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Warm-Up Effect on Active and Passive Arthrometric of Knee Laxity Julie R. Steele, PhD, Peter D. Milburn,

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PhD, Gregory J. Roger, MBBS

ABSTRACT. Steele JR, Milburn PD, Roger GJ. Warm-up effect on active and passive arthrometric assessment of knee laxity. Arch Phys Med Rehabil 1999;80:829-36. Objective: To determine the influence of a warm-up protocol suitable for use in clinical settings on tibia1 displacement and muscle activity during arthrometric knee laxity assessment. Design: Intervention study in which the subjects served as their own controls. Setting: The Biomechanics Research Laboratory, University of Wollongong, Wollongong, New South Wales, Australia. Subjects: Ten volunteers who reported no history of knee trauma or disease. Intervention: A warm-up consisting of 10 minutes of ergometer cycling (60rpm) followed by two sets of three hamstring muscle stretches. Main Outcome Measures: Outcome measures were: (1) anterior tibia1 translation and knee extension force assessed using a Dynamic Cruciate Tester@for each subject’s right knee during active and passive testing, and (2) intensity of quadriceps and hamstring muscle activity during knee laxity testing. Results: There was significantly less quadriceps activity after warm-up (t = 2.419, p = .039). However, there was no significant difference between anterior tibia1 translation, knee extension force, or hamstring muscle activity results before and after warm-up in either active or passive tests. Conclusion: A warm-up suitable for use in a clinical setting is not required before arthrometric assessmentof knee laxity. 0 1999 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

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ARM-UP IS USUALLY defined as preparation undertaken immediately before engaging in physical activity to enhance or produce optimum work performance.’ Warm-ups vary from active exercises that involve voluntary muscles to passive activities that attempt to increase soft tissue temperature using external methods such as massage, hot baths, or radio diathermy. Despite variations in preparatory activities, the principal objective of warming up is usually to increase muscle or core body temperature to optimum working temperature2,3 without causing fatigue.4 Many studies have investigated the effects of warm-up regimes on the performance of various motor tasks or on physiologic responses to warm-up. Despite conflicting results From the Department of Biomedical Science, University of Wollongong, Wollongong, New South Wales, Australia. Submitted for publication July 20, 1998. Accepted in revised form December 11, 1999. Presented in part at the National Annual Scientific Conference in Sports Medicine, October 1993, Melbourne, Australia. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprints are not available. 0 1999 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation 0003-9993/99/8007-5137$3.00/O

among these studies, investigators now agree that warm-up protocols, if of sufficient intensity, can facilitate optimum physical performance by increasing activity of the cardiorespiratory and circulatory systems in readiness for exercise.3s4 Although physiologic adjustments in response to warm-up are clearly documented, 5,6 less attention has been directed to mechanical adjustments of connective tissues or joint mobility to warm-up. Only one study was located that examined the effect of active warm-up on knee motion wherein changes in the knee joint range of motion (ROM) in response to 20 minutes of submaximal treadmill running and passive stretching were evaluated.7 Neither treadmill running alone nor combined running and stretching were found to significantly increaseknee joint ROM. In a more generalized study, Wiktorsson-Moller and colleagues8 examined the effect of 15 minutes of ergometer cycling (5OW) followed by a series of stretches on lower extremity joint ROM. The authors reported that cycling alone resulted in a significant increase in dorsiflexion ROM but had no significant effect on the ROM for hip abduction or adduction, hip flexion, hip extension, or knee flexion. On the other hand, cycling combined with stretching exercises significantly increased the ROM for all the aforementioned joints. Although no studies documented the effect of warm-up or submaximal exercise on knee ligament laxity, several studies have examined the relation between vigorous exercise and knee ligament laxity.9-16The results of these studies are equivocal, however. Some authors reported a consistent increase in passive knee laxity in response to exercise, whereas others documented no significant difference in knee laxity before and after exercise. It appeared that the degree of postexercise laxity depended on a combination of these and other factors, including the type and intensity of exercise performed, duration between exercise cessation and laxity measurement, temperature levels reached during exercise, and resting muscle tone after exercise.14 No adequate mechanical or physiologic model exists to explain the contributing factors in transient knee laxity changes that, as indicated in previous studies, occur after exercise.13 Knee stability is maintained through a complex interaction of dynamic muscular control, joint surface contact forces, and static restraints including ligamentous and other soft connective tissue.9,11Therefore, although the exact mechanism is still unknown, transient increases in knee laxity after participation in physical activity have been attributed to a lengthening of either static or dynamic restraints to motion, a decrease in muscle tone, or a combination of factors9 Being primarily composed of collagen and other structural proteins, the viscoelastic muscular and ligamentous restraints in and around the knee lengthen in response to repetitive loading experienced during physical activity in a time-dependent and stress-dependent manner.9%11,13,15 Transient increases in the laxity of connective tissues in response to cyclic loading have also been shown in laboratory studies.15 Several studies have shown temperature fluctuations can alter the mechanical behavior of viscoelastic connective tissues such as tendons and ligaments. 17-21In general, these studies have shown that elevated tissue temperature increased the extensibility or elongation of the viscoelastic tissue. For Arch

