Limitations of oxygen uptake and leg muscle activity during ascending evacuation in stairways

- Stair ascending at self-preferred speed is constrained by both aerobic and muscle capacity.
- Heart rate and oxygen uptake reached 89-96% of the maximal capacity.
- Leg EMG amplitudes decreased significantly reflecting speed reduction for fatigue prevention.
- Muscle activity interpretation squares (MAIS) were developed to interpret muscle fatigue.
- A reduced pace in an affordable level avoids fatigue resulting in an extended ascent.

Stair ascending performance is critical during evacuation from buildings and underground infrastructures. Healthy subjects performed self-paced ascent in three settings: 13 floor building, 31 floor building, 33 m stationary subway escalator. To investigate leg muscle and cardiorespiratory capacities and how they constrain performance, oxygen uptake (VO2), heart rate (HR) and ascending speed were measured in all three; electromyography (EMG) in the first two. The VO2 and HR ranged from 89 to 96% of the maximum capacity reported in the literature. The average highest VO2 and HR ranged from 39 to 41 mL·kg−1·min−1 and 162 to 174 b·min−1, respectively. The subjects were able to sustain their initial preferred maximum pace for a short duration, while the average step rate was 92-95 steps·min−1. In average, VO2 reached relatively stable values at ≈37 mL·kg−1·min−1. EMG amplitudes decreased significantly and frequencies were unchanged. Speed reductions indicate that climbing capacity declined in the process of fatigue development. In the two buildings, the reduction of muscle power allowed the subjects to extend their tolerance and complete ascents in the 48 m and 109 m high stairways in 2.9 and 7.8 min, respectively. Muscle activity interpretation squares were developed and proved advantageous to observe fatigue and recovery over time.