The Fight on the Flights : Emergency evacuations – human physiological performance, leg muscle activity and gait biomechanics during exhaustive stair and slope ascent

Physical exhaustion can constrain stair ascending capacity during emergency evacuation. The overall aim of this research was to explore and compare stair ascending capacities and physiological limitations when using two different modes: 1) self-preferred pace on three different public stairways, and 2) four machine-controlled paces on a stair machine corresponding to different percentages of maximal aerobic capacity (V̇O2max). After the exhaustive stair ascent, gait biomechanics were also studied when walking on an inclined metal walkway in the laboratory. Participants of different ages, genders and body sizes were recruited from social media. The specific objective was to determine, through the combined analysis of oxygen uptake (V̇O2) and electromyography (EMG), how cardiorespiratory capacity and local muscle fatigue (LMF) in the leg constrain the ascending capacity and affect walking gait kinetics and kinematics.The results showed that the average relative maximum oxygen uptake during stair ascent (V̇O2highest) reached 39-41 mL·min-1·kg-1 at the self-preferred pace in the field, and 44-45 mL·min-1·kg-1 at the controlled step rate (SR) corresponding to 90-100% V̇O2max in the laboratory. During ascent at the self-preferred pace, both V̇O2highest and heart rate (HRhighest) reached about 83-95% level of average human capacity reported in literature. During ascent at 90-100% V̇O2max SRs, the V̇O2highest reached about 92-94% of V̇O2max, while HRhighest peaked between 91 and 97% of HRmax. The SR was sustained at 92-95 steps·min-1 at the self-preferred pace on the stairs to complete the ascents in a 13-floor and 31-floor building. The average ascending durations of 4.3 and 3.5 minutes were recorded at an average SR of 109 and 122 steps·min-1 corresponding to 90 and 100% V̇O2max, on the stair machine. A physiological evacuation model was developed based on individual V̇O2max. The model proved to be useful in estimating step rate and vertical displacement, thus it is recommended for calculating the performance as such speed, height during stair ascent evacuation. The EMG amplitudes (AMPs) were different between the self-paced and controlled ascending speeds. During self-preferred ascent, the leg muscle AMPs showed a decreasing trend and the median frequencies (MDFs) were unchanged or slightly decreased indicating reductions of muscle power production and possible fatigue compensation by speed reduction. This allowed recovery to complete the ascents. In contrast, a significant increase of AMPs and decrease of MDFs were observed in the controlled paces evidencing the leg LMF. A muscle activity interpretation squares (MAIS) model was developed by plotting the muscle activity rate change (MARC) percentile points to interpret dynamic muscle activity changes and fatigue over time. At the self-preferred paces, the MARC points in the MAIS reflected recovery from muscle fatigue through power decrease and pace reduction. At the controlled paces, in contrast, the MARC points reflected muscle fatigue. Thus, MARC and MAIS are useful for observing muscle activity changes during repetitive tasks. Constant ascents at maximal intensity (90-100% V̇O2max) resulted in high lactate production and leg LMF due to high demand and insufficient recovery. This forced the subjects to stop within 5-min. The results infer that the combined effect of cardiorespiratory capacity exaggerated by leg LMF constrained stair ascending capacities, durations and vertical distances, thus restricting the V̇O2 uptake from reaching the V̇O2max, while any recovery can extend the tolerance. Finally, when walking up a 10° inclined surface after exhaustive stair ascent, the peak gait ground reaction forces, peak and minimum foot absolute angles, peak foot angular velocity and acceleration all significantly decreased with an increased required coefficient of friction. The altered gait biomechanics on inclines can affect the human locomotion and impede the evacuation process during emergencies