Toe-walking in children with cerebral palsy: contributions of contracture and excessive contraction of triceps surae muscle.

Toe-Walking in Children with Cerebral Palsy: Contributions of Contracture and Excessive Contraction of Triceps Surae Muscle

The study was designed to provide a quantitative analysis of toe-walking in children with cerebral palsy (CP). The total internal moment developed about the ankle joint during locomotion and the passive component of this internal moment were measured. The contributions of the active and passive components were expressed as the ratio (R) between the passive moment and the total internal moment. Measurements were compared for 13 children with CP and 5 healthy children. For the data analysis, the children with CP, exhibiting apparently similar toe-walking, were divided into two groups: 1) Group CPI and 2) Group CPII. Group CPI was characterized by a small ratio R value, which indicated the presence of excessive contractions of the triceps surae muscle during locomotion. In Group CPII, the ratio R value was abnormally high, which indicated that a contracture (ie, structural change of the muscle or the tendon) was entirely or at least partly responsible for toe-walking. Each group requires a different therapeutic strategy. [Tardieu C, Lespargot A, Tabary C, et al: Toe-walking in children with cerebral palsy: Contributions of contracture and excessive contraction of triceps surae muscle. Phys Ther 69:656-662, 1989] Key Words: Cerebral palsy, gait; Contracture; Kinesiology/biomechanics, gait analysis; Locomotion. Toe-walking in children with cerebral palsy (CP) has been examined in several studies, most of which have used a combination of electromyography and temporal changes in the angles of the ankle, knee, and hip joints.[1-3] Unfortunately, these authors used the term "spasticity" without clearly defining the term's specific meaning. Some surgeons have even presented their results on the correction of "toe-walking" without any analysis of the disorder. Lee and Bleck have clearly stated that only "toe-walking due to spasticity without contracture was treated surgically,"[4] and Conrad and Bleck reported on "children with dynamic equinus who exhibit toe-walking but who have no fixed equinus deformity."[5] These authors made judgments about gait based on contracture measurement when the hindfoot was inverted and the knee fully extended, even though these knee and inversion angles are different from those that occur in locomotion. Measurements made under these conditions render diagnosis difficult. During locomotion, the segments of the lower limb are subjected to external and internal forces that produce moments about the involved joints. The relationship between the moments attributable to external forces in healthy adult subjects has been studied thoroughly by several groups since Bresler and Frankel.[6] The total internal moment developed about a joint results from two different mechanisms: 1) the contraction of muscles crossing the joint and 2) the viscoelastic properties of the muscles, ligaments, and soft tissues surrounding it. Until recently, little information was available on the contribution of these components to the total internal moment about a joint. Morrison emphasized the part played by the passive component (PM),[7] whereas Siegler et al clearly distinguished between the active and the passive components of the internal moment about the ankle joint.[8] Siegler et al measured these variables on four healthy adults and one adult with a "mild equinus ankle deformity."[8] They found that the ratio between the passive component of the total moment and the total moment was very high in the subject with the pathological condition. This study was designed to provide a quantitative analysis of toe-walking in children with CP. The total internal moment during locomotion and the passive component of the total internal moment were measured. The contributions of the active and passive components are expressed as the ratio between the passive component and the total internal moment. These data were used to examine the following hypotheses: 1. In certain cases, contracture may

explain toe-walking; in these

subjects, the passive moment:total

moment ratio should be high, as

observed by Siegler et al.[8] 2. In other cases, toe-walking may

result from excessive contractions

of the triceps surae muscle,

producing a low passive moment:total

moment ratio. Examination of the

hypothesis required the

comparison of children with CP and

healthy children, the latter of

whom were required first to walk

normally and then to walk on their

toes. As toe-walking of healthy

children with structurally normal

muscle is only possible with a strong

contraction of their triceps surae

muscle, we believe that such

walking is a good model for children

with CP having excessive

contractions of the triceps surae muscle

during locomotion.

Method

Subjects

A total of 18 children (9 boys, 9 girls), aged 6 to 15 years, were examined. The 13 children with CP were accustomed to walking unaided. They suffered from hemiparesis (n = 6), mild diparesis (n = 6), and mild quadriparesis (n = 1). All of them exhibited a toe-walking gait pattern. Hemiparetic subjects were checked to ensure that any difference in the length of the two lower limbs was not the cause of toe-walking. Five healthy children were tested, first in normal walking and then in toe-walking. We believe that this latter condition, as we previously stated, is a suitable model for certain children with CP. All subjects walked barefooted. Informed consent was obtained from each subject's parent or guardian.

