Author: Antonio Alonso et al
Journal: Sports Health: A Multidisciplinary Approach (2025)

AI written summary

This pilot study investigated how different resisted sprint training (RST) loads: 0% (control), 20% (light load), and 80% body mass (heavy load) — affect sprint performance and the force–velocity profile (FVP) in professional male and female football players and referees who had no prior RST experience.

Participants & Protocol

• 50 participants: U19 male elite players, professional female players, and male referees.
• 6-week intervention: two RST sessions per week, each consisting of 4 × 30 m maximal sprints with assigned load.
• Pre–post testing via 1080 Sprint to capture full sprint mechanics and FVP variables.

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Key Findings

1. Heavy loads (80% body mass) produced the largest and broadest performance gains

The HL group showed clear improvements across the entire sprint profile:

• Faster 30 m sprint times and all 10 m splits (0–10 m, 10–20 m, 20–30 m).
• Increased maximal velocity (Vmax) and theoretical maximal velocity (V0).
• Increased theoretical maximal force (F0) and maximal power (Pmax).
• Higher force and power outputs across sprint segments (see Figure 2 on p.5).
• Improved force application at all velocities, visualized in the FVP shift on p.7.

Overall, heavy loads improved both ends of the FVP:

• Acceleration force capacity (F0)
• Maximal velocity qualities (V0, Vmax)

This contradicts traditional advice that heavy loads only benefit acceleration.

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2. Light loads (20% body mass) gave smaller, more specific benefits

LL training led to:

• Improvements mainly in mid-to-late sprint segments (10–20 m and 20–30 m).
• Small increases in V0.

But LL did not improve force production variables (Fpeak, Favg) across segments.

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3. Control group (0% load) improved slightly only in F0

Likely due to:

• Regular football training demands (accelerations, decelerations).
• The additional unresisted sprinting volume during interventions.

No improvements were observed in sprint times or velocity variables.

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Interpretation

Heavy RST (80% BM) is optimal for athletes new to resisted sprint training

Because these athletes likely have:

• Underdeveloped maximal horizontal force capabilities
• Limited exposure to heavy external loading

HL RST seems to provide the strongest stimulus for neuromuscular improvement and mechanical efficiency, improving both:

• Acceleration, and
• Maximal sprinting speed

This contrasts with older recommendations where:

• LL was preferred for max-velocity development
• HL was recommended only for acceleration

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Practical Recommendations

For football players and referees without prior RST experience:

• Use ~80% body mass resisted loads.
• Perform 2 sessions per week, 4 × 30 m maximal sprints.
• Expect improvements in:
  • Acceleration
  • Maximum speed
  • Force–velocity profile
  • Overall sprint performance

The study demonstrates that heavy-load RST is safe and highly effective, including for professional female football players, expanding the applicability of these findings.

AI comparison with existing FVP / resisted sprint training (RST) literature in women’s football

How this study is unique

This is the first RST study using very heavy resisted loads (80% body mass) that includes professional female football players, alongside male players and referees, and reports complete force–velocity profile (FVP) data. alonso-callejo-et-al-2025-first…

Most previous very-heavy sled studies were performed only with male athletes (professional or amateur), and none included female professionals in a mixed design.

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What we already knew from previous (mostly male) RST and FVP studies

1. Very heavy loads (~80% body mass)

Earlier literature in male footballers shows that heavy or very-heavy sled loads:

• Increase horizontal force production (F₀)
• Improve the acceleration phase (0–10 m)
• Shift the force–velocity profile by increasing force at low velocities
• Do not necessarily disrupt sprint mechanics when coached well

Biomechanics research also indicates that athletes can maintain key technical patterns even under heavy loading if posture and intent are coached.

2. Systematic reviews

• Petrakos et al. (2016) concluded that RST with 12–43% BM improves sprint performance, but evidence for very heavy loads was scarce and coaches were cautious due to potential technical disruption.
• Ward et al. (2024) found RST to be consistently effective for acceleration in team sports but that the optimal load remained unclear; most included studies involved male athletes.

3. What exists for women’s football specifically

Only a handful of sprint training studies focus on female footballers:

• Upton (2011): Combined assisted + resisted sprint training improved acceleration and max velocity in NCAA female soccer athletes more than unassisted sprinting.
  → No FVP data, and no very-heavy sled loads.

Most other female-focused work centers on plyometrics, gym-based strength, or general sprint training—not FVP-guided resisted sprinting.

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What Alonso-Callejo et al. (2025) adds—specifically for women

This study shows that professional female players respond very similarly to male players when exposed to 80% BM resisted sprint training, even as complete RST novices. Key findings:

• Heavy-load RST improves both ends of the FVP in women:
  • ↑ F₀ (horizontal force capacity)
  • ↑ V₀ and Vmax (maximal speed qualities)
  • ↑ Pmax (maximal power)
• Sprint times improve across all splits (0–10, 10–20, 20–30 m)
• Light loads (20% BM) do not produce the same global gains
  alonso-callejo-et-al-2025-first…

The authors note that prior research shows men and women have comparable mechanical responses to increasing sled loads (e.g., constant F₀ and Pmax across loads, linear reduction in V₀), suggesting that sex does not fundamentally change the load–response relationship when training structure and context are equal. alonso-callejo-et-al-2025-first…

Implication

The traditional caution (“avoid very heavy sled loads in female players”) is not supported by these findings. When well-coached:

Heavy-load RST (~80% BM) is safe, effective, and highly beneficial for professional female football players.

This is a major addition to the applied sprint literature in women’s football.

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Practical implications for women’s football practitioners

• Heavy-load RST offers a field-based alternative to gym strength training for teams with limited S&C facilities.
• The approach can simultaneously target acceleration, maximum speed, and FVP development in female players.
• Future research should explore individualized FVP deficits (force-deficit vs velocity-deficit) in women, which has been explored heavily in male cohorts but not yet in professional women.