## 4 Discussion
A second mechanism to explain an improvement in muscle strength, without an increase in muscle fiber CSA, suggests that the force generated by each cross-bridge would increase [45]. However, these mechanisms are difficult to investigate on a microscopic level. At baseline, we demonstrated that biopsy samples of the paraspinal muscles contain less glycolytic muscle fibers compared to healthy controls [14]. This suggests that a 12-week combined training program, consisting of a cardiorespiratory, general full body resistance and core muscle training protocol at high intensity could counteract the muscle fiber shift that was observed in NSCLC, at least for the MF. Although [PERSON] et al. [46] stated that the MF and ES were activated similarly during core muscle exercises, [PERSON] et al. [47] demonstrated higher activation of the MF compared to the ES during core muscle training. In addition, in our core muscle training protocol, participants were educated in the first sessions on isolated activation of the MF to minimize compensatory muscle work during the exercises. This might have resulted in a higher activation of the MF compared to the ES, which warrants the conclusion that muscle fiber adaptations in response to HIT would probably be more apparent in the MF. Interrelationships between muscle fiber alterations and clinical improvements after training  
Participants of this randomized controlled trial showed improvements in disability, exercise capacity and pain in both intervention groups. The improvements in disability and exercise capacity were greater in participants in the HIT group compared to the participants in the MIT group [24]. This implies that rehabilitation of participants with NSCLBP would reduce pain and disability, with a greater effect for rehabilitation at high intensity. Despite these clinical improvements, we could only demonstrate a muscle fiber shift towards a more glycolytic (type II) profile in the MF in response to HIT. As training at moderate intensity provided lower effect sizes for clinical outcomes [24] and did not result in changes in muscle fiber characteristics, the physiological effects of the increased intensity could have caused the increased effectiveness. However, no other changes in muscle morphology, such as muscle fiber hypertrophy, could be observed. A possible explanation for the lack of changes in muscle fiber CSA could be the fact that participants in our study were recruited by local advertisement. This type of advertisement may have attracted a group of participants who are more than average physically active in daily life, which might correlate with the fact that we could not observe a difference in muscle fiber CSA between healthy controls and persons with NSCLBP [14]. However, this hypothesis cannot be substantiated, because physical activity was not measured. The clinical improvements in the HIT group of our previous study (within the same cohort) exceeded clinically relevant cutoff values, which underlines the clinical importance of treatment effects [24]. It is unknown if these positive effects of HIT remain after cessation of the training; therefore, it is important that future studies address the long-term effects of HIT. Hornone levelsOur study demonstrated no changes in IGF-1 serum levels after a 12-week training protocol, while there was a decrease in GH after a training period at high intensity. Several studies have found that serum GH levels increase acutely after a single exercise bout [48; 49; 50]. However, the effect of a longer training period on GH serum levels is not very clear. Traditionally, no long-term changes in resting GH after a training period are reported [51; 52; 53; 54; 55]. However, GH serum levels were significantly elevated immediately after exercise, but returned to normal levels one to two hours after exercise [30]. The results of the present study must be interpreted carefully, since we only measured resting GHserum levels. Most studies have measured at multiple timepoint and investigated the GH response to training.