Forty-six male soldiers participated in this study. The average (standard deviation) age was 26.1 (4.5) years, height was 1.75 (0.62) m and weight was 80 (10.6) kg. All participants were volunteers and belonged to the Spanish infantry. They completed an informed consent form before their inclusion in the study. They did not have recent orthopaedic trauma, neurologic problems, or had any restrictions in relation to walking or load carriage. The Ethics Committee of the University of Granada approved this study.
Each soldier was heighted and weighted with a scale and measuring rod (SECA769, Hamburg, Germany). From this point on, in each of the 2 experimental sessions, the participants completed the following three phases in a continuous way. The first phase was the application of two cognitive tests for attention analysis. The second phase was to complete, as fast as possible, an obstacle course that they used for training their combat skills in the military headquarters. The third and last phase was, again, the application of the cognitive test that was performed in the first phase.
In the control condition, soldiers completed the obstacle course wearing military socks and boots, a camouflage uniform, and light body armour. In the combat condition, soldiers had to wear the same equipment as the control condition, as well as a helmet, body armour with ammunition clip, battle rifle (HK-42) with 5 empty chargers, combat gloves, and joint protectors. They also carried a battle backpack containing spare clothes, a rigid 1-litre bottle of water, an American blanket, individual medical kit, and various survival accessories. The approximate weight of the equipment in the combat condition was 19 kg. Each experimental session was completed on different test days, and at least 48 hours of rest were allowed between conditions. Each soldier carried out each condition following a randomized order.
During the obstacle course, a Tomtom Runner 3 (TomTom international, Amsterdam, Holland) was used to measure the obstacle course competition time in minutes. A Zephyr BioHarness system at 100 Hz (Zephyr Technology, Auckland, New Zealand) was placed in the thorax of each participant to obtain the mean heart rate (HR) during the obstacle course with the software OmniSense 5.1 (Zephyr Tech. Corp., MD, USA).
Cognitive tests: the Determination Test (DT) and the Divided Attention Test (WAFG)
The DT is a complex, multi-stimuli reaction test involving the presentation of both coloured stimuli and acoustic signals. The respondent reacts by pressing the appropriate buttons on the response panel that assessed the divided attention (Gurd et al., 2010). The DT is used to measure reactive stress tolerance and the associated ability to react (Schuhfried, 2017). Stress tolerance refers to the individual’s ability to resist the effect of the stimuli; that is, their ability to use modes of behaviour that enable them to cope as effectively as possible with the situation (Brickenkamp et al., 1986). The stress element of the DT arises from the need to sustain continuous, rapid and varying responses to rapidly changing stimuli (Schuhfried, 2017).
The DT form chosen was the “Rostock Form” that included colours and tones. The respondent’s task was to react as quickly as possible to visual or acoustic stimuli. There were 5 visual stimulus colours (white, yellow, red, green and blue), which appeared in an upper and a lower row. The reaction buttons assigned to these five colours were arranged on the response panel in such a way that the respondent could use both hands. There were 2 additional acoustic stimuli (high and low tone) assigned to the “sound” buttons in the middle of the panel. The visual stimuli were presented on the screen and the acoustic stimuli via headphones. Each stimulus appeared when a correct response was made to the current one, so the speed of the stimulus presentation was determined by the respondent. The administration time was approximately 8 minutes.
In addition, after the DT, the divided attention of the perception functions were measured using the WAFG test. This test is based on the paradigm of Zomeren and Brouwer (1994), where according to this model, the central factors of attention include the distinguishing of intensity and selectivity aspects. The intensity aspect of attention integrated alertness and vigilance, while the selectivity aspect included the focused-selective and divided attention.
In the short form of the WAFG Test, the respondent received stimulus on a visual and auditory channel presented in a cross-modal way. They had to respond if the signal on the visual stimulus channel became brighter twice in succession or if that on the auditory stimulus channel became quieter twice in succession. For that, 85 stimuli (visual and auditory) were presented. The easy short form of the WAFG test was chosen, which takes approximately 8 minutes. The stimuli were presented for 1500 ms, while a change took place after 500 ms. There was an interstimulus interval of 1000 ms between stimuli.
Both tests were completed in a warm, quiet and light room before and after the obstacle course. The participants were comfortably seated, and the computer´s screen was adapted to each subject’s eye level. In the test that required sound, the volume of the headphones was chosen by each participant. The instructions were provided at the start of the test and followed independently by the respondent on the screen. The 2 tests included a training mode before starting, completing a familiarization trial before the data collection. The researcher was not required to provide any further explanation. The variables analysed are presented in Table 1.
The Obstacle Course
The obstacle course was approximately 1 km from the start to the finish and returned with an altitude slope of 50 meters. The course included the following 6 obstacles:
- Cargo net climb: soldiers climbed up and over a 4.5 meter vertical cargo net.
- Balance log: soldiers had to traverse a balance log without touching the ground with their feet.
- Low crawling: soldiers low-crawled across 15 meters underneath a barbed wire obstacle located 0.5 meters over the ground.
- Steel wire balance: soldiers maintained their balance crossing 7 meters of steel wire. The steel wire was placed 2 meters above the ground. Soldiers had another steel wire at the height of their head to pass the obstacle.
- Underground tunnel: soldiers crawled through a 10-metre-long tunnel.
- Climbing with cord: taking advantage of a natural slope of the terrain, a rope was attached. Soldiers had to climb up the 15-metre slope using that rope. The slope had a gradient of approximately 50º.
Soldiers completed the obstacle course under control and combat conditions as quickly as they could. Before the data collection, each soldier completed the course once by walking to ensure that the itinerary of the course was clear.
SPSS software v.24 (IBM SPSS, Armonk, NY) was used for the data analysis. The variables of the 2 attentional tests were analysed with a repeated measured two-way ANOVA (equipment and fatigue). Tukey’s post hoc test was used whenever significant differences were found. The Kolmogorov–Smirnov test was used to test data normality, and Levene’s test was used to test the homogeneity of variance. Mauchly’s test was used to assess sphericity. In addition, a dependent-samples Student’s t test was used to analyse differences between the control and combat conditions in the obstacle course time and mean HR. The level of significance was set at p<0.05.