THE ETIOLOGY OF JUVENILE idiopathic arthritis (JIA) Plantar- and Dorsiflexor Strength in Prepubertal Girls With Juvenile Idiopathic Arthritis

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1 1224 Plantar- and Dorsiflexor Strength in Prepubertal Girls With Juvenile Idiopathic Arthritis Eva Broström, PT, PhD, Maria M. Nordlund, PT, Andrew G. Cresswell, PhD ABSTRACT. Broström E, Nordlund MM, Cresswell AG. Plantar- and dorsiflexor strength in prepubertal girls with juvenile idiopathic arthritis. Arch Phys Med Rehabil 2004;85: Objective: To compare lower-leg strength of young girls with polyarticular juvenile idiopathic arthritis (JIA) with that of healthy, age-matched controls. Design: Isometric and isokinetic strength tests of the plantarand dorsiflexors. All strength measures were made at an ankle angle of 90. Isokinetic plantar- and dorsiflexor measures were made at 15 /s during shortening (concentric) and lengthening (eccentric) actions. Setting: Strength testing laboratory. Participants: Ten prepubertal girls diagnosed with JIA and 10 healthy girls. Interventions: Not applicable. Main Outcome Measures: Isometric and isokinetic plantarand dorsiflexor strength. Results: Isometric plantar- and dorsiflexion torques were significantly lower (48% and 38% respectively; P.05) for the children with JIA than for the controls. The JIA group also produced lower shortening plantarflexion torques (52%, P.05). Lengthening plantarflexor torques did not differ significantly between the 2 groups (P.05). Controls were stronger than the JIA group for both shortening and lengthening maximal dorsiflexor actions (P.05). All children were 4 to 5 times stronger in plantarflexion than in dorsiflexion. Conclusions: Girls with JIA had significantly less plantarand dorsiflexor strength than age-matched, healthy peers. The reduced strength of children with JIA is likely to affect function in daily activities and probably contributes to reduced levels of physical activity. Key Words: Arthritis, juvenile rheumatoid; Muscle contraction; Muscles; Rehabilitation by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation THE ETIOLOGY OF JUVENILE idiopathic arthritis (JIA) is largely unknown but appears to differ substantially from adult rheumatoid arthritis. The long-term prognosis of 40% of See commentary p From the Woman and Child Health (Broström) and Department of Neuroscience (Nordlund, Cresswell), Karolinska Institutet, Stockholm, Sweden; and University College of Physical Education and Sports, Stockholm (Nordlund, Cresswell), Sweden. Presented in part at the Third International Conference on Strength Training, November 2002, Budapest, Hungary. Supported by the Swedish Research Council, the Swedish Centre for Sports Research, the Swedish Rheumatism Association, and Sunnedahl Foundation. 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 author(s) or upon any organization with which the author(s) is/are associated. Reprint requests to Andrew G. Cresswell, PhD, Dept of Neuroscience, Karolinska Institutet, Box 5626, Stockholm, SE , Sweden, Andrew.Cresswell@ neuro.ki.se /04/ $30.00/0 doi: /j.apmr the children with JIA is good, 1,2 with the disease often being self-limiting. It is not possible at the onset of the disease to predict accurately which children will recover and which will have a long-standing and destructive disease. Early diagnoses and active therapeutic interventions seem to be essential to minimize residual deformity and disability due to joint contractures and asymmetric bone growth. 3 Girls are more often affected than boys, and symptoms at the onset of the disease are often diffuse pain and swelling which lead to an avoidance of joint loading. The disease shows 7 distinct types of involvement. 3,4 The polyarticular type, which accounts for 25% to 35% of all patients, is defined as having 5 or more joints involved and is usually symmetric between limbs. It also generally involves the large joints of the knees, ankles, elbows, and wrists; the small joints of the hands and feet; and the cervical spine. 4-6 Apart from articular symptoms, weight loss and fatigue may also occur in all 3 subtypes of JIA. 7 Long duration of active disease is also associated with a reduction in height, a discrepancy in leg length, 8,9 and a shortening of muscle and tendon length that gives rise to flexion contractures Muscle atrophy, with an associated reduction of strength, is characteristic of children with JIA It also appears that children with early onset of unilateral knee arthritis are at greater risk for muscle atrophy. A study 14 of children with this disease has shown a reduced cross-sectional area of the knee extensor muscles and a reduction in knee extensor strength, regardless of active knee inflammation. Furthermore, muscle weakness and an associated reduction in strength have been shown to have a profound effect on the level of physical activity of children with JIA Assessments of muscle strength and muscle function are used regularly in the clinic to chart the progress of the disease. 