Velocity Loss During Resistance Training: Implications for Concurrent Training Adaptations.

All concurrent training groups significantly increased muscle cross-sectional area, with VL40 achieving the greatest hypertrophic gains.

• Forty-one moderately trained men were randomly assigned to VL0 (n=10), VL15 (n=10), VL40 (n=11), or ET alone (n=10) groups over 8 weeks.
• Group × time interaction was significant (p < 0.05) for vastus lateralis cross-sectional area.
• The endurance-only group (ET) showed no changes in muscle mass.
• Higher velocity loss thresholds during resistance training produced greater hypertrophic outcomes, with VL40 achieving the greatest gains.

Maximal aerobic speed (MAS) improved in all groups, with ET achieving the greatest gains, and within concurrent training groups, lower velocity loss was associated with higher effect sizes for MAS improvement.

• MAS improved significantly across all groups (p < 0.001).
• Group × time interaction for MAS was significant (p = 0.04).
• ET showed the greatest MAS gains compared to all concurrent training groups.
• Within CT groups, the pattern was: lower VL threshold corresponded to higher effect size for MAS improvement, suggesting greater RT-induced fatigue attenuated aerobic adaptations.

VL15 and VL40 concurrent training groups achieved greater one-repetition maximum (1RM) gains than the endurance-only group.

• Group × time interaction for 1RM was significant (p = 0.009).
• VL15 and VL40 both significantly improved 1RM strength.
• ET showed no strength gains.
• VL0 did not reach significance compared to ET for 1RM gains.

Endurance-only training significantly reduced the rate of force development at 400 ms.

• ET showed a significant reduction in rate of force development at 400 ms (p = 0.01).
• ET showed no strength gains across any strength-related variables.
• This finding suggests that endurance training alone impairs neuromuscular force production capacity.

VL0 increased EMG amplitude across loads during squat tests, while ET reduced EMG amplitude.

• Group × time interactions for EMG amplitude were significant (p < 0.05).
• VL0 (0% velocity loss, minimal fatigue) increased electromyographic amplitude across loads.
• ET reduced EMG amplitude, indicating impaired neuromuscular activation following endurance-only training.
• EMG was assessed during progressive loading squat tests.

VL15 and VL40 significantly improved multiple strength-related variables beyond 1RM, while ET showed no improvements in any strength measure.

• Strength-related variables assessed included maximal isometric squat, progressive loading squat performance, countermovement jump, and sprinting.
• VL15 and VL40 showed significant improvements in strength-related variables.
• ET failed to improve any strength or neuromuscular performance outcome.
• VL0 showed more limited strength adaptations compared to higher VL thresholds.

The concurrent training protocol involved squat-based resistance training followed by high-intensity interval running, separated by only 10 minutes, performed twice per week for 8 weeks.

• Resistance training was performed at 70%–85% of one-repetition maximum.
• Endurance training consisted of running at 90%–105% of maximal aerobic speed, decreasing from 18 to 8 minutes of total volume across the 8-week period.
• The two training modalities within each concurrent session were separated by only 10 minutes.
• Training frequency was two sessions per week for all groups.

Greater velocity loss thresholds during resistance training within concurrent training attenuated endurance adaptations relative to lower velocity loss thresholds.

• Within CT groups, VL0 showed the highest effect size for MAS improvement and VL40 the lowest.
• Group × time interaction for MAS was p = 0.04, confirming differential endurance adaptation based on VL threshold.
• This pattern suggests that higher neuromuscular fatigue accumulated during resistance training (higher VL) interferes with subsequent endurance adaptation.
• The finding supports the concept that fatigue induced during RT attenuates endurance adaptations in concurrent training.