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 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 7  |  Issue : 2  |  Page : 204-210

Effect of long-distance load marching on physiological responses at desert environment


Department of Ergonomics, Defence Institute of Physiology and Allied Sciences (DIPAS), Defence Research and Defence Organisation (DRDO), New Delhi, India

Date of Submission09-May-2022
Date of Decision05-Jun-2022
Date of Acceptance10-Jun-2022
Date of Web Publication06-Dec-2022

Correspondence Address:
Madhusudan Pal
Defence Institute of Physiology and Allied Sciences (DIPAS), Defence Research and Defence Organisation (DRDO), Lucknow Road, Timarpur, New Delhi - 110 054
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/bjhs.bjhs_73_22

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  Abstract 


AIM: The present study was undertaken to examine the effect of long-distance load marching on the physiological responses, walking efficiency, and mobility of soldiers at sandy desert environment.
METHODS: A total of nine physically fit soldiers (age, 30.00 [SE 0.9] years; height, 175.56 [SE 1.18] cm; and weight, 77.83 [SE 1.37] kg) volunteered for long-distance (6 km) load marching by self-selected speed when carrying 22 kg (28.27% of body weight [BW]) load and no load (NL). Heart rate (HR), respiratory frequency (RF), and core body temperature (CBT) were recorded. Relative workload (RWL) and Physiological Cost Index (PCI) were calculated to assess the work intensity and mobility.
RESULTS: It was observed that HR (P = 0.01), RWL (P = 0.01), and PCI (P = 0.01) were significantly increased by 23.79, 39.71, and 77.73%, respectively, and mobility was significantly reduced (P = 0.01) by 18.56% during marching with 22 kg load as compared to NL.
CONCLUSION: It may be that marching without external load at speed of 4.85 ± 0.54 km/h should not be continued for more than 2 h, whereas marching with load of 28.27% of BW at speed of 3.95 ± 0.55 km/h should be restricted to maximum 30 min. This combination of load, speed, and duration provides information about soldiers' routine load carriage tasks, which may help in optimizing the load carriage task to reduce the chances of cumulative carriage-related injuries and, therefore, may contribute to improved soldiers' operational readiness and mobility.

Keywords: Load marching, long distance, mobility, physiological responses, sandy terrain, soldiers


How to cite this article:
Arya K, Yadav A, Bhattacharyya D, Chatterjee S, Pal M. Effect of long-distance load marching on physiological responses at desert environment. BLDE Univ J Health Sci 2022;7:204-10

How to cite this URL:
Arya K, Yadav A, Bhattacharyya D, Chatterjee S, Pal M. Effect of long-distance load marching on physiological responses at desert environment. BLDE Univ J Health Sci [serial online] 2022 [cited 2023 Jan 28];7:204-10. Available from: https://www.bldeujournalhs.in/text.asp?2022/7/2/204/362841



Load carriage plays a fundamental role in military operations. Soldiers are required to carry load, which comprises protective ensemble, combat equipment, and sustainment stores. When carrying this load, the military personnel are expected to patrol for longer durations in challenging terrain conditions.[1] Various studies have established that an individual's load-carrying capacity is substantially influenced by multiple factors such as gender,[2],[3] body characteristics and composition,[4],[5],[6] familiarization of the task,[6] training,[3] gradient,[7] load distribution,[6],[8] load magnitude,[3] speed,[9],[10] and load carriage equipment design.[11],[12],[13],[14] The surface characteristics (e.g., bitumen, sand, and swamp) of the terrain also play an important role when considering load carriage capacity and energy cost of the military personnel.[6],[10],[15]

Soldiers are exposed to multiple physical stressors when deployed to harsh environmental conditions, which increases their risk of musculoskeletal injuries. According to Roy et al.,[16] the most common cause of musculoskeletal injuries is lifting and carrying heavy loads. Overexertion caused by excessive weight reduces a soldier's perceived ability to fight,[16] resulting in higher causalities and mission failure. Furthermore, thermal environment of the area of deployment also has a significant impact on the military personnel's ability to carry the required load. Studies have shown that hyperthermia directly affects energy metabolism of the human body[17] irrespective of other additional physical stressors. Load carriage in hot climatic condition increases the thermoregulatory pressure, leading to vigorous physical exertion and reduced work capacity,[18],[19] thus causing an increased metabolic demand of the personnel as they march through the hot desert carrying external load. Furthermore, the sandy terrain in the desert further contributes to the increasing metabolic cost of an individual.[20]

