Three dimensional preparatory trunk motion precedes asymmetrical upper limb movement
Introduction
Control strategies used by the central nervous system (CNS) to maintain stability of the trunk are complicated by the complex arrangement of muscles and the multiple forces acting on the trunk as a result of normal body movement. For instance, upper limb motion produces reactive moments (opposite in direction and equal in magnitude to those producing the limb movement) that are transferred to adjacent body segments [1], [2]. Recent evidence suggests that the strategy adopted by the CNS to counteract this perturbation involves preparatory motion of the trunk in the sagittal plane when upper limb movements are performed in a bilateral and symmetrical manner [3]. These preparatory adjustments were in a direction opposite to those produced by the reactive moments generated by limb movements [2], [4], [5]. It is unknown whether preparatory trunk movement also occurs in other planes when limb movements are asymmetrical (e.g. unilateral upper limb movement).
The complex arrangement of muscles of the trunk can control trunk motion directly and indirectly (e.g. by generation of intra-abdominal pressure). Superficial trunk muscles (rectus abdominis, obliquus externus abdominis and erector spinae) have been found to act in a manner which varies between limb movement directions [6], [7], [8], [9], [10] and is consistent with the initiation of preparatory trunk motion prior to symmetrical bilateral upper limb movement [3]. In contrast, the response of transversus abdominis is not influenced by the direction of reactive moments and may contribute to a non-direction specific control of trunk stiffness [3], [6]. In addition, generation of intra-abdominal pressure by transversus abdominis has been suggested to contribute to spinal control [3], [11], [12], [13], [14], [15].
The present study was designed to investigate, first, whether preparatory trunk motion occurs in all three orthogonal planes with asymmetrical upper limb movement and, secondly, if such preparatory motion occurs, to identify whether trunk muscle EMG is consistent with this motion and, thus, the control of trunk orientation. The final aim was to determine if the intra-abdominal pressure contributes to the preparatory processes associated with upper limb movement.
Section snippets
Subjects
Eight habitually active males volunteered for this experiment. Their mean (SD) age, body mass and height were 25 (4) years, 78 (8) kg and 1.82 (0.04) m, respectively. Subjects were excluded if they had any major respiratory or neurological condition or had a history of low back pain. The study was approved by the institutional ethics committee.
Measurement of trunk and limb movement
Kinematic data were collected using a Selspot II optoelectric movement analysis system (Selspot, Sweden) involving two cameras situated 1.5–2 m behind
Trunk movement
Unilateral flexion of the left upper limb was accompanied by a small but consistent movement between trunk segments prior to limb movement onset in the opposite direction to that expected from reactive moments (Fig. 3). This preparatory motion occurred between all trunk segments in the sagittal (extension), frontal (right lateral flexion) and transverse (clockwise rotation) planes and started prior to limb movement onset (Fig. 3, Fig. 4). Motion between trunk segments in the predicted resultant
Discussion
This study shows that three-dimensional preparatory trunk motion precedes unilateral upper limb movements. These preparatory movements were opposite in direction to those caused by the reactive moments resulting from movement of the upper limb. Furthermore, the pattern of trunk muscle activity is appropriate to initiate this movement in the majority of directions. The results confirm that anticipatory postural adjustments involve movements and not rigidification of the trunk.
Acknowledgements
We thank Åke Tisell for assistance with the electrode insertions, Alexander Ovendal, Mats Nygren and Merryn Hodges for assistance with the data collection. Financial assistance was provided by the Swedish Council for Work Life Research (96-0834) and the Swedish Medical Research Council (K99 04X). PH was supported by a Wenner-Gren Foundation visiting scientist Fellowship.
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