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example, LaBan17 showed that an elevation in temperature from 37°C to 42.5”C increased the elastic response of canine calcaneal tendon samples by approximately 25%. Despite this related work, no studies have confirmed the effect of participating in preliminary activity on results obtained during arthrometric assessmentof knee laxity. Furthermore, no studies have examined changes in hamstring muscle tension following warm-up and its influence on dynamic stabilization of knee motion. However, it is clearly documented that changes in hamstring muscle tension during assessmentof knee laxity can significantly alter anterior tibial displacement.22,23If participating in a bout of preliminary activity does influence knee laxity, via elongation of connective tissues of the knee or changes in muscle tone, such activity would need to be standardized before any assessment of knee laxity to ensure reliable and valid results. Therefore, the purpose this study was to determine the influence of a standard warm-up protocol suitable for use in clinical settings on tibia1 displacement and muscle activity during arthrometric assessmentof knee laxity. METHODS Five male and five female volunteers (mean age = 21.2 years (standard deviation [SD] 1.7 years) who reported no history of trauma or disease in either knee and had no evidence of abnormality on clinical examination were selected as subjects. Ten subjects were considered sufficient to demonstrate a difference between the two experimental conditions with adequate power (80% at p 5 .05).24 Mean and SD data used to represent the expected differences in knee function to estimate the appropriate sample size were based on previously published results.25 Written informed consent was obtained from each subject before testing and all testing was conducted according to the Statement on Human Experimentation, National Health and Medical Research Council, Australia. Assessment of Knee Laxity Active and passive sagittal plane knee laxity were assessed using a calibrated Dynamic Cruciate Tester?(DCT) arthrometer that was placed on a firm level bench, anterior to a modified dental chair. The DCT and dental chair were adjustable to accommodate for different lower limb lengths and to enable accurate body positioning required for testing. Component parts of the DCT and how it quantifies anterior tibia1 displacement relative to the patella and knee extension force are described in further detail by Steele and colleagues.25 Once positioned supine in the dental chair,25the subject’s test limb was rested on the femoral support and ankle cradle of the DCT with the knee hexed 30”. Care was taken to ensure precise knee llexion because anterior tibial translation is influenced by knee flexion angle.26An ankle strap and cam-lock system was used to restrain vertical leg motion during testing. However, foot motion and tibial rotation were not constrained during testing because imposing either internal or external rotation has been shown to influence measurement of anterior-posterior tibial translationz7 After familiarization with the experimental protocol, each subject performed five active tests and five passive tests on the right lower extremity before and immediately after the warm-up protocol. During active tests subjects performed a maximum isometric knee extension, relaxing immediately upon achieving maximal effort. To minimize reflex hamstring activation, the knee was extended using a smooth increase in effort. During passive tests an anteriorly directed force (240N) was applied to the posterior leg for 4 seconds using a 7.5-cm-wide brace and double pulley system while a strap restrained movement of the subject’s thigh (fig 1). The passive test was designed to simulate Arch

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Fig 1. Subject assessment.

