Author(s): PSARRAS, I., PAÏZIS, C., TERZIS, G., ZAVVOS, C.A., LEKATSAS, N., STIMOS, I., BOGDANIS, C.G., Institution: NATIONAL & KAPODISTRIAN UNIVERSITY OF ATHENS, Country: GREECE, Abstract-ID: 633

INTRODUCTION:
The interference effect is a well-known concern among practitioners due to its potential negative impact on athletic performance. Although concurrent training does not appear to impair muscle hypertrophy, its effects on strength and power adaptations remain debated. Given that concurrent training is an integral component of the in-season weekly schedule in team sports, the purpose of this study was to compare an optimally structured concurrent training approach with a strength-only approach on key muscle power performance metrics in elite youth soccer players during the in-season period.
METHODS:
Forty-eight elite youth soccer players were allocated to concurrent training (CG, n=16), strength-only training (SG, n=16) or control groups (CONTR, n=16). The SG performed an in-season individualized strength training program once per week for six weeks. Load and exercise selection was based on players individual force–velocity profiles, aiming to reduce force–velocity imbalance and enhance muscle power performance. The CG followed the same strength program but additionally performed either a high-intensity interval session (2 × 5 min with 15 s running and 15 s passive recovery at 10% of anaerobic speed reserve) or sprint interval training (six all-out sprints over 30–50 m) immediately after strength training. The CONTR continued their regular strength, power and endurance training programs. GPS data were collected only in experimental groups.
RESULTS:
Following the intervention, both experimental groups demonstrated similar improvements in neuromuscular performance, whereas CONTR showed non-significant changes. Countermovement jump height increased by 9.2% in the CG and 6.9% in the SG (both p < 0.001). Isometric force during the squat exercise (knee angle: 125º – 135°) at 100 ms increased significantly in both the CG and SG (20% and 23%, p < 0.001), while no changes were observed in the CONTR. Also, sprint performance over 10 m improved by 1.9% in the CG (p < 0.01) and SG by 1.5% (p = 0.03) with no changes in CONTR. Aerobic fitness improved only in the CG, as shown by the reduction of heart rate responses during the submaximal Yo-Yo Intermittent Recovery Level 1 test (−10.2%; p < 0.001), compared with the SG (−2.7%; p < 0.02). Total weekly training distance was similar in both experimental groups (p > 0.05); however, the CG covered 17.4% greater high-speed running distance compared with the SG (p < 0.01).
CONCLUSION:
Concurrent training does not induce an interference effect on muscle power–related performance and acceleration in elite youth soccer players. The inclusion of low-volume, high-intensity endurance training improves aerobic performance, as reflected by heart rate responses during the submaximal YYIR1 test, without compromising neuromuscular development when strength training was individualized based on force–velocity imbalance.