H10 sensor

The pre-test subjects performed an incremental exercise on a treadmill (LT test), which consisted of 5-min stages with 30 s breaks between stages. The first stage was started at 1.0 m∙s⁻¹, with increments of 0.5 m∙s⁻¹ every 5 min. The criteria for ending testing were a blood lactate concentration over 4 mmol∙L⁻¹ after each running speed or until volitional exhaustion [12 ,16 (link),37 (link)]. After four weeks of LIE, the same set-up was used for the post-test. HR and jogging/running speed at the 1.5, 2.0, and 4.0 mmol∙L⁻¹ blood lactate concentration levels (La⁻) were estimated using a mathematical model of the interpolation that has previously been explained in detail [16 (link),41 (link),42 (link),43 (link)]. Delta (∆) jogging/running speed and ∆HR at the 1.5, 2.0, and 4.0 mmol∙L⁻¹ La⁻ between pre- and post-testing were calculated (S_1.5, S₂, S₄, ∆S_1.5, ∆S₂, ∆S₄ and HR_1.5, HR₂, HR₄, ∆HR_1.5, ∆HR₂, ∆HR₄). The HR of all subjects was recorded using HR telemetry (H10 sensor, Polar Electro, Finland). The mean value of HR over the last 30 s of each stage was determined for statistical analysis. Capillary blood sampling for lactate analysis was taken from the earlobe (0.2 µL) immediately after each 5-min stage. All blood lactate levels were determined by an enzymatic-amperometric method (Lactate Scout 4, EKF-diagnostics GmbH, Germany) [44 (link),45 (link)].

The heart rate was measured using the polar monitoring system V800 and the H10 sensor (Polar, Finland; sample rate: 1000 Hz). Based on the recorded R-R intervals, the heart rate was analyzed with Kubios software (Kubios Oy, Kuopio, Finland). An automatic filtering process method was applied to eliminate artifacts and extra beats in the R-R interval (threshold: 0.45 s). The heart rate was analyzed in the following 5 min intervals: condition 1 (0–5 min), condition 2, (5–10 min), condition 3 (10–15), recovery 1 (0–5 min after condition), and recovery 2 (5–10 min after condition).