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a manual Lachman test used in clinical evaluation of knee laxity. The system used to apply the passive load and the reliability of the active and passive test protocols are described in further detail by Steele and colleagues.25,28The order in which active and passive tests were administered was counterbalanced. Limb dominance was not accounted for because side dominance has been shown to make no significant difference in anterior or posterior laxity of control knees before or after exercise.29 Testing was restricted to one limb to ensure a standard duration between cessation of warm-up and assessment of knee laxity. Post-warm-up measurements were taken within 5 minutes of completing the warm-up to minimize any effects of changes in ligament or muscle tone during the recovery period.14 Adequate standardized rest periods (1.5 minutes) were provided between each trial to minimize possible fatigue or viscoelastic effects. However, the duration of rest periods was limited to minimize loss of any warm-up effects. Throughout testing subjects were encouraged to remain as relaxed as possible to enable collection of valid knee laxity data. Apprehension felt by patients when they are tested might influence force-displacement curves via muscle guarding through tensing of the hamstring muscles that may limit anterior tibia1 displacement. Detection of Electromyographic Activity Electromyographic activity of rectus femoris (RF), vastus lateralis (VL), semimembranosus (SM), and biceps femoris (BF) were recorded during knee laxity testing using bipolar

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silver-silver chloride Medi-trace pellet disposable surface electrodesb After standard preparation to reduce electrical impedance levels of the skin below 6kOhms (CardioMetric Artifact Eliminator”), electrodes were adhered over the relevant muscle bellies (interdetection-surface spacing of approximately 10mm). Electrode placement sites were confirmed by palpating the muscles while the subjects performed isometric contractions. Reference electrodes for the hamstring and quadriceps muscles were positioned on the medial and lateral femoral epicondyles, respectively. Electrical potentials were relayed from the electrodes to Qantec MG differential preamplifiers Model 821d (gain, X20) and then to four isolated Qantec EMG amplifiers Model 810d via 2-m leads. The preamplifiers and leads were taped directly to the dental chair to minimize movement artifacts. Signals were suitably amplified to enable onscreen monitoring during maximal contractions. During the 4-second DCT data collection period, raw electromyogram (EMG) signals for the four muscles were sampled (1,OOOHz;bandwidth = 10 to 500Hz) and recorded using the Qantec Waveform Analysis System Package (WASP)d (version 2.0) on a personal computer. The computer was interfaced to the amplifiers via a WASP Interface WlO connected to a WASP A-D Converter Card using a Qantec W201 multiway cabled. To enable later normalization of the electromyographic data, intensity for each muscle was recorded during maximum voluntary isometric contractions (MVIC) of the subject performing one knee extension and three knee llexion efforts. The MVIC were performed in the same position required for knee laxity assessment.The knee extension MVIC served as a pretest to familiarize subjects with the testing protocol and to check signal amplification. It was not used in further data analysis. Warm-Up Protocol The warm-up was designed to simulate a standard protocol that could be undertaken by anterior cruciate ligament (ACL)deficient patients before arthrometric assessmentof knee joint laxity in a clinical setting. It was not intended to achieve maximal increases in tissue temperature or to include vigorous exercises that could not be accomplished by an ACL-deficient subject. The warm up protocol consisted of 10 minutes of cycling on a calibrated Monark Ergomedic 818E cycle ergometeP followed by a set of hamstring muscle stretches. Although core or muscle temperature changes were not measured in this study, it is well established that 10 minutes of cycling is sufficient to elicit moderate increases (0.4” to 0.6”C) in core temperature.30 Furthermore, the temperature of active muscles rises more rapidly than core or rectal temperature during cycling, with muscle temperatures reaching relative equilibrium at 10 to 20 minutes.31-33Hoffman and coworkers33reported mean increases in quadriceps muscle temperature of 2.1”C for men cycling for 10 minutes at 60% of their maximal oxygen uptake from rest. The onset of sweating, under normal environmental conditions, is also considered a good indicator of sufficient temperature elevation to achieve an appropriate warm-up effect.4,5 Therefore, 10 minutes of cycling at a work rate, which caused mild sweating, was considered sufficient indication subjects were adequately warmed up. Cycling was selected as the mode of physical exercise because it actively contracts the muscles surrounding the knee that were involved in the subsequent assessmentof knee laxity. Furthermore, ACL loading during the knee extension phase of ergometer cycling (pedal down) are lower than those incurred during other knee extension exercises.This is because the quadriceps muscles ceaseto be active when the pedal is in the horizontal position of the down stroke

