Physiological Capacity During Simulated Stair Climbing Evacuation at Maximum Speed Until Exhaustion

Eighteen healthy university students evenly divided by gender without any musculoskeletal problems, history of any major illness or disabilities were selected (Table 1). To determine eligibility, the test subjects completed a questionnaire that revealed the basic information about their health including history of diseases and medications. Written informed consent was obtained from all participants. A health declaration form was also delivered in advance and filled in by the subjects.

The subjects received verbal and written information about the test procedures twice: once during the recruiting period and once before the tests. The experiments were conducted on two different days: on day 1, the \( {\dot{V}}{\text{O}}_{\rm 2max} \) was measured on a treadmill; on day 2, the main experiment involving stair-ascending movement to simulate evacuation was conducted. The protocol is described in the schematic flowchart in Fig. 1. The subjects were asked to refrain from drinking alcohol and exercising at least 24 h before the tests. They were also requested to avoid heavy meals, or drinking coffee or tea for at least 2 h before the test. They were asked to do a trial walk on the treadmill and a trial ascent on the stair machine before the actual tests, if they were not familiar with the instruments. The subjects were instructed that they might terminate the test at any time without providing any reason. The methods and procedures used in this study comply with the [56]. The Regional Ethical Review Authority in Lund, Sweden also approved the protocol (Dnr. 2016/1061).

Three isometric maximum voluntary contractions (MVCs) of the dominant knee extensors (VL and RF), ankle plantar (GL) and dorsiflexor (TA) were performed and the EMG recorded prior to the stair-ascending test. To obtain the MVCs of the individual subject, one of the investigators applied maximal manual isometric resistance at midrange for the respective muscle. Each MVC duration lasted for 3–5 s with a development of the contraction for about 1–2 s for the RF, GL, and TA muscles when laying supine on a plinth except the VL, which was obtained in sitting.

Ascending durations (ADs) were varied depending on the individuals’ capacity. Most of the participants reported pain in their legs, breathlessness, and rated the simulated evacuation as “extremely tough”. The post-ascent physiological and related values including \( {\dot{V}}{\text{O}}_{\rm 2highest} \), t (17) = − 3.35, p > .01; HR_highest, t (17) = − 9.79, p > .01; breathing frequency (BF), t (17) = − 3.35, p > .01 were significantly lower than the values during \( {\dot{V}}{\text{O}}_{\rm 2max} \) test; but not the minute ventilation (\( {\dot{V}}E \)). On the contrary, blood lactate (BLa⁻), rating of perceived exertion (RPE), and respiratory exchange ratio (RER) were higher without significant differences than those during the \( {\dot{V}}{\text{O}}_{\rm 2max} \) tests. The main results of the stair-ascending tests are listed in Table 2.

The main findings of the study showed that the subject could manage an ascending for an average of 3.5 (1.2) min, at the constant SR corresponding to 100% of individual \( {\dot{V}}{\text{O}}_{\rm 2max} \) in a simulated evacuation. This result supports the first hypothesis of the ascending duration (AD). In present study, the mean (standard deviation, SD) SR was 122.2 (9.7) steps min⁻¹. An average vertical distance (V_height) reached was 85.5 (32.1) m at a vertical displacement of 24.4 (1.9) m min⁻¹, when the calculated ascending speed (AS) was 0.66 (0.05) m s⁻¹. This calculated V_height is approximately 29 floors. These are in agreement with the previous results of average AD 4.3 min when the subjects reached a calculated V_height 95.0 m at an AS of 0.59 m s⁻¹ at the 90% of \( {\dot{V}}{\text{O}}_{\rm 2max} \) level with reasonably a lower SR 109.4 steps min⁻¹ [23]. The novel datasets presented in these current and referenced papers [23, 24] provide worthwhile information for both fire safety researchers and engineers to conduct fire safety design as well as the calibration of evacuation modelling tools. The results of this study also validate the previously developed evacuation model, that is being used to calculate the SR and V_height [33].

In this controlled study, the speed of the stair machine was fixed due to the stair machine’s technical limitations. The subjects might have been able to prolong their effort in a real life emergency by slowing down according to the individual physiological capacities, situations, conditions and knowledge of the evacuation path. These results could help to estimate the appropriate V_height for arranging safety measures, such as where to place resting planes or egress facilities in long staircases of deep underground constructions to ensure a safe evacuation. However, these results should be applied carefully to real life situations when considering the step height and depth. In the recent field stair-ascending study on 13 and 31 floor building stairways, the average SRs were 92 and 95 steps min⁻¹, respectively, in the self-preferred pace. The continuous reduction of ASs through SR were observed about or after 1 min of the onset [24]. Therefore, the suggested tolerance limit of ascending could be about 2–6 min at maximum intensity and constant rate. \( {\dot{V}}{\text{O}}_{\rm 2highest} \) can be achieved very shortly too (within ~ 1 min) during this study’s constant and very-fast exercise [42, 53]. Vanhatalo et al.’s study recommended that a constant and high intensity work or an all-out effort duration should be 3 min to reach exhaustion. In addition, Holmér and Gavhed [27] reported that people could manage to maintain an activity level for about 5 min at their maximum intensity [27]. Moreover, the results of this specific study are limited to healthy and young adult participants. It is therefore, expected that the observed performance might be different on a more diverse population, but the previous study at 90% \( {\dot{V}}{\text{O}}_{\rm 2max} \) SR with multiple demographic characteristics supported this and was in-line with these present results.