Procedure and Data Analysis

Two tests were performed. The first test was a locomotion experiment in which the kinetic and dynamic variables required for the calculation of the total internal moment about the ankle joint were measured. In the second test, the passive component of the internal moment was determined. Moments were measured at the ankle joint, perpendicular to the sagittal plane of the foot and the motions of plantar flexion and dorsiflexion.

Measurement of total internal moment during stance phase of gait cycle. Each child was required to cross the walkway at his or her normal speed. A strain-gauge force plate was incorporated into the walkway. At foot contact, it provided measurement of the vertical and the anterior-posterior forces as well as the anterior-posterior position of the center of pressure (Fig. 1). Strobophotogrammetry was chosen for measurement of cinematic variables. Cinematic and force-plate measurements were synchronized ([+ or -] 5 msec). Strobophotogrammetry was chosen because the reflective markers borne by the child are small (1.0 [mm.sup.2]) and cause no discomfort.[9] These considerations are important because some children with CP respond to constraint with involuntary contractions. A marker was placed at the approximate axis of rotation of the ankle and another at the head of the fifth metatarsal bone. The coordinates of these points were necessary for the calculation of moment. Other markers were placed to measure hindfoot inversion or eversion and knee angles during locomotion (the terms "inversion" and "eversion" are applied to posterior-view rotation between the shank and the heel). These angles were used in the second test. The method allows measurement of the inversion-eversion angles, which are in a plane perpendicular to the sagittal plane. The method has two drawbacks: 1) The marker coordinates can be measured only at 50msec intervals; and 2) coordinate measurements are very cumbersome, thus only a few steps were analyzed for each child (the fourth step of a 10-step sequence and 2 or 3 sequences of a total of 10 to 15 sequences). We analyzed the gait of a child with CP only if the child's parents or physical therapist stated that the gait was typical for that particular child. Figure 1 shows simplified models of the lower limb and the foot. In these models, the lower limb is composed of rigid links and the ankle joint is an ideal frictionless hinge joint. Movement is only in the sagittal plane. Measurements and calculations were made for all subjects, which allowed us to verify that this approximation was acceptable for the 13 children with CP tested. The analytical expression of the total internal moment (TM) on the lateral axis in the approximate center of rotation of the ankle (A) is [Mathematical Expression Omitted] where XA and ZA are coordinates of [Mathematical Expression Omitted] in A, RX and RZ arae components of ground reaction forces, Xp is the abscissa of the center of pressure, m is the mass of the foot, g is the acceleration of gravity, I is the moment of inertia of the foot about its center of gravity, [Theta] is the angle between the [Mathematical Expression Omitted] acceleration of the foot, and d is the distance between the center of rotation of the ankle joint and the center of gravity of the foot. At a specific instant (designated as "instant S") occurring at approximately 55% through the [Mathematical Expression Omitted] [Mathematical Expressions Omitted] photogrammetric coordinates, were zero or very small. The components m, I, and d were calculated from direct measurements and from anthropometric tables for children.[10] The calculations [Mathematical Expressions Omitted] [Mathematical Expressions Omitted] between 0.01 and 0.06 N.m, and thus negligible. The term mdgsin [Theta], attributable to foot weight, was 0.4 to 0.9 N.m. This term was calculated for each total moment. The components Rz and m were standardized to the average body weight (30 kg) so that subjects of different weights could be compared. At instant S, the conditions were quasi-static and the absolute value of the total internal moment could be reduced to the moment of the ground reaction minus the moment attributable to the weight of the foot. The total internal moment was the sum of the passive and active components. The active component at instant S essentially was due to the triceps surae muscle; the moments of the synergists were comparatively very small; and, in the healthy children, the principal antagonist (tibialis anterior muscle) moment was zero. This muscle had no electrical activity at this instant.[1] The presence of activity and muscle moment cannot be excluded for children with CP. For the locomotion of a child with CP to be a true walk, the body weight had to be transferred onto the studied lower limb. This transfer was checked by calculating the Rz:body weight ratio for each stance phase and comparing it to the ratio obtained for healthy children. Measurement of passive component of internal moment. The test was performed with the subject lying on his or her side. The leg to be studied was placed in an apparatus previously described.[11] The passive moment of the triceps surae muscle was measured as a function of ankle angle in a horizontal plane. The state of relaxation was confirmed by the absence of myoelectric activity in the triceps surae, soleus, and tibialis anterior muscles. The horizontal plane chosen for each subject was the same as the sagittal plane used in the walking test, and the passive moment was not dependent on the body weight. The approximate axis of rotation of the ankle joint was aligned with the axis of rotation of the apparatus throughout the test. The calcaneum was placed in the position of the inversion or eversion angle, and the knee was maintained at the flexion angle measured during locomotion at instant S. The moment selected on the passive moment-ankle angle curve was the corresponding to the ankle angle at instant S. The moment on this curve corresponding to 10 degrees of plantar flexion ("moment C") also was examined. Moment C gave an approximate measurement of the structural contracture of the triceps surae muscle.