10,11,14,15 Single muscle groups are typically assessed, with the knee extensors being the most commonly evaluated. Little attention has been given to the strength of the ankle flexors and extensors in these patients; however, one might expect that these muscles would be equally affected by the disease. As such, knowing the strength of the plantar- and dorsiflexor muscles may also be useful in tracking the development of the disease, particularly because these muscles are used extensively during daily activities, such as walking, running, and standing As partial support, it has also been proposed from clinical observations that strength training may improve the functional mobility of children with JIA. 21 Traditionally, physical therapists have used manual muscle testing to evaluate muscle strength in children. 22 The subjective character of manual muscle testing often results in large measurement variability, thereby questioning its suitability for accurate strength assessment. 23 Strength assessment using a hand-held dynamometer 12,14,24-29 has become a more popular and somewhat more objective method to measure isometric strength. It has been used to measure lower-extremity isometric strength in healthy children 12,13,27 and in children with JIA. 12,13 However, there are several significant limitations of the hand-held dynamometer as a tool for accurately measuring strength. Hence, more accurate and reliable methods of strength assessment need to be found

2 ANKLE STRENGTH IN JIA, Broström 1225 Table 1: Characteristics of the Children With JIA and of the Controls Age (y) Body Mass (kg) Height (cm) JIA (n 10) (8 12) ( ) ( ) Controls (n 10) (8 12) ( ) ( ) NOTE. Values are mean standard deviation (SD) (range). to evaluate the strength of the lower extremities of children with JIA. Using computer-controlled isokinetic (constant-velocity) dynamometers, which show highly objective and reliable measures of strength in both healthy and diseased populations, some 40 strength measurement studies have been performed on children. 10,11,15,36-41 Only a few studies have investigated ankle strength in healthy children 12,40,41 and children with JIA, 12 and most of the studies have not measured the plantarflexors. Moreover, the lack of reporting strength measures in units of torque 12,40 makes quantitative comparisons impossible. Therefore, the aims of our study were to assess comprehensively the isometric and dynamic (shortening, lengthening) plantar- and dorsiflexor strength of young girls with JIA, using a custom-built isokinetic dynamometer and to compare their lower leg strength with that of healthy, age-matched controls. METHODS Participants Twelve prepubertal girls, who were diagnosed with JIA according to the criteria set by the International League of Associations for Rheumatology, 4 participated in the study. All girls had lower-extremity polyarticular involvement and a disease duration of years. Two of the children were subsequently excluded because of excessive pain during preliminary measurements. The characteristics of the remaining 10 children with JIA are given in table 1. All children were independent walkers and were taking standard medication for example, nonsteroidal anti-inflammatory drugs (NSAIDs), methotrexate, and tumor necrosis factor (TNF) inhibitor (table 2). None of the children had known neurologic problems or a history of gross motor delay. Ten healthy children, matched for gender and age, participated in the study as a control group. Their characteristics are also presented in table 1. Written information about the experiment was given to the children and their parents, after which verbal consent was obtained. The study was approved by the human ethics committee of the local research institute and was conducted according to the Declaration of Helsinki. Experimental Set-Up All children underwent an examination of plantar- and dorsiflexor strength, using a microprocessor-controlled torque motor. Subjects lay prone on a bench with their left foot securely strapped to a rigid plate, attached to the motor, using nonelastic sports strapping tape and Velcro bands. The ankle joint was aligned with the axis of the torque motor, and straps were used to secure the thigh to reduce the chance of knee flexion when performing the strength tasks. Plantar- and dorsiflexion torques were measured using a torque transducer, a located in the axle of the torque motor, that accurately controlled ankle displacement and angular velocity. 