## 5 Conclusions and Further Research  
The need for quantifying the accuracy of individuals in executing reaching motor tasks of different difficulty is a problem of relevant importance in different human activities. In work environment, operations' tolerance needs to be consistent with human accuracy (e.g., human-robot interaction). In rehabilitation programs, individuals with disabilities are required to follow a nominal trajectory while minimizing the distance between actual and nominal positions so as to indirectly estimate the psychomotor abilities.  
In this paper, the authors propose a 'Speed-ID-Accuracy' model extending previous studies on accuracy of individuals in executing simple motor tasks referred to the Fitts' law. The model proposed extend the investigations toward two directions: firstly, it considers a general geometry of the trajectory which is constrained by continuously variable width of targets; furthermore, the accuracy of individuals in executing such a general motor task is considered to be dependent not only on task speed but also on the ID calculated for the assigned geometry of the motor task. A measure of accuracy is proposed; it is defined as the ratio between the standard deviation of the endpoint position and the final target width so as to take into account the precision of individuals in meeting target of different sizes.  
The model proposed can be applied to a general case provided the geometry of the nominal trajectory (s = s(x)) and the shape of continuously variable target width (W = W(s)) as well as the related experimental data on average speed and on errors of individuals in acquiring the final target are given.  
The model has been applied to two numerical cases concerning two different types of reaching motor tasks: the 'Tunnel Traveling Task' and the 'Circular Traveling Tasks'. The model parameters have been obtained by regressing experimental data available in the literature. In both cases the model well fitted experimental data and revealed robust in appreciating the dependency of accuracy on task speed execution and ID values. The classical 'Speed-Accuracy' model proved to be inaccurate in assessing the accuracy of more complex reaching motor tasks as it neglects the effect of task complexity.  
To extend the model applicability, further research should be focused on experiments on differently aged and sexed individuals in executing predefined movements along trajectories of whatever geometry. Characterizing the accuracy along complex trajectory could reveal a resource- and time-consuming task. To overcome such a limit, the model proposed could be fed by experimental data captured by motion tracking systems in both labs and working environments.  
All authors contributed equally to the manuscript, in particular as regards conceptualization, methodology, software, investigation, writing--original draft, and writing--review, and editing. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Italian Ministry of Education, Universities and Research (MIUR), SO4 SIMS project (Smart Operators 4.0 based on Simulation for Industry and Manufacturing Systems-Project PRIN-2017 FW8 BB4).  
The authors declare no conflict of interest.