Impact of machine-based core muscle strengthening on back muscle function in chronic low back pain across age groups and sexes: Results of a prospective surface electromyographic fatigue study

Introduction: The impact of aging on core muscle training outcomes on chronic low back pain (CLBP) remains poorly understood. This study investigated whether a 6-month machine-based core muscle strengthening (MB-CMS) program would produce differential surface electromyography (SEMG) fatigue-related responses in younger versus older males and females with CLBP. Methods: Four cohorts with CLBP (younger males [n= 53; mean age 35.3 ± 9.6], younger females [n= 43 mean age 32.9 ±10.4]; older males [n= 48; mean age: 64.8 ± 8.4]; older females [n= 48, mean age: 64.3 ± 7.2]) completed a 6-month rehabilitation program with 40 MB-CMS sessions. SEMG monitoring was performed during back extensions at 80% of maximum effort from the multifidus (L5), longissimus (L2), and iliolumbalis lumborum (L1). The primary outcome was median frequency fatigue slope (MDF-SEMG), normalized to onset, measured bilaterally. Secondary outcomes included MDF-SEMG and root mean square (RMS)-SEMG at fatigue-test onset. A mixed-effects model analyzed SEMG changes across recording sites. Results: At baseline, older individuals had significantly higher Body Mass Index scores than younger ones [2.2 (95%CI:1.1;3.4)]. Maximum back extension strength and trunk flexibility were comparable at baseline and improved significantly, without relevant differences between age- and sex-specific groups [younger vs older: 1.4 (95%CI:-11.2;14.0); younger vs older males: 1.8 (95%CI:-18.4;22.0), younger vs older females: 2.1 (95%CI:-12.6;16.8)]. Initial fear avoidance behavior and pain rating scores were similar across groups and improved after the intervention in all groups except older females(younger males: -0.4 (95%CI:-0.6;-0.2); older males: -0.4 (95%CI:-0.6;-0.2); younger females: -0.4 (95%CI:-0.5;-0.2); older females: -0.1 (95%CI:-0.3;+0.1). Baseline endurance behavior was similar across groups and remained unchanged after intervention. Roland Morris Disability Questionnaire scores were higher in older than in younger male and female participants and improved significantly within groups, without notable differences in age and sex. After intervention, MDF-SEMG fatigue changed toward steeper slopes in younger males (L5, L2) and older females (L5), without significant age- or sex-related between-group effects. Pooled sites showed greater changes in younger individuals. RMS-SEMG initial values showed a significant age-group effect (p<0.001), with increases in younger participants, except L2 in females, but not in older ones. MDF-SEMG fatigue changes correlated positively with enhanced spine sagittal flexibility (p<0.001). Changes in MB-CMS-related RMS-SEMG initial values indicate greater increases in back extensor muscle excitability in younger people with CLBP than in older ones. Conclusions: MDF-SEMG fatigue findings remain ambiguous. Because age-related disuse rather than hypertrophic muscle alterations may have increased MDF-SEMG fatigue in multifidi, particularly in older females with CLBP, MB-CMS protocols may require adaptation to prevent spinal degeneration during long-term training in these patients. Disclosure: *R.H. and G.E. contributed equally to the study.