Results The influence of hindfoot inversion or eversion on the passive component of the internal moment was analyzed first. Two positions were imposed on a healthy child with the same knee flexion angle of 15 degrees: 1) hindfoot eversion of 8 degrees and 2) hindfoot inversion of 5 degrees. Figure 2 shows the two passive moment-ankle angle curves. For an ankle angle of 9 degrees of dorsiflexion (instant S), the passive moments indicated by the curves are very different (4 and 10 N*m). The importance of measuring this angle during walking at instant S and of passively imposing it for the second test was demonstrated. This measurement is even more necessary for children with CP because very abnormal values can be measured in this group; the extreme values were a hindfoot inversion of 25 degrees and a hindfoot eversion of 20 degrees (average eversion value in healthy subjects was 9 [degrees] [plus or minus] 3 [degrees]). The influence of the knee angle could be shown in the same way. The extreme values measured in the children with CP were a genu recurvatum of 12 degrees and a flexion of 53 degrees (average flexion value in healthy subjects was 14 [degrees] [plus or minus] 5 [degrees]). Figure 3 shows that the movement of the center of pressure throughout the stance phase of the gait cycle was very different in the healthy children and the children with CP. In the children with CP, the body weight was transferred during locomotion onto the studied lower limb in 23 of the 24 stance phases studied. The Rz:body weight ratio was not different from the ratio found for the healthy children. Only one sequence was eliminated because of such a difference. The Table presents the means and standard deviations for passive moment:total moment ratios for all the children. The most pertinent measurement is the ratio (R) between the passive moment (PM) and the total internal moment (TM) measured at the instant S (where R = 100 x PM/TM). For Group HI (healthy children, normal walking), the ratio R was 14.4 [plus or minus] 3.0. For Group HII (healthy children, toe walking), the passive moment (R numerator) was greatly reduced. The total internal moment (R denominator) was greatly reduced. The total internal moment (R denominator) was also reduced because, despite an equal ground reaction force, the difference between the abscissas of the center of rotation (XA) and of the center of pressure (Xp) decreased. These results were expressed by the ratio R of 3.5 [plus or minus] 4.0. For Groups HI and HII, the value of the passive moment C (an approximate measurement of the structural contracture of the muscle) was 0.3 [plus or minus] 0.2. The children with CP were divided into two groups according to the value of ratio R. Group CPI was characterized by a small value of ratio R (R = 1.4 [plus or minus] 2.0). The Table shows that this result essentially was due to a very low passive moment. The R numerator was very small, and the denominator was only slightly smaller than that of the reference group (Group HI). Toe-walking in this group was due to excessive contraction of the triceps surae muscle. This conclusion is confirmed by two other results: 1) Toe-walking healthy children, who obviously contracted their triceps surae muscle, produced a low ratio R, and 2) the value of the passive moment C was very low (about 0), as in the healthy children. Group CPII was characterized by an abnormally high value of the ratio R. The children in the group can be further classified by their ratio R values as Group CPII-1 (R = 43.0 [plus or minus] 12.0) and Group CPII-2 (R = 25.0 [plus or minus] 6.0). For the children in Group CPII-1, the passive moment was greater than that of the reference group (Group HI) and the total moment was decreased, as in Group HII. Contracture (ie, structural change of the muscle or the tendon) may have been entirely responsible for the toe-walking. The high value of the passive moment C supports this conclusion (5.0 [plus or minus] 2.6). For the children in Group CPII-2, the data require more detailed analysis than those for Group CPII-1. The passive moment is as great as in Group CPII-1, but the total moment is much greater. The passive moment C value was also high (7.0 [plus or minus] 3.5). Toe-walking may be explained by a strong contracture, which is aggravated by the presence of excessive contractions. Two subjects with CP are classified separately in the Table (Ci and P). They show that a child may choose a self-strategy of gait, which we had not expected. Their passive moments and total moments were almost normal; consequently, their ratio R values were normal. This result was obtained despite an ankle angle in strong plantar flexion at instant S. Their passive moment C values were 2.1 and 5.8, respectively, demonstrating a contracture. These children had chosen an intense toe-walking gait pattern, as if they did not want to stretch their contractured triceps surae muscle.

