Effects of Different Sled Loads on Sprint and Neuromuscular Performance in Young Male Football Players: An 8-Week Periodized Field Study

Background and Rationale:

Sprint performance, acceleration capacity, and change of direction speed are key determinants of athletic success in association football. The age period of 13-14 years represents a "critical window of opportunity" characterized by rapid adaptations in neuromuscular control, motor unit recruitment, and myelination. Resisted sprint methods, particularly sled towing, allow athletes to overload horizontal force production at sprint-specific joint angles without altering overall running kinematics. However, there is a methodological gap in the literature regarding the optimal and safe acute load prescription (%BW) for pediatric and youth populations.

Study Design and Methodology:

This study is an 8-week, parallel-group randomized controlled field trial involving 30 licensed male youth football players. Participants are randomly allocated into three homogeneous intervention groups using a stratified block randomization method based on baseline 15-meter sprint performance:

Light Load Group: Sled towing with 10% Body Weight (BW)

Moderate Load Group: Sled towing with 15% Body Weight (BW)

Heavy Load Group: Sled towing with 20% Body Weight (BW)

Training Protocol:

Sled sprint training sessions will be conducted twice per week (minimum 48 hours of recovery between sessions) over 8 weeks (16 total sessions) at the beginning of regular football practice, immediately following a standardized 15-18 minute warm-up protocol. Running distance (15 meters per repetition), inter-repetition rest (2 minutes), and inter-set rest (3 minutes) will be held constant across all groups. Training progression will be achieved through gradual volume increases:

Weeks 1-2: 2 sets × 3 reps (90 m/session)

Weeks 3-4: 2 sets × 4 reps (120 m/session)

Weeks 5-6: 3 sets × 3 reps (135 m/session)

Weeks 7-8: 3 sets × 4 reps (180 m/session)

Outcome Assessments:

Baseline (pre-test) and post-intervention (post-test) evaluations will be conducted on non-fatigued days. Primary outcome measures include:

Sprint Performance: Linear sprint times over 5m, 10m, and 15m measured using a high-precision wireless photocell timing system.

Neuromuscular Performance: Vertical jump heights (cm) assessed via Squat Jump (SJ) and Countermovement Jump (CMJ) tests on a digital contact mat/force platform.

Reactive Strength: Drop Jump (DJ) test from a 30 cm box to calculate the Reactive Strength Index (RSI = jump height / contact time).

• Resisted Sled Sprint Training (Other), also known as Sled Towing: An 8-week periodized resisted sprint training protocol using weighted sleds. Sessions are performed twice weekly over a 15-meter sprint distance, immediately following a standardized warm-up. Participants are assigned to sled towing loads equivalent to 10%, 15%, or 20% of their individual body weight (BW). Training volume progresses systematically from 2 sets of 3 repetitions (Weeks 1-2) to 3 sets of 4 repetitions (Weeks 7-8) with 2 minutes of inter-repetition rest and 3 minutes of inter-set rest.

Primary outcomes

• Linear Sprint Performance (5m, 10m, and 15m Sprint Times) [Baseline (Pre-test) and 8 weeks (Post-test)]: Linear sprint performance measured over 5 meters (acceleration), 10 meters, and 15 meters using a high-precision wireless photocell timing system. Times are measured in seconds (s), where lower values indicate better sprint performance.
• Squat Jump (SJ) Height [Baseline (Pre-test) and 8 weeks (Post-test)]: Lower-limb concentric explosive power output measured via Squat Jump test on a digital contact mat/force platform. Surcharge from arm swing is isolated by keeping hands on hips. Jump height is measured in centimeters (cm), where higher values indicate greater explosive power output.
• Countermovement Jump (CMJ) Height [Baseline (Pre-test) and 8 weeks (Post-test)]: Stretch-shortening cycle (SSC) performance and lower-limb reactive power measured via Countermovement Jump test on a digital contact mat/force platform. Jump height is recorded in centimeters (cm), where higher values represent superior vertical jump capacity.
• Reactive Strength Index (RSI) [Baseline (Pre-test) and 8 weeks (Post-test)]: Reactive strength capacity and neuromuscular stiffness assessed via Drop Jump (DJ) test from a 30 cm box. Calculated as jump height (meters) divided by ground contact time (seconds) \[RSI = Jump Height (m) / Contact Time (s)\]. Higher index scores reflect better reactive strength and shorter contact time.

Sex: Male, ages: 13 Years to 14 Years, accepts healthy volunteers

Inclusion Criteria:

* Male football players aged chronologically between 13 and 14 years.
* Minimum of 2 years of active, licensed competitive football experience in a club or academy.
* Regular attendance at scheduled team training sessions.
* Written Informed Consent/Assent signed by both the participant (Child Assent Form) and parent/legal guardian (Parental Consent Form).
* Free of severe lower-limb musculoskeletal injuries (e.g., ligament tears, fractures, meniscus injuries) in the past 6 months.

Exclusion Criteria:

* Chronological age <13 or >14 years.
* Presence of chronic, cardiovascular, metabolic, or respiratory conditions (e.g., asthma) that pose risks during high-intensity maximal exercise.
* Regular use of performance-enhancing supplements or medications affecting physiological and neuromuscular responses.
* Absence from more than 10% of prescribed sled training sessions (2 or more sessions out of 16).
* Failure to complete pre- or post-intervention performance testing batteries, or failure to demonstrate maximal effort during testing.
* Concurrent participation in external individual strength, speed, or fitness programs outside the football academy.

• Muhammed Z Kahraman, PhD, Principal investigator, Bitlis Eren University