Spinal proprioception and back muscle activation are impaired by spinal “creep” but not by fatigue

Track 2. Musculoskeletal Mechanics-Joint ISB Track of the same finger. Movements were either voluntarily produced by the subject or passively imposed by a torque motor. Results show a systematic impact of the rhythmic movement on the discrete movement irrespective of how these movements were generated (voluntarily or passively). This includes phase entrainment (i.e., a systematic modulation of the onset time of the discrete movement phase-locked to the oscillation) and gating (i.e., the ability of one movement to constrain or even impede the execution of the other). Comparison of the experimental findings with predictions computed with an extended version of Feldman's X-model showed that these effects can be completely reproduced when the threshold properties of the muscles are explicitly taken into account. These findings support the view that the muscles and their reflex circuitry introduce specific discontinuous operations at the level of motor execution, which should be considered in models of human motor control. 5277 Tu, 17:15-17:30 (P23) Muscle redundancy enables the transition between, and adjustments of, complex motor tasks F.A. Medina 1, R.V. McNamara 1, S.L. Backus 2, M. Venkadesan 1, V.J. Santos 1, F.J. Valero-Cuevas 1,2. 1Neuromuscular Biomechanical Laboratory, Cornell

University, Ithaca, NY; 2 The Hospital for Special Surgery, New York, USA We present evidence supporting suggestions that muscle redundancy, often called the central "problem" of motor control, is instead an evolutionary tool to produce everyday complex motor tasks [1]. We recorded fine-wire EMG from all 7 muscles of the index finger while eight subjects produced isometric fingertip force ramps following a tapping motion. We characterized the muscle coordination pattern as a time-varying 7-D vector of EMGs, and dotted the unit vector at each sample time with the average unit vector over the 80 ms of peak force [2]. This time series of correlation coefficients quantifies the similarity of the coordination pattern during the task compared to the pattern at peak force. First, we find that the coordination patterns for motion are distinct to those for force production. Transitioning from downward motion to isometric force increased r from <0.5 before contact to >0.9 after contact. In addition, r typically reached a first plateau >0.9, only to drop significantly to between 0.7 and 0.95, and then recover to >0.97. Linear regression shows that the lower the initial plateau, the larger the drop in r. Importantly, force vector magnitude increased monotonically throughout, and the dramatic drop in r was not correlated with changes in force vector direction or orientation of the distal phalanx. We conclude that that the nervous system utilizes muscle redundancy to travel in the null space of the transformation from muscle forces to fingertip force and posture (i.e., the drop) to adjust the coordination pattern used for the transition (the first plateau) to align it with the coordination pattern for peak force. This would not be possible if musculature were not redundant for the production of static fingertip force. Acknowledgements: Dr. Marilynn Price. NSF 0237258, NIH R01-AR050520, and NIH R01-AR052345.

References [1] Loeb, G. E. Motor Control 4, 81-3; discussion 97-116 (2000) [2] Valero-Cuevas, E J. J Neurophysiol 83, 1469-1479 (2000) 7427 We, 08:15-08:30 (P27) Spinal proprioception and back muscle activation are impaired by spinal "creep" but not by fatigue D. Sanchez 2, M. Adams 1, P. Dolan 1. 1Department of Anatomy, University of Bristol, Bristol, UK, 2Faculty of Medicine, University of Valencia, Valencia, Spain

Introduction: Creep in spinal tissues can diminish or delay activation of back muscles during spinal flexion movements [1,2]. This may be related to changes in the relationship between tissue stress and strain which could affect sensory feedback and motor control mechanisms. This study compared the effects of creep and muscle fatigue on spinal proprioception and muscle activation during forward bending tasks. Methods: Thirteen healthy subjects underwent one of two treatments on separate days: a. sitting flexed in a low chair for 1 hour (to induce creep in spinal tissues) b. the Biering Sorensen test (to induce back muscle fatigue). Proprioception and muscle activation patterns were assessed before and after each treatment. Proprioception was measured using the 3-Space Fastrak to determine repositioning error, at L1 and $1, when subjects attempted to reproduce upright and flexed postures. Muscle activation was evaluated by identifying the onset of EMG activity in thoracic and lumbar erector spinae muscles during forward bending. Lumbar flexion at the onset of muscle activation was used to assess latency. Results: Following creep, proprioceptive acuity was impaired (p < 0.0001), particularly at L1, where repositioning errors increased from 0.9±1.00 to 1.6±1.50 in standing, and from 2.1±1.70 to 4.3±2.40 in flexion. In addition,

