$504 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1) Poster Presentations

spines were tested intact first, then flexible stabilization, then after spinal fusion to compare the effect of the restabilization to the adjacent segments. A spinal motion tester was developed to apply displacement to the end of the cranial vertebra to simulate flexion-extension motion continuously. Motion of the spine was measured to track makers mounted on the every vertebral body with CCD and intradiscal pressure was measured with the pressure sensor inserted anteriorly in the mid sagittal plane. In flexion, significant increases of intradiscal pressure between intact and flexible stabilization was observed in L3/4 and L4/5. Though, any significant difference was not observed other cases. In extension, ROM of adjacent segment increased significantly after instrumentation. In flexion, significant increase was found in ROM of adjacent segment after restabilization except for the L5/6 after flexible stabilization. While the column inclined up to 3 degrees, there is a slight increase of angle at flexible stabilized segments. Though, remarkable increase of the segmental angle was recorded as the column bended over 3 degrees. In the spinal fusion, notable increase of angle was not found. It suggested that the mobility of the restabilized segment is important difference between the fusion and the flexible stabilization.

7065 Mo-Tu, no. 20 (P57) Muscle effort of the upper extremity during pushing up and keeping balance in a wheelie activity

P.-C.P. Lin 1 , L.-'~ Guo 2, C.-J. Lin 1 , F.-C. Su 1 . 1Institute of Biomedical Engineering, National Cheng Kung University, Tainan, Taiwan, 2Faculty of Sport Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan

A wheelie, a difficult trick, is performed when the user pops the front casters off the ground and keeps balance on the rear wheels. The purpose of this study was to identify electromyography (EMG) activity of functional muscle groups in a wheelie activity. The whole wheelie activity was divided into four phases, preparing, push-up, adjusting, and balance phases, using the pitch angle of the wheelchair. In the preparing phase, the antagonist contracted to prepare for the coming of agonist contraction. Until the push-up phase, muscles weren't fired for the first forward striking to push the front casters off ground. Then, wrist muscle force, especially the extensor, and hand holding force increased, and elbow extensor and shoulder flexor muscles contracted at the same time to push the wheel forward. In the adjusting phase, the wheel was pulled backward to let the center of mass back to the base of support while EMG recordings showed muscle activities on extensors of the wrist, elbow and shoulder joints. In the balance phase, the wrist extensors were mainly involved in holding the handrim. In the last two phases, the wrist flexor played an important role in rotating the wheel backward slightly, but the wrist extensor initiated to let the wheel rotate forward somewhat for balance keeping. However, these should accompany with the triceps to complete the sufficient rotation angle of wheel for maintaining balance if the change of wrist joint angle cannot provide an enough wheel rotating angle for wheelie balance. The concentric contraction of triceps causes the elbow extension and stretches the biceps which is an inactive and biarticular muscle to pull the humerus flexion around the shoulder joint and push the wheel rotated forward, and its eccentric contraction leads elbow flexion and shoulder extension to push the wheel to rotate backward. No mater what phase it was in, the pectoralis major P. Clavic. activated to compensate the trunk control. Consequently, this study helps us understand the pattern and timing of muscle activation in a wheelie performing.

6892 Mo-Tu, no. 22 (P57) Influence of pregnancy on the musculotendinous stiffness of wrist flexors C. Bisch 1,2, E. Stephan 1 , J. Gondry 3, J.-P. Libert 1 , C. P6rot 2, F. Telliez 1 . 1Equipe DMAG-INERIS (EA 3901), Universit6 de Picardie Jules Verne, Amiens, France, 2D6partement de G#nie Biologique, UMR CNRS 6600, Universit~ Technologique de Compiegne, France, 3Centre Gyn6cologique et Obst#trique, CHU, Amiens, France

In spite of the well-known influence of hormonal status on the musculo- tendinous system, no study has investigated the influence of pregnancy on biomechanical characteristics of musculotendinous stiffness at wrist level. 25 pregnant and 13 non-pregnant women were investigated. The quick-release technique was used to characterise the stiffness of wrist flexor muscles by an ambulatory wrist ergometer. Quick-release movements during wrist flexion were performed three times at 3 submaximal isometric torques (75, 50 and 25% of maximal torque). Stiffness was calculated during the first 15 ms of the movement, as the ratio between changes in angular acceleration and angular displacement, multiplied by the corresponding inertia value. Stiffness of the wrist flexors increased linearly with torque, irrespective of the subject (correlation coefficients ranging between 0.63 and 0.97; p < 0.05). The stiffness index, which corresponds to the slope of the individual stiffness-torque relationship (Lambertz et al., J. Appl. PhysioL, 2001), was higher (p < 0.01) in pregnant women (5.9 ± 2.3 rad q ) than in women (4.4 ± 1.6 rad -1 ).