30 The torque signal was amplified b and low-pass filtered at 10Hz (Neurolog NL 125 c ) before undergoing analogto-digital conversion at a sample rate of 100Hz, using a CED Power 1401 d and Signal d data collection software. Initially, the subjects performed 3 maximum isometric plantarflexor actions followed by 3 maximum isometric dorsiflexor actions at an ankle angle of 90 (defined as the neutral position). Thereafter, lengthening and shortening plantar- and dorsiflexor maximum voluntary actions were randomly performed. All lengthening and shortening trials were performed twice, at a constant angular velocity of 15 /s over the prescribed range of motion (ROM) (see next section). During shortening trials, the subjects worked in the same direction as the motion of the motor; whereas during lengthening trials, the subjects worked against the motion of the motor (fig 1). A rest period of at least 2 minutes was given, to reduce the risk of fatigue. Assessment of ROM Ankle ROM was assessed before commencing the strength measurements. A 30 ROM was used for strength testing if the subjects were able to comfortably move through 15 of plantarflexion and 15 of dorsiflexion from the neutral position. All children were able to tolerate their ankle being moved through Age (y) Body Mass (kg) Table 2: Characteristics of the Children With JIA Diagnosed in the Polyarticular Subgroup Height (cm) Joint Involvement in Lower Extremities Treatment Ankle* NSAIDs, SSZ Ankle,* knee* NSAIDs Ankle,* subtalar,* right hip NSAIDs Ankle,* subtalar,* knee* TNF inhibitor Ankle,* subtalar,* knee,* hip* NSAIDs, MTX Ankle,* subtalar* NSAIDs, MTX Ankle,* subtalar* MTX Ankle,* knee* TNF inhibitor, NSAIDs Ankle,* subtalar,* knee,* hip* TNF inhibitor, NSAIDs Ankle,* subtalar,* knee* MTX 0 Abbreviations: MTX, methotrexat; SSZ, sulfasalazine; VAS, visual analog scale. *Bilateral joint involvement. There was no active arthritis in the lower extremities (left leg) at the time of testing. VAS (mm)

3 1226 ANKLE STRENGTH IN JIA, Broström RESULTS Fig 1. A schematic diagram of the 30 ROM through which the subjects attempted to exert maximal isometric, lengthening, and shortening actions of the plantar- and dorsiflexors. Angle-specific torque (strength) measures were made for all tests at an ankle angle of 90 that is, as the ankle passed through 90 for the shortening and lengthening measures and statically at 90 for the isometric measures. 15 of plantarflexion. Five of the 10 JIA children could not endure ankle movement through 15 of dorsiflexion. One of those 5 children managed 10 of dorsiflexion; the other 4 managed only 5 of dorsiflexion. All the control children were able to move through the full 30 ROM. Data Analysis Plantarflexor torque was defined as being positive and dorsiflexor torque as negative. For isometric maximal voluntary actions, the mean of the highest 1-second period of torque was used. Angle-specific torque (the neutral position of 90 ) was used for all lengthening and shortening trials. For every action type, only the trial giving the highest torque value was used for further analysis. A 3-way (subject group, action type, muscle) analysis of variance (ANOVA) with repeated measures on 2 factors (action type, muscle group) was used. This test was first applied to the children with JIA who were divided into 2 groups: those who had a full dorsiflexion ROM and those who did not. Because no significant main effect was found between the JIA children with different ROMs, all children with JIA were subsequently considered as 1 group. The same type of ANOVA was then performed on the children with JIA and the healthy controls as 2 groups. The level of significance for main effects and interactions within the ANOVA was set to P equal to.05 or less. Planned comparisons were performed where a significant main effect or interaction was found. The level of significance was Bonferroni adjusted to correct for multiple comparisons. All statistical analyses were performed using the Statistica software package. e Torque Comparisons Between JIA Children With an ROM Less Than 30 and 30 ROM There were no significant angle-specific torque differences for isometric, shortening, and lengthening plantar- and dorsiflexion actions between the children with JIA and an ROM less than 30 and the children with JIA with an ROM of 30 (table 3). Because of the lack of strength difference between the 2 JIA subgroups, all children with JIA were then placed in the same group, for comparison with healthy, age- and gender-matched controls. Plantar- and Dorsiflexion Torque Comparisons Between Children With JIA and Controls Maximal voluntary isometric plantar- and dorsiflexion torques were significantly lower (48% and 38%, respectively) for the children with JIA than for the controls (table 4, fig 2). For shortening plantarflexions, the JIA group produced significantly lower torques (52%) than the controls (table 4, fig 2). However, for lengthening plantarflexor actions, the torques did not differ significantly between the 2 groups, although there was a trend (P.07) for the children with JIA to be weaker (controls, Nm; children with JIA, Nm). Controls were significantly stronger than the JIA group for both shortening (52%) and lengthening (37%) maximal dorsiflexor actions (table 4, fig 2). There was no significant difference between the 2 groups for body mass. As such, no between-group comparisons differed from those described above, when torque values were normalized to body mass. Torque Comparisons Between Action Types in Children With JIA and in Controls For the children with JIA, angle-specific 90 maximal isometric plantarflexor torques were 34% greater than the maximal shortening torques. JIA lengthening torques of the plantarflexors were significantly greater