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and the hamstring muscles are active as the knee approaches maximum extension.34 Seat height was adjusted so each subject’s knee was flexed 20” when the pedal was in the down position and the ball of the foot was on the pedal. A knee flexion angle of 20” was selected to reduce ACL loading during cycling because ACL-injured patients are usually discouraged from performing isotonic quadriceps contractions near full extension in the first year following injury or reconstruction to protect the reconstruction of the partially torn ACL and to minimize stretching of secondary restraints.23s35,36 Subjects cycled at 60 revolutions per minute (rpm) with the resistance (in kilograms) set to achieve a work rate equivalent to 5 metabolic equivalents (METS). This work rate was selected since pilot testing had established it as sufficient intensity to elicit mild sweating without inducing fatigue after cycling for 10 minutes. The resistance set for each subject was calculated using the procedures described by the American College of Sports Medicine. 37 The purpose of calculating the work rate based on METS was to achieve the same relative heat production for each subject during cycling by having the subjects work at the same relative energy cost per kilogram of body weight. The workload calculations have been shown to provide reasonable estimates of MET values for exercise intensities of 300 to 1,20Okg/m/min. Immediately after completing cycling, the subjects were positioned supine on a mat beside the DCT testing rig where they performed two sets of stretching exercises. Each set consisted of three hamstring stretches based on a modified contract-relax proprioceptive neuromuscular facilitation stretching technique. With the subject supine and the knee extended, the examiner raised the subject’s right lower extremity off the mat in the sagittal plane until the subject said it had reached the limit of range of hip flexion motion. The subject’s limb was held at this point for 3 seconds after which time the subject performed a maximal isometric contraction of the hip extensors for 6 seconds.After the isometric contraction, the subject’s right thigh was again passively flexed at the hip to the limit of the ROM. The actions of passive hip flexion and active hip extension were performed three times for each set of stretches with a S-second relaxation break between sets. The total stretching time therefore approximated 1 to 1.5 minutes. Stretching activities were included in the warm-up to maximize elongation of the connective tissue elements of the hamstring muscle-tendon unit and to minimize hamstring muscle tension. Laboratory studies have shown that cyclic repetitive stretching can reduce the amount of tension placed on a muscle at a given length of the muscle-tendon unit and increase muscle lengm3* Stretches were performed after cycling when the connective tissue temperatures of the test limb were elevated. Stretching exercises have been shown to more effectively increase extensibility of connective tissue when the stretch is coupled with elevated tissue temperatures20 Furthermore, loading collagenous tissues has been shown to produce significantly less tissue damage if the tissue temperature is raised before initiating stretching.21 After completing the hamstring stretches, the subjects were immediately repositioned in the dental chair and DCT rig to enable collection of post-warm-up DCT and EMG data. Previous marking of the locations of all relevant landmarks on the subject’s limb to position the DCT facilitated speed and accuracy in subject repositioning. Transition from the stretching mat to initiating knee laxity data collection did not exceed 5 minutes. A blanket was also placed over the subject’s left lower extremity and torso during post-warm-up testing to minimize Arch

