Reliability of isokinetic strength measurements of the knee in children with cerebral palsy

Transcription

1 Reliability of isokinetic strength measurements of the knee in children with cerebral palsy Moshe Ayalon* PhD; David Ben-Sira PhD; Yeshayahu Hutzler PhD, The Zinman College of Physical Education and Sport Sciences at the Wingate Institute; Talila Gilad MD, Hilel Yafe Medical Center, Hadera, Israel. *Correspondence to first author at Life Science Department, The Zinman College of Physical Education and Sport Sciences, Wingate Institute 42902, Israel. This study evaluates the reliability of isokinetic testing of the knee flexors and extensors in children with cerebral palsy (CP). Twelve children (seven girls, five boys), aged 9 to 15 years, participated in this study. The children s strength was measured bilaterally, using a dynamometer. Each participant was tested twice at an interval of 1 week. During each session, the participant performed five consecutive cycles of knee extension and flexion. Testing velocity was set at 90 /s, and the range of motion was 80, starting with the knee flexed at 80 and ending in full extension. The measured variable was the peak torque. Reliabilities were determined using intraclass coefficient (ICC) with two-way ANOVA model. The ICCs for individual sessions range from 0.90 to ICCs for the eight repetitions over the two sessions range from 0.95 to The findings indicate that measuring isokinetic strength in the tested population is highly reliable and should be considered in rehabilitation protocols. Manual muscular testing is widely used by clinicians as one of the procedures to assess motor dysfunction. However, when an exact definition of muscular force is required, employment of quantitative methods is recommended for either diagnosis or follow-up, especially in populations with muscular pathology (Cabry 1991). For example, selective dorsal rhizotomy (SDR) has been used to reduce spasticity in the lower limbs among children with cerebral palsy (CP) (Cahan et al. 1987, Oppenheim 1990, Park 1993). Oppenheim (1990) reported an association between weakness of the antigravity muscles and unsatisfactory outcome results of SDR and suggested it may be a contraindication to surgery. Thus, an accurate and reliable assessment of preoperative muscular function is warranted. In recent years physical activity has increased in both adults and children with CP as a result of better equipment and training protocols. This trend includes an increase in the proportion of the population with CP taking part in strengthtraining programs. Holland and colleagues (1990) reported that intensive strength training, combined with appropriate stretching routines before and after training, resulted in an improved performance among athletes with CP. McCubbin et al. (1985) found a positive effect of isokinetic strength training on reaction time and the rate of torque development among individuals with CP aged 10 to 20 years. Such effects are similar to those frequently observed in participants without neurological disorders. Isometric evaluation, using a hand-held dynamometer, was conducted by Damiano and Abel (1998) to assess the outcome of strength training in children with CP. Although isometric resistance can be used to assess strength in a muscle group around a joint with a limited range of motion (ROM), it does not provide detailed information on the dynamic characteristics of the muscular strength throughout the full ROM. Such information can be obtained by dynamic devices such as isokinetic machines. Isokinetic devices provide exact and reliable monitoring of individual progress during a training program even at very limited ranges of muscular strength. Isokinetic muscular training has a distinct advantage over other modes of strength training as maximal torque can be generated throughout the whole ROM. In addition, isokinetic dynamometers are relatively safe because the resistance is adjusted to the participant's effort by a measuring device. Thus, when limiting factors such as pain or discomfort are suddenly felt, the resistance is immediately reduced and the risk of injury minimized. Force curves over the ROM are displayed in most isokinetic devices, providing visual feedback to the exercising individual. This quality is particularly valuable for motor control and motivational purposes in children with CP who often exhibit proprioceptive and attention disorders. A critical aspect in selecting a measuring procedure is its reliability. The reliability of isokinetic strength testing of knee flexors and extensors is widely documented in the literature, especially for adult participants without CP. Testing procedures for concentric contractions of knee extensors and flexors were reported to be highly reliable across a wide range of angular velocities (Perrin 1993). Molnar et al. (1979) investigated the reliability of isokinetic testing on a variety of muscle groups of the lower and upper extremities in children aged 7 to 15 years. They concluded that isokinetic testing is simple to conduct and highly reliable for typically 398 Developmental Medicine & Child Neurology 2000, 42:

2 developing children with normal intelligence as well as children with minor learning disability. Burnett et al. (1990) reported an intraclass correlation coefficient (ICC) of 0.84 for hip extensors in children aged 6 to 10 years. Very limited information is available on the reliability of isokinetic testing among participants with neurological disorders. For example, Holland et al. (1994) concluded that isokinetic testing is reliable for assessing the knee flexors and extensors at 60 /s among adults with CP. To date, only moderate reliability has been established for isokinetic assessment of the knee extension and knee flexion of children with CP, tested at 30 /s (van den Berg-Emons et al. 1996). The reliability of isokinetic strength measurement at higher angular velocities has not been documented. High angular velocities are typical in everyday and sporting activities and, therefore, should be included in exercise protocols. This study aimed to estimate the reliability of isokinetic testing of the knee flexors and extensors in children with CP during concentric contraction at an angular velocity of 90 /s. Method PARTICIPANTS Seven girls and five boys aged 9 to 15 years participated in the study, after their parents gave written consent. The children were diagnosed with varying degrees of CP classified according to the criteria employed by Bax (1964) and Bleck and Nagel (1982). The characteristics of the participants are specified in Table I. At the time of the study the participants were enrolled in a specially designed physical activity program at the Zinman College for Physical Education and Sport Sciences. Each child attended a 2-hour exercise session twice a week. The program included swimming, rehabilitative gymnastics, and aerobic training. TEST PROTOCOL The strength of the knee flexor and extensor was measured bilaterally, using a Cybex II isokinetic dynamometer (Division of Lumex, Inc., Bay Shore, NY, USA), interfaced with a microcomputer. The dynamometer shaft was aligned with the axis of rotation while the participant sat with their back at a vertical position and the hip at 90. The participants were strapped to the chair at the chest, pelvis, and thigh. Additional back support was provided to accommodate the participant's size. The support was stabilized with a strap. The lever of the dynamometer was individually adjusted to the size of the shank to match the standard Cybex protocol. The measured variable was the peak torque generated during maximal effort. Software developed by the authors was employed to record the changes in angles and torques as a function of time. Torque measurements were corrected for gravity using the procedure suggested by Herzog (1988). Each participant reported to the laboratory for two testing sessions at an interval of 1 week. The same examiner (MA) conducted all tests. At each session, participants initially made 15 to 20 submaximal repetitions to familiarize themselves with the equipment and the specific testing velocity. After a resting period of 3 minutes, the participant performed five consecutive cycles of knee extension and flexion movements. Testing velocity was set at 90 /s, and the ROM was 80, starting with the knee flexed at 80 and ending in full extension. In a study by van den Berg-Emons et al. (1996), a large portion of the sample of children with CP failed to perform a meaningful isokinetic effort at ranges of 30 /s to 120 /s. To select an appropriate velocity for this study, a pilot study was conducted in which 90 /s was found to be most comfortable for the participants. Participants were instructed to perform both flexion and extension movements at their maximal effort. DATA ANALYSIS Analysis was based only on the last four repetitions. The first repetition was omitted because it characteristically produces lower peak torques during extension, as it starts from a static position. Data were analysed separately for the non-affected and affected legs. In the participants with diplegia, the highest extension peak torque determined dominance. However, the focus of the analysis was on the affected leg, which is the extremity that is affected by neurological disorders among the participants with hemiplegia, and is most affected by neurological disorders among those with diplegia. Reliabilities were determined using ICC with a two-way ANOVA (Baumgartner 1989). This was used for each of the two