As already mentioned, and within certain limits, the heart rate increases linearly with the work performed, provided this is dynamic not static and is performed with a steady rhythm.
When the work is comparatively light, the pulse rate increases quickly to a level appropriate to the effort, then remains constant for the duration of the work. When work ceases, the heart returns to its normal beat after a few minutes.
With more strenuous work, however, the heart rate goes on increasing until either the work is interrupted or the operator is forced to stop from exhaustion. Figure 6.9 shows diagrammatically the behavior of the pulse during certain work studies.