Evaluation of physical work capacity and leg muscle fatigue during exhaustive stair ascending evacuation

AbstractPhysical exhaustion can constrain human ascending work capacity and performance during emergency evacuation on stairs. The overall aim of this research was to explore and compare stair ascending capacities and physiological limitations during two different strategies: self-preferred pace in three different public stairways in the field, and controlled pace in a laboratory on a stair machine. Subjects of different ages, genders and body sizes were recruited from social media to simulate evacuation scenarios. The specific objective was to determine, through the combined analysis of oxygen uptake (VO2) and electromyography (EMG), whether local muscle fatigue (LMF) in the legs due to repetitive activity rather than cardiorespiratory capacity is the factor that limits ascending capacity.The average relative VO2highest reached 39-41 mL·kg-1·min-1 in the field tests, and 44 mL·kg-1·min-1 in the laboratory at 90% VO2max step rate (SR) L3. In the field tests, the VO2 and heart rate (HR) reached 89 to 95% and 89 to 96%, respectively, of the human capacity reported in literature. In the lab, the average %VO2max and %HRmax reached 94 and 97%, respectively. At the self-preferred pace, an ascent could be continued at a SR about 92-95 steps·min-1 on the stairs; while in the controlled pace ascent, the maximum duration could be sustained for about 2-6 minutes at 90% VO2max related to average SR 109 steps·min-1 on the stair machine. The EMG amplitudes (AMPs) were different in the two the ascending strategies, while neither of the VO2 uptake patterns were affected. During self-preferred ascent, the leg muscle AMPs showed a decreasing trend and median frequencies (MDFs) were unchanged or small decrease. In controlled ascent, the AMPs tended to increase and MDFs to decrease. The significant AMP decrease and the unchanged MDF in self-preferred ascent indicated reductions of muscle power production and possible fatigue compensation by reduction of speeds, which allowed subjects to continue their ascents until the end. In contrast, the significant increase of AMP and MDF decrease in the controlled pace were evidence of leg LMF. In order to interpret dynamic muscle activity changes over time, we developed the muscle activity interpretation squares (MAIS) to plot the muscle activity rate change (MARC) percentile points. In the self-preferred pace, the muscle fatigue recovery through power decrease and pace reduction was reflected in the MARC percentile points that appeared in the MAIS. In the controlled pace ascent, the MARC points were in the muscle fatigue square. Thus, MARC and MAIS are found to be useful to observe muscle activity changes during dynamic tasks. High and constant intensity (97% of HRmax) ascents were evidenced by hyperventilation and LMF due to insufficient recovery that forced the subjects to stop the ascents before 5-min. During 90% VO2max, LMF presumably prevented the VO2 from reaching VO2max and limited the ascent duration to 4.32 minutes. The results infer that leg LMF combined with cardiorespiratory capacity constrain the stair ascending capacities while any recovery can extend the tolerance. (Less)