sessions separately (four repetitions for each session) as well as to all eight repetitions over the two testing sessions. Reliabilities were estimated for each session separately to determine consistency within a single session, while the overall reliability was selected to reflect consistency across sessions. Reliabilities were computed for both absolute peak torques (N m) as well as relative peak torques (N m body weight 1 ). Interclass reliability is designed to estimate the quality of the measuring instrument in discriminating among different participants. However, it is not an index of the internal consistency within each participant. Due to the uniqueness of the population, which is known to have difficulty in reproducing gross motor behavior, the individual s standard deviation (SD) and coefficient of variation (CV) were computed to evaluate consistency for each of the participants. These measures were computed for each session separately (four repetitions per session) as well as for the two sessions together (eight repetitions over two sessions). In addition, two-way ANOVAs (trials by sessions) were used to determine whether a training effect existed from the first session to the second one or within the testing sessions. Table I: Characteristics of the participants Participant Age Weight Height Sex Type of CP Classifi- (y) (kg) (cm) cation F Right hemiplegia M Left hemiplegia M Right hemiplegia M Right hemiplegia M Ataxia F Spastic diplegia F Spastic diplegia F Dyskinesia F Spastic diplegia M Dyskinesia F Left hemiplegia F Left hemiplegia 7 Isokinetic Strength Measurements in Children with Cerebral Palsy Moshe Ayalon et al. 399

3 Results Personal mean peak torque (MPT), SD, and CV were calculated for each of the two muscle groups at the two sessions (Table II). The correlation between overall MPT and body weight were 0.75 and 0.69 for extension and flexion of the affected leg respectively. To accommodate for this relation, MPTs are also presented as relative mean peak torque (RMPT), i.e. MPT body weight 1. MPTs ranged between 6.21 to N m and 5.06 to N m for extension and flexion respectively. The RMPTs are in the range of 0.27 to 1.36 N m body weight 1 and 0.13 to 0.65 N m body weight 1 for extension and flexion respectively. Participant 9 was not able to develop measurable flexion torques at both legs on any of the sessions. This participant was omitted from the analysis of reliability to avoid a bias that may cause an overestimation of the actual reliability coefficient. The descriptive individual statistics for eight trials are presented in Table III. The CVs were in the range of 8.03% to 41.53% (mean 19.0%) and 8.12% to 55.03% (mean 24.37%) for extension and flexion of the affected leg respectively. ICCs between the four trials of each of the sessions as well as for all eight trials are presented in Table IV, for both MPT and RMPT. The ICCs for individual sessions range from 0.90 to ICCs for the eight repetitions over the two sessions range from 0.95 to Two-way ANOVAs with repeated measures (four repetitions two sessions) were conducted separately for flexion and extension using both MPT and RMPT. Neither significant differences between sessions or trials, nor any significant interaction effects, were found. Discussion This investigation was designed to estimate the reliability of isokinetic testing in children with CP to establish the appropriateness of such testing in clinical practice. It was not intended to develop norms of strength for this population. Participants did not report any pain or discomfort during or following the testing session. In some cases participants had difficulty retaining a smooth, repetitive movement across the range of motion during the initial trials of the first training session but overcame this difficulty before the recorded tests took place. The moderately high correlations between MPT and body weight suggest that relative strength should be preferred over absolute values. Such practices are also recommended for healthy populations to even out the strength body size relationship (Dvir 1995). The findings indicate that measuring isokinetic strength in the tested population is highly reliable. The difference in Table II: Mean (SD) and coefficient of variation of extension and flexion peak torque of affected leg at two sessions of trial for each participant Subject Mean Peak Torque Relative Mean Peak Torque Session I Session II Session I Session II (Nm) CV (Nm) CV (Nm BW 1 ) (Nm BW 1 ) Mean (SD) (%) Mean (SD) (%) Mean (SD) Mean (SD) Extension (4.50) (2.00) (0.09) 1.02 (0.04) (3.28) (2.46) (0.08) 0.71 (0.06) (2.88) (7.92) (0.04) 0.65 (0.11) (0.66) (6.27) (0.02) 0.58 (0.19) (2.34) (3.64) (0.09) 0.89 (0.14) (2.76) (1.38) (0.12) 1.00 (0.06) (0.88) (0.88) (0.04) 0.39 (0.04) (3.08) (3.52) (0.14) 1.36 (0.16) (0.23) (1.38) (0.01) 0.36 (0.06) (6.63) (0.78) (0.17) 1.26 (0.02) (2.88) (0.96) (0.06) 0.81 (0.02) (2.94) (3.92) (0.06) 0.77 (0.08) Mean (2.76) (2.93) (0.08) 0.82 (0.08) Flexion (3.50) (6.00) (0.07) 0.65 (0.12) (0.41) (0.82) (0.01) 0.42 (0.02) (4.32) (3.60) (0.06) 0.35 (0.05) (3.30) (7.26) (0.10) 0.59 (0.22) (1.04) (3.64) (0.04) 0.73 (0.14) (0.92) (2.30) (0.04) 0.47 (0.10) (1.10) (0.66) (0.05) 0.56 (0.03) (3.96) (5.72) (0.18) 0.38 (0.26) 9 0 (0) 0 (0) 0 (0) 0 (0) (1.17) (1.17) (0.03) 0.60 (0.03) (1.44) (3.36) (0.03) 0.22 (0.07) (4.41) (2.45) (0.09) 0.37 (0.05) Mean (2.13) (3.08) (0.06) 0.45 (0.09) CV, coefficient of variation; BW, body weight. 