Combat gear shifts exercise transitions and raises heart rate at VT1

Wearing combat gear and body armor shifted the treadmill stages at which trained female and male soldiers reached two ventilatory transitions, while producing little change in absolute oxygen uptake at the first transition. In a randomized crossover study of 18 Swedish soldiers, the first ventilatory threshold, VT1, appeared at stage 5.2 ± 0.8 during loaded exercise in men, compared with 6.4 ± 1.7 without equipment; in women, it appeared at 5.1 ± 0.6 rather than 7.1 ± 1.5. The second threshold, VT2, likewise occurred at lower stages under load. Despite the lower treadmill settings, absolute oxygen consumption at VT1 changed little, while heart rate was higher with equipment. The measurements therefore point to a changed relationship among treadmill demand, transported mass and cardiovascular response—not proof that the soldiers’ underlying metabolic threshold moved to a lower physiological level.

The study involved nine women and nine men with one to five years of military service. Each participant completed the same laboratory graded treadmill test twice, in randomized order and separated by 2–14 days. One trial was a no-load condition consisting of body mass, running shoes and light clothing. The other added each soldier’s fitted personal combat equipment, armored plates and a dummy AK5 weapon. Equipment was individually fitted rather than standardized by body mass. The external load averaged 16.34 ± 2.51 kg in women and 15.70 ± 2.04 kg in men, but because the groups had different body masses, it represented a greater proportion of body mass in women: 24.84% ± 4.56% versus 20.08% ± 1.65%. This difference limits interpretation of comparisons between women and men.

Exercise began with five minutes of marching at 5.4 km·h⁻¹ on a level treadmill, followed by five minutes at 8 km·h⁻¹. The speed then remained at 8 km·h⁻¹ while treadmill grade increased by two percentage points every three minutes until voluntary exhaustion. Heart rate, oxygen uptake, carbon dioxide production, minute ventilation and muscle oxygen saturation were measured continuously. Mean values from the final 30 seconds of each stage were used for analysis. The authors identified VT1 with the V-slope method, locating the breakpoint in the relationship between carbon dioxide production and oxygen uptake. VT2, also called the respiratory compensation point, was identified from a secondary rise in ventilation relative to oxygen uptake together with a marked rise in ventilation relative to carbon dioxide production. One investigator, blinded to sex and load condition, identified both transitions visually.

The threshold-stage shifts appeared in both sexes. Relative to no-load exercise, the ordinal stage at VT1 fell by 19% in men and 28% in women, while the corresponding reductions at VT2 were 29% and 32%. Those percentages describe stage numbers, not percentage decreases in treadmill grade, and the staged protocol provided relatively coarse temporal resolution. Elapsed time to each threshold was not retained as a separate outcome. The main load effect was nevertheless clear: VT1 occurred at a lower stage with equipment in both groups, and the same pattern held for VT2. Neither threshold showed a statistically significant load-by-sex interaction, so the results do not establish that women and men responded differently to the added equipment.

Absolute oxygen uptake tells a more nuanced story. At VT1, it changed from 2.78 to 2.72 L·min⁻¹ in men and from 2.24 to 2.22 L·min⁻¹ in women between no-load and loaded trials, and the analysis detected no load effect on this measure. Heart rate, however, rose with equipment. It increased from 168.8 ± 13.3 to 180.3 ± 6.5 beats·min⁻¹ in men and from 175.9 ± 9.8 to 183.6 ± 10.8 beats·min⁻¹ in women; the overall load effect was p < 0.001. Thus, the soldiers reached a broadly similar absolute oxygen uptake at a lower treadmill setting, but with a higher cardiovascular response while carrying more mass. At VT2, absolute oxygen uptake was about 5% lower with equipment in both sexes, while no significant load effects were detected for heart rate or the threshold as a percentage of condition-specific peak oxygen uptake. These nonsignificant findings were imprecise rather than evidence of equivalence.