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subject cooling. Loss of warm-up effect was considered minimal because it had been shown that inactivity for 15 minutes had minimal inthtence on initial increases in hip ROM gained from cycling and stretching warm up activities.3g Wet and dry bulb temperature readings were taken throughout the testing sessions using an Ogawaseiki Model Osk7159~SW sling psychrometerf to provide an indication of consistency in environmental conditions (relative humidity and ambient temperature) between the test sessions.Over the 10 test sessions (3 days), the ambient temperature averaged 19.3”C (SD, 1.4”C) whereas relative humidity averaged 57.8% (SD, 7.6%). Data Analysis From the DCT force-displacement curves, peak tibial displacement (in millimeters) and peak knee extension force (in kilograms) results were determined using a custom software package. Subject’s peak values were determined for each of the five active and five passive trials performed before and after warm-up. The raw EMG data were initially visually inspected to confirm that the signals were suitable for further analysis based on minimum signal contamination or distortion. A 500-msec maximal intensity sample of RF activity and of VL activity from each of the 4,000-msec active trials was then analyzed by removing any signal offset and then rectifying, integrating (IEMG), and normalizing the data for time. The RF and VL activity results were averaged to provide one representative value of the intensity at which the quadriceps muscles were activated during each active trial. Because each active trial represented a MVIC, the processed EMG signals were compared directly before and after warm-up and further normalization of the EMG data was not required. During active and passive trials the total 4,000-msec samples for the BF and SM were analyzed: signal offset removed, full-wave rectified, integrated, normalized for time, and then expressed as a percentage of a MVIC of each muscle. The BF and SM results were averaged to provide a representative value of the intensity of activity of the hamstring muscles during each knee laxity active and passive trial. Statistical Analysis DCT and EMG results for the five trials for each subject for each test condition were averaged. This provided representative values of the four dependent variables (peak tibial displacement, peak knee extension force, and hamstring and quadriceps muscle activity) for each subject during active and passive test conditions before and after warm-up. The dependent variables, grouped according to test type (active versus passive), were analyzed using t tests for paired samples (dependent means). The main purpose of the design was to determine whether there were any significant differences (p < .05, df = 9, two-tailed test) between the before and after warm-up results for each dependent variables. Analyses were conducted using SPSSI PC+ statistical package.s

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measured before warm-up (3.2mm) compared with after warm-up (3.2mn-1,t = -.07,p = .945). Knee Extension Force No significant difference was found between mean active knee extension force recorded before warm-up (3858kg) and after warm-up (37.47kg, t = 1.18, p = .268) (fig 3). Furthermore, mean pre-warm-up passive knee extension force results (6.97kg) did not differ significantly from passive knee extension force recorded after warm-up (6.70, t = 1.09, p = .302). Therefore, the warm-up protocol used in the study did not significantly influence the force actively exerted by subjects in a maximum isometric quadriceps contraction or the force applied by the leg segment to the ankle strap of the DCT test rig when the passive load was applied to the posterior calf.

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Tibia1 Displacement Anterior tibia1 displacement descriptive data grouped as a function of warm up status and test type are presented in figure 2. No significant difference was found in the mean active tibia1 displacement data post-warm-up (11.2mm) compared with the pre-warm-up data (10.9mm, t = -.56, p = .587). Furthermore, there was no change in mean passive tibia1 displacement Arch

