Muscular and neuromuscular control following soccer-specific exercise in male youth: changes in injury risk mechanisms

Key points

1. 01Reactive strength index and leg stiffness both dropped significantly (p < 0.001) after the soccer fatigue protocol
2. 02Muscle activation was selectively impaired in medial thigh muscles (rectus femoris, vastus medialis, semitendinosus) but not lateral muscles (vastus lateralis, biceps femoris)
3. 03Functional hamstring-to-quadriceps ratio was unchanged at all velocities and joint angles tested
4. 04Reduced leg stiffness implies increased ground contact time, greater centre-of-mass displacement, and greater shear force at the knee - all linked to ACL injury risk
5. 05This is the first study to document these combined neuromuscular deficits in fatigued male youth soccer players

How it was conducted

Design

Pre-post experimental study using a simulated soccer fatigue protocol (SAFT)

Participants

18 elite male youth soccer players; mean age 14.4 +/- 0.5 years, maturity offset +0.86 years (post-PHV)

Fatigue protocol

Soccer Activity Field Test (SAFT) validated to replicate Premier League match-play fatigue; duration matched Czech FA youth match rules

Outcome measures

Reactive strength index (drop jump), leg stiffness (submaximal hopping at 2.5 Hz), isokinetic dynamometry with surface EMG for five thigh muscles, functional H/Q ratio

Analysis

Wilcoxon matched-pairs tests; effect sizes reported as Cohen's d; significance set at p < 0.05

What they found

• RSI decreased from 1.57 +/- 0.42 pre to 1.42 +/- 0.32 post (p < 0.001; ES = 0.40)
• Absolute leg stiffness decreased from 24.4 +/- 4.7 to 21.7 +/- 5.16 kN/m (p < 0.001; ES = 0.55)
• Relative leg stiffness decreased from 34.6 +/- 5.3 to 30.7 +/- 6.1 kN/m (p < 0.001; ES = 0.68)
• Statistically significant muscle activation decreases were observed in 9 of the measured variables, primarily in medial muscles; effect sizes ranged from 0.33 to 0.97
• Vastus medialis activation at 60 deg/s: pre 0.50 +/- 0.19, post 0.44 +/- 0.17 (ES = 0.33, p < 0.05); rectus femoris at 60 deg/s: pre 0.47 +/- 0.17, post 0.38 +/- 0.10 (ES = 0.64, p < 0.05)
• No statistically significant changes in functional H/Q ratio at any velocity (60, 120, 180 deg/s) or joint angle; effect sizes small (range 0.04 to 0.46)

Limitations

• Small sample (n = 18) from a single Czech club limits generalisability
• Hip was tested in a flexed, non-functional position due to dynamometer constraints, not the position typical of injury
• SAFT excludes kicking actions, so localized medial fatigue may be underestimated in real match conditions
• No direct measurement of hip abductor or gluteal muscles, preventing full kinetic-chain interpretation

Why it matters

For patients

Youth soccer players are at heightened risk of knee and ACL injury toward the end of a match when fatigue compromises their leg stiffness and muscle coordination, even if their strength balance appears intact.

For clinicians

Targeting neuromuscular robustness and stretch-shortening cycle training early in adolescent athletic development may reduce the injury-risk window identified here; the H/Q ratio alone is insufficient to assess fatigue-related risk.

For readers

This study provides evidence that standard strength-balance measures miss important fatigue-induced deficits in young athletes; future prevention programs should include tests of reactive strength and leg stiffness.