Individualized High-Intensity and Low-Intensity Cycling Training

Subjects were randomized into two training groups, applying minimization method (Hu and Hu, 2012 (link)) after the first three visits to laboratory. Training zones were determined based on lactate thresholds. The subjects were instructed to continue their daily physical activities (commuting, non-physical hobbies, etc.), but all strenuous exercise in addition to LIT or HIT was not allowed. The exact training programs are given in Supplementary Material 1.
LIT consisted of cycling under LT₁-power. The weekly 5–6 training days included long (1.5–4 h), medium (1–1.5 h), and short (45–60 min) exercises. Weekly training hours progressed individually (see Progression-paragraph below) based on perceived exertion from 4.5 up to maximum 12.5 h. Subjects did their training mostly outdoors with their own bicycles with Rally RK200 dual-sensing power meters (Garmin Ltd., Taiwan). A possibility for indoors cycling with trainer or Wattbike Trainer (Wattbike Ltd., Nottingham, UK) was given. Three (out from 16) subjects did their training completely indoors, and the others did 3% of their training indoors.
HIT consisted of 2—3 weekly indoor training sessions with Wattbike Trainers, or indoor trainer with their own bicycles with Rally RK200 dual-sensing power meters. Training consisted of high-intensity work intervals 3–7 min long with recovery periods ¾ of the work interval duration. In the first training week, there were 15 min of cumulative high-intensity time in a training session, and it progressed individually (see Progression-paragraph below) up to 30 min per session. Each session included 10 min warm up and cool down. These, as well as recovery periods, were done with power <60 W, and high-intensity segments were initially 110% LT₂ power ( $\pm 15$ W).
In both groups, RPE (0–10) was reported from each training session. HR from each session was recorded with the Garmin HRM-Pro heart belt. In the LIT group, power data were recorded with power meters (Rally RK200), and in the HIT group, power data were collected from the Wattbike Trainer or power meters (Rally RK200). All data was transferred after the session to AthleteMonitoring app (AthleteMonitoring, FITSTATS Technologies, Inc., Moncton, Canada), from which training realization was actively monitored weekly by the first author. In the HIT group, if all weekly training had RPE $\le$ 6 and HR did not rise above LT₂-threshold, the power was increased by 10%. In the LIT group, the subjects were actively given feedback whether training was at the prescribed level. Apart from the first HIT-session, all sessions were unsupervised. Training power was modified in both groups according to VO_2max -test (visit 4) at training week 6.
Daily physical activity was gathered by measuring heart rate from wrist continuously by Garmin Forerunner 945 for two to 4 weeks before the start of the intervention. This was done to estimate the baseline endurance training before training intervention.
Training zones and load. Training was monitored by distributing cycling power output to five zones (Cejuela-Anta and Esteve-Lanao, 2011 (link)): Z1 (below LT₁—10 W); Z2 (LT₁—10 W to LT₁ + 10 W); Z3 (LT₁ + 10 W to LT₂—10 W); Z4 (LT₂—10 W to LT₂ + 10 W); Z5 (above LT₂ + 10 W). For each zone, a weighting factor was linked (in an ascending order: 1, 2, 3, 4, 7.5) and training load was calculated by multiplying the factor by the time spent in the zone (Cejuela-Anta and Esteve-Lanao, 2011 (link)).
Targeted power of LIT was designed to be at Z1 and Z2, and HIT at Z4 and Z5. Heart rate was divided into three zones (HR Zones 1—3) separated by LT₁ and LT₂. In addition, for the LIT group, it was calculated how many times their 30 s -average power exceeded Z2 -zone. For baseline physical activity before the start of the training intervention, Z0 was defined to be a zone below halfway between resting HR and LT₁, and Z1b above Z0 and below LT₁. During physical activity heart rate measurement part of the data was dismissed, 12 $\pm$ 15% of the gathered data, because of heart rate was not adequately recorded from wrist.
Training progression. Both groups had 10 training weeks. Weeks 3 and 7 were load reduction weeks for enhancing recovery. Subjects’ training progression was linked to the perceived exertion. After each training week (excluding load reduction weeks), subjects were asked ‘How much the training has strained your week on a scale of 0–10?’, and the training time was increased more with lower exertion (see Supplementary Digital Content 1). For those finishing the study, adherence rate in training sessions was 98.4%. The weekly training intensity distribution is shown in Figure 2 and training realization in Table 2.