This study examines how the body's stress (sympathetic) nervous system responds to 8 weeks of either strength training alone or strength training combined with endurance (treadmill running) training in previously untrained young men. Skin sympathetic nerve activity, a noninvasive marker of sympathetic nervous system activity, is measured before and after training, along with muscle strength, jumping ability, and heart rate variability. The study investigates whether changes in sympathetic nervous system activity are associated with changes in strength and jumping ability, and whether this relationship differs between men who perform strength training alone versus those who combine strength and endurance training.

This study examined how the body's stress (sympathetic) nervous system responds to 8 weeks of either strength training alone or strength training combined with endurance (treadmill running) training in previously untrained young men. Skin sympathetic nerve activity, a noninvasive marker of sympathetic nervous system activity, was measured before and after training, along with muscle strength, jumping ability, and heart rate variability. The study found that in men who did strength training only, those whose sympathetic nervous system activity increased the most also gained the most strength and jumping ability. This relationship was not seen, and in some cases reversed, in men who combined strength training with endurance training. The findings suggest that adding endurance exercise changes how the nervous system adapts to a strength training program, alongside the well-documented reduction in strength gains seen with combined training (the "interference effect").

Inclusion Criteria:

• Healthy young men
• No hypertension or cardiovascular/vascular disease
• No migraine or neurodegenerative disease
• No smoking or alcohol use
• No prior structured exercise training
• No musculoskeletal injury
• Willing to abstain from caffeine, alcohol, and strenuous exercise for 24 hours before testing

Exclusion Criteria:

• Hypertension or cardiovascular/vascular disease
• Migraine or neurodegenerative disease
• Current smoking or alcohol use
• Prior structured exercise training history
• Musculoskeletal injury precluding resistance or endurance exercise

Individual participant data that underlie the results reported in this article will be made available upon reasonable request to the corresponding author.

Participants and trainers could not be blinded to group allocation given the nature of the exercise interventions. All autonomic, neuromuscular and reflex outcome data were acquired, processed and statistically analyzed by investigators blinded to group allocation, using coded, de-identified records until analyses were complete.