A study conducted by Kamon and Belding[21] observed that carrying 20 kg load in temperate and hot climatic condition (35°C and 45°C) resulted in increased heart rate (HR) of 7–10 beats for every 10°C increase in the temperature. Further, the study also confirmed the thermal stress-related onset of early fatigue, mobility reduction, and potentially increased risk of musculoskeletal strains, leading to heat-related injuries. Another study showed that the strenuous activities like load carriage in hot and humid conditions and sandy terrain can augment the metabolic and cardiorespiratory demands and decrease mobility.[22] Therefore, there is an ardent need to explore the actual challenges faced by Indian soldiers, when marching long distances while carrying heavier loads in their line of duty in order to maximize their performance and maintain combat readiness for the soldiers' general well-being and safety.

Till date, there have been limited studies focusing on soldiers' long-distance marching in sandy desert conditions with load carriage, and its further effects on their physiology. There are just two load carriage studies currently available; one was conducted on the tourist population by Imangulova et al.[23] that investigated the cardiorespiratory reactions of the tourist in a desert environment. The study found that walking on a sandy surface cost 20%–25% more energy than walking on a firm surface. Another study including the military troops carrying weights up to 32% of their body weight (BW) and marching for a short distance of 800 m in a desert environment predicted the ideal load of 6.27, 13.7, and 24.86 kg for the duration of 8 h, 2 h, and 30 min, respectively.[15] Considering the paucity in the research, the current study was designed to assess troops' physiological responses, walking efficiency, and mobility across extended distances.


  Methods Top


Study location

This study was conducted at desert environment (the Thar Desert of northwestern region of India). A sandy terrain track of 6 km was chosen, which is regularly used by soldiers for their day-to-day load marching activity. The terrain track was rough consisting of combination of level and undulating surfaces (36.66%) along with combination of up and downhill inclination of 9.6°, 7.8°, and 9.3°. The physical properties of sand were tested and are shown in [Table 1].
Table 1: Details of sand physical properties

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Participants

Nine physically fit, SHAPE-I category male Indian soldiers volunteered for the present study. SHAPE-I category is the only acceptable standard criteria for armed forces, wherein the medical classification of soldier is done based on their fitness level. Only those individuals who had been deployed to the study location for at least 1 year prior to the commencement of the current study were considered in this investigation to ensure that all participants were heat acclimatized. They were thoroughly explained about the purpose and need of the study. Physical characteristics of the participants, i.e., mean ± SE of age, height, and weight, were recorded as 30.00 ± 0.9 years, 175.56 ± 1.18 cm, and 77.83 ± 1.37 kg, respectively. All the volunteers provided written informed consent to participate in the study. The study protocol conformed to the principles outlined by the Declaration of Helsinki protocol, 2013, on the use of humans as a study subject and was approved by the Institutional Ethical Committee.

Load selection

The control condition was no load (NL), while the magnitude for functional load condition was 22 kg. As a part of their combat readiness, Indian soldiers are regularly trained with the similar load known as the combat load, which corresponded to 28.27% of the average BW of the participants for the present study.

The following are the specifics of the load components, the magnitude of load filled in each component, and their location in the body:

0 kg (0% of BW) – Without any load.

22 kg (28.27% of BW) – 11 kg in backpack + 4.7 kg haversack on the left lateral side of the waist + 2.1 kg web in front of the waist, and finally 4.2 kg INSAS rifle in the right hand.

Maximum aerobic capacity (VO2 max.) estimation

Due to a lack of well-equipped laboratory infrastructure in the desert, indirect estimation of cardiorespiratory fitness was explored. The most frequently used Cooper's 12-minute run test[24] methodology was utilized to calculate each participant's VO2 max. A 400 m round track was designed for running with proper markings to ensure that correct distance is recorded. The participants were divided into three groups having three individuals in each group and all of them did warm up before the actual running test. After the exhaustion of 12 min, the exact distance travelled by each individual was measured. The VO2 max was calculated using the following Cooper's test-based equation.

VO2 max = (Distance covered in meters-504.9)/44.73.

The average VO2 max value of participants was found to be 40.57 ± 11.31 ml/kg/min.

Experimental details

The participants were given an overview of the study's significance and experimental protocol on the 1st day as well as their basic demographic details were collected.

Participants were instructed to report to scientific team daily at 0830 h. All participants wore full combat uniform including combat boots and helmet. Physiological sensors (Zephyr bio-harness, M/s Medtronic, USA) were attached as instructed by the OEM's manual followed by load placement for functional load carriage experiment. Following that, they were instructed to form a line at the track's start point and then march on the designated track at their own pace when carrying 22 kg load and (NL, 0 kg). Each load condition was applied on separate day. Total completion time was recorded by a stopwatch for the calculation of walking speed.

