$538 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Poster Presentations

heel callosities when driving a 4 4 style vehicle. The driver's foot position was influenced by the seat geometry of the vehicle. Cessation of driving the 4 4 style vehicle and driving of an alternative automobile for a period of 4 weeks resolved the condition. On return to the 4 4 style vehicle, the abnormal callus patterns redeveloped while using the same footwear and no change in any other parameters. It is therefore suggested that seat and consequent foot position is an important ergonomic factor that should be addressed in the future design of automobile seating.

5.11 Occupational Disorders, Repetitive Strain Injury 6968 We-Th, no. 33 (P60) Assessment of neuromuscular efficiency and fatigability of back extensor muscles in pregnant women with low back pain C. Bisch 1,2, J. Gondry 3, C. Vanhoutte 1 , C. P6rot 1 . 1D#partement de G6nie Biologique, UMR CNRS 6600, UTC, Compiegne, France, 2 Laboratoire DMAG (EA 3901), UPJV, Amiens, France, 3Centre gyn#cologique et obst6trique, CHU, Amiens, France

During pregnancy the majority of women suffer from low back pain (LBP). This pathology is currently associated with muscle fatigability. The present study aimed at assessing neuromuscular efficiency (NME) and fatigability of erector spinae muscles in these women. Eighteen pregnant with LBP and 9 non-pregnant women (control group; C) par- ticipated to the study. Electrical surface activities were recorded bilaterally from the Spinalis thoracis (StM) and Longissimus thoracis (LtM) muscles. Women sitting in a standardized position performed 3 maximal voluntary isometric trunk extensions to determine maximal torque (MVC), maximal muscle activity (ENR) and NME (MVC/ENR). They performed submaximal trunk extensions at different MVC levels before and after muscle fatigability test (Ftest). The %MVC/%ENR slope determined NME under submaximal conditions. Ftest consisted to maintain the standardized trunk position despite a load (8kg) placed over the shoulders. RMS and MPF initial values (IRMS; IMPF) and time relationship slopes (RMSslope; MPFslope) were determined. Under maximal contractions, pregnancy increased NME (+107%; p=0.036) as a result of a lower StM activity (-57%; p=0.002). Under submaximal contractions, pregnancy increased %MVC/%ENR linear correlation (R 2) for Stm before Ftest (+67%; p=0.034). In C only, Ftest modified R 2 (StM: +22%; p =0.046; LtM: -8%; p=0.042). At the beginning of Ftest, StM showed lower IRMS (-37%; p<0.001) for pregnant women. Typical fatigue criteria were obtained for the group C (positive RMSslope; negative MPFslope), but a negative RMSslop e (p=0.005) and positive MPFslop e (p=0.063) were found for the majority of pregnant women. For LtM, LBP in pregnancy decreased IRMS (-19%; p=0.018) and increased IMPF (+21%; p=0.001). No difference for EMG fatigue criteria was observed between the 2 groups. During pregnancy, these results suggest that LBP is not explained by a greater extensor muscle fatigability; furthermore, pregnancy and/or LBP modify the synergy and coactivation pattern between back muscles.

6566 We-Th, no. 34 (P60) The influence of individual geometry on the results of a numerical model of the lumbar spine - Experimental data and simulation C. Mischke 1, G. Huber 2, M.M. Morlock 2, H.E W61fel 1 . 1Darmstadt University of Technology, Dept. of Structural Dynamics, Darmstadt, Germany, 2Hamburg University of Technology, Biomechanics Section, Hamburg, Germany

Introduction: Whole body vibrations and repetitive shocks to the body repre- sent a health risk, especially with regard to the lumbar spine [1]. Prescriptive limits do not take into account individual loading capacity due to skeletal differ- ences. The aim of this study is to illustrate the impact of skeletal parameters on the mechanical behaviour of lumbar spines. Method: A detailed dynamic FE-model of the lumbar spine had been devel- oped to determine the damaging effects of vibration and shock [2]. However, this model could not be individualized. In the present study the possibilities for the individualization of this previous model have been extended. It is now possible to adapt the model to individuals, using 22 geometrical parameters. These parameters were derived from three-dimensional geometric models that were created based on CT data from three excised lumbar spines. The results of in vitro experiments on the excised human lumbar spines were then used for validation purposes. These data were obtained under realistic boundary conditions (frequencies, number of cycles, etc.), as the behaviour of the materials comprising the spine is non linear. Results: The material laws and material parameters of the model were improved based on the in-vitro measurements. It was shown that the geometry of the intervertebral disc in particular has a great deal of influence on the results. Conclusion: Although the specimens were harvested from donors represent- ing an average-sized male, the mechanical behaviour varied. Consequently, individualized models are crucial to precisely estimate individual health risk.

