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Vladimir M. Zatsiorsky

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Vladimir M. Zatsiorsky

A rather messy situation exists in the motor control literature with the notion of muscle and joint viscosity. The source of the confusion lies in attempts to study human movement using a simple mechanical concept that was proposed for uncomplicated objects such as a gel or gas. Although A.V. Hill (1938) wrote, “The 'viscosity' hypothesis must be dismissed” (p. 193), the concept of muscle/joint viscosity is still very popular in the literature. Distressingly, the reported values of muscle/joint viscosity differ greatly. Eleven dissimilar “viscosities” are briefly discussed. They represent disparate characteristics of the human motion system. It seems that the concept of viscosity is inadequate for describing motor control phenomena.

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Vladimir M. Zatsiorsky and Nikolai Yakunin

Despite relatively numerous studies, the biomechanics of rowing remains poorly understood. Much of the data is contradictory and, worse still, there appears to be a lack of consensus on the mechanical background of rowing, which leads to different approaches to the measurement of a number of parameters known by the same terms. This makes direct comparison impossible. For this reason, the present review divides the discussion of relevant biomechanical problems into two stages. The first stage involves the construction of mechanical models of the rowing element under consideration, compilation of equations, and determination of parameters used in these equations. In the second stage, experimentally obtained empirical data on these parameters are discussed.

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Vladimir M. Zatsiorsky and Marcos Duarte

The goal of this study was to explore the rambling-trembling decomposition in quiet standing. The center of pressure (COP) and the horizontal ground reaction force (Fhor) were registered in healthy subjects standing in an upright bipedal posture on a force platform. The COP positions at the instants when Fhor = 0 were identified (instant equilibrium points, IEP) for the anterior-posterior direction, then the COP time series, were partitioned into its components using 2 different techniques, rambling-trembling decomposition and gravity line decomposition. The two decomposition techniques provided very similar results. An unexpectedly large correlation between the trembling trajectory and the difference between COP and gravity line was found, r = 0.91 (range, 0.83 < r < 0.98). The correlation implies that the GL moves from an IEP to the subsequent IEP along a smooth trajectory that can be predicted by the spline approximation. A substantial negative cross-correlation at a zero time lag was observed between the trembling and the Fhor, -0.90 < r < -0.75. For the rambling trajectory, the coefficients of correlation with Fhor were low, -0.33 < r < -0.05. The data support the hypothesis that during quiet standing the body sways for two reasons: the migration of the reference point (rambling) and the deviation away from that point (trembling).

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Vladimir M. Zatsiorsky and Marcos Duarte

A method of decomposing stabilograms into two components, termed rambling and trembling, was developed. The rambling component reveals the motion of a moving reference point with respect to which the body's equilibrium is instantantly maintained. The trembling component reflects body oscillation around the reference point trajectory. The concepts of instant equilibrium point (IEP) and discrete IEP trajectory are introduced. The rambling trajectory was computed by interpolating the discrete IEP trajectory with cubic spline functions. The trembling trajectory is found as a difference between the approximated rambling trajectory and the COP trajectory. Instant values of the trembling trajectory are negatively correlated with the values of the horizontal ground reaction force at a zero time lag. It suggests that trembling is strongly influenced by a restoring force proportional to the magnitude of COP deviation from the rambling trajectory and acts without a time delay. An increment in relative COP position per unit of the restoring force, in mm/N, was on average 1.4 ± 0.4. The contribution of rambling and trembling components in the stabilogram was ascertained. The rambling variability is approximately three times larger than the trembling variability.

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Michael M. Morlock and Vladimir M. Zatsiorsky

The performance in bobsledding is influenced by several factors. This study concentrated on influences of the environment and the bobsled crew on the final time of a bobsled run. The analysis was performed with data collected during the four-man competition at the 1988 Winter Olympic Games in Calgary. It was shown that the start order, the ice temperature, and the push time together explain about 50% of the variance in the performance (α=0.05). It is suggested that the existing rule concerning the start order in a heat be modified to guarantee a fair competition. Selected speed and turn time variables were shown to give an indication of the characteristics and the important sections of the bobsled track at Canada Olympic Park. It is speculated that the optimization of turn times is more important than the increase in speed in a turn.

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Gert-Peter Brüggemann, Michael Morlock and Vladimir M. Zatsiorsky

Performance in bobsled and luge events is influenced by several environmental, material/equipment, and team-related factors. This study concentrated on the influences of equipment and athlete on overall performance and compared the luge, 2-man bobsled, and 4-man bobsled competitions at the 1994 Lillehammer Winter Olympic Games. Start time and overall acceleration in the analyzed straight section showed significant correlations with the final time. It was concluded that for the top teams in bobsled and luge, fast start time and high speed at the end of the start section were prerequisites for an excellent overall performance. Driving capacities in the most difficult sections of the track were more statistically important among the top 15 competitors, especially in the luge. The influence of the runners could not be identified in either bobsled or luge competitions.

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Mark L. Latash, Fan Gao and Vladimir M. Zatsiorsky

The method of multidimensional scaling was applied to matrices of finger interaction (IFM) computed for individual participants for finger force production tasks. When IFMs for young controls, elderly, and persons with Down syndrome were pooled, only two dimensions described interpersonal differences; these were related to total force and to the total amount of enslaving. When IFMs for each group were analyzed separately, subpopulation-specific dimensions were found. Potentially, this analysis can be applied to discover meaningful dimensions that reflect differences in indices of finger interaction across and within subpopulations which differ in their apparent ability to use the hand. It may also be useful for tracking changes in finger interaction that occur in the process of specialized training or motor rehabilitation.

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Koichiro Kanatani-Fujimoto, Betty V. Lazareva and Vladimir M. Zatsiorsky

A method for analysis of time-series data, local proportional scaling (LPS), is proposed and its applications in motor control and biomechanics are discussed. The method is based on comparison of two time curves: a reference curve x(t) and a test curve x'(t'). By assumption, x'(t') is received from x(t) by local affine transformations, local extensions/compressions along the x and t axes [x(t)→x'(t'), where → stands for the local extensions/compressions along the x and t axes]. The aim of the LPS method is to discover the underlying transformations, including gain indexes, time epochs, velocity quotients, time segments, and time quotients. The LPS method can be used for (a) comparing the time-series curves in a concise transparent manner; (b) scaling the curves, bringing x'(t') in conformity with x(t); (c) automatic segmentation of the time series data; and (d) data classification.

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Joel R. Martin, Mark L. Latash and Vladimir M. Zatsiorsky

This study investigated the effects of modifying contact finger forces in one direction—normal or tangential—on the entire set of the contact forces, while statically holding an object. Subjects grasped a handle instrumented with finger force-moment sensors, maintained it at rest in the air, and then slowly: (1) increased the grasping force, (2) tried to spread fingers apart, and (3) tried to squeeze fingers together. Analysis was mostly performed at the virtual finger (VF) level (the VF is an imaginable finger that generates the same force and moment as the four fingers combined). For all three tasks there were statistically significant changes in the VF normal and tangential forces. For finger spreading/squeezing the tangential force neutral point was located between the index and middle fingers. We conclude that the internal forces are regulated as a whole, including adjustments in both normal and tangential force, instead of only a subset of forces (normal or tangential). The effects of such factors as EFFORT and TORQUE were additive; their interaction was not statistically significant, thus supporting the principle of superposition in human prehension.