than both isometric (28%) and shortening (52%) plantarflexor torques (table 4, fig 2). Angle-specific maximal isometric dorsiflexor actions were 57% stronger than shortening dorsiflexor actions. Maximal dorsiflexion torques were significantly greater during lengthening than during isometric (23%) as well as maximal shortening (56%) actions (table 4, fig 2). For the controls, maximal isometric plantarflexor actions were 29% greater than the maximal shortening actions. Anglespecific lengthening torques of the plantarflexors were significantly greater (30%) than shortening plantarflexion torques at Table 3: Maximal Torque (Strength) for Isometric, Shortening, and Lengthening Actions of the Plantar- and Dorsiflexors for the Children With Full ROM and for Those With Less Than 15 of Dorsiflexion Action Type JIA With Full ROM (n 5) JIA With 15 Dorsiflexion (n 5) Isometric plantarflexion Shortening plantarflexion Lengthening plantarflexion Isometric dorsiflexion to to 13.9 Shortening dorsiflexion to to 5.8 Lengthening dorsiflexion to to 20.1 NOTE. There were no significant torque differences between the groups for any action type.

4 ANKLE STRENGTH IN JIA, Broström 1227 Table 4: Maximal Torque (Strength) for Isometric, Shortening, and Lengthening Actions of the Plantar- and Dorsiflexors for 10 Children With JIA and 10 Healthy Children Matched for Gender and Age Action Type JIA Controls Isometric plantarflexion Shortening plantarflexion * * Lengthening plantarflexion * Isometric dorsiflexion to to Shortening dorsiflexion * 0 to * 7.3 to Lengthening dorsiflexion to to NOTE. ANOVA revealed a significant interaction (F 2, ) between group, muscle group, and action type. P values between the 2 groups were significant at P.025 after Bonferroni adjustment. *Shortening torques were significantly less than isometric torques; lengthening torques were significantly greater than both isometric and shortening torques. P 90 ; no significant difference was observed between isometric and lengthening plantarflexion torques (table 4, fig 2). Dorsiflexion torques were significantly greater during lengthening than during shortening actions (42%), whereas maximal isometric dorsiflexion torques were 28% greater than those produced during shortening dorsiflexion. Torque Comparisons Between Muscle Groups in Children With JIA and in Controls Both the children with JIA and the controls were approximately 4 times stronger when performing isometric plantarflexions than when performing isometric dorsiflexions (table 4). Shortening plantarflexion torques for both groups also exerted approximately 5 times more torque than shortening dorsiflexions, and lengthening plantarflexions also exerted approximately 4 times more torque than lengthening dorsiflexions (table 4). DISCUSSION The main finding of the study was that ankle extensor and flexor strength in children with JIA was approximately 40% to Fig 2. Angle-specific maximal voluntary torque (strength) measured at an ankle angle of 90 during isometric, shortening, and lengthening plantar- and dorsiflexor actions in children with JIA (n 10, black bars) and healthy, age- and gender-matched controls (n 10, white bars). 50% less than that of healthy, age- and gender-matched controls. Furthermore, it appears that previously measured isometric strength of the ankle plantarflexors in both healthy children and children with JIA has been considerably underestimated. Strength Differences in JIA Children With Different ROM The reduced dorsiflexion ROM in 5 children with JIA was clearly related to the severity of the disease, where contractures of the plantarflexors had reduced the ability of the children to flex their ankle joint beyond the 90 position. Although the differences between the 2 JIA groups did not reach statistical significance, the children with the reduced ROM were consistently weaker in all measurements. This may be a consequence of the shorter time given to develop maximal torque before the measurement was taken. Strength Differences Between Children With JIA and Controls Apart from lengthening (eccentric) plantarflexion, the children with JIA exerted less maximal voluntary torque for all strength measures than the age- and gender-matched healthy controls. It is important to note that the 2 groups of children were matched not only for gender and age; but also for body mass. As such, the lower strength measures of the children with JIA point toward a significant reduction in their functional strength compared with the healthy children. If the JIA children had proportionally lower body mass than the controls, then it could have been argued that, despite being weaker, the children with JIA had the same relative strength. The finding of reduced isometric and shortening (concentric) ankle dorsiflexor strength in children with ankle arthritis has been reported by Lindehammar and Backman. 12 In that study, the assumed reduced strength was thought to be related to the reduced muscle thickness of those muscles closest to the inflamed joint. However, the presentation of their results as measures of force, and not torque, makes any direct strength comparisons to the study unfeasible. A more recent study 13 has also shown a significant decrease in both isometric plantar- and dorsiflexion torque in children with JIA compared with healthy children. In that study, age- and gender-matched children with JIA were significantly weaker when attempting to produce maximal isometric plantar- and dorsiflexions against a handheld dynamometer. Care should be taken in interpreting those data, 13 because the strength values reported were approximately 50% lower than those found in our study for isometric dorsiflexion torque and 4 times lower for isometric plantar-

