## 4 Discussion  
This pilot study evaluated changes in paraspinal muscle fiber characteristics in response to a combined training program consisting of a cardiorespiratory, general full body resistance and core muscle training protocol at different intensities (high intensity vs. moderate intensity). We hypothesized that there would be an increase in muscle fiber size combined with a shift towards a more glycolytic muscle profile in the ES and MF after HIT. However, our data demonstrate that biopsy samples of the MF of participants in the HIT group have a larger muscle area occupied by type II fibers (higher RCSA) in response to the training program. In contrast to our hypothesis, we found that on average, muscle biopsies of the ES and MF did not demonstrate increased muscle fiber CSA in both intervention groups. _Muscle fiber characteristics_  
Participants in both intervention groups of this randomized controlled trial showed improved back muscle strength, without differences between groups [24]. Despite this increased back muscle strength, our data could not demonstrate significant changes in muscle fiber CSA. According to our knowledge, there are only two studies that have investigated the effect of exercise training on muscle fiber characteristics in patients with low back pain: [PERSON] et al. [39], who focused on the ES, and [PERSON] et al. [40], who investigated the MF. Our results are in line with the study of [PERSON] et al., who also found no differences in the muscle fiber size of the ES [39] in response to different low to moderate intense exercise programs, although there was an increase in back muscle strength [41]. In contrast, [PERSON] et al. [40] found an increase in the size of type II muscle fibers of the MF after a three-month high intensity training. A possible explanation for these differences is that the type II muscle fibers were atrophic at baseline in the study of [PERSON] et al. [40], while in our study [14] and the study of [PERSON] et al. [39], the muscle fiber CSA was comparable to those of healthy controls. Since it is well known that atrophic muscle fibers are more prone to muscle fiber hypertrophy induced by exercise training, this could explain the increase in muscle fiber size of type II fibers in the study of [PERSON] et al. [40]. The increase in muscle strength could result from a higher proportional area occupied by type II muscle fibers, since these fibers produce greater force [42; 43]. [PERSON] et al. [40] found no differences in muscle fiber type distribution for the MF after a three-month training program at high intensity and [PERSON] et al. [39] found no differences in muscle fiber type distribution for the ES after low to moderated exercise programs. These results are in line with our results, since we could also not demonstrate significant changes in muscle fiber distribution. However, our results showed that a training program at high intensity resulted in an increase in RCSA of type II muscle fibers, while these changes were not found after a training program at moderate intensity. This increase in RCSA of type II fibers results in a shift towards a more glycolytic profile. During resistance training, motor units, and hence muscle fibers, are recruited according to the'size-principle.' Smaller motor units are associated with type I fibers, while larger motor units are associated with type II fibers. According to the'size-principle,' type I muscle fibers are recruited at lower intensity exercises and type IImuscle fibers at high intensity exercises [44]. Therefore, training at high intensity increases the muscle area occupied by type II fibers. This might explain why we could only find an increase in RCSA of type II after HIT. There are two other mechanisms that could contribute to an increased muscle strength while the muscle fiber size remains stable. First, there may be an increase in myofibrillar density. As such, the number of cross-bridges, acting in parallel to generate a contraction, increases independently of any increase in muscle fiber [45].