###### Abstract  
Low back pain is a prevalent musculoskeletal disorder with high disability. Although exercise therapy is an important part of the multidisciplinary treatment of non-specific chronic low back pain (NSCLBP), the impact of exercise therapy on muscle morphology of the paraspinal muscles is largely unknown. The main aim of this study is to evaluate the effects of a high intensity training (HIT) program on muscle fiber characteristics of the multifidus (MF) and the erector spinea (ES) musculature in persons with NSCLBP and to compare these results with the effects of a moderate intensity training (MIT) program, which is used as a control group. In total, 38 participants with NSCLBP were randomly assigned to a HIT or MIT intervention group. Participants performed a 12-week training program. At baseline and after the training program, biopsy samples from the lumbar MF and ES were obtained. Muscle fiber characteristics were analyzed by immunofluorescence. For the ES, no significant changes were found in muscle fiber CSA and distribution of muscle fiber types in response to a training program at moderate or at high intensity. For the MF, a significant higher muscle area occupied by type II muscle fibers, referred to as the relative cross-sectional area (RCSA), was found in the HIT group. Based on muscle fiber characteristics in biopsy samples, a HIT program of 12 weeks seems to result in a more glycolytic profile in the MF. multifidus; erector spinea; low back pain; high intensity training; muscle biopsy; muscle fiber characteristics +
Footnote †: journal:creased muscle strength and self-assessed functional disability [6; 7; 8; 9; 10; 11]. Muscle dysfunction is often the result of muscle degeneration which is characterized by two main macroscopic signs: muscle atrophy and fatty infiltration. Several studies have documented a smaller anatomical cross-sectional area (CSA) [6; 8; 9] and an increase in fat content [6; 11] of the paraspinal muscles. Morphological changes to the paraspinal muscles can impair spinal stability which might contribute to causing/maintaining NSCLBP [12]. Evidence on muscle fiber characteristics at a microscopic level in low back pain is scarce and conflicting. According to [PERSON] et al., 2004, there were no differences in muscle fiber type distribution between persons with NSCLBP and healthy controls (HC) [13], while [PERSON] et al., 2020 showed that persons with NSCLBP have a more oxidative muscle fiber type profile compared to HC [14]. An important feature that affects muscle morphology and muscle (dys)function is exercise training [15]. Surprisingly and although exercise therapy plays an important role in the multidisciplinary treatment of NSCLBP, the overall treatment effects remain low [16]. Several training methods are used in the treatment of NSCLBP, including training protocols that aim to recruit the lumbar paraspinal muscles by using core stability and endurance training. These exercise therapies focus on strengthening and decreasing the fatigability of these muscles [17; 18; 19]. Since slow-contracting muscle fibers are more resistant to fatigue, exercise training to decrease fatiguability might result in a more oxidative and slow-contracting muscle fiber profile [20; 21; 22]. However, [PERSON] et al., 2020 [14] showed that paraspinal muscles of persons with NSCLBP are represented with a more oxidative and slow-contracting profile. Furthermore, it has been shown that exercise therapy at low or moderate intensity in NSCLBP rehabilitation programs could be insufficient to have therapeutic effects [16; 23], while a high intensity training (HIT) program consisting of cardiorespiratory interval, general resistance and core muscle training has been shown to have greater improvements on reducing disability and increasing exercise capacity [24]. This improvement in disability and increased exercise capacity might be the result of morphological changes in the paraspinal muscles.