Abstract
Several types of continuous human movements comply with the so-called Two-Thirds Power Law (2/3-PL) stating that velocity (V) is a power function of the radius of curvature (R) of the endpoint trajectory. The origin of the 2/3-PL has been the object of much debate. An experiment investigated further this issue by comparing two-dimensional drawing movements performed in air and water. In both conditions, participants traced continuously quasi-elliptic trajectories (period T = 1.5 s). Other experimental factors were the movement plane (horizontal/vertical), and whether the movement was performed free-hand, or by following the edge of a template. In all cases a power function provided a good approximation to the V–R relation. The main result was that the exponent of the power function in water was significantly smaller than in air. This appears incompatible with the idea that the power relationship depends only on kinematic constraints and suggests a significant contribution of dynamic factors. We argue that a satisfactory explanation of the observed behavior must take into account the interplay between the properties of the central motor commands and the visco-elastic nature of the mechanical plant that implements the commands.
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Acknowledgments
The work was supported by the Italian Ministry of University and Research (PRIN Grant 2010MEFNF7_002), Italian Space Agency (COREA Grant 2013-084-R.0) and Horizon 2020 robotics program (ICT-23-2014 under Grant Agreement 644727—CogIMon). We wish to thank Roberto Catavitello for designing and realizing the tank used for the experiments.
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Catavitello, G., Ivanenko, Y.P., Lacquaniti, F. et al. Drawing ellipses in water: evidence for dynamic constraints in the relation between velocity and path curvature. Exp Brain Res 234, 1649–1657 (2016). https://doi.org/10.1007/s00221-016-4569-9
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DOI: https://doi.org/10.1007/s00221-016-4569-9