The mean \( {\dot{V}}{\text{O}}_{\rm 2} \) kinetics showed that it reached a relative stable state following the exponential increase during the 60-70% normalized time (Fig. 3a); however, the termination of the ascents was obvious and happened immediately. Thus, this result rejects the second hypothesis of this simulated evacuation study. The obtained \( {\dot{V}}{\text{O}}_{\rm 2} \) pattern demonstrated that a relative stable state could be reached, even at the end of this constant and intense step rate (SR) while reaching exhaustion. This \( {\dot{V}}{\text{O}}_{\rm 2} \) kinetics evidenced that most of these ADs were sufficiently long ≥ 1½–2 min to achieve the relative stable states [26, 41]. This \( {\dot{V}}{\text{O}}_{\rm 2} \) kinetics after the exponential increase showed several fluctuations suggesting that the sustainment of the SR or work tolerance [17] within this heavy work intensity domain [9, 10, 29, 44], extended thus the AD from about 70 to 100% periods. The relative \( {\dot{V}}{\text{O}}_{\rm 2} \) stabilization at the end was related to the fact that the subjects could tolerate this constant load of stair-ascents for a short period as the fatigue process had already started [13, 30, 53, 59]. Thus, the ascending SR influences the \( {\dot{V}}{\text{O}}_{\rm 2} \) kinetics and the V_height. The estimated SR was based on the ramp responses (\( {\dot{V}}{\text{O}}_{\rm 2max} \) tests). These obtained results suggest that \( {\dot{V}}{\text{O}}_{\rm 2} \) component of any heavy-intensity activity, which was based on the ramp responses, should be further studied.

In this study, neither the \( {\dot{V}}{\text{O}}_{\rm 2highest} \) nor the HR_highest reached 100% of \( {\dot{V}}{\text{O}}_{\rm 2max} \), supporting the hypothesis. These post-ascents lower cardiorespiratory capacities than the \( {\dot{V}}{\text{O}}_{\rm 2max} \) test evidenced that the ascent was ended before reaching the subjects maximal level (Table 2). Only the \( {\dot{V}}{\text{O}}_{\rm 2highest} \) of the six subjects’ were ≥\( {\dot{V}}{\text{O}}_{\rm 2max} \). However, the achieved average \( {\dot{V}}{\text{O}}_{\rm 2highest} \), 44.8 mL min⁻¹ kg⁻¹ was reasonably higher than the value of 43.9 mL min⁻¹ kg⁻¹ from the previous evacuation experiment at 90% of \( {\dot{V}}{\text{O}}_{\rm 2max} \) SR [23]. In the evacuation study in different buildings at self-selected ascending pace, the average \( {\dot{V}}{\text{O}}_{\rm 2highest} \) and HR_highest ranged from 39 to 41 mL min⁻¹ kg⁻¹, and 163 to 174 b min⁻¹, respectively. Moreover, the mean stable \( {\dot{V}}{\text{O}}_{\rm 2} \) was found to be between 37.2 and 38.5 mL min⁻¹ kg⁻¹, and while the mean stable HR was between 163 and 168 b min⁻¹, when the subjects managed ascending both 13 floor (3 landings/floor) and 31 floor (2 landings/floor) buildings at 92–95 steps min⁻¹ upon completion [24]. These stable \( {\dot{V}}{\text{O}}_{\rm 2} \) and HR values for ascending on those two buildings were found ≈ 75–80% of \( {\dot{V}}{\text{O}}_{\rm 2max} \) corresponding SR tests those were used in the previous lab study [23], and ≥ 85% of average maximal capacities reported in the databases [34]. The intensity of those field study’s’ SRs (≥ ventilatory threshold) seemed tolerable enough to be able to continue ascending for a long duration without being exhausted, contrary to the SR used in this study.

The reasons for the achieved lower levels of physiological values in the stair-ascent evacuation than those in the \( {\dot{V}}{\text{O}}_{\rm 2max} \) tests were because of the two different types of activities with different durations: incremental ramp test to the simulated stair-ascending movement test for evacuation maintained at a constant work rate. However, stair-ascending movement required working against gravity and local muscle fatigue (LMF) might precede the whole body exhaustion due to very highly repetitive activity of the leg muscles. The static posture of the upper extremities might have reduced oxygen demand and contributed to a lower \( {\dot{V}}{\text{O}}_{\rm 2highest} \) values during the ascents than the \( {\dot{V}}{\text{O}}_{\rm 2max} \) tests [32, 42]. These healthy subjects were allowed to hold the handles for support and the engaged muscles were somewhat different in stair-ascending compared to running on a treadmill.