5 1228 ANKLE STRENGTH IN JIA, Broström flexor torque. Such low torque values are likely due to the methodologic difficulty of accurately measuring ankle flexor and extensor strength using a hand-held dynamometer. 42 The only additional study assessing isokinetic ankle plantar- and dorsiflexion strength in children 41 reported peak lengthening torques for healthy children of about the same age and weight to be approximately 18% lower than the angle-specific values found here. Damiano et al 41 also reported peak torque values for children with spastic cerebral palsy (CP) that were approximately 40% lower than the angle-specific torques for the children with JIA measured in our study. Their reported shortening torques were also lower ( 28% for controls, 20% for children with CP) than the shortening torques presented here. However, this reduction was to be expected, because the test velocity for their shortening and lengthening trials was twice that of our study (ie, 30 /s). Unfortunately, in the Damiano study, no isometric plantar- or dorsiflexion tests were made. Strength Differences Among Isometric, Shortening, and Lengthening Actions Measuring the torque isometrically at 90, or as the ankle passed through 90 for the dynamic measures, makes certain that all measurements are made at the same muscle length. Torque comparisons can therefore be made between action types without the confounding effect of different muscle forces being produced at different lengths. It is well known that it is possible for adults to develop greater maximal plantar- and dorsiflexor torque measures isometrically than when the same active muscle is undergoing shortening. 30 This phenomenon is also well established for other muscle groups, such as the knee and elbow flexors and extensors. 31,43 There is, however, limited information on this relationship in both healthy children and children with musculoskeletal disease. The significant reduction in shortening torque is velocity dependent 41 and the 29% to 34% lower shortening torques at a shortening velocity of 15 /s for the children with JIA and healthy controls, respectively, are relatively comparable to the reduction of the torques seen in healthy adults at the same velocity. 30 Interestingly, there was a much larger reduction in shortening torque compared with isometric for dorsiflexion for both groups ( 60% 70%). Although it is clear from the literature that maximal voluntary lengthening actions produce significantly greater torque than shortening actions at the same velocity, there is still a great deal of controversy as to whether, under voluntary activation, maximal lengthening actions produce greater torques than maximal isometric actions. In our study, the children with JIA were able to produce significantly higher lengthening torques than isometric torques for both plantar- and dorsiflexions, whereas the healthy control children could not. This finding may indicate a removal of involuntary inhibition during lengthening actions in children with JIA. However, caution must be taken when interpreting this result, because it may be that the maximal voluntary isometric torques produced by the children with JIA were influenced by inhibitory factors and were lower than what the muscle was truly capable of producing. Strength Comparisons Between Muscle Groups The strength similarity or difference between groups of muscles crossing opposite sides of the same joint has been of interest to many researchers. Both injury prediction and rehabilitation programs have focused on strength ratios between muscle groups, with the quadriceps/hamstring strength ratio being the most commonly investigated. 37,44-46 To our knowledge, only limited information is available about the relationship between plantar- and dorsiflexor strength in children. 