The present study points out an increase in the musculotendinous stiffness of the wrist flexors in pregnant women (+34%). Recently, it has been pointed out in animal that oestrogen replacement therapy caused greatest reduction in type lib fibre size in muscles composed predominantly of fast fibres (Piccone et al., Exp. Physiol., 2005). Therefore, in pregnant women, the increase in oestrogen release could induce a fibre type transition phenomenon, namely an increase in the proportion of slower, stiffer muscle fibres. The higher stiffness measured in pregnant women may contribute to increase the occurrence of musculoskeletal disorders, as often observed during pregnancy.

5247 Mo-Tu, no. 23 (P57) Relative moment potential balance in the metacarpal-phalangeal joints of the hand W.L. Buford Jr., S. Koh, C.R. Andersen, S.E Viegas. University of Texas Medical Branch, Galveston, TX, USA

This study extends prior investigations of muscle moment arms at finger metacarpo-phalangeal joints (MCPJ) to provide a more detailed description of muscle balance. By combining moment arm data for flexion-extension (FE) and ulnar-radial (UR) motion with known tension fractions a renewed understanding of moment potential balance helps to describe clinical conditions as well as provide knowledge relevant to reconstruction and joint arthroplasty. Twelve fresh cadaver arms (6 male, 6 female, average age 71) were acquired through the Texas willed body program. Specimens were dissected through limited incisions but sufficiently to attach nylon cable to the finger tendons using previously described methods. [2] The muscles studied were the flexor digitorum profundi, flexor digitorum sublimi, extensor digitorum communi, ex- tensor indicis proprius, palmar interossei, lumbricales, two paths of the first dorsal interosseous based on origination on the first metacarpal and the second metacarpal, the extensor digiti quinti, abductor digiti quinti, and flexor digit quinti. Muscle excursions, flexion-extension (FE) and ulnar-radial (UR) MCPJ angles were measured and moment arms determined using methods previously described. [1,2] The average UR moment arm values for each muscle with the MCPJ at neutral are combined with the same for FE as described in an earlier study. [2] To provide an indication of moment potential balance, the moment arms are mul- tiplied by the respective muscle tension fractions [3] and combined to indicate the potential moment in the principal rotation directions (flexion, extension, ulnar and radial deviation). The total vector sum provides an indicator of the moment potential assuming maximal contraction of all muscles. These results combining improved investigations of moment arms for the MCP joints in FE and UR motion with previously described tension fractions provide a clear graphical understanding of joint muscle balance. For example, it is readily seen by comparing the moment balance for the normal and intrinsic minus MCPJ that the intrinsic minus joint will eventually migrate ulnarly.

References [1] An KN, et al. J. Biomech. 1983; 16(6): 419-425. [2] Buford WL, et al. In: Proceedings 59th Annual Meeting, ASSH, New York, Sep

9-11, 2004. [3] Brand PW, et al. J Hand Surg. 1981; 6: 209-219.

5818 Mo-Tu, no. 24 (P57) Improvement of the ISB joint coordinate system to describe shoulder joint kinematics A. Levasseur 1 , E T6treault 1,2, J.A. de Guise 1 , N. NuSo 1 , N. Hagemeister 1. 1Laboratoire de recherche en imagerie et orthop6die, Ecole de technologie sup#rieure, Montr6al, Canada, 2H6pital Notre-Dame, CHUM, Montreal, Canada

Important aspects in motion analysis methods are the coherence between the performed and computed movement, and the facility of interpreting the data collected. The methods used in movement analysis often rely on the definition of joint coordinate systems (JCS). In 2005, the International Society of Biome- chanics (ISB) has proposed recommendation on the definition of shoulder JCS with the aim to standardize shoulder kinematics analysis. Recently, it was demonstrated by Senk and Ch6ze (2006) that movement computed with the ISB recommendation was incoherent. Despite improvement due to the use of a new rotation sequence, clinical interpretation of the movement calculated with the ISB JCS still remains unsatisfactory. Therefore, the aim of this research project was to improve the JCS recommended by the ISB in order to facilitate clinical interpretation of shoulder joint motion. The experimentation was carried out on 8 cadaveric shoulder specimens. Elevation of the arm in the scapular plane and three-dimensional (3D) humeral head excursion was recorded using an electromagnetic tracking device. Differences in computed abduction angle and humeral head excursion were analysed using the ISB and the improved JCS (two parallel JCS and localized at the glenohumeral rotation center). The results obtained reveal difference in the initial abduction and humeral head position. The use of the improved JCS was found to reduce inter- subject variability for both initial positions hence providing much easier clinical