Human tolerance to physical work: the relationship of the power output, the metabolic condition and the attentive state in the performance of exhausting physical work

Homeostasis and disequilibrium characterised some of the responses of male racing cyclists working against a cycle ergometer load that increased continuously to exhaustion. Normal linear work states of adequate compensation and limited work states of disequilibrium were identified mathematically for the metabolic variables (HR, VE, O2F, VEO2). Also associated were changes in the variables describing the mental state (laboratory pain, threshold shift for awareness of sound and pressure at exhaustion, blood cH and extraversion). These latter were developed on the hypothesis of a biological filter for sensory impulses which was tentatively identified with the reticular activating system of the brain stem. The statistics for the rates of change of the metabolic variables in the two states and for their maximum values, and the measures of the mental state were both very predictive of the criterion of the maximum work ability. On this work criterion there was a multiple correlation of 0.89 with the maximum oxygen fraction and the resistance to limitation of this fraction, and of 0.70 with the two measures of threshold shift. From the multiple interrelationships of the variables, efficient oxygen provision was the outstanding factor. Measures of the mental state and the metabolic condition interrelated on two factors that appeared to derive from the modification of afferent impulses by the central nervous system during disequilibrium. Four statements follow: (1) with increasing demand, homeostasis and disequilibrium are ordered hierarchically over the physiological and mental functions; (2) when a metabolic demand may be met by a number of functions the most efficient is utilised, that is, the one which requires the least energy for its own mechanics; (3) the appearance of disequilibrium sets a finite limit to the satisfaction of a demand; (4) over the healthy human range of competence for exhausting physical work the predictive limiting factors may alter from thermal regulation in the poorer workers to the ability of the higher efficiency metabolic processes and eventually to that of the lower efficiency processes in the best workers. The filtering of impulses through the central nervous system is involved at all of these levels but the detail is not clear.