This argument is mainly due to later writing rather than Taylor's work as Taylor stated "The task is always so regulated that the man who is well suited to his job will thrive while working at this rate during a long term of years and grow happier and more prosperous, instead of being overworked."  Taylor's concept of motivation left something to be desired when compared to later ideas.His methods of motivation started and finished at monetary incentives.
Taylor places the blame squarely on management and their inability "to do their share of the work in cooperating with the workmen."  Taylor's attitudes towards workers were laden with negative bias "in the majority of cases this man deliberately plans to do as little as he safely can."  The methods that Taylor adopted were directed solely towards the uneducated. This type of behaviour towards workers appears barbaric in the extreme to the modern reader, however, Taylor used the example of Schmidt at the Bethlehem Steel Company to test his theories. And indeed it would be if applied to an educated mechanic, or even an intelligent labourer."  The fact that Taylor took the effort to firstly know the workers name and to cite it is some indication that he empathized with the workforce.
In Taylor's view, supervisors and workers with such low levels of education were not qualified to plan how work should be done. Perhaps the key idea of Scientific management and the one which has drawn the most criticism was the concept of task allocation.
Taylor's solution was to separate planning from execution. Task allocation  is the concept that breaking task into smaller and smaller tasks allows the determination of the optimum solution to the task.
The figure of 21 pounds  was arrived at by the study.
To ensure that this shovel load was adhered to, a series of different shovels were purchased for different types of material.