## 2 Materials and Methods
### Subjects
The experimental group (hereafter referred to as HIT-group) performed all exercises at high intensity. The cardiorespiratory training consisted of an interval training protocol on a cycle ergometer. The interval training consisted of five 1-min bouts (110 repetitions per minute at 100% \(\text{VO}_{\text{2 max}}\) workload), with 1 min of active rest (75 repetitions per minute at 50% \(\text{VO}_{\text{2 max}}\) workload) in between. The high intensity cycling bouts increased every two sessions by 10 s, up to 1 min 50 s after 12 sessions. The active recovery time remained constant at 1 min. The protocol was repeated from session 13 to 24 but with an updated workload. The general resistance training consisted of three upper body and three lower body exercises executed on fitness equipment. In the first sessions, the exercises were explained by the physiotherapist and a one-repetition maximum (1 RM) test was performed. In the following sessions, one set of a maximum of 12 repetitions were performed at 80% of 1 RM for each exercise. The workload was progressively increased to ensure a HIT. Core muscle training consisted of six core exercises. In the first sessions, the exercises were explained and participants were educated to activate specific or isolated core muscles (m. multifidus, mm. glutei, m. transversus abdominis). In the following sessions, participants performed one set of 10 repetitions of a 10 s static hold. The participants were encouraged to hold the last repetition as long as possible. Exercise difficulty was progressively increased. The control group (hereafter referred to as MIT-group) performed all exercises at moderate intensity. The cardiorespiratory training consisted of a continuous training protocol on a cycle ergometer. The training program started with 14 min of cycling at continuous workload (90 repetitions per minute at 60% \(\text{VO}_{\text{2 max}}\) workload). Duration increased every two sessions with 1 min 50 s up to 22 min 40 s after 12 sessions. The protocol was repeated from session 13 to 24 but with an updated workload. General resistance exercises and core muscle training were identical to the experimental group, except for the intensity. For the general resistance training, one set of 15 repetitions were executed at 60% of 1 RM. For the core muscle training, one set of 10 repetitions of a 10 s static hold were done. Exercises remained the same; only the time of a static hold was increased each six sessions. Primary outcomes:  
Biopsy sampling  
Muscle samples were obtained from the right erector spinae (ES) and multifidus (MF) muscle at the level of spinous process vertebra L4 according to the procedure of [PERSON] et al., 2018 [36] in the week before the start of the intervention and in the week after the end of the intervention. These muscle samples were obtained while the participant was in prone position with a small amount of lumbar flexion. First, the puncture site was determined by ultrasound as the point of separation between ES and MF. This site was locally anesthetized (Xylocaine 1% ()) before the skin was incised over a length of 2 mm. The thoracolumbar fascia was pierced with a coaxial needle, through which the biopsy needles were inserted. Afterwards, a muscle sample of the ES was obtained by a biopsy needle that was directed laterally. A muscle sample of the MF was obtained by a second biopsy needle that was directed medically. Both muscle samples were removed from the needle and placed on a piece of cork. Samples covered with optimum-cutting temperature compound (Tissue-Tek) were frozen in precooled isopentane and stored in \(-80\)\({}^{\circ}\)C until further analysis. All samples were given a unique identification code.