400 Developmental Medicine & Child Neurology 2000, 42:

4 reliability between the RMPT and the MPT demonstrate that MPT variance is affected by variance in body size. On the other hand, RMPT values reflect strength capacity after adjusting for differences in body size. In spite of the lower reliability coefficients relative to those of MPT, their order of is as high as other values reported in the literature on normally developing populations (Perrin 1993). Reliability of such high magnitude suggests using isokinetic testing and training in children with CP at an angular velocity of 90 /s. This finding contradicts those of van den Berg-Emons et al. (1996). Their study included measurements at 30 /s, 60 /s, and 120 /s with two repetitions at each velocity. They reported moderate and non-significant reliabilities at angular velocities of 60 /s (r=0.75 for flexion, r=0.55 for extension) and 120 /s (r=0.65 for flexion, r=0.42 for extension). A possible explanation for this contradiction is that reliability in their study was based only on two repetitions, which were performed on the same day. In contrast, reliability in this study is based on a larger number of repetitions performed on two different days. The larger sampling of the participants capacity in this study is the source of the higher reliability that was manifested by the ICC. Their report includes tests of aerobic and anaerobic power for the same participants. It is possible that the multitude of tests might have caused fatigue or affected the concentration of the participants. The participants in this study represent a heterogeneous sample of children with CP. Caution is required when using muscle strength as a measure of changes in functional capability. In these cases, group norms may prove irrelevant to the patient s progress. It is recommended that each child with CP is monitored in relation to a baseline established at the start of their individual program. When measuring strength in a population without CP, a CV up to 13% is expected over different sessions (Gleeson and Mercer 1992). Table II shows that lower mean CV values were obtained in the MPT of the extensors than in the MPT values of the flexors. The latter exceed the expected CV values. It is possible that the flexors are less frequently used at high levels of contraction, especially among the participants with CP. High personal CV indices indicate instability in the performance of an individual participant. Such instability is a result of one or more extreme values. Extreme MPT values indicate that the participant could not perform consistently at their full potential throughout all the trials. An extremely low MPT value may indicate a temporary neurological disorder, or lack of concentration as a limiting factor. If the results are characterized by a gradual decrease of the peak torque, it may be attributed to fatigue. Fatigue trends were not observed in any of the participants of the current study. The individual CVs in Table II reveal that some participants exhibit large inconsistency in their performance either on the same day or between the two testing sessions. Some of these participants exhibited improvement from the initial trials to the later ones. Such improvement probably signifies late habituation on the part of the participant. Other participants maintained a relatively constant level of MPT while in others no specific trend could be established. The interindividual variability in the MPT pattern across trials results in obstruction of any general trend between trials or sessions, as reflected in the non-significant results of the ANOVA. The wide range of intraindividual inconsistencies are not large enough to mask differences between participants with different MPT levels. This is reflected in the high reliability coefficients. CV values can be used to measure the efficiency of treatment of a rehabilitation process. It can be said that the response to a treatment (training) is better when the MPT is higher and the CV values lower. This parameter coincides with the definition of motor skill, which includes a high level of