Environmental parameters (temperature and relative humidity) of the study location's surroundings were recorded with the help of temperature and humidity data logger sensors, Model name-i Button Hygrochron (M/s Maxim Integrated, USA) and Kestrel 4000 weather meter (Nielsen-Kellerman, USA). The minimum and maximum values of temperature and relative humidity obtained were 40.35°C ± 0.35°C and 47.55°C ± 3.46°C and 14.35% ± 9.55% and 23.45% ± 12.94%, respectively.

RWL was calculated using the equation by Swain et al.:[25]

% VO2 max = (% Max. HR-37.18)/0.6463.

Physiological Cost Index (PCI) was calculated using the formula by Fredrickson et al.[26] and Bhise:[27]

PCI (beats/m) = (Mean HR at Work-Mean HR at Rest)/Walking Speed (m/min).

Calculation of walking speed (NL and 22 kg load condition)

Individual dataset of each participant was noted from start time and stop time, and based on the time taken to complete the 6 km marching, the average speed (s) was calculated by dividing the total distance travelled (d) by total time taken to complete that distance (t), [S = d/t].

Statistical analysis of the data

The last 7 min values of the physiological data (HR, respiratory frequency [RF], and core body temperature [CBT]) were averaged and considered as the individual value and further statistically analyzed. All data were presented as mean (SE). The Shapiro–Wilk test was conducted to check the normality of the data. Non parametric test i.e., Wilcoxon signed-rank test was performed to compare and find out the effect of load (Independent factor) on various dependent variables (HR, RF, CBT, PCI and %VO2max). Statistical analysis was done using Statistical Package for Social Sciences for Windows V 20.0 (SPSS, Nikiski, AK 99635, USA). For each statistical test, significance level was fixed at P ≤ 0.05. All the graphs were created using GraphPad Prism version 5.00 for Windows, GraphPad Software, San Diego California USA, www.graphpad.com.


  Results Top


Graphical representation of physiological responses, namely, HR, RF, CBT, PCI, %VO2 max, and marching speed when carrying different loads (NL and 22 Kg) during the present study is shown in [Figure 1]a,[Figure 1]b,[Figure 1]c,[Figure 1]d,[Figure 1]e,[Figure 1]f. All the studied parameters showed an increasing trend with load. A significant increase was observed in HR from 137.81 to 170.60 beats/min (P = 0.01), in RWL from 60.28%–84.22% (P = 0.01), and in PCI from 0.94–1.68 beats/min (P = 0.01) from NL to 22 kg load carriage condition. On the other hand, increase in RF (36.86–39.59 breaths/min, P = 0.09) and CBT (38.43°C–38.55°C, P = 0.62) from NL to 22 kg did not find significant. It was found that reduction in the marching speed or mobility was also statistically significant (P = 0.01) between 22 kg load and NL.
Figure 1: (a-f) Physiological responses: (a) HR (beats/min), (b) RF (breaths/min), (c) RWL (%), (d) PCI (beats/min), (e) CBT (°C), and (f) walking speed (km/h) during marching while carrying 22 kg load and NL. No load: NL, HR: Heart rate, RF: Respiratory frequency, CBT: Core body temperature; PCI: Physiological Cost Index; ns: Not significant, level of significance: P < 0.01(**)

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  Discussion Top


The objective of the present study was to evaluate soldiers' physiological responses to long distance marching of 6 km when carrying NL and a combat load of 22 kg, i.e., 28.27% of BW in a desert environment with sandy terrain. The physiological responses in terms of HR, RF, CBT, %VO2 max, and PCI increased during the load carriage condition relative to the NL condition. The trends of physiological response observed in the soldiers are consistent with the work of earlier studies at normal sea level.[28],[29],[30] temperature simulated conditions,[31] and real-time desert environments.[15],[20],[23]

The increase in HR is the primary response of the body under any stressful condition.[32] In the present study, the combination of load magnitude and marching speed led to significant increase in HR by 23.79% in load versus NL condition. In the previous study[20] having similar methodological approach of carrying load (22.68 kg) and marching (4.83 km/h) in desert condition, the increase observed was 17.32%, i.e., 127–149 beats/min in firm versus sandy terrain. Therefore, findings of the present study have established that marching in the sandy terrain when carrying similar load led to 14.50% increase, which is nearly 22 beats/min more than the response of sandy surface observed by Strydom et al.[20] This established that the present study's environmental as well as terrain condition were much more challenging. In addition, on comparing the HR response of carrying NL in sandy surface of the present study with carrying 22.68 kg load on a firm surface of the previous study,[20] the HR response was 8.05% higher, implying that terrain has an additional impact on the body.

A previous study[23] has stated that the HR response increases up to 15.4 and 20.8% when the load magnitude increases by 35 and 50% of the BW, respectively; however, in that study, volunteers carried load in flat, hard surface taking a 20–30 min rest interval between walking, and the air samples for the last minute of marching were collected in the Douglas bag and analyzed. In the present study, the volunteers were continuously marching on a sandy surface, resulting in 23.79% increase in HR when carrying lesser load magnitude of 28.27% of BW, which is regularly carried by the soldiers. Pal et al.[15] reported similar increase in HR by 24.84% when carrying 21.4 kg in comparison with response of NL condition.

Heat stress has an effect on the respiratory system as well, a rise in ambient temperature resulting in increased ventilation.[33] When exercising in the heat, total ventilation is modulated by output from central control and inputs from the muscle mechanoreceptors and metaboreceptors via group III and IV muscle afferents contributing to hyperthermia induced hyperventilation.[34] In the present study, a similar trend was observed with an increase in RF by 7.41%, although this increase was not statistically significant (Z = −1.718, P = 0.09).

Hyperventilation is also triggered by rise in CBT threshold during exercise in heat.[34] CBT values for the present study were found to be higher than the normal CBT ranges of 36.5°C–37.5°C in both NL and 22 kg load condition. The increase from NL to 22 kg load condition was 0.32%, i.e., 0.12°C; the increase was not statistically significant (Z = −0.491, P = 0.62). The average CBT was 38.43 ± 0.23 throughout marching when carrying NL, with the highest recorded at 39.34°C. Similarly, when carrying 22 kg load and marching, the average CBT recorded was 38.55 ± 0.19, with the highest being 39.80°C. Similar to the observations of a previous study,[35] no participant reported any sign of heat exhaustion/heat stroke. This indicates that the soldiers were well acclimatized with the environment of the study location as the core temperature was less than 40°C, reducing the risk of heat stroke.[36] However, previous studies[37],[38],[39] have reported that a rise in core body temperature above 39.0°C is detrimental to human performance. The elevated internal temperature, causes the heart to pump faster.[39] As the blood vessels dilate, blood flow is increased to the extremities causing sweating to help cool the body.[38] The increased fluid loss through sweating along with redistribution of blood flow to periphery can reduce the blood supply to the brain by compromising central venous pressure and mean arterial pressure.[35] The increase in internal temperature leads to cell death as proteins may start to unfold.[39] Therefore, extreme caution must be taken when increasing both load and marching duration, as the effects are unknown and may cause the CBT to rise to dangerously high levels.

The energy cost of any task is highest in the loose sandy terrain.[40] According to Strydom et al.,[20] walking on the sandy surface increases energy cost by 60%–200%, at least for speed/walking pace >3 km/h. Walking on sand surface, both unloaded and loaded (20%–50% of BW) increase the energy cost of walking by an average of 20%–25% when compared to walking on a solid surface.[23] In the present study, we analyzed every participant's individual speed for both load condition to check for any difference in marching speed. Reduction in the speed from 4.85 to 3.95 km/h was obtained for NL and 22 kg load, respectively, and the decrease reported is statistically significant (Z = −2.524, P = 0.01). This similar observation was found in earlier load carriage studies also, where participants were allowed to carry the load at a self-selected pace, and walking speeds have decreased as loads increased.[41],[42],[43]

The terrain properties are critical for determining the ease of movement on a surface. There is a scarcity of research assessing the sand properties of the sandy surface used in human motion studies. The physical properties of the sand sample collected from the study location are reported in [Table 1]. Sand has a fineness modulus of 2.2–3.2 mm, which represents the average size of the sand particles. The present study's fineness modulus had a value of 2.4 mm, which is within the fine sand range (2.2–2.6 mm). The bulk density of the sand was 1524 kg/m3, which is greater than loose sand and clay soil and closer to the density of dry sand. Bulk density is inversely related to the porosity of soil. The friction coefficient of sand is less in comparison to concrete surface; therefore, walking on loose sand in desert areas becomes quite challenging. Sand has a lot more pore space in between its granules, and during walking, the pressure by foot is applied on the sand, which forces the sand particles to realign themself to minimize porosity. The addition of small amounts of water can further decrease the friction coefficient of sand by almost a factor of two.[44] The combination of sand's physical properties, along with high temperature and humidity, poses a unique challenge to the human body.

Macgregor[45],[46] recommended PCI as one of the basic methods to estimate the energy cost using HR, its value reflects the increased HR required for walking, and is expressed as heartbeats per meter. It provides a measure of overall walking performance, as it includes both a physiologic measurement and velocity. PCI of marching when carrying NL and 22 kg load was studied to find out the cost of long-duration marching in the desert environment. When marching with a 22 kg load, there was a significant increase in PCI value (Z = −2.547, P = 0.01), which was found to be 77.73% more than NL condition. As a result, we conclude that carrying an additional 28.27% of BW in a desert environment with similar climatic condition may result in poor walking economy, i.e., higher energy expenditure or oxygen uptake of walking. Furthermore, the PCI value varies with walking speeds. Carrying an additional 22 kg load in a desert environment resulted in an 18.56% reduction in mobility, with the PCI value being higher at slow marching speeds. Wu[47] observed a similar pattern stating that PCI values are high at slower speeds, implying poor economy.

RWL showed a significant increase by 39.71% (Z = −2.547, P = 0.01) in NL versus 22 kg. Vogel et al.[48] suggested that any task that requires a workload of 50% VO2 max should not exceed beyond 8 h, 60% VO2 max should be sustained for approximately 2 h, and RWL of 75% VO2 max should not be exceeded beyond 30 min duration. As per the findings of the present study, the RWL for NL and 22 kg load was 60.28 and 84.22%, respectively, and according to the workload criteria by Vogel et al.,[48] the workload is more than 60% of the VO2 max in NL condition; thus, marching in the hot desert environment (with similar environmental conditions) without carrying any load should not be done for more than 2 h continuously. The RWL elicited by carrying 22 kg load in sandy surface exceeded the 75% VO2 max criterion, indicating that this severe activity should not be sustained for more than thirty min to maintain efficiency and combat readiness of soldiers. This explains why it is critical to develop load carriage guidelines for long duration marching activity in the current terrain-specific context.

Soldiers posted at extreme locations having harsh environment conditions are supposed to carry external load due to operational requirements. Therefore, at similar type of terrain alike the present study, soldiers when carrying load of 28.27% of BW should not exceed the time duration as mentioned in order to maintain physiological or physical strain within acceptable limits. There is a critical need to explore scientific data on the physiological responses of soldiers exposed under harsh environmental conditions. Future research must concentrate on developing terrain specific load carriage guidelines to maintain operational readiness and soldiers' health.


  Conclusion Top


The present study stated that carrying 22 kg load, i.e., 28.27% of BW and marching at a speed of 3.95 ± 0.55 km/h in sandy desert terrain condition, caused intense exertion, as the RWL value exceeded the physiological limit of <75% VO2 max, thereby limiting the activity to 30 min. Carrying NL and marching at a self-selected speed of 4.85 ± 0.54 km/h can be continued for 2 h for similar environment condition. All the physiological parameters increased from NL to 22 kg load condition and the increase was significant in HR, RWL, and PCI. The mobility of the soldiers was reduced by 18.56% due to carrying external load in sandy terrain in comparison with NL condition. The right combination of load, duration, and speed would minimize the risk of injuries and will be helpful in maintaining the operational readiness and mobility of the soldiers at real desert environmental condition.

Limitations of the study

The study was limited for the following factors due to lack of few logistics support as the study location was situated in extremely remote desert:

  1. A smaller sample size
  2. Sweating rate/level of hydration during pre-, post-, and intermittent periods of the exercise
  3. Direct measurement of CBT
  4. Lack of direct assessment of VO2 max.


Acknowledgment

The authors would like to express their sincere gratitude to the Defence Research and Development Organisation, Ministry of Defence, Government of India, for providing the permission and financial assistance for the study. They are also thankful to the Ministry of Home affairs for providing volunteers, logistics, and continuous administrative support throughout the study duration. They would like to express their profound gratitude to the volunteers of the study for their unconditional support, valuable time, and effort for successful completion of this study. The authors would like to thank University Grants Commission for providing research scholarship to Kavita Arya. They would also like to thank Sh. Amarpal for assisting in the field study. The authors are grateful to Director, DIPAS, and other members of Ergonomics Laboratory for their constant encouragement.

Financial support and sponsorship

This research work is funded by the Defence Research and Development Organisation, Ministry of Defence, Government of India. One of the authors, Kavita Arya, got UGC NET-JRF scholarship for continuing her research.

Conflicts of interest

There are no conflicts of interest.



 
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