13,41 In the study by Hedengren et al, 13 using isometric measurements of the plantar- and dorsiflexors with a hand-held dynamometer, it was reported that the plantarflexor strength in children with JIA and in healthy controls was approximately 30% to 35% greater than that of the dorsiflexors. However, these values appear to be severely underestimated; 2 studies using isokinetic dynamometry to measure lower leg strength in children (our study and the study by Damiano 41 ) show the plantarflexors as being capable of producing approximately 4 to 5 times more torque (400% to 500% stronger) than the dorsiflexors. Interestingly, this ratio was more or less the same for isometric and dynamic muscle actions. Furthermore, the ratios were essentially the same for the children with JIA and for the healthy controls. This could be interpreted to mean that the arthritic disease affects muscle strength equally on both sides of the joint, and therefore any rehabilitation of the lower leg in children with JIA should include approximately equal activation of both muscle groups. Functional Implications of Lower-Leg Weakness Increased functional weakness of the lower-limb muscles is likely to induce a vicious circle of further inactivity and even more muscle weakness. Regrettably, such a downward spiral of strength and inactivity is likely to be greatest for the lower-limb muscles, because it has been shown that these muscles are the most prone to atrophy with inactivity. 47 As partial evidence to support this speculation, children with JIA have been reported to have significantly lower levels of physical activity than healthy children of the same age and, in our study, reduced lower-limb strength. Unfortunately, it is unclear whether strength training or the performance of weight-bearing exercises reduces inflammatory activity or radiologic progression of joint disease in either adults or children with arthritis. It is therefore suggested that future studies evaluate the feasibility and potential effect of lower-limb strength training exercises, during nonactive periods of inflammatory disease, on lower-limb strength and overall physical activity levels in children with JIA. Strength Measurement Considerations Although the strength values given here are several times greater than those reported earlier, 13 it must be recognized that the present values may still underestimate the maximum torque that the investigated muscle groups can produce. Assessment of the absolute torque-producing capacity of the muscle can be done only by using involuntary supramaximal electric stimulation of the innervating peripheral nerve superimposed on an ongoing voluntary muscle action. 44 Unfortunately, such stimulation protocols are associated with short but intense discomfort due to the electric stimulation of sensory skin receptors. It should be pointed out that measurement of voluntary strength in the clinic needs to be made in a manner that has high validity and reliability. Although the commonly used hand-held dynamometer has been shown to have satisfactory test-retest reliability, 13,27 the problems associated with test validity cannot be overlooked. Difficulties in stabilizing the patient, accurately assessing and maintaining joint position, measurements of lever arms, and, above all, using the examiner s own strength as resistance add to large inaccuracies in strength measurement. Hence, if strength assessment is to be used as a valid tool for measuring the severity and time course of a disease, then more accurate methods of strength assessment need to be used.

6 ANKLE STRENGTH IN JIA, Broström 1229 CONCLUSIONS Girls with JIA have significantly less plantar- and dorsiflexor strength than age-matched healthy peers. Because this weakness cannot be explained by a relatively lower body mass, it is believed to be of functional relevance. The reduced strength in the children with JIA is likely to affect function in daily activities, such as walking, running, and standing, and probably contributes to the reduced levels of physical activity seen in children with this disease. Although the functional benefits of strength training of the plantar- and dorsiflexors have yet to be evaluated, accurate strength assessments need to be made throughout the disease period to assess the progress of the disease and the efficacy of treatments. References 1. Arguedas O, Fasth A, Andersson-Gare B, Porras O. Juvenile chronic arthritis in urban San Jose, Costa Rica: a 2 year prospective study. J Rheumatol 1998;25: Andersson GB. 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7 1230 ANKLE STRENGTH IN JIA, Broström 43. Kawakami Y, Kanehisa H, Ikegawa S, Fukunaga T. Concentric and eccentric muscle strength before, during and after fatigue in 13-yearold boys. Eur J Appl Physiol Occup Physiol 1993;67: Kannus P. Hamstring/quadriceps strength ratios in knees with medial collateral ligament insufficiency. J Sports Med Phys Fitness 1989;29: Aagaard P, Simonsen EB, Magnusson SP, Larsson B, Dyhre- Poulsen P. A new concept for isokinetic hamstring: quadriceps muscle strength ratio. Am J Sports Med 1998;26: Burnie J, Brodie DA. Isokinetic measurement in preadolescent males. Int J Sports Med 1986;7: Akima H, Kuno S, Suzuki Y, Gunji A, Fukunaga T. Effects of 20 days of bed rest on physiological cross-sectional area of human thigh and leg muscles evaluated by magnetic resonance imaging. J Gravit Physiol 1997;4: Suppliers a. Maywood Instruments Ltd, 17 Stadium Way, Tilehurst, Reading, RG30 6BX, England. b. Vishay Nobel, AB Box 423, Karlskoga, Sweden. c. Digitimer Ltd, 37 Hydeway, Welwyn Garden City, Hertfordshire, AL7 3BE, England. d. Cambridge Electronic Design Ltd, Science Pk, Milton Rd, Cambridge, CB4 0FE, England. e. StatSoft Inc, 2300 E 14th St, Tulsa, OK