PubMed · 13575464
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S M MILNER, S L BAKER. 1958. Disappearing bones.. https://doi.org/10.1302/0301-620x.40b3.502
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The purpose of this study was to explore whether global efforts stop as a consequence of postural muscle exhaustion. To this end, seated adults were asked to exert 75% maximal voluntary contractions bimanual push efforts until exhaustion. A dynamometer was used to measure the horizontal force exerted on a bar (Fx) and a custom-designed force plate measured the antero-posterior displacement of the centre of pressure (Xp). Electromyograms were picked up by bipolar surface electrodes from the primum movens (serratus anterior) and four postural muscles (trapezius superior, erectores spinae, rectus abdominis, rectus femoris). Root mean square and mean power frequency were calculated over 2-s intervals and compared to corresponding Fx and Xp values. It was shown that the effort stops as a consequence of exhaustion of postural muscles (rectus abdominis and rectus femoris), and not of the primum movens. It is concluded that postural muscles make a major contribution to global efforts, in that they allow compliance to biomechanical requirements, that is, to preserve the distance between the centre of pressure and the centre of gravity, which must be proportional to the external force.
Arguably, the act of reaching constitutes one of the most devoted lines of contemporary developmental research. In addition to the underlying dynamical characteristics of motor coordination, a key element in programming is limb selection, a phenomenon (handedness) that has so far resisted any reasonable unified explanation. From a more contemporary view, two factors appear to have the most influence on hand selection for a given task: motor dominance and attentional information related to task demands. This study was designed to determine what factor(s) influence choice of limb for reaching in hemispace in reference to motor dominance, object proximity, and a hemispheric bias favoring use of the hand on the same side as the stimulus. Strong right-handed children were asked to reach and retrieve a small object across right and left hemispace locations beginning with the arms uncrossed and arms-crossed. With the arms-crossed condition, an imagined and actual movement execution was administered. Results from the uncrossed condition supported previous reported findings for adults and children. That is, participants responded ipsilaterally using the hand on the same side as the stimulus, thus supporting the case for object proximity and hemispheric bias. However, in the arms-crossed condition the vast majority of participants preferred keeping the limbs crossed in response to right and left hemispace stimuli, which leads to the suggestion that object proximity rather than hemispheric bias was the driving factor in this context. The behavioral pattern for imagined and actual movement was not significantly different. Overall, the findings add to the growing acceptance that limb selection is task and context dependent, rather than a biologically based invariant feature of motor behavior.
Steal phenomena associated with brachial bridge grafts for hemodialysis access may compromise blood flow to the forearm. This work is designed to investigate and compare, by means of a simple mathematical model, the potential of six surgical procedures to alleviate steal. A flow model based on an electrical analogue was developed. An untapered 6-mm prosthetic brachial-axillary access (PBAA) was selected as the prototype configuration, and the theoretical effect of six access modifications on forearm flow was analyzed. Major simplifications include the use of Poiseuille's law for estimating arterial resistance and ignoring the contribution of collateral circulation. Intra-operative flow measurements using a Transonic flowmeter were obtained in two individuals undergoing treatment for a steal syndrome. The flow model predicts that the greatest increase in distal flow is achieved by the distal revascularization-interval ligation (DRIL) procedure, followed by a 6-mm axillobrachial artery bypass graft without interval ligation, the conversion of the PBAA to an axillary-axillary loop access, and the conversion to an axillary-brachial access. Intra-operative measurements in two patients agreed closely with theoretical calculations. A simple flow model provides a tractable framework for comparing procedures designed to avoid or treat steal phenomena. Distal revascularization without interval ligation and the conversion of a PBAA to an axillary-axillary loop access or an axillary brachial access can be effective alternatives to the DRIL procedure in selected clinical settings.