Oxygen uptake and muscle activity limitations during stepping on a stair machine at three different climbing speeds

This laboratory study examined human stair ascending capacity and constraining factors including legs' local muscle fatigue (LMF) and cardiorespiratory capacity. Twenty-five healthy volunteers, with mean age 35.3 years, maximal oxygen uptake (VO_2max) of 46.7 mL·min^-1·kg^-1 and maximal heart rate (HR) of 190 bpm, ascended on a stair machine at 60 and 75% (3 min each) and 90% of VO_2max (5 min or until exhaustion). The VO₂, maximal heart rate (HR_max) and electromyography (EMG) of the leg muscles were measured. The average VO_2highest reached 43.9 mL·min^-1·kg^-1, and HR_highest peaked at 185 bpm at 90% of VO_2max step rate (SR). EMG amplitudes significantly increased at all three levels, p < .05, and median frequencies decreased mostly at 90% of VO_2max SR evidencing leg LMF. Muscle activity interpretation squares were developed and effectively used to observe changes over time, confirming LMF. The combined effects of LMF and cardiorespiratory constraints reduced ascending tolerance and constrained the duration to 4.32 min. Practitioner Summary: To expedite ascending evacuation from high-rise buildings and deep underground structures, it is necessary to consider human physical load. This study investigated the limiting physiological factors and muscle activity rate changes (MARC) used in the muscle activity interpretation squares (MAIS) to evaluate leg local muscle fatigue (LMF). LMF and cardiorespiratory capacity significantly constrain human stair ascending capacities at high, constant step rates.