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1.
Bilateral synergies in foot force production tasks
Nejc Šarabon, Goran Marković, Pavle Mikulić, Mark L. Latash, 2013, original scientific article

Abstract: We analysed the effects of task symmetry during bilateral accurate force production tasks performed by the two feet. In particular, we tested a hypothesis that bilateral deficit would lead to higher indices of synergies defined as co-varied adjustments in the two forces across trials that reduced total force variability. The subjects produced steady-state force followed by a quick force pulse into the target. The two feet could be acting both into plantar flexion and into dorsiflexion (symmetrical tasks), or in opposite directions (asymmetrical task). We used the framework of the uncontrolled manifold hypothesis to quantify two variance components, one of which did not change total force (VUCM), while the other did (VORT). Synergy indices during the asymmetrical task were higher than in either symmetrical task. The difference was due to higher VUCM (compared to the symmetrical plantar flexion task) or lower VORT (compared to the symmetrical dorsiflexion task). The synergy index showed a drop (anticipatory synergy adjustment, ASA) starting 100150 ms prior to the force pulse initiation. The ASA tended to be shorter and of a smaller magnitude for the asymmetrical task. This is the first demonstration of bilateral synergies during accurate force production by the legs. We conclude that bilateral deficit has no or weak effects on two-leg synergies. The results fit the earlier introduced scheme with two groups of neural variables defining average performance of a redundant system and patterns of co-variation among its elemental variables, respectively
Keywords: kinesiology, force, synergy, legs, bilateral deficit, symmetry
Published in RUP: 15.10.2013; Views: 5154; Downloads: 241
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2.
The effects of aging on the rambling and trembling components of postural sway
Nejc Šarabon, Andrej Panjan, Mark L. Latash, 2013, original scientific article

Abstract: The effects of healthy aging on postural sway and its rambling and trembling components were studied. Young and elderly subjects stood quietly for 1 min in different postures, and with eyes open and closed. We found that age-related changes in postural sway and its components were similar to those observed in young participants in challenging conditions. These changes may therefore be viewed as secondary to the increased subjective perception of the complexity of postural tasks. Contrary to our expectations, stronger effects of age were seen in characteristics of rambling, not trembling. The commonly accepted hypothesis that older persons rely on vision more was not supported by this study: we found no significant interaction effects of age and vision on any of the sway characteristics. It was concluded that the reported higher reliance on vision in older persons may be task-specific. The results are compatible with the ideas that much of the age-related changes in postural sway emerge at the level of exploring the limits of stability and using the drift-and-act strategy. Our results suggest that the dominant view on rambling and trembling as reflecting supraspinal and peripheral mechanisms, respectively, may be too simplistic
Keywords: postural control, aging, sway, rambling, trembling
Published in RUP: 15.10.2013; Views: 3163; Downloads: 79
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