Background In cross-country mountain biking (MTB), performance is primarily determined by aerobic capacity. However, shortened race durations from 150 to 80 min increased the relevance of anaerobic capacity, muscle strength, and power. Adequate levels of trunk muscle strength (TMS) enable force transfer between lower and upper extremities during starts, inclines, and finishes. Cycling economy (CE) may benefit from TMS training. This study examined the effects of TMS training on TMS, TMS endurance, and CE in MTB athletes. Methods Twenty-four trained or highly-trained female (Tier 2-3) MTB athletes aged 14-22 years were pair-matched by age and randomly assigned to a TMS group or an active control. Control exercised lower limb but not trunk muscles. Over eight weeks during the off-season, both groups completed three weekly 30-min sessions. Pre-, post training, assessments included maximal isometric TMS, lateral trunk endurance, and CE. Hormonal measures were included to account for menstrual cycle-related variability. CE was assessed during an MTB race course treadmill simulation via physiological (O2/CO2·min?¹·kg?¹) and CE-related mechanical parameters. Results TMS training improved maximal trunk flexor (p<0.001, d = 1.1), extensor strength (p<0.001, d = 1.7), and lateral TMS endurance (p=0.03, d = 0.74), compared with controls. Lateral bike displacement was reduced in the TMS group (p=0.001, d = 0.8). Physiological CE parameters showed no between-group differences. Hormonal measures did not moderate effects (p>0.05). Conclusions Eight weeks of global TMS training improved maximal isometric trunk flexor, extensor strength and lateral bike displacement in female MTB athletes. These findings suggest that off-season TMS training may improve MTB performance. Key Points - This study found improvements in maximal isometric trunk muscle flexor and extensor strength, lateral trunk muscle endurance, and mechanical factors affecting cycling economy, reflected in reduced lateral bike displacement after 8 weeks of trunk muscle strength training, compared to traditional strength training that did not target the trunk muscles. - Trunk muscle strength training is an effective training modality that can be implemented during the off-season to improve performance related factors in female mountain bikers.
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