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Hamstring Muscle Activity Descriptive data for hamstring muscle activity recorded during active and passive tests before and after warm-up are presented in figure 4. There was no significant difference between the mean hamstring activity recorded in active trials before warm-up (5.03% MVIC) compared with after warm-up (4.93% MVIC, t = .51, p = .623). Furthermore, there was no significant difference in mean hamstring activity recorded during passive trials before warm-up (1.41% MVIC) compared with during passive trials after warm-up (1.53%, t = -.94, p = .371). Quadriceps Muscle Activity Mean and SD results for quadriceps activity grouped according to warm-up status are shown in figure 5. On average, subjects showed significantly less quadriceps activity after the warm-up period (.32OV msec) compared with the quadriceps activity recorded before warm-up (.365V. msec, t = 2.419, p = .039). This represented a 12.3% reduction in quadriceps activity recorded during the maximum isometric knee extension efforts after warm-up. This reduction in quadriceps activity, however, had no significant influence on active knee laxity or active knee extension force results.

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DISCUSSION Knee Laxity Skinnern and Johannsen9 and their colleagues reported consistent, although not all significant, increases in passive anterior knee laxity measurements of subjects who had participated in vigorous exercise protocols. However, no significant increases in active anterior laxity were found after exercise in either study. In contrast, in this study no change was found in either passive or active knee laxity (tibia1 displacement) after warm-up. Inconsistency in changes in knee laxity after exercise has also been reported within studies for subjects with intact knees. For example, Steiner and colleagues12 reported that although basketball players and distance runners displayed significant increases in passive anterior-posterior laxity after exercise,

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sedentary controls and power lifters did not show any significant changes. The authors attributed different laxity responses to variations in the types of exercises the subject groups participated in. Activities that placed repetitive stresseson the knee, such as distance running and basketball, led to significant laxity increases after exercise, whereas activities that caused high compressive joint loads for only a few cycles, such as squat power lifts, did not influence anterior-posterior laxity.r2 Loading of the primary restraints to knee motion with sufficient intensity and frequency appeared necessary to provide an adequate stimulus for increased knee laxity. The cycling protocol in this study was designed to be suitable for ACL-deficient subjects. Therefore, ACL loading was minimized during cycling by restricting leg movement through the final 20” of knee extension. However, minimizing cyclic ACL loading throughout the warm-up may have prevented sufficient loading stimuli to either the ligament or the secondary restraints to anterior-posterior motion required to elicit any increase in knee laxity after warm-up. This is consistent with the work of Lehmann and colleaguesuswho showed that application of heat alone did not contribute to elongation of collagenous rat tail tendons. A combination of heat and loading were required to achieve residual lengthening of collagenous tissue.ls ACL-deficient subjects have been shown to demonstrate, on average, a smaller increase in anterior laxity following exercise compared with subjects with intact kneesz9 This was attributed to an absence of the ACL that meant any increase in laxity following exercise for ACL-deficient subjects was caused by changes in laxity of the secondary restraints. On the other hand, subjects with an intact knee could demonstrate changes to the ACL, the primary restraint to anterior tibia1 motion, and to the secondary restraints. Based on these results it is assumed ACL-deficient subjects would also demonstrate no significant increase in knee laxity in response to the warm-up protocol used in this study. However, further research is recommended to confirm this assumption. Gillette and coworkers7 evaluated changes in body core temperature and changes in knee ROM in response to 20 minutes of submaximal treadmill running and passive stretching. The energy expenditure required to achieve an increase Arch

