Phase Transition Defines Steady State Beyond Threshold

SIGNIFICANCE: Research describes the importance of threshold identification in performance as an indication of maximal steady state beyond which the indices of fatigue are soon realized. The rate change is inappropriately identified as a specific point leading to the onset of fatigue. It is hypothesized that physiological rate changes are more appropriately identified as phase transitions of acute adaptations. This identification would provide new paradigms for endurance athletic performance. The purpose of this study is twofold: 1) to determine the physiological kinetics during steady work beyond physiological thresholds, and 2) to establish a paradigm where physiological thresholds are viewed as phase transitions accommodating a greater workload. DESIGN: Subjects were nineteen male (n=13) and female (n=6) subjects. Data analyses included the non-invasive measures: expired gases (VO₂, VCO₂, RER), time to exhaustion (TE) and heart rate (HR). Approval by the University IRB and informed consent were obtained. Subjects performed two tests on a cycle ergometer: 1) graded exercise stress test (GXT) and 2) performance test (PT) to volitional fatigue. The GXT established peak capacities and physiological thresholds. Thresholds were ventilatory (VT) utilizing the V-slope method, and heart rate (HRT) utilizing a crossover of a logarithmic regression line of best-fit with a linear regression line of best fit. During PT, subjects pedaled at a self-selected cadence at a prescribed power output. PT power was calculated from the GXT as the median between the power at threshold and the power at volitional fatigue. Measures during PT included gas analysis, heart rate and TE. Descriptive statistics and linear regressions for determining slope of rate change during the PT were performed for data analyses. Significance was set at p < 0.05. RESULTS: Mean (SD) GXT values indicated a fit population: VO₂max= 56.7 (9.2) ml/kg/min.; HRmax=191 (8.2) bpm; VT as % of max= 74.4 (9.8). PT measures at steady state indicated high intensity work output: HR= 181.6 (9.3) bpm; VO₂= 46.6 (7.1) ml/kg/min.; RER=1.1 (.1); TE= 9.9 (4.2) min.; steady state VO₂ as % of max= 83.1 (9.8); steady state HR as % of max=95.2 (4.7). CONCLUSIONS: The following conclusions were established: 1) physiological kinetics indicate low slope increases in VO₂ (.07), VCO₂ (.09), RER (0) and HR (2.1); 2) high intensity work indicated by VO₂ and HR during PT, and mean TE of ~10 min. show adaptation responses to maintain steady state. Therefore, physiological thresholds and work beyond these thresholds define a phase transition.
Keyword(s): exercise/fitness, performance, research