Heart rate and oxygen consumption during active psychological challenge: the effects of level of difficulty.

Heart rate and various metabolic and ventilatory indices were monitored while 24 young males engaged in two psychologically challenging tasks, mental arithmetic and Raven's matrices. Each task was structured to present subjects with three levels of difficulty: easy, hard, and impossible. Measurements were also made while subjects undertook graded isotonic exercise on a bicycle ergometer; for each subject, heart rate was plotted against oxygen consumption over the various exercise loads. Knowing oxygen consumption during the psychological tasks, these regression equations permitted the calculation of expected heart rates during each task condition, and thus the computation of ‘additional heart rate’ as the difference between actual and predicted heart rate values. Cardiac activity, whether represented as additional heart rate or as the difference between resting and task heart rate levels, was sensitive to variations in difficulty level. In both tasks the easy condition elicited significantly less cardiac activity than both the hard and the impossible conditions. Changes in oxygen consumption and carbon dioxide production did not vary significantly with difficulty. Subjects' self-reports of active engagement and arousal paralleled the cardiac effects; in both tasks the easy condition was experienced as relatively unengaging and unarousing.