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activation of lumbar erector spinae was delayed after creep (p < 0.01 ), indicated by a 4.50 increase in lumbar flexion at EMG onset. Following fatigue, there were no significant changes in proprioception or in back muscle activation. Discussion: This study has shown that proprioceptive acuity and muscle activation during forward bending tasks are impaired by creep in spinal tissues but not by back muscle fatigue. These findings suggest a mechanism whereby ligamentous creep as a result of prolonged flexion can alter motor control strategies in back muscles. This may increase the risk of bending injury to the lumbar spine.

References [1] Solomonow et al. Spine 1999; 24: 2426-34. [2] Dolan et al. Orthopaedic Research Society, Washington, 2005. 7575 We, 08:30-08:45 (P27) Postural mechanisms during unipedal quiet stance on compliant surfaces J.L. Croft, V. von Tscharner, R.E Zernicke. Human Performance Lab, Faculty

of Kinesiology, University of Calgary, Alberta, Canada Human stance is inherently unstable due to the small base of support and high center of mass. Previous studies have shown that activity in the plantar flexors precedes anterior-posterior shifts in the center of pressure (COP) on firm surfaces. It is unknown if muscle activity onset is affected by increased instability. We investigated the control strategy required to maintain upright unipedal stance on a compliant surface. Subjects stood for 10 trials of 30s on each of three surfaces (ground, foam, air-filled disc). Kinematics, COP, and EMG from leg muscles were collected. Traditional time-summed measures of COP (path length, area of best-fit ellipse, medial-lateral range, anterior-posterior range) were calculated for each trial. Cross correlation functions (CCF) were calculated to assess the relationship between COP, EMG intensity and joint angles. Average CCFs were determined for each subject for each surface, and those with significant peaks were analyzed for time shifts. Medial gastrocnemius and soleus activity led COP by 216 to 237 ms and 169 to 191 ms respectively, and was not significantly affected by surface. Anteriorposterior center of mass (COM) position led COP by 11 to 37 ms, and was not affected by surface. Peroneus activity led medial-lateral COP by 140 to 169 ms. The time shift on the disc was significantly greater than both ground and foam. The results indicated that stability was controlled by a feed forward mechanism whereby the muscles get systematically activated before COP changes. A critical limit of instability seems to be required before the time from muscle activation to COP response changes. This limit may have been exceeded in the medial-lateral direction from foam to disc, but not in the anterior-posterior direction, where stability is greater. Ankle extensors anticipate change in COM position on all surfaces, but ankle evertors do not. 7477 We, 09:00-09:15 (P27) A modal analysis of movement control strategies in a maximal standing reach A.A. Ahmed, J.A. Ashton-Miller. Biomechanics Research Laboratory,

University of Michigan, Ann Arbor, USA Among the elderly, forward reaching is considered one of the three most challenging activities of daily living. Moreover, the task becomes more challenging with increasing reach distance. It is unclear, however, how the central nervous system reconciles the conflicting requirements of postural stability and reaching goals. Modal analysis was used to explore these mechanisms in a task in which stability and reach distance are both maximally challenged. Eleven healthy young women reached forward, while standing, to a target positioned from 95% to 125% of their maximum reach distance. They were allowed to lift both heels but not to bend their knees. Kinematic data were recorded unilaterally from the upper body, leg and foot at 100Hz. The task was modeled as a triple inverted pendulum, consisting of the FOOT, shankthigh (LEG), and head-arms-torso (HAT) segments. Using modal analysis, movements in joint angle coordinates were transformed into three generalized modal coordinates, each representing movement corresponding to one of the system eigenvectors. Each eigenvector, or mode shape, describes a relative configuration for the three segments. The results show that two mode shapes were consistently dominant, together accounting for over 95% of the movement variance in 108 trials. One mode represents a 'forward' angular movement of all segments, whereas the second is a 'braking mode', in which the LEG moves in the opposite direction of the HAT and FOOT. The 'forward' mode clearly dominates, and advances the center of mass and fingertip horizontally. The 'braking' mode, however, contributes with movement in the opposite direction. The counteracting influence of the 'braking' mode was significantly greater on the center of mass than on the fingertip motion (P<0.0001). The 'braking' mode highlights the surprisingly critical role of LEG segment control during a maximum reach. We conclude