Discussion The results of this study show that separate measurement of the total internal moment in locomotion and the passive component of the total internal moment can be used to analyze toe-walking in children with CP. We believe that the ratio of these two variables gives a good indication of the nature of the disorder. These measurements can only be valid, however, if two conditions are fulfilled. First, there must be no reflex component measured at the same time as the passive component with the patient lying on his or her side. This condition was carefully verified in both this study and in that of Siegler et al[8] by monitoring electro-myographic activity. In neither study was any such activity recorded. We concluded, therefore, that the moment measured in this test accurately reflects the passive resistance of the joint and that this passive resistance is the same as that involved in locomotion. Second, the contribution of the tibialis anterior muscle must be assessed in children with CP at the instant S Berger et al found that there was minimal activity of this muscle in children with CP.[1] Nevertheless, we assumed that an active moment of the tibialis anterior muscle may occur and examined its possible effects upon our conclusions. The values in the Table would remain unchanged, but the total internal moment would be the difference between the absolute values of the moments for the triceps surae and tibialis anterior muscles. As a result, the moment of the triceps surae muscle would be increased. This alteration does not affect the grouping of the children with CP, and the triceps surae muscle therapy would not be changed. A fundamental assumption was made in considering that the movement of locomotion was restricted to the sagittal plane. We stated previously that measurements and calculations indicated that such an assumption was acceptable for the 13 children with CP studied. This approximation, however, was not appropriate for one child; in this case, a three-dimensional study would have been necessary. Despite this single exception, the method used does demonstrate that the apparently similar toe-walking exhibited by a group of children with CP may have different underlying causes and consequently may require a different therapeutic intervention. In Group CPII-1, the therapy that we propose is to apply plaster casts, progressively lengthening the triceps surae muscle.[12] In practice, plaster casts, placed just below the knee allowing gait, are renewed each week. According to the results obtained, the therapy lasts two to four weeks. If necessary, surgical lengthening of the tendon is postponed until after the subject is fully grown. In Group CPII-2, the therapy we propose is similar, but contractions could make the plaster cast unbearable for the child. The contractions may be transiently suppressed by surgically crushing the nerves that innervate the gastrocnemius muscle and, if necessary, the soleus muscle. In this group, the risk of recurrence is high, and night braces may be helpful in lengthening the triceps surae muscle.[13] In Group CPI, therapy will concentrate on reducing excessive contractions in the triceps surae muscle because this muscle is structurally normal. We think that surgical lengthening of the tendon is contraindicated. In some children of this group, the contractions are present even when the child is in the supine position and is instructed to relax. The therapy that we apply is myorelaxant drugs if the contractions are present in numerous muscles and partial neurectomy if the contractions are localized. In other children of the group, the contractions are present only during gait. Control leg braces (Perlstein type) allow the user to prolong the conscious control obtained during physical therapy sessions. In this group, the risk of recurrence is high, and it is necessary to keep watch over the child over a long period of time. For children Ci and P, contracture therapy will not be sufficient; it will be necessary to try to understand why this walking strategy was chosen. In some cases, toe-walking helps the child to compensate for joint and muscle disorders above the ankle. For example, some children may have knee flexor muscle contracture or quadriceps femoris muscle weakness. This condition may provoke a backward fall when the body weight is applied on the heel; therefore, the child is obliged to apply his or her body weight forward by toe-walking. If a brief paralysis of the nerves supplying the gastrocnemius muscles is induced by injecting a local anesthetic drug, this paralysis suppresses toe-walking but also makes gait worse, if not impossible. Therefore, this fact tends to demonstrate that motor disorders above the ankle are involved, which requires further analysis. Although the examinations described in this study are not suitable, in themselves, for routine clinical practice, they do reveal essential differences. They clearly show that an analysis of toe-walking in patients with CP is necessary, and they indicate that a supplementary clinical test should be included that measures contracture at the angles of knee and hindfoot inversion-eversion observed in the stance phase of the gait cycle.

