Track 2. Musculoskeletal Mechanics-Joint ISB Track 2.3 Motor Control of Human Movement $481

the aggregate modulus (HA) and hydraulic permeability (k) have not been determined beyond strains of 20% (P6ri6 et al., J. Biomech., 2005; 38: 2164- 2171). The compression mechanics of the nucleus pulposus has been rarely studied, with experiments adopting a multiple-step testing regime; however the validity of this approach has not been assessed. Therefore, this study aimed to determine the variation of HA and k of the nucleus pulposus with confined compression up to 70%, in single-step and multiple-step testing regimens. Bovine coccygeal nucleus pulposus plugs (10 mm diameter, -1 mm thickness) were tested in confined compression, in 0.15M NaCI phosphate buffered saline, at a strain rate of 2~tm.s -1, to single strains of 10% (N=3), 25% (N=3) , 40% (N=3), 55% (N=3) and 70% (N=4) . Four additional samples were subjected to all the above strains, incrementally administered. HA and k were determined from fitting the linear biphasic model to each stress relaxation phase of the stress-time curve. HA and In(k) were analysed with respect to strain and condition using 2-way ANOVA. HA varied neither with strain nor condition, nor their interaction: HA = 168.9±21.8kPa (pooled mean±s.e., N=36) . Ln(k) decreased linearly with strain (p <0.001), corresponding to a mean k =3.51 10 -15 m4/Ns at 10% strain to 5.50 10 -17 m4/Ns at 70%. In addition, mean In(k) was less in the multiple-step group (p=0.001). These results suggest that HA remains constant at high strain, without densifi- cation affecting the modulus. Permeability varied in accordance with previous exponential models of the relationship between permeability and strain (e.g. Ateshian et al., J. Biomech., 1997; 30:1157-1164). The significant effect of loading condition suggests that caution should be adopted when performing multiple cycles on single specimens.

5503 We-Th, no. 24 (P56) The neutral zone in lumbar joint movements and how it is affected by preload H. de Visser, C. Adam, M. Pearcy. Institute for Health and Biomedical Innovation, School of Engineering Systems, Queensland University of Technology, Brisbane, Australia

Introduction: It is important to understand lumbar spine mechanics, as it has been shown that much low back pain is attributable to mechanical factors. One important aspect of spinal mechanics is the neutral zone, defined as a region of little or no resistance to motion (absolute rotational stiffness less than 0.05 Nm/°) around the joint's neutral position. This study was performed to characterise the size of the neutral zone and the effect of axial preload for different spinal motions. Methods: Using a 6 degree-of-freedom (DOF) robot plus 6-DOF force sensor, 6 isolated ovine lumbar joints were subjected to 5 repetitive movements in 3 directions (6°/-150 extension/flexion, ±70 lateral bend,±3 ° axial twist) with 4 axial loads (0, 150, 300, 450 N) under 2 anatomical conditions (zygapophysial joints intact and removed). For each direction, the fixed axis, about which the joint would rotate with a minimal motion-opposing moment, was determined in advance. Results: In flexion/extension there usually was a neutral zone extending over 9-12 °. Increasing preload usually made the joint stiffer in regions outside the neutral zone, but did not affect the neutral zone itself. In lateral bend and axial twist no neutral zone was generally observed. For all movements, the only effect of zygapophysial joint removal was a constant stiffness reduction over the whole movement and often the creation of a neutral zone over several degrees in lateral bend. Discussion and Conclusion: For flexion/extension ovine spinal joints have a neutral zone that can be in excess of 10 °. This means in their neutral position, the individual joints have virtually no stability and the spine depends on other measures such as muscle activation to maintain stability in the sagittal plane. For lateral bend there is also a region of little resistance, but not as profound as in flexion/extension. Preload has no effect on the neutral zone, only causing a stiffness increase outside the neutral zone.

2.3 Motor Control of Human Movement

5774 We-Th, no. 25 (P56) Chaos theory applied to posturographic data in parkinsonian patients

L. Ladislao 1, M. Guidi 2, G. Ghetti 3, S. Fioretti 1. 1Department of Electromagnetism and Bioengineering, Universita Poiltecnica delle Marche, Ancona, Italy, 2Neurology Operating Unit, INRCA Hospital, Ancona, Italy, 3posture and Movement Analysis Laboratory, INRCA Hospital, Ancona, Italy

Aim of this work was to evaluate parkinsonian patients (PARK) by static posturography tests in order to study the stability of posture system, the role of visual input and the influence of an acute administration of levodopa. The problem of characterizing postural control is approached from the perspec- tive of nonlinear dynamics and chaos theory. The Largest Lyapunov Exponent (LLE) is a parameter that nonlinear analysis methods allow us to determine

in a reliable manner. For a nonlinear deterministic system, a positive value of this parameter implies that this system is chaotic. In order to estimate LLE, as first step, it was necessary to confute the hypothesis of linear structure of data through a determinism test: in particular the surrogate data test has been applied. As second step, attention was paid on the choice of appropriate nonlinear filtering methods related to nonlinear analysis. Analysed subjects were 14 PARK and 20 control subjects (CTRL). Subjects were asked to maintain the orthostatic posture for 60 seconds on a dynamo- metric platform. Every trial was repeated twice and was performed in Eyes- Open and in Eyes-Closed conditions. PARK were analysed both before (PRE) and after (POST) levodopa administration. Results show positive LLE values that, in the case of PARK, tend to be higher than LLE estimated for CTRL. This is particularly true in the PRE condition. From these findings one can argue that PARK, if compared with CTRL, show less stability, mainly in PRE condition. This instability is not always evident looking at the centre of pressure geometric characteristics because these latter do not take into account the dynamical properties of the posture control system.

5852 We-Th, no. 26 (P56) The influence of sport on sensory and motor abi l i t ies in children B. Pr~torius, T.L. Milani. Chemnitz University of Technology - Institute of sport science, Chemnitz, Germany

Decreasing sensorimotor abilities of children by lack of exercise have many consequences: obesity, bad posture, hypertension, psychological strain etc. [1]. To understand the influencing factors on this phenomenom, the sensorimotor abilities have to be investigated. The main goal of this study is to analyze the various effects of different kinds of sport on motor and sensory abilities of children. The KTK (Coordination Test for Children) was used to inverstigate the motor abilities [2]. Sensory abilities of the plantar foot were determined by detecting the vibration thresholds [3]. Anthropometrical data and information about participation in sports clubs were collected. 531 children (age: 5-13yrs.) took part in this study. 49% reported to be active member of a sports club. Within this group 23% took exercises in individual sport (e.g. Gymnastics), 20% in team sport (e.g. Soccer) and 6% in martial art (e.g. Judo). The results show that motor abilities are significantly better of children who are members in sport clubs than of those are not active (p < 0.001). Taking the different kinds of sport into account, the results show statistically significant positive effects of individual sports on motor abilities. Sensory thresholds show no statistical significance (p=0.84). Nevertheless there are tendencies that team sport and martial art affect the foot sensitivity in a positive way. Sport in general has positive effects on motor abilities. The results have shown that exercises in martial art and team sports have the same effects on motor and sensory abilities of children. This could be attributed to similar needs like eqilibrate movements on one leg. Especially sensory abilities are required in these sports. The abilities of children performing individual sports are the best in KTK and the worst in the sensitivity test.

References [1] Lou JE, G. T., Flynn JM, Exercise and children's health. Current Sports Medicine

Reports 2002; I: 349. [2] Schilling F, K. E. K6rperkoordinationstest fur Kinder - KTK, Belz test, 2000,

G6ttingen. [3] Pr~torius B, M. T. Barfuss gegen Koordinationsst6rungen! In: Motorik angenom-

men.

5381 We-Th, no. 27 (P56) Investigation of shock waves at tibia and L3 in walking-running transition K.S. Hwang 1 , C.G. de Oliveira 1,2,3, J. Nadal 1 . 1Biomedical Engineering Program, COPPEIUFRJ, Rio de Janeiro, Brazil, 2Biomechanics Laboratory, EEFDIUFRJ, Rio de Janeiro, Brazil, 3physical Activity Science Institute of Aeronautics, Rio de Janeiro, Brazil

The phenomenon known as walking to running transition (WRT) are not well understood, whereas it has been postulated that WRT occurs as a mechanism of energy saving [1]. However, studies with quadrupeds show an increase of energetic expenditure, but a decreasing of deleterious mechanical loads [2], conducting some authors to postulate that the WRT minimizes injurious loads, as bone stresses. This study investigates if minimizing the impacts at tibia and/or at L3 could be considered a mechanism of triggering WRT. Thirty health subjects, 19 males and 11 females, 23.2±3.2 years old (mean±SD) were monitored through skin-mounted accelerometers. They walked and ran in a treadmill at speeds 80%, 90%, 100%, 110% and 120% of their WRT speeds (WRTS) during 10s each. For a given test and subject, the peaks related to