vs. vs. vs. vs. vs.
2: 3: 4: 3: 4: 4:
0.737 0.163 0.141 0.108 0.300 0.008*
Rate of change of stiffness (N/mm2)
25.742 (33.669)
18.226 (27.084)
33.086 (43.260)
4.685 (26.811)
0.034*
1 1 1 2 2 3
vs. vs. vs. vs. vs. vs.
2: 3: 4: 3: 4: 4:
0.437 0.470 0.023* 0.171 0.170 0.007*
displacement was because these parameters had been shown to have higher sensitivity than laxity when assessing subjects with ligamentous injuries (Maitland et al. 1995). The loaddisplacement characteristics of an articulation can be expressed with different mathematical functions. Laxity is a parameter that measures the magnitude of displacement under a given level of force. The limitation of this measure is that it does not provide information on the quality of displacement throughout the loading phase. Stiffness ( ®rst derivative of loaddisplacement curve) is a functional parameter that signi®es the biomechanical response of the joint to external load during a prescribed level of force. It is more indicative of the mechanical joint function at a submaximal loading condition. The rate of change in stiffness (second derivative of the load-displacement curve) signi®es the onset of constraints and the sharpness with which constraints come into play. The rate of change in stiffness has also been reported to contain important information regarding the end-feel assessment of a joint (Maitland & Kawchuk, 1997). We did not ®nd a signi®cant difference between the running group and control group in contrast to the previous ®ndings of Wishart (1995), in which the runners were found to have signi®cantly less laxity than the control subjects.
c 2001 Harcourt Publishers Ltd *
MANOVA P values
Post-hoc linear contrasts (P values)
We recruited subjects with at least 2 years of training in running. Most of our subjects were trained between 2 and 4 years. For the previous study, the training intensity was decidedly higher than our study, as most of the subjects had more than 5 years of training. The relatively lower training intensity of our subjects could explain the insigni®cant difference between the runners and control subjects. Considering the other two sports groups, they had signi®cantly lower laxity and higher stiffness than the control (Table 1). From the point of mechanical consideration, the nature of basketball would impose very high intermittent compressive and shear loading to the joint structures due to the high speed running, jumping and cutting actions. In swimming, the compressive loading to the joint would not be very high, but the action of the quadriceps muscles could create high anterior shearing loading to the joint thus stretching the ACL (O'Connor et al. 1990). Actually, the shear force that crosses the knee is more important than the joint compression in ligament loading, as the shear force to the joint due to muscle contraction requires the restraining action of the ligaments to maintain joint integrity. When considering the swimming action, it is an open kinetic chain activity, in which the shear force to the joint puts direct tensile loading to the
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(2001) 2, 66±70
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ACL that provides the sagittal restraining forces. It is possible that the ACL adapted by collagen remodelling (Cabaud et al. 1980; Oakes et al. 1982; Sakuma et al. 1993) and therefore this group of subjects presented with the least laxity and highest stiffness. The stiffness in the soft tissues is important for injury prevention and functional restraint to excessive movements of the joint. Our ®nding provides insight to coaches in their training of athletes in order to improve knee joint stability. However, our study was conducted with ablebodied subjects, it is not known whether similar effects would also exhibit in patients. Further studies need to be performed in subjects with ACL insuf®ciency so as to examine the effects of exercises in the patient population.
Acknowledgements The authors gratefully acknowledge the Sports Development Board Research Fund for ®nancial support of this study, Mr Simon Yeung of Hong Kong Polytechnic University, Mr Kunloi Tang of Hong Kong Amateur Athletic Association and Mr Martin Lam of Hong Kong Sports Institute for administrative support.
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