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greater than l.O”C in body core temperature was found to be 86% (SD, 1.6%) of a subject’s age-predicted maximum heart rate. The authors claimed that 20 minutes of exercise at 86% of maximum heart rate would not be practical as a therapeutic regime and negated any potential enhancement of ROM through combined exercise and passive stretch protocol7 Although active muscle temperature rises more rapidly than core temperature during cycling,31,32if a work rate of 86% of maximum heart rate for 20 minutes was required to achieve increases in tibia1 translation, it would also be impractical as an exercise regime before assessing clinical knee laxity in ACLdeficient patients. That is, such a vigorous regime would be inappropriate for many patients because of symptoms associated with their injury. Knee Extension Force Elam5 claimed that after warm-up, a muscle would contract more forcefully than without prior exercise because of the treppe phenomenon. That is, active muscles require decreasing degrees of succeeding stimuli to elicit maximal contraction than muscles that have been at rest. Muscle therefore contracts more forcefully after it has contracted a few times compared to its initial contractions.5 In this study there was a time lag between cessation of cycling and the first active quadriceps contraction for laxity assessment because of the hamstring stretching protocol (which did not involve quadriceps contractions) and subject repositioning in the DCT test rig. Therefore, any treppe phenomenon resulting from the active warm-up in this study appears to have been minimized because the quadriceps muscles were rested before the post-warm-up active laxity assessment. This is consistent with the lack of any significant difference between the pre-warm-up and post-warm-up active knee extension forces. Most changes in muscle response to warm-up are attributed to increasesin muscle tissue temperature after physical activity, where muscle temperature has been shown to have a profound affect on isometric muscle function.40 Increased muscle tissue temperature is due in part to the dissipation of heat produced by friction from sliding filaments during contraction and nonutilized (chemical) energy.5 It has been claimed that increased muscle temperature results in an increased speed or strength of muscular contraction.41 However, others have stated that increasedmuscle temperature primarily influences muscle contractile velocity or muscular endurance and generally had minimal effect on maximal isometric forces.40,42,43 In a study specifically focusing on active warm-up effects, Wiktorsson-Moller and colleagues8 found no significant effect from 15 minutes of ergometer cycling (50W) and/or proprioceptive neuromuscular facilitation stretching on isometric or isokinetic hamstring or quadriceps muscle strength results as measured by the Cybex II dynamometer for eight male subjects. Furthermore, Hoffman and coworkers33 found no significant effect of ergometer cycling (50rpm) of varying durations (1,3, 10, and 20 minutes) on quadriceps strength when seven men cycled at lower intensities (20% to 60% of their maximum oxygen uptake). Significant reductions in quadriceps strength, assessed with subjects supine and the knee flexed to 90”, was only evident after the subjects cycled at 80% of the maximum oxygen uptake.33 Results of our study were consistent with these latter two studies where no significant difference in maximal isometric knee extension was found after warm-up or lower intensity exercise. Hoffman and colleagues33 postulated that reductions in muscular strength following rhythmic activities such as cycling were attributable to muscle contractile failure caused by Arch

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high-intensity exercise and were not evident at lower intensity exercise levels. It can therefore be assumed that the lowintensity warm-up performed in this study did not cause any muscle contractile failure. Consequently, force applied to the tibia via isometric quadriceps contraction during active knee laxity assessment did not significantly differ during prewarm-up knee assessment trials compared with the postwarm-up trials. Applying an equivalent load would, in turn, have contributed to the similar tibial displacement results found in the before and after warm-up assessmentsin our study. Hamstring Muscle Activity In addition to the static ligamentous restraints to anterior tibia1 translation, muscular forces play a role in dynamic stabilization of knee motion. lo Enoka4 claimed an increase in muscle temperature affected muscle relaxation more substantially than force development. The effects of warm-up on hamstring muscle activity were therefore of primary interest because changes in hamstring tension can significantly alter anterior tibia1 displacement results.22,23 Weesner and colleagues14 stated muscle tone would be expected to be greater in knees tested immediately after exercise than at any time during the recovery period. Such an increase in resting muscle tone could increase joint load and thereby mask any increase in postexercise laxity of the ligaments9 However, others have speculated that physical activity, particularly exercise that causes fatigue, could decrease resting muscle tone, resulting in apparent increased knee laxity.“J3 Davies and Young 43 demonstrated an increase in muscle temperature of 3.1”C caused a decrease in contraction time by 7% and a decrease in one-half relaxation time by 22%. (Half relaxation time is the time between attainment of peak twitch and return to 50% of peak value.45) Furthermore, laboratory studies have shown that cyclic repetitive stretching reduced the amount of tension placed on a muscle at a given length of the muscle-tendon unit.37,46 It could therefore be assumed that increases in muscle temperature from participating in active warm-up before knee laxity assessment could increase hamstring relaxation. Furthermore, as a viscoelastic tissue, the repetitive submaximal stress during physical activity could have a lengthening effect on muscle, particularly when combined with stretching exercises.ll For these reasons, active warm-up could potentially contribute to gaining more reliable and valid assessmentof anterior tibial displacement by reducing both voluntary and involuntary muscular guarding during knee laxity testing. Results of this study indicated that 10 minutes of cycling at submaximal intensity followed by hamstring stretching had no measurable effect on hamstring muscle tension during knee laxity assessment. In fact, hamstring activation levels both before and after warm-up were negligible (less than 5% MVIC) indicating successful subject relaxation during testing. Therefore, warming up did not significantly alter hamstring muscle tone in this study and, as such, provided no benefit in increasing the validity or reliability of knee laxity assessmentresults. Quadriceps Muscle Activity A frequently cited benefit of warming up is improved mechanical efficiency of muscular contraction associated with increased tissue temperatures. This improved efficiency is the result of many factors, including a reduction in internal viscosity of the muscle.4-6,46,47 The physiologic basis of improved mechanical efficiency of muscular contraction following warm up is further described by Shellock.6 Although subjects in this study demonstrated a significant