Conclusions Measurements of the passive and active components of the internal moment at the ankle joint were used to investigate the mechanisms of toe-walking in children with CP. Although children with CP may exhibit similar toe-walking patterns, their gait patterns may be attributable to different disorders. Excessive contractions of triceps surae muscle observed in Group CPI require entirely different therapeutic intervention than the structural contracture of this muscle observed in Group CPII. In certain cases, toe-walking is due to upper motoneuron disorders, which require further analysis. Although the small size of groups in this study prohibits definitive conclusions, these conclusions are confirmed by our 10 years of clinical experience with children with CP. [Figure 1 to 3 Omitted] [Tabular Data Omitted]

References [1]Berger W, Quintern J, Dietz V: Pathophysiology of gait in children with cerebral palsy. Electroencephalogr Clin Neurophysiol 53:538-548, 1982. [2]Dietz V, Berger W: Normal and impaired regulation of muscle stiffness in gait: A new hypothesis about muscle hypertonia. Exp Neurol 79:680-687, 1983 [3]Winters TF, Gage JR, Hicks R: Gait patterns in spastic hemiplegia in children and young adults. J Bone Joint Surg [Am] 69:437-441, 1987 [4]Lee CL, Bleck EE: Surgical correction of equinus deformity in cerebral palsy. Dev Med Child Neurol 22:287-292, 1980 [5]Conrad L, Bleck EE: Augmented auditory feedback in the treatment of equinus gait in children. Dev Med Child Neurol 22:713-718, 1980 [6]Bresler B, Frankel JP: The forces and moments in the leg during level walking. Trans Am Soc Mech Engrs 72:27-36, 1950 [7]Morrison JB: The mechanics of muscle function in locomotion. J Biomech 3:431-451, 1970 [8]Siegler S, Moskowitz GD, Freedman W: Passive and active components of the internal moment developed about the ankle joint during human ambulation. J Biomech 17:647-652, 1984 [9]Tardieu C, Tabary C, Tardieu G: Mesure strobo-photogrammetrique des angles tibiocalcaneens et de voute plantaire pendant la phase d'appui de la marche. J Biophys Biomecan 10:99-108, 1986 (French) [10]Jensen RK: Body segment mass, radius and radius of gyration proportions of children. J Biomech 19:359-368, 1986 [11]Tardieu C, Huet de la Tour E, Bret MD, et al: Muscle hypoextensibility in children with cerebral palsy: I. Clinical and experimental observations. Arch Phys Med Rehabil 63:97-102, 1982 [12]Tardieu G, Tardieu C: Cerebral palsy: Mechanical evaluation and conservative correction of limb joint contractures. Clin Orthop 219:63-69, 1987 [13]Tardieu C, Lespargot A, Tabary C, et al: For how long must the soleus muscle be stretched each day to prevent contracture? Dev Med Child Neurol 30:3-10, 1988

Catherine Tardieu, PhD, is Directeur de Recherche, Institut National de la Sante et de la Recherche Medicale (INSERM), and Director, INSERM Unite 215, Handicaps moteurs neurologiques et croissance, Hopital Raymond Poincare, 92380 Garches, France. Address correspondence to Dr Tardieu. Alain Lespargot, MD, is Researcher, INSERM Unite 215, and Chief, Centre d'education motrice Guy Tardieu, 45330 Augerville la Riviere, France. Chantal Tabary was Ingenieur de Recherche, INSERM Unite 215, when this study was conducted. Ms Tabary died on May 12, 1988. Marie-Dominique Bret, MD, is Researcher, INSERM Unite 215, and Chief, Centre pour enfants CP, 78870 Bailly, France.