EMG activations of all four muscles represented how the leg muscle activity influenced the stair-ascending performance at maximum and constant intensity. High velocity muscular efforts were required to maintain the intense step rate (SR) that were also reflected by the initial high and progressive AMP values. The development of \( {\dot{V}}{\text{O}}_{\rm 2} \) appears to be associated with the gradual recruitment and rate coding of motor units of fast-twitch type II. However, type IIx motor units, although have the largest number of fibers, which are also metabolically fatigable, and thus less efficient for prolonged exertion. The significant increase in EMG AMP 10th percentile values (Tables 3, 4) were found for all four major leg muscles including two thigh (VL and RF); calf (GL) and TA of the anterior leg measured in this study, while the MDFs were significant decreased mainly the antagonists muscles, vastus lateralis (VL) and tibialis anterior (TA). The different fatigue patterns were observed between the mono- (VL) and bi-articular, rectus femoris (RF) muscles in EMG recording, which was similar to the study by Ebenbichler et al. [20] (Fig. 4). The initial AMP increase within 1 min indicated the recruitment of fast-twitch fibers characterized by high force production but low endurance, and that the fatigued motor units had been replaced spontaneously to maintain the high intensity ascending [8]. The subjects tried to minimize the use of the calf, gastrocnemius lateralis (GL) and other thigh muscles in this experiment during exhaustion by off-loading their body weight through their forearms while holding on to the handles of the stair machine. The decreased MDF results of this study were similar to the progressive decrease of MDF results that were found during a fast ramp cycling exercise [52].

The MARC points at the 90–100% periods of all four measured muscles appeared in or at least close to the muscle fatigue squares in the MAIS. This supports the muscle activity assumptions [15]. The MARC points were distributed between the muscle force increase and muscle fatigue squares. Some MARC points of the VL, RF and GL muscles during the 60–80% period were not found in the muscle fatigue square but appeared in the muscle force increase square, showing the evidence of decreasing MDFs (Fig. 5). This suggests that the subjects were maintaining the highest possible force productions to comply with the machine-controlled high SR before reaching fatigue. This interpretation advocates that the MARC points in the MAIS may be used for observing muscle activity changes over time.

The achieved physiological results of ascents showed the extreme nature of the tasks. The subjects’ blood lactate (BLa⁻), respiratory exchange ratio (RER), breathing frequency (BF), and rating of perceived exertion (RPE) values suggest that they were super-exhausted. The average post stair-ascending BLa⁻ level increase was similar to the post-\( {\dot{V}}{\text{O}}_{\rm 2max} \) test, both were 14 mmol l⁻¹. The BLa⁻ range obtained in this study is also comparable to Goodwin et al.’s reported value (15–25 mmol l⁻¹) during 3-8 min long “all-out” maximal exercise intensity [22]. The high repetitive SR increased the energy demand and provoked the high BLa⁻ productions, leading to the progressive increase of \( {\dot{V}}{\text{O}}_{\rm 2} \), eventually termination of the ascents just immediately after reaching the relative stable state [50, 51, 55]. The subjects’ mean RER calculated 1.2 at the end and their mean RPE calculated on average 18.2 out of 20 in Borg’s scale, which is maximal exertion. These high values were higher for the post-ascending test (Tables 1, 2) provide the evidence of hyperventilation [54], when BF reached 51 min⁻¹. These above results suggest that this constant SR was above the lactate threshold at which the subjects reached their limits [38]. As the SR rate was above the lactate threshold, the attainment of a steady state was reached slowly before quitting the ascents. Thus, those high BLa⁻, RER and RPE values also met the required level of the secondary but disputable [42, 43] \( {\dot{V}}{\text{O}}_{\rm 2max} \) criteria, which also indicates the subjects reached the fatigue threshold [7, 28].

Most of the subjects also reported “burning and painful legs”, in the thighs and/or calfs. The very-fast SR induced local muscle fatigue (LMF) of the legs in advance of the anticipated maximum capacity leading to the discontinuation of the ascents and restricted the \( {\dot{V}}{\text{O}}_{\rm 2highest} \) to reach \( {\dot{V}}{\text{O}}_{\rm 2max} \) [11]. Additionally, the recorded mean minute ventilation (\( {\dot{V}}E\)) value at the end of ascents was also lower than the \( {\dot{V}}{\text{O}}_{\rm 2max} \) tests mean value indicating an intensive ascending activity. These results suggest that the developed anaerobiosis in the leg muscles constrained the subjects’ ascending performance as well as the \( {\dot{V}}{\text{O}}_{\rm 2} \) capacity. The unprecedented stop of the ascents by the subjects was due to lack of energy supply to the working muscles because of the impaired oxygen supply or metabolism in response to the excessive demand to maintain the high SRs. The results revealed that physical exhaustion takes place very quickly, thus impaired evacuation performance, if the speed is at the maximum level and constant speed. This should be considered when deciding the SR in case of long ascending evacuation on the stairs. In order to simulate a more relevant and practical evacuation situation for determining the stair ascending endurance, a laboratory study is recommended allowing subjects to control their own speeds after starting the ascents at their maximum levels. Further experiments are required too to ascertain the recovery periods after the unprecedented stops due to exhaustion at different strategies or speeds in order to prolong the onset of fatigue or continue ascending at different heights for evacuation.