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decrease in quadriceps muscle activity during maximal isometric knee extensions after warm-up, there was no reduction in knee extension force. From these results it is suggested that the increase in muscle temperature achieved during warming up was sufficient to enhance efficiency of muscle contraction. Therefore, subjects generated the same level of knee extension force during maximum isometric quadriceps contractions but with fewer motor units activated after warm-up compared with before warm-up. In addition to changes in tissue temperature on muscle contractile efficiency, warm-up can increase muscular efficiency by the mere intrinsic characteristic movements undertaken.5 That is, repeated muscular contractions during the warm-up protocol or during the five pre-warm-up active knee laxity assessment trials may have provided sufficient familiarization with the required test protocol to increase their eff& ciency at performing the active test by enhancing synchronization and recruitment of motor units. This familiarization, however, did not influence tibia1 displacement or knee extension force during active tests and therefore did not increase reliability of knee assessmentprotocols. CONCLUSIONS There would be many potential problems associated with defining an appropriate warm-up protocol to be followed before assessingknee laxity for individual patients and under varying environmental conditions. Sedgwick2 and Clarke and colleagues40 cautioned that muscle temperatures associated with optimum performance lay within a highly critical range. Insufficient active warm-up may have no measurable influence on performance and excessiveactive warm-up may be detrimental because of fatigue. Furthermore, achieving this critical temperature during a warm-up protocol is influenced by many factors including environmental conditions, exposure, and clothing.2 The warm-up protocol used in this study, designed to be appropriate for use in before assessing knee laxity in ACLdeficient patients, had no significant effect on knee laxity assessment results during either active or passive testing. Therefore, such a warm-up protocol is not required to be incorporated as standard procedure before knee laxity assessment using the DCT.

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10. Maitland ME, Lowe R, Stewart S, Fung T, Bell GD. Does Cybex testing increase knee laxity after anterior cruciate ligament recon-

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Suppliers a. Dynamic Cruciate Tester; Smith & Nephew Richards Pty Ltd, West Ryde, NSW, Australia. b. Medi-trace pellet disposable surface electrodes; Graphic Controls Corporation, Medical Productes Division, Buffalo, NY. C. CardioMetric Artifact Eliminator@; Victoria, Australia. d. Qantec Systems, University of Queensland, Queensland, Australia. e. Monark Ergomedic 818E cycle ergometer; Medos Company Pty Ltd, Lidcombe, NSW, Australia. f. Ogawaseiki sling psychrometer; John Morris Scientific Pty Ltd, Chatswood, NSW, Australia. g. SPSS, Chicago, IL.