Durability is improved by both low and high intensity endurance training

Introduction: This is one of the first intervention studies to examine how low- (LIT) and high-intensity endurance training (HIT) affect durability, defined as ‘time of onset and magnitude of deterioration in physiological-profiling characteristics over time during prolonged exercise’. Methods: Sedentary and recreationally active men (n = 16) and women (n = 19) completed either LIT (average weekly training time 6.8 ± 0.7 h) or HIT (1.6 ± 0.2 h) cycling for 10 weeks. Durability was analyzed before and after the training period from three factors during 3-h cycling at 48% of pretraining maximal oxygen uptake (VO2max): 1) by the magnitude and 2) onset of drifts (i.e. gradual change in energy expenditure, heart rate, rate of perceived exertion, ventilation, left ventricular ejection time, and stroke volume), 3) by the ‘physiological strain’, defined to be the absolute responses of heart rate and its variability, lactate, and rate of perceived exertion. Results: When all three factors were averaged the durability was improved similarly (time x group p = 0.42) in both groups (LIT: p = 0.03, g = 0.49; HIT: p = 0.01, g = 0.62). In the LIT group, magnitude of average of drifts and their onset did not reach statistically significance level of p < 0.05 (magnitude: 7.7 ± 6.8% vs. 6.3 ± 6.0%, p = 0.09, g = 0.27; onset: 106 ± 57 min vs. 131 ± 59 min, p = 0.08, g = 0.58), while averaged physiological strain improved (p = 0.01, g = 0.60). In HIT, both magnitude and onset decreased (magnitude: 8.8 ± 7.9% vs. 5.4 ± 6.7%, p = 0.03, g = 0.49; onset: 108 ± 54 min vs. 137 ± 57 min, p = 0.03, g = 0.61), and physiological strain improved (p = 0.005, g = 0.78). VO2max increased only after HIT (time x group p < 0.001, g = 1.51). Conclusion: Durability improved similarly by both LIT and HIT based on reduced physiological drifts, their postponed onsets, and changes in physiological strain. Despite durability enhanced among untrained people, a 10-week intervention did not alter drifts and their onsets in a large amount, even though it attenuated physiological strain.


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Training details There were two groups: low intensity training (LIT) and high intensity training (HIT) groups for which the subjects were randomly divided. Both groups had 10 training weeks. During week 6, a follow-up VO2max test was done to check training intensities, and weeks 3 and 7 were load reduction weeks for enhancing recovery.
There were pre-scheduled, 22-line progression table for training for LIT (Supplementary Table 3) and HIT (Table 4) groups with equal calculated training loads, in an ascending order (see Supplementary Figure 1). Each subject started with progression line 1. After each training week (excluding load reduction weeks), subjects were asked 'How much the training has strained your week in the scale 0-10?', and 10-scale RPE- Weekly RPE Progression 0 -2 + 4 3 -4 + 3 5 -6 + 2 7 -8 + 1 9 -10 0 (the same week was repeated) Training was monitored by distributing cycling power output to five zones, and for each zone a weighting factor was linked (Cejuela-Anta and Esteve-Lanao, 2011) (Supplementary Table 2). Training load was calculated multiplying the factor by the time spend in zone. The subjects were encouraged to consume enough carbohydrates during the intervention.

Equalizing training
Although training scheduling table was equalized by training load calculation, the realization of training loads were 88 (14) in LIT and 55 (8) in HIT (p < 0.001, g = 3.0). There is no clear consensus how the training loads between LIT and HIT should be equalized (Normand-Gravier et al., 2022;Passfield et al., 2022). We used individualized training load progression, where the training load was equalized individually so that subjects would have similar training load relative to their maximum tolerable level. All in all, subjects in the LIT group felt training easier and thus progressed at faster pace than subjects in the HIT group.

Training in LIT group
In Supplementary Table 3 below 22-line scheduled progression table for LIT group is given. After the first training week (which was the first line in the progression table) the progression was individualized moving forward 0-4 lines based on the weekly RPE (Supplementary Table 1). Five (out from 16) subjects progressed to the last progression line 22.
LIT group trained outdoors with their own bikes. A possibility for indoors cycling with trainer or Wattbike Trainer (Wattbike Ltd., Nottingham, UK) were given. Three (out from 16) subjects did their training completely indoors, and the others did 3 % of their training indoors.
Training power in LIT group was solely below first lactate threshold (LT1), and the training power was monitored and feedback given weekly. Especially, if steep hills were not possible to cycle below LT1 -power, subjects were adviced to walk up the hills.

Training in HIT group
In Supplementary Table 4  HIT group trained indoors with Wattbike Trainer (Wattbike Ltd., Nottingham, UK) or trainer with their own bicycles with Rally RK200 dual-sensing power meters. Each session started with 10 minutes warm up (< 60 W) after which the prescribed intervals were made. The exercise ended in 10 minutes cool down (<60 W).
In the interval section, the work intensity was initially 110 % of second threshold power (LT2) (±15 ), and recovery intensity was < 60 W. The length of recovery interval was ¾ of the work interval. The training power, heart rate, and RPE were monitored, and feedback given weekly. If all training during the week had RPE ≤ 6 and HR did not rise above LT2 -threshold, then the power of work intervals was increased by 10 %. In the lines 15-22 of the scheduled training progression table, the intensity of the work interval was "maximal sustainable effort".
During training week 6, a follow-up VO2max -test was done, which replaced the interval exercise from that week with the lowest training load.