Abstract
The hand trajectory of motion during the performance of one-dimensional point-to-point movements has been shown to be marked by motor primitives with a bell-shaped velocity profile. Researchers have investigated if motor primitives with the same shape mark also complex upper-limb movements. They have done so by analyzing the magnitude of the hand trajectory velocity vector. This approach has failed to identify motor primitives with a bell-shaped velocity profile as the basic elements underlying the generation of complex upper-limb movements. In this study, we examined upper-limb movements by analyzing instead the movement components defined according to a Cartesian coordinate system with axes oriented in the medio-lateral, antero-posterior, and vertical directions. To our surprise, we found out that a broad set of complex upper-limb movements can be modeled as a combination of motor primitives with a bell-shaped velocity profile defined according to the axes of the above-defined coordinate system. Most notably, we discovered that these motor primitives scale with the size of movement according to a power law. These results provide a novel key to the interpretation of brain and muscle synergy studies suggesting that human subjects use a scale-invariant encoding of movement patterns when performing upper-limb movements.
Highlights
Our results are in line with experimental observations by Sosnik et al.[46], who provided evidence of the existence of basic units of movement that subjects perform to completion even if told to stop right after the initiation of a movement unit
Table 1 provide a summary of the results of the study
Figure 6A shows the values of the correlation coefficients estimated to assess if the movement elements matched the values predicted by
Summary
Our results are in line with experimental observations by Sosnik et al.[46], who provided evidence of the existence of basic units of movement that subjects perform to completion even if told to stop right after the initiation of a movement unit. Stopping the movement only after completion of the movement element that has been already initiated may be a strategy to preserve the smoothness of movement This observation suggests that the movement elements discovered in our study are not an epiphenomenon of more complex, high-level control mechanisms associated with the generation of human movements, but they are basic building blocks utilized by the central nervous system to generate complex upper-limb movement
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