2.2. Procedures
During the 12-week intervention period, the LF group performed two interval sessions per week, whilst the HF group performed four sessions per week, at the same intensity and matched for weekly training volume (
Figure 1). Oxygen consumption (VO
2), heart rate (HR), and blood lactate were measured, and exercise economy was calculated pre- and post-intervention. In addition, a performance test (8 km roller-skiing time trial) was conducted before and after the intervention period.
2.2.1. Interval Training
The intervention was performed as dry land training during the summer period from May to August. The exercise intensity scale developed by the Norwegian Olympic Centre (NOC) was used for classification of training intensity (
Table 2). Interval durations and work intensity for both interventions were selected to correspond with what the literature suggests to be an effective model to elicit performance adaptations in endurance athletes.
Outside of the planned intervention sessions, athletes were instructed to train normally, with low-intensity training (<82% HRmax I-zones 1–2) and a maximum of one additional session per week of high-intensity interval training (>87% HRmax, I-zones 4–5). To ensure that the training load during the intervention period outside of prescribed training did not differ between groups, subjects were provided with an electronic training diary with both written and verbal instructions on how to record their training. Data recorded in these diaries were used to calculate the training volume and intensity distribution.
During the intervention period, the LF group completed one session per week of 8 × 8 min intervals in I-zone 3 with 2 min recoveries (total time 64 min) and one session of 6 × 12 min intervals in I-zone 3 with 3 min recoveries (total time 72 min). The HF group performed twice-weekly sessions of 4 × 8 min in I-zone 3 separated by 2 min recoveries (total time 32 min) and two sessions of 3 × 12 min in I-zone 3 with 3 min recoveries (total time 36 min). All interval sessions were performed as rollerski skating. For both groups, the total prescribed weekly training time in I-zone 3 was 136 min. The LF group had a minimum of one day, and typically two days, between each session in I-zone 3. The HF group performed two I-zone 3 sessions on two consecutive days, while there was one day off before the remaining I-zone 3 session.
Each interval session included a self-selected warm-up and warm-down. During the first four weeks of the intervention period, 1–2 interval sessions per week were organized as group training sessions, during which blood lactate and HR were monitored and controlled to ensure that athletes were training at the planned intensity. During the remaining weeks of the intervention, once participants had become accustomed to maintaining the correct intensity during training sessions, only a selection of sessions were monitored.
2.2.2. Test Day 1: Submaximal Exercise Test and VO2max
Pre- and post-intervention, subjects performed a submaximal incremental rollerskiing (skating) test on a motorized treadmill (Lode Valiant Special, Lode B.V. Groningen, Netherlands) in a laboratory maintained at 17–21 °C and 25%–40% relative humidity. Participants completed a ~15-min standardized warm-up. The test started at 5% incline and 10.8 km/h for men and 9 km/h for women. Each stage lasted 5 min, followed by a one-minute recovery. The treadmill incline was increased by 2% between each stage. The test was terminated when blood lactate concentration exceeded 4.0 mmol/L (Lactate Pro LT-1710t, ArkRay Inc., Kyoto, Japan). Expired air was analyzed continuously with VO2 determined at 30 s intervals throughout the test (Oxycon Pro, Erich Jaeger, Hoechberg, Germany). HR was measured continuously using short-range telemetry (Polar S620i, Kempele, Finland). For both HR and VO2, the average values recorded during the final three minutes of each stage were used for analyses. A finger-prick blood sample was taken during the recovery between each stage for blood lactate determination. Exercise economy was calculated from body mass-adjusted oxygen consumption, and HR and lactate recorded for the lowest test workload (5% incline 10.8 km/h for men and 9 km/h for women). This workload was chosen to ensure aerobic metabolism, indicated by a lactate level < 2.5 mmol/L.
VO
2 at an estimated lactate concentration of 4.0 mmol/L was calculated by a forecast algorithm. We used a built-in algorithm in Excel 2016, using the measured values beyond and above an intensity corresponding to 4.0 mmol/L to predict a given
y-value (VO
2) at a given
x-value (4.0 mmol/L), based on the measured values in the data set, using the algorithm
a +
bx:
Here, x and y are the sample means average (x = lactate, y = VO2).
Following ~10 min recovery, VO
2max was measured via a continuous, incremental rollerskiing (skating) test to volitional exhaustion. The test started at a 5% incline and 10.8 km/h for men and 9 km/h for women. Every minute, the treadmill incline was increased by 2%. When an incline of 15% was reached, treadmill speed was then increased by 0.5 km/h every minute. VO
2 was measured continuously, and the average of the two highest consecutive 30 s measurements was defined as VO
2max. Respiratory exchange ratio ≥ 1.05, a plateau in VO
2 with increasing workload, and blood lactate concentration ≥ 8 mmol/L were used as criteria to evaluate if VO
2max was obtained [
18]. HR was measured continuously, with the highest value during the test defined as HRpeak.
2.2.3. Test Day 2: Rollerski Time Trial Performance
Pre- and post-intervention, each athlete completed an 8 km rollerski (skating) uphill time-trial outdoors. This had a typical duration of 30–35 min and was preceded by a 30 min standardized warm-up. Athletes started in a random order at 30 s intervals pre-intervention, and again in this same order post-intervention to prevent any possible motivational effects. Performance times were recorded by two synchronized stopwatches (Regnly RT3, Emit AS, Oslo, Norway). The weather conditions pre- and post-intervention were stable, with ambient temperature between 17–20 °C, no wind and dry asphalt. All athletes were familiar with the 8 km course.
Subjects used identical rollerskis (Swenor Skate, Sport Import AS. Sarpsborg, Norway), with each athlete using the same pair of skis pre- and post-intervention. These were not used between pre- and post-testing to avoid changes in rolling resistance. Participants also used the same skating poles (SWIX CT3, Swix Sport AS. Lillehammer, Norway) of self-selected length. The two test days were separated by a minimum of 48 h and a maximum of 6 days. Prior to testing, participants underwent a standardized 2 day tapering period. Tests were performed at the same time of day (± 1 h) and following a 1 h fast. Participants were required to abstain from strenuous exercise, alcohol, and caffeine during the 24 h leading up to each test. Athletes not able to participate in whole intervention period were excluded from the study (n = 1 in the LF group and n = 4 in the HF group).
2.3. Statistical Analyses
Since not all variables passed the normality test, all results are presented as median values with the corresponding inter-quartile range (IQR). A Wilcoxon matched-pairs signed rank test was used to test for differences between each parameter before and after the 12-week intervention period. In all comparisons, two-tailed tests were used. Differences were considered significant at p < 0.05. Due to the small sample size and expectations of small changes in these already well-trained athletes, the data were further analyzed with mean effects size (ES). ES was calculated as Cohen’s d to compare the practical significance of the performance improvements among the two groups. The criteria to interpret the magnitude of the ES were 0.0–0.2 trivial, 0.2–0.6 small, 0.6–1.2 moderate, 1.2–2.0 large, and >2.0 very large. The calculations were performed in Microsoft Excel 2010 (Microsoft Corporation), and the statistical analyses were conducted in SigmaPlot for Windows, version 12.2 (Systat Software GmbH, Erkrath, Germany).