The distinction between a treadmill stage and a metabolic transition is central to interpreting the result. Loaded and unloaded trials did not represent equivalent external work: the equipment increased total transported mass and could also have altered gait, movement mechanics, ventilation, weapon carriage and local fatigue. The study design could not separate those mechanisms. A lower stage at which VT1 appeared therefore does not, by itself, demonstrate that the physiological boundary between moderate and heavy exercise shifted. Instead, the measurements show that the equipment changed the mapping between the imposed locomotor task and the internal response. The same absolute oxygen uptake at VT1 could be reached at a different treadmill setting, with more cardiovascular demand, because the soldiers were moving a different combined body-and-equipment system.

Near-infrared spectroscopy added a local muscle signal but also introduced important interpretive limits. A sensor was positioned over the right vastus lateralis, midway between the greater trochanter and lateral femoral epicondyle, and muscle oxygen saturation was referenced to a 30-second standing value. The analysis found a time-by-sex interaction, with less baseline-referenced desaturation in women at VT1, VT2 and the temporal minimum when results were averaged across load conditions. The largest descriptive difference occurred during the loaded trial: baseline-referenced desaturation amplitude was −39.0 ± 23.9 percentage points in men and −17.1 ± 14.5 percentage points in women, with a between-sex contrast of p < 0.01. In no-load exercise, the corresponding contrast was not statistically significant. These observations cannot be generalized to whole-body or whole-limb oxygenation, and the analysis cannot cleanly separate physiological differences from measurement effects.

Right-leg fat averaged 31.4% ± 3.8% in women and 13.4% ± 4.7% in men, a large group difference, and the thickness of adipose tissue directly beneath the sensor was not measured. Subtracting each participant’s standing value does not remove that influence because the optical path through the sampled tissue can differ between individuals. Differences in muscle mass, recruitment, perfusion and oxygen utilization could also contribute to the observed pattern. With only nine participants per sex and exploratory between-sex analyses, the muscle-oxygenation result was appropriately characterized as site-specific and preliminary rather than as evidence that men intrinsically sustain greater muscle deoxygenation under load. The source also noted that the temporal minimum in this muscle should not automatically be equated with whole-body maximal effort or the response of other locomotor muscles.

Recovery measurements were similarly inconclusive. Heart-rate recovery was calculated as the decline during the first minute after exercise, while an exploratory index captured the rise in right-vastus-lateralis oxygen saturation during the first 10 seconds after participants sat down. No significant load, sex or interaction effects were detected for either outcome. This does not show that recovery was equivalent: the sample was small, and no equivalence testing was performed. The 10-second muscle signal is also sensitive to posture, movement and the muscle pump, and cannot represent systemic or functional recovery. The absence of a detected effect is therefore best read as an unresolved question rather than as a reassuring result.

Several other limitations constrain what the experiment can establish. The sample was too small for reliable sex-by-load inference, and the available data did not retain the randomized test-sequence variable, preventing an independent assessment of order effects on peak oxygen uptake. A single blinded assessor identified the ventilatory transitions without a second reviewer, consensus procedure or formal intra-rater reliability assessment. Small differences in oxygen uptake also need to be considered against plausible instrument, day-to-day and time-of-day variation. Most importantly, this was a laboratory study of 18 soldiers using one treadmill protocol, one equipment ensemble and a single sampled muscle. Its results show that combat gear alters the task-to-physiology relationship during graded running, but they do not identify which component of the equipment is responsible or establish how the same responses would occur during prolonged field missions. The clearest conclusion is a measured one: equipment changed where the transitions appeared and raised heart rate at VT1, while absolute oxygen uptake there remained essentially unchanged.

Subject of Research: Combat gear, ventilatory thresholds and local muscle oxygenation during graded treadmill exercise in soldiers

Article Title: Effect of combat gear and body armor on ventilatory thresholds and muscle oxygenation during graded treadmill exercise in female and male soldiers

Article References: Larsson, J., Dencker, M., Bremander, A., & Olsson, M. C. (2026). Effect of combat gear and body armor on ventilatory thresholds and muscle oxygenation during graded treadmill exercise in female and male soldiers. Frontiers in Physiology, 17, Article 1943048. https://doi.org/10.3389/fphys.2026.1943048

Image Credits: AI Generated

DOI: 10.3389/fphys.2026.1943048

Keywords: combat gear, body armor, load carriage, soldiers, ventilatory thresholds, muscle oxygenation, near-infrared spectroscopy, treadmill exercise, cardiovascular strain, sex differences, Effect, combat