consistency in performance. Table III: Mean (SD) of extension and flexion peak torque of affected leg for all eight trials of two sessions for each participant Participant Mean peak Relative mean CV torque peak torque (Nm) (Nm BW 1 ) Mean (SD) Mean (SD) Extension (3.97) 0.99 (0.08) (3.73) 0.66 (0.09) (10.56) 0.77 (0.15) (7.50) 0.76 (0.23) (3.14) 0.85 (0.12) (4.74) 0.83 (0.21) (5.78) 0.63 (0.26) (3.46) 1.30 (0.16) (1.46) 0.32 (0.06) (5.14) 1.21 (0.13) (7.75) 0.67 (0.16) (4.04) 0.82 (0.08) Mean (5.11) 0.82 (0.14) Flexion (5.55) 0.60 (0.11) (1.62) 0.39 (0.04) (4.46) 0.38 (0.06) (5.34) 0.62 (0.16) (3.41) 0.66 (0.13) (3.44) 0.35 (0.15) (0.98) 0.55 (0.04) (4.65) 0.38 (0.21) (0) 0 (0) (2.77) 0.55 (0.07) (3.38) 0.18 (0.07) (3.64) 0.38 (0.07) Mean (3.27) 0.42 (0.09) CV, coefficient of variation; BW, body weight. Table IV: Intraclass coefficient (ICC) between four repetitions during each session Session I II I+II MPT extension <0.001 MPT flexion <0.001 RMPT extension <0.001 RMPT flexion <0.001 MPT, mean peak torque; RMPT, relative mean peak torque. p Isokinetic Strength Measurements in Children with Cerebral Palsy Moshe Ayalon et al. 401

5 The results of this study are reported in terms of MPT or RMPT. An alternative measure of performance is total work (defined as the product of torque ROM). It can be argued that this variable is more relevant under the assumption that it better represents the participant's capacity throughout the whole ROM. MPT was preferred, however, because in certain cases participants demonstrated difficulties in maintaining maximal effort across the full ROM without affecting MPT. This phenomenon is known as work deficiency (Perrin 1993). However, this observation does not exclude the potential usefulness of work to be clinically used in following the participant's progress in a rehabilitation program. The clinical potential of this parameter should be carefully considered in future work. Accepted for publication 19th November References Baumgartner TA. (1989) Norm-referenced measurement: reliability. In: Safrit MJ, Wood TM, editors. Measurement Concepts in Physical Education and Exercise Sciences. Champain, IL: Human Kinetics Books. Bax MCO. (1964) Terminology and classification of cerebral palsy. Developmental Medicine & Child Neurology 6: Bleck EE, Nagel RA. (1982) Physically Handicapped Children A Medical Atlas for Teachers. Boston, MA: Allyn and Bacon. Burnett CN, Betts EF, King WM. (1990) Reliability of isokinetic measurements of hip muscle torque in young boys. Physical Therapy 70: Cabry JMH. (1991) Isokinetic strength aspects in human joints and muscles. Applied Ergonomics 22: Cahan L, Kundi M, McPherson D, Starr A, Peacock W. (1987) Elecrophysiologic studies in selective dorsal rhizotomy for spasticity in children with cerebral palsy. Applied Neurophysiology 50: Damiano DL, Abel MF. (1998) Functional outcomes of strength training in spastic cerebral palsy. Archives of Physical Medicine and Rehabilitation 79: Dvir Z. (1995) Isokinetics: Muscle Testing, Interpretation and Clinical Applications. Edinburgh: Churchill Livingstone. Gleeson NP, Mercer TH. (1992) Reproducibility of isokinetic leg strength and endurance characteristics of adult men and women. European Journal of Applied Physiology and Occupational Physiology 65: Herzog W. (1988) The relation between the resultant moments at a joint and the moments measured by an isokinetic dynamometer. Journal of Biomechanics 21: Holland L, McCubbin J. (1994) Reliability of concentric and eccentric muscle testing of adults with cerebral palsy. Physical Activity Quarterly 11: Holland LJ, Steadward RD. (1990) Effects of resistance and flexibility training on strength, spasticity/muscle tone and range of motion of elite athletes with cerebral palsy. Palaestra 6: McCubbin JA, Shasby GB. (1985) Effects of isokinetic exercise on adolescents with cerebral palsy. Adapted Physical Activity Quarterly 2: Molnar GE, Alexander J, Gutfeld N. (1979) Reliability of quantitative strength measurements in children. Archives of Physical Medicine and Rehabilitation 60: Oppenheim W. (1990) Selective posterior rhizotomy for spastic cerebral palsy. Clinical Orthopaedics and Related Research 253: Park TS, Gaffney PE, Kaufman BA, Molleston MC. (1993) Selective lumbosacral dorsal rhizotomy immediately caudal to conus medullaris for cerebral palsy spasticity. Neurosurgery 22: Perrin DH. (1993) Isokinetic Exercise and Assessment. Champain, IL: Human Kinetics Publishers. Van den Berg-Emons HJG, van Baak MA, de Barbanson DC, Speth L, Saris WHM. (1996) Reliability of tests to determine peak aerobic power, anaerobic power, and isokinetic muscle strength in children with spastic cerebral palsy. Developmental Medicine & Child Neurology 38: Developmental Medicine & Child Neurology 2000, 42: