Heart rate

After the sprint test and additional 10 min of recovery, the athletes performed a maximal incremental exercise test (initial load 100 W + 20 W 1 min⁻¹) until exhaustion to determine the VO_2max and the corresponding workload (PPO). Heart rate (Polar, Kempele, Finland), VO₂ and carbon dioxide output (VCO₂) (Cortex Metalyzer II, Leipzig, Germany) were continuously measured during the test.
Afterward, the VO_2max and dLa/d_tmax were used to calculate the maximal lactate steady state (MLSS_c) and the lactate turn point 1 (LT1) according to Hauser et al. (2014a (link)).

A modified lactate minimum test (Knöpfli-Lenzin and Boutellier, 2011 (link)) was performed exactly 7 min after finishing the VO_2max test. After these 7 min of passive recovery, RPE, lactate and Heart rate were measured. The drop in Heart rate within this time was used to calculate a Heart rate recovery index (HRR). Subjects started a second incremental test, beginning with a workload 50 W below the MLSS_c. The workload was increased by 10 w every 90 s, until complete exhaustion. Heart rate (Polar, Kempele, Finland) was continuously measured during the test, RPE and lactate samples were obtained out of the earlobe at the end of each step. Power output at the step which elicited lactate minimum (LM) was considered as maximal lactate steady state power (MLSS_P), according to Knöpfli-Lenzin and Boutellier (2011 (link)).

During all phases of treadmill walking, individuals wore a ceiling mounted harness (that did not provide body weight support) and held a handrail to prevent falls. Additionally, we monitored heart rate (Polar, Kempele, Finland) for safety, and determined perceived exertion using the Borg Rate of Perceived Exertion (RPE) scale after each walking phase. If participants exceeded >80% their age-predicted max heart rate, the treadmill was stopped, and the participant was provided with a seated rest break until their heart rate recovered. However, no included participants needed the treadmill to stop during any of the walking phases.
Participants walked on a dual belt treadmill that captured kinetic data through two force plates, one under each belt, at 1000 Hz (Bertec, Columbus, OH, USA). Kinematic data were captured and recorded at 100 Hz using a Vicon MX40, 8-camera motion capture system, and time-synchronized with the kinetic data in Nexus software (v2.8.2, Vicon Motion Systems, Inc., London, UK). We used a custom marker set with seven retroreflective markers, one for each heel, lateral malleolus, and fifth metatarsal head, and the left medial malleolus.

To determine exercise performance, participants performed a weighted ruck TTE test on a motorized treadmill (TMX425C treadmill). Participants were instructed to walk for as long as possible. Participants were provided with feedback on the time (at regular 10 min intervals) covered during each TTE test but were not informed of the overall performance time until the completion of the study. The timing devices were concealed from the participant's view throughout the test. The TTE test consisted of walking on a treadmill wearing a military backpack weighing 30% of the subject's body weight (23.3 ± 3.5 kg), as accepted during operational missions (Army, 2022 ) in comfortable environmental conditions (19–21°C, 35–40% relative humidity). After a 5 min warm‐up (2 mi/h; 3.2 km/h; 0.89 m/s; 2% slope), the pace remained the same and the slope increased to 14%, until reaching volitional exhaustion. Heart rate (Polar Electro) and metabolic gases were continuously collected during the entire protocol using a metabolic cart for assessment of RER, V˙O2, V˙CO2, V˙E, HR and substrate oxidation.

The exercise protocol consisted of a 10-km time trial run on a treadmill (Woodway, ergo ELG 55, Weil am Rhein, Germany), where the athletes had to complete the distance in the shortest amount of time. Two exercise trials were performed in a randomized order, one at 9 am and the other at 4 pm, with at least a 48-h interval between the trials. The trials were performed in an environmental chamber (Weis-Gallenkamp, UK) where the temperature was controlled at 6 °C with 60% relative humidity (the average British winter temperature 4.4 °C, http://metoffice.gov.uk). During the run, subjective ‘Ratings of Perceived Exertion’ (RPE), on a scale of 6–20 (Borg, 1998 ), heart rate (Polar Electro, Finland), and running speed were recorded at the end of each km. The athletes had free control of their running speed, without having access to the value of the speed. Also, they were allowed water ad libitum throughout the trial and were advised to refrain from intense physical activity 24 h prior to the exercise trials.