Acknowledgements: This study was financed by the German Federal Institute for Occupational Safety and Health (research projects F 1899 and F 2028) and was supported by the University Medical Center Hamburg-Eppendorf (Forensic Medicine, Diagnostic and Interventional Radiology).

References [1] ISO 2631-1. Mechanical vibration and shock - Evaluation of human exposure

to whole-body vibration, 1997. [2] Hofmann, Pankoke, W61fel. Individualisierbares finite-elemente-modell des

sitzenden menschen zur berechnung der beanspruchungen bei vibrationsan- regung in verschiedenen raumrichtungen und sto6anregung an einer reihe von k6rperstellen, In: Wirtschaftsverlag NW, Fb 994, Dortmund, 2003.

5.12. Rehabilitation Mechanics 5029 We-Th, no. 35 (P60) Shoulder muscle function in females with trapezius-myalgia L.L. Andersen, EB. Hansen, C. Andersen, K. Skovholm, L. Rosendal, K. S~gaard, G. Sj~gaard. National-Institute-of-Occupational-Health, Copenhagen, Denmark

Trapezius-myalgia is frequent in occupational groups engaged with monotonous low-force work tasks. It can be hypothesized that one factor in the development of trapezius-myalgia may a more strenuous pattern of trapezius muscle activation during low-force tasks in myalgic subjects compared to healthy controls. Purpose: To investigate shoulder muscle activation with EMG during a stan- dardized low-force work task (LOW) and during brief maximal voluntary muscle contraction (MAX). Methods: 13 females clinically diagnosed with trapezius-myalgia (TM; 45±6yrs, 166±6cm, 72±14kg) and 9 healthy female controls (CON; 45±8yrs, 168±5cm, 68 ±9kg). MAX: Maximal voluntary static muscle con- traction during shoulder-elevation and shoulder-abduction to obtain torque and maximal EMG amplitude in the trapezius and deltoideus muscles, respectively. LOW: Bilateral static arm abduction at 900 performed for 20sec, with no external resistance besides the weight of the subject's arms. EMG during LOW was normalized to MAX EMG. "Weight" of the subjects arm was measured in a Biodex dynamometer as the passive moment at 900 abduction. Results: MAX EMG among TM compared to CON was lower in the trapezius muscle during shoulder-elevation (439 vs. 769~tV, p<0.01) but not in the deltoideus muscle during shoulder-abduction (312 vs. 416 ~tV, p > 0.05). Max- imal torque tended to be lower during shoulder-elevation (59 vs 70 Nm) but not shoulder-abduction (36 vs. 38Nm). During LOW, normalized EMG in TM compared to CON was higher in the trapezius (24.2 vs. 9.9%, p<0.01) but not deltoideus muscle (28.6 vs. 26.2%, p >0.05). The weight of the subject's arms was similar between the two groups (17.2 vs. 16.2 Nm). Discussion: Maximal shoulder muscle activation appears to be specifically reduced in the trapezius muscle in subjects with trapezius-myalgia. During static lifting of the arms with no additional resistance, a higher percentage of the trapezius muscle strength capacity is used, which can not be explained by differences in arm weight. Thus, in subjects with trapezius-myalgia a greater relative workload is imposed specifically on the trapezius but not deltoideus muscle during low-force tasks. Speculatively, this may be contribute to a more pronounced development of fatigue in the trapezius muscle during prolonged low-force work tasks which again may lead to further accumulation of local pain and reduction of strength, i.e. a vicious circle is induced.

4361 We-Th, no. 36 (P60) Frequency-dependent vibration penetration into soft tissue in a fingertip J.Z. Wu, D.E. Welcome, R.G. Dong. National Institute for Occupational Safety and Health, Morgantown, W~, USA

Excessive dyamic loading of soft tissues during vibration exposure has been proposed to induce the degeneration of the neural and vascular systems, and finally, lead to the development of many occupation-related disorders in operators' hand/fingers. Dynamic distributions of stress/strain in finger tissues have never been evaluated experimentally due to technical difficulties. The goal of this study is to investigate the frequency-dependent vibration penetration into the soft tissue in the fingertip. The dynamic responses of the fingertip were an- alyzed using a multi-layered, nonlinear 2D finite element model, which includes skin layer, subcutaneous tissue, bone, and nail. The simulation procedure was performed in two stages: static pre-compression and steady-state dynamic analysis. The fingertip is assumed to undergo small harmonic vibrations (mag- nitude 0.5 mm) around the deformed, stressed state. The dynamic analysis is performed in a frequency domain range of 16-2000Hz. The predicted distributions of the vibration magnitude show that the vibration magnitude at the contact surface is constant at 0.5 mm (specified), while the internal vibration magnitudes are location- and frequency-dependent. It is clear that the fingertip has a resonant frequency around 125 Hz, at which the vibration magnitudes in the soft tissues are much greater than that of the contact plate. At very high